Reciprocating Internal

Combustion Engines

Michele Manno
Department of Industrial Engineering
University of Rome Tor Vergata
Last update: October 30th, 2015

Internal Combustion Engines

Contents
1. General remarks and engine classification
2. Main operating parameters
3. Air intake
4. Supercharging and turbocharging
5. Fuel metering in spark ignition engines
6. Fuel injection in compression ignition engines
7. Operating characteristics and performance maps
8. Load matching: torque and rotational speed requirements
9. Pollutant formation and control

Internal Combustion Engines

General remarks and engine classification

Engine components

Image sources:
(left) M. Ehsani, Y. Gao, S.E. Gay, A. Emadi, Modern Electric, Hybrid
Electric, and Fuel Cell Vehicles, CRC Press LLC, New York, 2005.
(right) J.B. Heywood, Internal Combustion Engine Fundamentals, McGrawHill, New York, 1988.

Internal Combustion Engines

General remarks and engine classification

Engine classification
1. Method of ignition
a) Spark Ignition, SI (Otto engines)
b) Compression Ignition, CI (Diesel engines)
2. Working cycle
a) full cycle in 4 piston strokes (four-stroke engine, 4S)
b) full cycle in 2 piston strokes (two-stroke engine, 2S)
3. Fuel
gasoline, fuel oil (diesel), natural gas, liquid petroleum gas (LPG), alcohols (methanol, ethanol)...
4. Air intake
a) Naturally aspirated engine
b) supercharged engine
c) turbocharged engine
5. Air/fuel mixture preparation
a) Carburetion
b) Indirect fuel injection
c) Direct fuel injection
6. Application
Propulsion (automobile, truck, light aircraft, marine), portable power systems, power generation
Internal Combustion Engines

General remarks and engine classification

Engine classification
Method of ignition
Spark ignition engines
A mixture of air and fuel (usually gasoline) vapor is ignited by an electrical discharge (spark) across
the spark plug.
Compression ignition engines
The fuel is injected directly into the engine cylinder just before the combustion process is required to
start. The liquid fuel jet is atomized into tiny droplets, and evaporates inside the hot compressed air;
after a short delay period, the air/fuel mixture spontaneously ignites, because temperature and
pressure are above the fuels ignition point (thanks to high compression ratios).
Working cycle
o Four-stroke engines
the working cycle takes four piston strokes, or two crankshaft revolutions, and more than half is
dedicated to exhaust gas expulsion (scavenging) from the cylinder and to fresh mixture intake inside
the cylinder: the working fluid is thus replaced efficiently at each cycle.
o Two-stroke engines
the working cycle takes just two piston strokes, i.e. one crankshaft revolution: power density is
therefore higher than in 4S engines, but the scavenging process is less efficient.

Internal Combustion Engines

General remarks and engine classification

Geometric and kinematic parameters
Bore (cylinder diameter)
Crank radius
Connecting rod length
Piston stroke
Unit displacement
Displacement
( : number of cylinders)

=2
=
=

Minimum chamber volume

Volumetric compression ratio

=(

Working cycle frequency

(2S: = 1; 4S: = 2)

= /

Rotational speed

=2

Crank angle

Mean piston velocity

=2

)/

TC or TDC: Top Dead Center

BC or BDC: Bottom Dead Center

=2

Image source: R. Stone, Introduction to Internal Combustion Engines, Palgrave Macmillan, 2012

Internal Combustion Engines

General remarks and engine classification

Geometric and kinematic parameters
Typical values of geometric and kinematic parameters:
= /

Ratio of cylinder bore to piston stroke

= /

Ratio of connecting rod length to crank radius

0.8 - 1.2
3.0 - 4.0

Volumetric compression ratio (SI engines)

8 - 12

Volumetric compression ratio (CI engines)

12 - 24

Mean piston speed

8 - 15 m/s
1.8
1.6
1.4

up /
up

1.2
1
0.8
0.6
0.4
0.2
0

Internal Combustion Engines

R = 3.0
R = 3.5
R = 4.0
0

45

90
[deg]

135

180

General remarks and engine classification

Four-stroke engine working cycle
1. Intake stroke: fresh mixture is drawn into the cylinder
by the depression induced by the piston stroke.
To increase the mass inducted, the inlet valve opens
shortly before the stroke starts and closes after it ends.
2. Compression stroke: air (or air/fuel mixture) is
compressed to a small fraction of its initial volume,
reaching pressure and temperature levels that depend
on initial pressure and volumetric compression ratio .
Toward the end of the compression stroke, combustion
is initiated and the cylinder pressure rises more rapidly.
3. Power stroke: high temperature and pressure gases
push the piston down, forcing the crank to rotate.
As the piston approaches BDC the exhaust valve opens
to initiate the exhaust process and drop the cylinder
pressure to close to the exhaust pressure.

4. Exhaust stroke: exhaust gases exit the cylinder, first

spontaneously because the pressure inside the cylinder
is higher than inside the exhaust manifold (blowdown
process), then because they are displaced by the piston
as it moves toward TDC. As the piston approaches TDC
the inlet valve opens, and just after TDC the exhaust
valve closes, so the cycle starts again.
Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

General remarks and engine classification

Two-stroke engine working cycle
I. Compression stroke: as the piston moves toward
TDC, the mixture inside the cylinder is
compressed, while at the same time the pressure
inside the crankcase decreases, so fresh air is
drawn as soon as the inlet ports (2) are uncovered
by the piston. As the piston approaches TDC,
combustion is initiated.
II. Power (or expansion) stroke: all ports are closed
by the piston in the first part of the stroke, then the
exhaust ports (1) are first uncovered, and most of
the burnt gases exit the cylinder in an exhaust
blowdown process. Then the transfer ports (3) are
uncovered and fresh charge, which has been
compressed in the crankcase during the
compression stroke, flows into the cylinder. The
piston and the ports are generally shaped to deflect
the incoming charge from flowing directly into the
exhaust ports, so as to achieve effective
scavenging of the residual gases.
Image source: (top) R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

General remarks and engine classification

Naturally aspirated SI 4S engine cutaway

1.
2.
3.
4.
5.
6.
7.
8.

Air filter
Carburetor
Engine head
Exhaust pipe
Cylinder block
Piston
Alternator
Connecting rod

9. Crankshaft
10.Sump
11.Oil pump
12.Camshaft
13.Pushrod
14.Coil ignition
15.Spark plug
16.Exhaust valve
17.Rocker arm

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

10

General remarks and engine classification

3-cylinder SI 4S engine cutaway

Image source: MTZ worldwide, February 2013

Internal Combustion Engines

11

General remarks and engine classification

Naturally aspirated CI (Diesel) 4S engine cutaway

1.
2.
3.
4.
5.
6.

Engine head
Piston
Cylinder block
Connecting rod
Crankshaft
Sump

7. Oil pump
8. Oil filter
9. Injection pump
10.Glow plug
11.Injector
12.Camshaft

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

12

General remarks and engine classification

Combustion process in SI engines
the fuel is completely vaporized before the start of combustion,
thanks to its high volatility: therefore the fuel/air mixture is physically
homogeneous (gas mixture);
the fuel/air mixture does not ignite spontaneously, because pressure
and temperature are below auto-ignition values;
the combustion is started by a spark, which provides the necessary
activation energy to the fuel/air mixture surrounding the spark plug;
after the ignition, the flame propagates regularly to the whole mixture;
the required activation energy is provided to the unburnt fuel/air
mixture by the flame itself;
flame propagation requires that air/fuel ratio is homogeneous in the
combustion chamber, and close to stoichiometric, otherwise the
flame could be extinguished;
the process is fast because it is controlled by chemical reaction rates
and turbulence -> high rotational speeds are possible; the ideal
representation is a constant-volume process in the Otto cycle.
Image: typical flame propagation images in the combustion chamber at 1000 rpm (ASOS: After Start of Spark; fuel: iso-octane)
Source: Merola, Vaglieco, Optical investigations of fuel deposition burning in ported fuel injection (PFI) spark-ignition (SI) engine, Energy 34 (2009) 21082115.

Internal Combustion Engines

13

General remarks and engine classification

Combustion process in CI engines
the combustion process starts after liquid fuel is injected, and air is at
temperature and pressure high enough for auto-ignition;
there is no premixing of air and fuel: the fuel must first vaporize and
diffuse through the surrounding air, in order to create a locally
stoichiometric air/fuel mixture that can ignite spontaneously
(diffusive combustion);
the overall air/fuel mixture can be lean (combustion is based on autoignition rather than flame propagation);
the combustion process is controlled both by physical (fuel
vaporization and diffusion) and chemical parameters (reaction rates):
hence it is slower than in spark ignition engines and the ideal
representation is a constant-pressure in the Diesel cycle.

Images: (right) conceptual schematic of conventional diesel combustion, from John E. Dec, Advanced compression-ignition enginesunderstanding the incylinder processes, Proceedings of the Combustion Institute, 32 (2009) 2727-2742; (bottom) visible flame images at several crank angles at 1500 rpm, from
Mancaruso, Vaglieco, An experimental comparison of n-Heptane, RME and diesel fuel on combustion characteristics in a compression ignition engine, Fuel
Processing Technology 107 (2013) 44-49.

Internal Combustion Engines

14

General remarks and engine classification

Comparison between SI and CI engines
CI engine limitations:
o heavier weight, due to higher compression ratios necessary to reach pressures and temperatures
required for fuel auto-ignition;
o lower specific power (with reference to engine displacement), and consequently higher footprint
for the same power, due to lower rotational speed;
o higher noise level, because of the different nature of the combustion process.
CI engine advantages:
o higher overall efficiency, thanks to higher compression ratios, which in SI engines are limited in
order to avoid combustion anomalies (knocking);
o better part-load performance, thanks to the different power control system (no need to throttle air
in the intake manifold: partialization is obtained simply through the control of the total amount of
fuel injected);
o lower quality fuel required (even though emission regulations have increased manufacture
costs).

Internal Combustion Engines

15

General remarks and engine classification

Comparison between 4S and 2S engines
2S engine limitations:
o lower efficiency, because work is not delivered during the whole expansion stroke, but only
during the first part of it, before exhaust ports are uncovered to allow exhaust gas blowdown and
scavenging;
o worse exchange of working fluid -> higher pollutant emissions;
o higher thermal and mechanical stresses, because every stroke (compression and expansion) is
marked by high pressures and temperatures, while in 4S engines two strokes out of four are
dedicated to the gas exchange process (air induction and exhaust gas expulsion), which takes
place at low pressures and temperatures.
2S engine advantages:
o higher specific power: theoretically double than 4S engines, because in 2S engines there is a
power stroke at every crankshaft revolution; in practice, specific power is only about 5060%
higher due to worse gas exchange process;
o simpler construction, because ports or automatic valves can be used to control air intake and
exhaust discharge, rather than cam-controlled valves necessary in 4S engines;
o more uniform torque, in particular for low-power engines, because a useful work phase takes
place for every crankshaft revolution.
Internal Combustion Engines

16

Operating parameters
Ideal thermodynamic cycles
Constant-volume
Otto

Thermodynamic processes
1-2 Adiabatic compression
2-3 Heat input:
Otto: constant volume
Diesel: constant pressure
Sabath: mixed (isochoric 2-3, isobaric 3-3)

Constant-pressure
Diesel

Limited-pressure
Sabath

Ideal cycles parameters

Volumetric compression ratio:

Heat input ratio at constant volume:

Heat input ratio at constant pressure:

/
/
/

3-4 Adiabatic expansion

4-1 Constant volume heat rejection
Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

17

Operating parameters
Ideal cycle analysis
Heat input:
=

Rejected heat:
=

Cycle efficiency:
=1

Reversible adiabatic compression:

=
Isochoric heat input:
=

Isobaric heat input:

Reversible adiabatic expansion:

=

Internal Combustion Engines

18

Operating parameters
Ideal cycle efficiency
Sabath cycle efficiency:
=1

1
1

1+

0.9

Otto cycle efficiency:

Otto
Diesel

0.8

=1

0.7
0.6

=1

1
1

id

Diesel cycle efficiency:

0.5
0.4

Ideal cycle efficiency:

0.3

depends on fluid properties though the ratio of specific

0.2

heats

0.1

increases with volumetric compression ratio ,

because it increases the average temperature of heat input
decreases as the ratio

Internal Combustion Engines

k = 1.4
b = 2.0
8

12

16
rv

20

24

increases

19

Operating parameters
Fuel/air cycle efficiency
In order to take into account the real
thermodynamic behavior of the working fluid,
a fuel/air cycle is defined. Its characteristics are:
air and combustion products are perfect gases,
with specific heat ( , ) and specific heat
ratio dependent on temperature;
the combustion process is complete and
instantaneous;
no heat transfer takes place with engine walls;
reversible compression and expansion.
The fuel/air cycle efficiency
(or / ) is
thus defined as the ratio between the work output
of such a cycle and its heat input:
=
/
The plot on the right shows the influence of volumetric compression ratio
on fuel/air cycle efficiency
for different equivalence ratios
(defined later as
/ ), in the case of a constant-volume (Otto) cycle.
is the absolute humidity; the fraction of exhaust gas residuals.
Image source: R. Stone, Introduction to Internal Combustion Engines, Palgrave Macmillan, 2012

Internal Combustion Engines

20

Operating parameters
Air/fuel cycle vs real thermodynamic cycle
Further differences between air/fuel cycle and real cycle are due to:
Finite combustion time: the combustion process usually lasts at least 50 crank angle degrees
Incomplete combustion and chemical dissociation
Heat transfer between burned gases and cylinder walls and between air and intake manifold
Crevice effects and leakage
Exhaust blowdown loss due to anticipated opening of exhaust valve
Pressure losses at intake and exhaust valves: in naturally aspirated engines, this means in
particular that work must be done by the piston on the gas during the intake and the exhaust
processes (pumping work)
Finite valve opening and closing time
The real working cycle is defined as indicated cycle (see the following slide), so an
indicated thermodynamic efficiency may be defined as the ratio of the actual work output and the
work output of a corresponding fuel/air cycle:
=

Internal Combustion Engines

21

Operating parameters
Indicated cycle
The real working cycle and engine performance are
measured with a dynamometer: the engine is clamped on
a test bed and the shaft is connected to the dynamometer
rotor, which is coupled to the stator by electromagnetic,
hydraulic or mechanical (friction) means.
The force required to balance the stator gives the
engine torque:
=
Engine power is then given by the product of torque and
rotational speed
=

This is the usable power that is delivered by the engine to

the load, which is in this case a brake (brake power):
hence the suffix b.
Besides, pressure inside the engine is measured by
sensors called indicators (hence the definition of
indicated cycle) as a function of crank angle and,
consequently, cylinder volume .
Internal Combustion Engines

22

Operating parameters
Real (indicated) cycle
Pressure vs. crank angle ( - ) diagram

Start of combustion (SOC)

Exhaust valve opening (EVO)

Exhaust valve closing (EVC)

TDC

BDC

TDC

BDC

IVO

Intake valve closing (IVC)

Exhaust

Intake

EVO

Expansion

IVC

Intake valve opening (IVO)

EVC

Compression

TDC

Indicated cycle
(pressure vs cylinder volume diagram, - )
Left: indicated cycle of a 4S, SI engine
Right: indicated cycle of a 4S, CI engine

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

23

Operating parameters
Indicated cycle
Indicated work per cycle (per cylinder)

Indicated mean effective pressure (imep)

=

bmep is the work available per cycle and unit

displacement
Friction mean effective pressure (fmep)
=

Indicated fuel conversion efficiency

=

Brake mean effective pressure (bmep)

fmep is here defined to include work spent to

drive accessories

Indicated cycle power output

=
=
/ =

Mechanical balance in terms of power

=

Mechanical efficiency
=

is the fraction of the engine power needed

to drive accessories and overcome friction

Internal Combustion Engines

Mechanical balance in terms of effective

pressures:
=
=

24

Operating parameters
Overall fuel conversion efficiency and specific consumption
Overall fuel conversion efficiency must take into account energy losses due to friction and work
necessary to drive accessories, so it is defined in terms of brake power:
=

It is therefore the product of indicated fuel conversion efficiency and mechanical efficiency:
=

The specific fuel consumption is, by definition, the fuel mass flow rate that must be burned in the
engine to obtain a unit power output: thus, it is given by the inverse of the product of fuel conversion
efficiency and heating value. It is usually expressed in [g/kWh]:
=

Internal Combustion Engines

25

Operating parameters
Volumetric efficiency
Volumetric efficiency is used to measure the effectiveness of an engines induction process, and is
that effectively flows into the intake system divided by the mass of air
defined as the mass of air
that would fill a volume equal to the displacement at inlet air conditions (inlet air density ):
=

In naturally aspirated engines volumetric efficiency is lower than 1 because of pressure losses in the
intake system (distributed losses in the intake manifold and concentrated losses in the intake valve).
Typical maximum values are in the range 8090% for SI engines, and somewhat higher in CI
engines (because there are no throttling losses, which in some measure are always present in SI
engines even at full load).
In supercharged and turbocharged engines inlet air density is higher than the ambient value, so
volumetric efficiency is higher than 1 (and for this reason it is not totally appropriate to talk about
efficiency in this case).
Inlet air mass flow rate can be expressed as follows:
=

Internal Combustion Engines

26

Operating parameters
Air/fuel ratio, torque and power output
An obviously important parameter for the
combustion process is the air/fuel ratio :
=

The energy input into the system per unit time

can thus be expressed as:
=

The fuel/air equivalence ratio is an even

more informative parameter for defining
mixture composition, because it compares the
stoichiometric air/fuel ratio to the actual one:

/ =

Power output:
=

Brake mean effective pressure

=
=
Its inverse is the relative air/fuel ratio :
Torque is proportional to bmep and engine
displacement:

=
According to these definitions:
o for fuel-lean mixtures:

< 1,

>1

o for stoichiometric mixtures:

= 1,

=1

o for fuel-rich mixtures:

> 1,

<1

Internal Combustion Engines

=
=

27

Operating parameters
Thermal energy balance
Fundamental energy balance equation:
=

Heat flux absorbed by refrigerating fluid and lubrication oil:

=
+

Heat flux rejected as sensible heat in the exhaust gases:

Other forms of heat dissipation (

):

o incomplete combustion;
o radiation.
Spark Ignition Engines

Compression Ignition Engines

20 30%

28 40%

16 33%

15 37%

30 50%

24 40%

4 20%

4 12%

Source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

28

Operating parameters
Influence of rotational speed on efficiencies
Taking into account indicated thermodynamic efficiency
working cycle gets shorter, so:

, if rotational speed increases the

leaks of working fluid and heat transfer between fluid and engine walls both decrease
energy losses due to imperfect and incomplete combustion increase
On the other hand, the air/fuel cycle efficiency
slightly increases with the rotational speed because
of higher dilution of fuel (the amount of residual gas is higher) and thus lower temperatures (which
reduce the effect of the specific heats variability).
In the case of mechanical efficiency

, if rotational speed increases:

friction losses and pumping work also increase

Therefore, varying rotational speeds:
indicated thermodynamic efficiency has a maximum, albeit with only a slight variation
mechanical efficiency decreases as rotational speed increases
fuel conversion efficiency

has therefore a maximum, but with a significant decrease only at

high rotational speeds, due to the marked drop in mechanical efficiency

Internal Combustion Engines

29

Operating parameters
Influence of air/fuel ratio on efficiencies in SI engines
Fuel/air cycle efficiency
: for fuel-rich mixtures ( <
,
> 1) a fraction of the fuel cannot burn, so it decreases almost
,
linearly with the air/fuel ratio, for fuel-lean mixtures, ( >
< 1), there is a slight increase due to higher dilution

=1

and

).

Rich mixture

Lean mixture

Air/fuel ratio
Image source (bottom): G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

30

Overall fuel conversion efficiency [%]

(because

Specific fuel consumption [g/MJ]

does not depend on air/fuel

Friction mean effective pressure
ratio: mechanical efficiency thus depends on air/fuel ratio only
through its influence on imep, and therefore it varies as the
indicated efficiency does

Equivalence ratio

Specific power output [kW/dm3]

: it is highly dependent
Indicated thermodynamic efficiency
on air/fuel ratio because of its influence on reaction speed. For
air/fuel ratios markedly higher or lower than the stoichiometric
value the speed of the chemical reactions decreases
significantly, bringing about higher energy losses and therefore
efficiency losses.
Maximum reaction speeds are obtained with slightly rich
mixtures ( 0,9); reactions are effectively frozen for < 0,5
or > 1,5.

Operating parameters
Influence of air/fuel ratio on efficiencies in CI engines
Since CI engines work exclusively with lean mixtures
( 0,7), increasing air/fuel ratios is always beneficial
with reference to dissociation and incomplete
combustion, so the indicated thermodynamic
increases.
efficiency
The different combustion mechanism makes the effects
related to reaction speeds much less important.
Only with extremely lean mixtures the indicated
thermodynamic efficiency drops significantly.
Regarding the influence of air/fuel ratio on mechanical
efficiency, the same considerations apply to CI and SI
engines, but the mechanical efficiency curve for CI
engines is different than for SI engine because imep
behaves differently.

Internal Combustion Engines

31

Operating parameters
Typical design and operating data for internal combustion engines

Source: J.B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, New York, 1988.

Internal Combustion Engines

32

Air intake
Polar valve timing diagram

Intake Valve
Intake valve opens (IVO) before TDC in order to have the valve completely open when the
induction stroke begins, so as to maximize air induction into the cylinder.
Intake valve close (IVC) after BDC in order to take advantage of kinetic energy of exhaust gases in
the intake manifold, so as to achieve a good air induction, thanks to inertial effects, even after BDC.

Image sources:
(left) R.N. Brady, Internal Combustion (Gasoline and Diesel) Engines, In: Encyclopedia of Energy, Elsevier, New York, 2004, Pages 515-528, ISBN 9780121764807.
(right) A. Paul, P.K. Bose, R. S. Panua, R. Banerjee, An experimental investigation of performance-emission trade off of a CI engine fueled by dieselcompressed natural gas (CNG) combination and
dieselethanol blends with CNG enrichment, Energy, Volume 55, 15 June 2013, Pages 787-802, ISSN 0360-5442, http://dx.doi.org/10.1016/j.energy.2013.04.002.

Internal Combustion Engines

33

Air intake
Polar valve timing diagram

Exhaust Valve
Exhaust Valve Closes (EVC) after TDC in order to take advantage of the inertia of the exhaust
gases, which draw even more fresh air into the cylinder thanks to the overlap period (time while
intake and exhaust valve are simultaneously open).
Exhaust Valve Opens (EVO) before BDC so as to discharge initially the burned gases due to the
pressure difference between the cylinder and the exhaust system (blowdown); after BDC the
cylinder is scavenged by the piston as it moves toward TDC (displacement process).
Advanced EVO allows to reduce pumping work (pressure decreases in the cylinder) but it also
reduces the power stroke, so an optimum value exists as a compromise between these effects.
Image sources:
(left) R.N. Brady, Internal Combustion (Gasoline and Diesel) Engines, In: Encyclopedia of Energy, Elsevier, New York, 2004, Pages 515-528, ISBN 9780121764807.
(right) A. Paul, P.K. Bose, R. S. Panua, R. Banerjee, An experimental investigation of performance-emission trade off of a CI engine fueled by dieselcompressed natural gas (CNG) combination and
dieselethanol blends with CNG enrichment, Energy, Volume 55, 15 June 2013, Pages 787-802, ISSN 0360-5442, http://dx.doi.org/10.1016/j.energy.2013.04.002.

Internal Combustion Engines

34

Air intake
Intake and exhaust valves
Volumetric efficiency increases as the cross-section
available to induced air and to exhaust gases
increases.
The influence of intake cross-section is much more
pronounced than that of the exhaust cross-section, by
a factor equal to (because it affects the fluid filling
the whole cylinder, and not only the dead volume).
In order to increase intake and exhaust cross-sections,
it is obviously better to adopt multivalve systems
rather than increasing the size of a single valve.
Therefore the following systems are commonly used:
o

3 valves (2 intake and 1 exhaust)

4 valves (2 intake and 2 exhaust)

5 valves (3 intake and 2 exhaust)

(anyway, exhaust valves are usually smaller)

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

35

Air intake
Phenomena influencing gas exchange processes
In naturally aspirated engines, the volumetric efficiency is
lower than 1 because of several phenomena taking place
in the gas exchange process. The most important are:

2. Pressure losses through the intake valve: when the

induction stroke ends, pressure inside the cylinder is
lower than atmospheric pressure ( < ) because of
energy losses taking place as the fresh air (or mixture)
flows through the valve, so the air density is also lower
than atmospheric air density ( < ).

Intake

Exhaust

Pressure [kPa]

1. Exhaust gas expansion: at the end of the exhaust

stroke, the dead volume is filled with exhaust gases,
whose pressure is higher than the atmospheric pressure
( > ): these gases expand as the induction process
begins and the pressure in the cylinder decreases, and
occupy a volume larger than the dead volume.

Volumes

3. Heat exchanges between engine walls and induced air

also make airs density lower than atmospheric density.

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

36

Air intake
Volumetric efficiency: quasi-static effects
Fuel vapor (and also water vapor) reduces the air partial pressure
below the mixture pressure. If is the mixture pressure (at the
beginning of the compression stroke), then = , + , + ,
(a -> air, f -> fuel, w -> water). Partial pressure of air is given by:
,

= 1+

This effect can be quite large for gaseous fuels.

Fuel vaporization: the mixture temperature decreases as liquid fuel is
vaporized. The temperature difference that occurs after evaporation is:
,
=
, + , /
with: fraction of fuel evaporated; , enthalpy of vaporization.
For isooctane, at = 1, 19 K; for alcohols, given their large
enthalpy of vaporization, the effect can be quite large ( 128 K
for methanol) and compensate for the reduction in air partial pressure.
As the pressure in the exhaust manifold increases, the volume
occupied by the residual gas in the cylinder also increases, so the
volumetric efficiency decreases.
Image source: J.B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, New York, 1988.

Internal Combustion Engines

37

Air intake
Volumetric efficiency: influence of ambient and refrigerant temperature

= 302,4 K
Volumetric efficiency ratio

Volumetric efficiency increases with ambient

temperature (according to a power 0,5),
because less heat is exchanged between
mixture and walls. In any case, the mass of
induced air decreases as ambient
temperature increases:

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

Volumetric efficiency ratio

Refrigerant temperature affects wall

temperature and thus heat transfer during
the gas exchange process: therefore,
volumetric efficiency slightly increases as
refrigerant temperature decreases.

Ambient temperature [K]

= 363 K

Refrigerant temperature [K]

38

Air intake
Volumetric efficiency: influence of rotational speed
Volumetric losses due to charge heating
decrease as
(and therefore ) increases
because the time available for heat exchange
between fluid and engine walls also decreases.
The effect of friction, both at the intake and at
the exhaust, is proportional to

at low-medium

speeds, and increases even more strongly with

at high speeds, when sonic flow conditions
(choking) are reached at the intake valve.
Backflow: because the inlet valve closes after
the start of the compression stroke, a reverse
flow of fresh charge from the cylinder back into
the intake can occur as the piston moves
toward TDC. This flow is larger at lower speeds.
Dynamic effects: inertial effects and wave
effects (tuning).

Image source: J.B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, New York, 1988.

Internal Combustion Engines

39

Air intake
Volumetric efficiency: effect of valve timing
EVO point
(degrees before BDC)

Pressure [MPa]

Early Exhaust Valve Opening (EEVO)

If the exhaust valve closes much earlier than BDC,
energy losses for incomplete expansion increase,
but the blowdown process is more effective, thus
reducing the work in the exhaust stroke.
Therefore, a compromise value must be sought.

TDC

BDC

Cylinder volume

Late Intake Valve Closing (LIVC)

As the difference between BDC and intake valve
closing increases, the volumetric efficiency vs
engine speed curve shifts towards higher rotational
speeds. With fixed valve timing, this also means
that backflow increases at low speeds.

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

Volumetric efficiency

[rpm]

IVC point
(degrees after BDC)

Engine speed [rps]

40

Air intake
Volumetric efficiency: inertial effect
The pressure in the inlet manifold varies during each cylinders
intake process due to the piston velocity variation, and the unsteady
gas-flow effects that result from these geometric variation.
At higher engine speeds, the inertia of the gas as the intake valve is
closing increases the pressure in the inlet port and continues the
charging process as the piston slows down around BDC and starts
the compression stroke. The inlet valve is closed some 40 to 60
after BDC, in part to take advantage of this ram phenomenon.

Pressure oscillations increase volumetric efficiency when the intake

systems natural frequency is twice the rotational speed: 2 .
Therefore, given the geometry of the intake system, the rotational
speed that maximizes volumetric efficiency is:

Volumetric efficiency ratio

The fluid filling intake manifold and cylinder can be approximated as

a 1-degree of freedom oscillator (Helmholtz resonator), whose
natural frequency is:

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

Frequency ratio

41

Air intake
Volumetric efficiency: wave effects (tuning)
Pressure oscillations due to wave propagation in the
intake system influence the gas exchange process.

If the reflected wave reaches the intake valve in the

second half of the induction phase (about 90 after TDC),
pressure at the valve increases just as the intake stroke is
almost complete, aiding the induction process.
In this case the intake system is said to be tuned.

Pressure [MPa]

When the intake valve opens, a rarefaction wave

propagates upstream; it is then reflected back as a
pressure wave at the first discontinuity.

Resulting pressure
Reflected
wave
TDC

BDC
LIVC

EIVO

Without reflection

Crankshaft angle [deg]

The time required for a wave to travel along the system is

= / , which, in terms of crankshaft angle, is:
=2 =2
/
The system is tuned if: 2 /2

Given actual values of sound speed , an intake system is

usually tuned at high rotational speeds; it could also be
tuned at relatively low speeds with very long pipes.

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

42

Air intake
Supercharging and turbocharging: layouts
a) Mechanical supercharging
b) Turbocharging
c) Engine-driven compressor and turbocharger
d) Two-stage turbocharging
e) Turbocharging with turbocompounding
f) Turbocharger with intercooler

C Compressor
E Engine
I

Intercooler

T Turbine

Image source: J.B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, New York, 1988

Internal Combustion Engines

43

Air intake
Supercharging: Roots compressor (positive displacement)

Image source:
(right) R. della Volpe, Macchine, Liguori Editore, Napoli, 2011
(left) P.W.Wetzel, J.P.Trudeau, New supercharger for downsized Engines, MTZ worldwide, February 2013

Internal Combustion Engines

44

Air intake
Turbocharger

Image source: E. Chebli et al., Development of an exhaust-gas turbocharger for HD Daimler CV engines, MTZ Worldwide, February 2013

Internal Combustion Engines

45

Air intake
Turbocharging with intercooler

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

46

Air intake
Turbocharging

pressure

Adiabatic expansion

TDC

BDC

Volume

Turbocharging is based on the residual energy of the exhaust gases

Backpressure increases as exhaust gases flow through the turbine
Area 2-3-4-5 represents the energy related to a spontaneous discharge in atmosphere, and it is the
maximum energy that could theoretically be extracted from the exhaust gases in an ideal impulse
layout (where each cylinder is directly connected to the turbine inlet)
In a constant pressure system, each cylinder discharges to an exhaust manifold that is large enough
to dampen pressure oscillations; kinetic energy is then lost while exhaust gases expand to pressure
(2-3 process), with an increase in total enthalpy and temperature
Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

47

Air intake
Supercharging vs. turbocharging
For positive displacement compressors, pressure ratio is almost independent on the engines
rotational speed: therefore, good performances can be achieved even at low rotational speeds and
when accelerating from low speeds.
The turbocharger on the other hand provides increasing pressure ratios as the engine speed
increases: inlet pressure may be either insufficient at low speeds or too high at high speeds.
The turbocharger is more reliable; installation and maintenance are easier.
The turbocharger weighs less and is smaller than a supercharger, other things being equal
(in particular, for the same pressure ratio and air flow rate).
Pressure ratio is approximately up to 3 for a turbocharger and up to 2 for a Roots compressor.

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

Mechanically driven

Torque

During transient response, turbochargers do not respond as fast

as mechanically driven compressors (turbo lag), due to the time
needed for the exhaust system and turbocharger to generate
the required boost.
Inertia, friction, and compressor load are the primary contributors
to turbo lag.
The mechanically driven compressor has a faster response
because its rotational speed is directly coupled to the engines speed.

Turbo
Naturally Aspirated

Time [s]

48

Air intake
Turbocharging: applications
Spark Ignition engines:
o The degree of supercharging in SI engines is mainly limited by the knock: supercharging reduces
ignition delay which increases the knocking tendency.
o Before the advent of direct injection systems, turbocharging required a decrease of the
compression ratio in order to reduce the risk of knocking -> higher specific fuel consumption.
o With the advent of direct injection systems and the availability of better fuels (higher octane
numbers), turbocharging could be more easily adopted, because fuel vaporization, together with a
wide use of charge cooling, allows to avoid the need to reduce compression ratios.
Compression Ignition engines:
o Since in CI engines there is no risk of knocking, supercharging is limited only by the maximum
permissible mechanical and thermal loads. Indeed, supercharging has even a positive influence on
the combustion process.
o Therefore, turbocharging is extensively used in CI engines, and it makes their performance reach
the same level of naturally aspirated SI engines.

Internal Combustion Engines

49

Air intake
Turbocharging: exhaust gas by-pass valve
Turbocharger

Exhaust gas
Fresh air

Turbine

Discharge

Inlet

Engine

Bypass valve

The turbocharger is sized in such a way that it can

provide maximum supercharging pressure at
4050% of the maximum rotational speed, in order
to achieve good torque output even at low speeds.
At high rotational speeds, the excess exhaust
gases are discharged through a bypass valve.

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

50

Air intake
Turbocharging: variable geometry turbine
As an alternative, in order to achieve
good performances on a wide range
of rotational speeds, a variablegeometry turbine can be used, which
is equipped with variable-pitch nozzle
blades.

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

51

Fuel metering in SI engines

Requirements of a 4S, SI engine

Rich mixture

Lean mixture

Overall fuel conversion efficiency [%]

Mixture homogeneity
The air/fuel mixture must be homogeneous, in
order to burn fuel rapidly and completely.

Specific fuel consumption [g/MJ]

Air/fuel ratio
The metering system must provide the
appropriate quantity of fuel, so as to obtain the
required air/fuel ratio for every operating point.

Equivalence ratio

Specific power output [kW/dm3]

Fuel volatility
Fuel must be premixed with air, before the
spark starts the combustion process.

Air/fuel ratio

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

52

Fuel metering in SI engines

Carburetor

1. Air filter
2. Carburetor
3. Throttle valve
4. Intake manifold
5. Fuel tank
6. Fuel filter
7. Cam
8. Diaphragm pump

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

53

Fuel metering in SI engines

Elementary carburetor
Fundamental components:
Venturi (convergent-divergent nozzle)
In its throat the depression originated by the
inlet air flow draws fuel from its tank.

Air
Fuel Pressure
equalizing
passage
Float
chamber

Fuel discharge tube

Connects the fuel tank to the Venturi throat.
The fuel flow is metered by a calibrated orifice.

Fuel discharge
nozzle

Venturi
throat

Calibrated
orifice

Throttle
plate

Float chamber
The fuel level is maintained at a constant
height in a float chamber: a pressure
equalizing passage makes the pressure inside
the float chamber equal to air pressure at the
Venturi inlet. Thus, hydrostatic pressure on the
calibrated orifice depends only on the flow rate
of air.
Throttle valve
It controls air flow rate, and as a consequence
power output, acting on volumetric efficiency.

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

54

Fuel metering in SI engines

Carburetor vs. injection systems
Limits of carburetors

Advantages of fuel injection

It is difficult to control precisely the required

air/fuel ratio in different operating conditions

better control on air/fuel ratio, both in terms of

precision and repeatability, in all operating
conditions; more uniform distribution among
cylinders; consequently, positive influence on:

Pressure loss in aspiration

It is difficult to control its operation in transient
conditions, due to the inertia of the fuel mass
Significant variation of specific fuel
consumption with load, particularly pronounced
in case of frequent transient operating conditions
It is necessary to heat the intake manifold in
particular operating conditions, with the
consequent decrease in volumetric efficiency, in
order to avoid that fuel condenses on engine walls
Limited control on pollutant emissions in the
whole operating range

Internal Combustion Engines

o specific fuel consumption

o performance
o pollutant emissions
better transient operation, because of lower fluid
volumes in the system (lower fluid inertia)
higher volumetric efficiency, thanks to lower
pressure losses and no intake manifold heating
it makes possible to increase compression ratio,
because the fuel/air mixture has less time
available for autoignition, or alternatively to use a
fuel with lower octane rating

55

Fuel metering in SI engines

Classification of injection systems
Position of fuel injectors
o Indirect injection (IDI): it takes place in the intake manifold
o Direct injection (DI): fuel is directly injected inside the cylinder
Control of injectors
o Mechanical injection: an engine-driven pump pressurizes the fuel and meters the injected volume
by means of an automatic mechanical injector
o Electronic injection: an electromagnetic fuel injector is used; metering and control of injection are
bestowed upon the ECU
Distribution among cylinders (only for indirect injection)
o Single-point systems: only one or two fuel injectors meter the fuel into the air flow directly above
the throttle body
o Multipoint port injection: a fuel injector is used for each cylinder; fuel is injected into the intake
port of each cylinder

Internal Combustion Engines

56

Fuel metering in SI engines

Injection systems

Direct Injection (DI, left)

and Indirect Injection (IDI, right)
1. Injector
2. Intake manifold
3. Intake valve

Bosch Electromagnetic Injector

(electronic injection system)
1. Filter
2. Electric wire
3. Winding
4. Winding armature
5. Needle valve
6. Needle tip
7. Fuel pipe
8. Tightening ring
9. Seal ring
10.Seal ring

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

57

Fuel metering in SI engines

MultiPoint Fuel Injection (MPFI)

1. Fuel tank

11.Throttle switch

2. Fuel pump

12.Air flow sensor

3. Fuel filter

13.Air temperature sensor

4. Pressure regulator

14.Lambda sensor

5. Electronic Control Unit (ECU)

15.Coolant temperature sensor

6. Ignition coil

16.Auxiliary air device

7. HT distributor

17.Crankshaft angle sensor

8. Spark plug

18.Battery

9. Injector

19.Ignition and starting switch

10.Throttle valve

20.HVAC switch

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

58

Fuel metering in SI engines

Direct injection system (Bosch)
Solenoid (HDEV5) and piezo (HDEV 4.1) injectors
Metering accuracy 2 mg/inj
up to 3 injections per cycle (up to 5 inj/cycle for
stratified-charge combustion)

HDEV5

HDEV4.1

Source: A. Heinstein et al., High-pressure Direct Injection Systems for Gasoline Engines , MTZ Worldwide, March 2013

Internal Combustion Engines

59

Fuel metering in SI engines

Direct vs. Indirect injection systems
Problems arising with direct injection systems:
Higher mechanical and thermal stress
The injectors must withstand pressure and temperature levels that are reached inside the cylinder;
furthermore, they are subject to dirt and soot particles originating from the combustion process.
Installation complexity
Direct injection requires that the injector be mounted directly on top of the cylinder, where spark plug
and intake and exhaust valves are placed too.
Higher injection pressure
Direct injection systems need at least 3540 bar in order to achieve the correct fuel vaporization and
mixing with the air inside the cylinder (modern injection systems reach pressures up to 200 bar).
In the case of indirect injection systems, an injection pressure of 45 bar is good enough.
More difficult mixture homogenization
In order to achieve a mixture homogenization comparable to indirect injection systems, the air intake
system needs to be more sophisticated, so as to increase turbulence (through swish, squish and
tumble motions): a high degree of turbulence speeds evaporation of fuel, enhances air-fuel mixing
and increases combustion speed and efficiency.

Internal Combustion Engines

60

Fuel metering in SI engines

Direct vs. Indirect injection systems
Advantages of direct injection systems:
Evaporative cooling
In indirect injections systems, fuel vaporization takes place in the intake manifold and it subtracts
heat from the manifold walls and from the intake valve; in direct injection systems, on the contrary,
fuel vaporization takes place inside the cylinder, so that it cools the induced air, with a double
benefit:
o higher density -> higher volumetric efficiency: torque output increases by approximately 56%;
o lower temperature -> lower risk of detonation (knocking) -> compression ratio can be increased
(by approx. 20%) -> significant improvements in efficiency and fuel consumption are possible.
Cold start and transients
Direct injection removes the problem of fuel condensation on the intake manifolds walls or on the
intake valve, which is particularly important during transient behaviors and at cold start: both fuel
consumption and CO and HC pollutant emissions are reduced.
Longer valve overlap period possible
Fuel injection is carried out when both valves are closed, so the induced air does not contain any
fuel, and thus in the overlap period there is no risk of fuel flowing back to the intake manifold (with
the possible risk of backfire) or of fuel loss with exhaust gases.

Internal Combustion Engines

61

Fuel metering in SI engines

Direct vs. Indirect injection systems
Indirect
injection

Direct
injection

3498

3498

10,7

12,2

[bar]

45

200

[kW]

200

215

[rpm]

6000

6400

[bar]

11,4

11,5

[kW/l]

57,2

61,5

[Nm]

350

365

[bar]

12,6

13,1

100,0

104,3

[g/kWh]

240

235

[g/kWh]

360

290

[cm3]

[Nm/l]

n [rpm]
,

(part load)

Source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

62

Fuel injection in CI engines

General remarks

The air/fuel ratio must be close to the stoichiometric

value only locally, i.e. close to the injector; overall,
the fuel/air mixture can be lean, which allows to
control power output just through the mass of fuel
injected, without the need of controlling also the
mass of air induced.
On the other hand, the fuel/air equivalence ratio
cannot exceed threshold levels given by the
following reasons:

Brake mean effective pressure (bmep) [MPa]

CI engines use low-volatility fuels: the fuel must

therefore be injected into the induced air in the form
of droplet spray, and the drops should be as
small as possible.

Power

o pollutant emission

rps

o mechanical stresses
o thermal stresses

Rotational speed n

rpm

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

63

Fuel injection in CI engines

Indirect injection (prechamber) and direct injection

Indirect injection (prechamber)

1. Injector
2. Prechamber
3. Glow plug (used as a cold-starting aid)

Direct injection
Reduction of heat losses (no heat exchange
with the prechamber walls) -> increase in fuel
efficiency
On the other hand, injection pressure must
be significantly higher

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

64

Fuel injection in CI engines

Common Rail Fuel Injection System

Internal Combustion Engines

65

Fuel injection in CI engines

Common Rail: injector
Open injector
Fuel leak back
(return)

Closed injector
Electrical
connection
High pressure
fuel inlet

High pressure
fuel inlet

Solenoid
actuator

Solenoid
actuator

Two-way valve

Two-way valve
Injector valve
open

Solenoid energized
Fuel pressure is relieved above
the valve control plunger
Balance of forces: Fa > Fc+Fe
Nozzle open

Internal Combustion Engines

Injector valve
closed

Solenoid not energized

Fuel pressure is the same above
and below the valve control plunger
Balance of forces: Fa < Fc+Fe
Nozzle closed

66

Fuel injection in CI engines

Common Rail: injector characteristics

Source: D. Schppe et al., Servo-Driven Piezo Common Rail Diesel Injection System, MTZ Worldwide, March 2012

Internal Combustion Engines

67

Fuel injection in CI engines

Common Rail: multiple injections

Internal Combustion Engines

68

Fuel injection in CI engines

Common Rail: application examples

Cursor 11 Euro VI engine series, used for commercial truck propulsion (FPT Industrial)

Internal Combustion Engines

69

Fuel injection in CI engines

Common Rail: application examples

Vector V20 engine series, used in power generators (FPT Industrial)

Internal Combustion Engines

70

Operating characteristics and performance maps

Power, torque and fuel specific consumption curves

Performance maps are usually drawn with reference

to full-load operation (full-load performance maps)
and represent power, torque and fuel consumption vs.
rotational speed.

Torque depends on
and , so its maximum lies
where volumetric efficiency is highest, and falls
rapidly because of the decrease in mechanical
efficiency.

Beyond a particular value of rotational speed, the

speed increase cannot compensate the decrease in
mechanical and volumetric efficiency: therefore that
) corresponds to the maximum
operating point (
power output of the engine.
The operating range of the engine is usually limited to
only slightly higher than
a maximum speed
, because there is no point in using the
decreasing part of the power curve.

Fuel specific consumption depends only on the

overall fuel conversion efficiency .

Power
Fuel specific consumption
Torque

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

71

Operating characteristics and performance maps

Power, torque and brake mean effective pressure curves of a SI engine
This chart shows the performance map of a
naturally aspirated 4S SI engine
(displacement 2525 cm3).
The brake mean effective pressure curve has
the same shape of the torque curve, because:

Curves do not start from zero power output

but from a minimum speed, below which
excessive vibrations and irregular operating
conditions would arise.
The operating range goes from the minimum
speed up to a maximum speed slightly higher
than the maximum power operating point.

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

72

Operating characteristics and performance maps

Power and torque of a turbocharged SI engine

Source: The New BMW Inline Six-cylinder Gasoline Engine, MTZ worldwide, October 2015

Internal Combustion Engines

73

Operating characteristics and performance maps

Performance map of a CI engine
Engine characteristics:
Turbocharged
Direct injection
Displacement

= 4134 cm

The torque rises very fast at low speeds,

then it is maintained constant over a wide
range of speed, and finally decreases at high
speeds. This behavior depends on:
the turbocharger characteristics;
the limits on the fuel/air equivalence ratio.

n [rpm]

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

74

Operating characteristics and performance maps

Influence of displacement and number of intake valves

The chart on top shows the performance map of two

similar SI 4S naturally aspirated engines of different
displacement.
Displacement does not affect bmep
(there is only a marginal influence on the volumetric
efficiency related to valve area).
Therefore, torque and power output increase linearly
with the increase in displacement, on the whole
operating range.

The bottom chart shows the performance map of two

CI 4S engines, same displacement ( = 2500 cm3),
different number of valves.
The volumetric efficiency increases significantly in the
4-valve engine: this results in a higher torque output.
The increase in power output is even more
pronounced because the 4-valve engine has a wider
speed range (the valves are smaller and lighter, so can
withstand higher velocities).

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

75

Operating characteristics and performance maps

SI automotive engine performances

Year
[cm3]
Supercharging
Injection

2001

2006

2002

2002

2005

2006

2007

1998

1598

1796

1796

1390

1798

2979

Turbo

Turbo

Mech.

Mech.

Mech. + Turbo

Turbo

2 Turbo in //

Indirect

Indirect

Indirect

Direct

Direct

Direct

Direct

8,8

8,8

8,7

10,5

10,0

9,4

10,2

120

120

150

200

[bar]
[kW]

140

132

141

125

125

118

225

/ [kW/l]

70

82,6

78,5

69,6

89,9

65,6

75,5

5400

5500

5800

5300

6000

5000

5800

250

230

260

250

240

250

400

/ [Nm/l]

125,1

144

144,8

139,1

172,7

139

134,3

(max) [bar]

15,7

18,1

18,2

17,5

21,7

17,4

16,8

(min) [g/kWh]

239

[rpm]
[Nm]

< 250

< 235

Design characteristics and performance of some SI 4S supercharged engines for automotive applications, built by European companies from 2001 to 2007
Source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

76

Operating characteristics and performance maps

CI automotive engine performances
1

10

11

12

13

1999

2001

2002

2003

2004

2004

2004

2004

2005

2005

2005

2005

2006

3900

1995

2148

3936

2460

2967

2497

1991

1493

2993

2987

4134

3996

Turbocharging

2
Turbo
in //

Turbo

Turbo

2
Turbo
in //

Turbo

Turbo

Turbo

Turbo

Turbo

2
Turbo,
series

Turbo

2
Turbo
in //

Turbo

Injection

CR
I gen.

CR
II gen.

CR
II gen.

CR
II gen.

CR
II gen.

CR
II gen.

CR
II gen.

CR
II gen.

CR

CR
III gen.

CR
III gen.

CR
III gen.

18

17

18

17,3

17,1

17

18

18

16,5

18

16,5

17

[bar]

1350

1600

1600

1600

1600

1600

1600

1600

1600

1600

1600

n. of injections

1 pilot

up to
5

2 pilot

2 pilot

>1

2 pilot

2 pilot

2+1+2

[kW]

175

110

110

202

128

171

130

103

70

200

165

240

231

/ [kW/l]

44,9

55,1

51,2

51,3

52,0

57,6

52,1

51,7

46,9

66,8

55,2

58,1

57,8

) [rpm]

4000

4000

4200

3750

3500

4000

4200

4000

4400

3800

3750

3600

560

330

340

650

400

450

400

300

210

560

510

650

730

/ [Nm/l]

143,6

165,4

158,3

165,1

162,6

151,7

160,2

150,7

140,7

187,1

170,7

157,2

182,7

) [rpm]

17502500

2000

2000

18002500

2000

14003250

2000

16003000

18002800

2000

16002800

16003500

2200

[bar]

18,0

20,8

19,9

20,8

20,4

19,1

20,1

18,9

17,7

23,5

21,5

19,8

23,0

[g/kWh]

207

202

205

198

202

202

206

208

Year
[cm3]

[Nm]

18

202

Design characteristics and performance of some CI 4S turbocharged engines for automotive applications, built by European companies from 1999 to 2006
Source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

77

Operating characteristics and performance maps

Performance of 4S, 4-cylinder automotive engines

Fuel
[cm3]

Turbocharging

10

Gasoline

Gasoline

Gasoline

Gasoline

Gasoline

LPG

Diesel

Diesel

Diesel

Diesel

1368

1368

1368

1368

1742

1368

1598

1956

1956

1956

Turbo,
intercooler

Turbo,
intercooler

Turbo,
intercooler

Turbo,
intercooler

Turbo,
intercooler

Turbo,
intercooler

Turbo,
intercooler

Turbo,
intercooler,
var.
geom.

Turbo,
intercooler,
var.
geom.

Turbo,
intercooler,
var.
geom.

Multiair*

Multiair*

Air inlet
Injection

MPFI

MPFI

MPFI

MPFI

DI

MPFI

Multijet 2

Multijet 2

Multijet 2

Multijet 2

9,8

9,8

9,8

9,8

9,25

9,8

16,5

16,5

16,5

16,5

1600

1600

1600

1600

[bar]
[kW]

77

88

125

125

173

88

77

103

125

125

/ [kW/l]

56,3

64,3

91,4

91,4

99,3

64,3

48,2

52,7

63,9

63,9

) [rpm]

5000

5000

5500

5500

5500

5000

4000

3750

4000

4000

206

206

250

250

340

206

320

350

350

350

/ [Nm/l]

150,6

150,6

182,7

182,7

195,2

150,6

200,3

178,9

178,9

178,9

) [rpm]

1750

1750

2500

2500

1900

1750

1750

1500

1750

1750

18,9

18,9

23,0

23,0

24,5

18,9

25,2

22,5

22,5

22,5

[Nm]

[bar]

* Direct control system of valve opening, with no throttle valve.

Design characteristics and performance of the Alfa Romeo Giulietta engines.
Source: http://www.alfaromeo.it/it/Documents/schede-tecniche/GiuliettaSchedaTecnica-ConsumiPrestazioniEmissioni.pdf (last retrieved December 2013).

Internal Combustion Engines

78

Operating characteristics and performance maps

Fuel economy characteristics

Naturally aspirated SI engine, direct injection,

displacement =

Turbocharged CI engine, direct injection,

displacement =

Full-load performance map: see Direct vs. Indirect injection systems

Full-load performance map: see Performance map of a SI engine

bmep

bmep

Rotational speed [rpm]

Rotational speed [rpm]

Image source: R. della Volpe, Macchine, Liguori Editore, Napoli, 2011

Internal Combustion Engines

79

Load matching
Typical applications
1.

Load requiring constant rotational speed

(as in the case of an electric generator, with the rotational speed linked to the grid frequency):
as a consequence, torque output depends on the external load.
In this case, in order to contain as much as possible any speed fluctuation, power output is divided
among many cylinders and a flywheel with a large moment of inertia is used.
Furthermore, it is possible to optimize the performance, in terms of fuel specific consumption, for
the required operating conditions, taking into account possible load variations.

2.

Load requiring torque output increasing as the square of the rotational speed ):
the engine is thus matched to a fluid machine such as compressors, pumps, aeronautical or
marine propellers, etc.
In this case the engine speed at design operating conditions should be as close as possible to the
optimal one for the external load, in order to reduce the size of the gearbox, or if possible avoid
altogether its use. For large marine 2S CI engines, engine speed has even been reduced down to
12 s-1, so as to couple directly the engine to the propeller.

3.

Load requiring a wide range of operating conditions, both in terms of speed and torque.
It is for example the case of ground propulsion.

Internal Combustion Engines

80

Load matching
Power generation: example of state-of-art engine performance
/#0865

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/#0)

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8


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9$

M,M9J

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0QOKPCNIGPGTCVQTGHEKGPE[

Source: MAN Diesel & Turbo Power Plant Programme

Internal Combustion Engines

81

Load matching
Power generation: example of state-of-art engine performance and dimensions
18V48/60TS engine

Operation mode
Performance data

Unit

Power per cylinder

kW

1050

1100

1150

1200

Tot. engine power

kW

18,900

19,800

20,700

21,600

Tot. el. genset power

kW

18,428

19,305

20,183

21,060

g/kWh

171

172

174

177

kJ/kWh

7,305

7,350

7,430

7,560

Spec. fuel oil consumption

acc. to ISO 3046, without pumps,
mech. Power output, +5% tolerance

Heat Rate
acc. to ISO 3046, without pumps,
mech. Power output, +5% tolerance

NOx emissions (dry at 15% O2)

Mean effective pressure
Spec. lube oil consumption

Dimensions (mm)

Dry mass (t)

mg/Nm3

1850

1740

1580

1480

bar

23.2/22.6

24.3/23.7

25.4/24.7

26.5/25.8

g/kWh

0,60

0,60

0,60

0,60

9625

5410

24510

9023

4694

407

407

407

407

407

Source: MAN Diesel & Turbo Power Plant Programme

Internal Combustion Engines

82

Load matching
Power generation: example of state-of-art engine performance and dimensions
With generator (genset)

Without generator

In-line engine L32/44CR

GenSet dimensions
A

mm

7,470

8,530

7,055

8,315

9,575

mm

4,328

4,328

4,376

4,376

4,376

mm

11,795

12,858

11,431

12,691

13,951

6L32/44CR
7L32/44CR

mm

2,676

2,676

4,200

4,260

4,260

mm

4,975

4,975

5,000

5,200

5,200

Dry mass

84

97

117

144

172

Engine type

No. of cyl.

L
mm

L1
mm

W
mm

H
mm

Weight
t

6,312

5,265

6,924

5,877

2,174

4,163

39.5

2,359

4,369

8L32/44CR

7,454

6,407

2,359

4,369

44.5
49.5

9L32/44CR

7,984

6,937

2,359

4,369

53.5

10L32/44CR

10

8,603

7,556

2,359

4,369

58.0

Engine type

No. of cyl.

L
mm

L1
mm

W
mm

H
mm

Weight
t

12V32/44CR

12

7,195

5,795

3,100

4,039

70

14V32/44CR

14

7,970

6,425

3,100

4,262

79

16V32/44CR

16

8,600

7,055

3,100

4,262

87

18V32/44CR

18

9,230

7,685

3,100

4,262

96

20V32/44CR

20

9,860

8,315

3,100

4,262

104

V-engine V32/44CR

Source: MAN Diesel & Turbo Power Plant Programme

Internal Combustion Engines

All weights and dimensions are for guidance only and apply to dry engines without flywheel. Masses include built-on lube oil automatic filter, fuel oil filter and electronic equipment.
Minimum centreline distance for twin engine installation: 2,500 mm (L32/44CR), 4,000 mm (V32/44CR). More information available upon request.

83

Load matching
Power generation: plant layout

Representation of the IPP3 plant in Jordan. Source: Wrtsil

Internal Combustion Engines

84

Load matching
Power generation
Facilities of 80 MW or more, operating or under construction
Name

Location

Aratu, Salvador

Brazil

Capacity MW

Fuel

Year1

Conguration

E/G supplier

1056

Diesel

U/C

120 x 18V32/40 in 6 units

MAN Diesel & Turbo

IPP3

Jordan

573

Tri-fuel

2014

38 x 50DF

Wrtsil

Quisqueya I+II

Dominican Republic

431

HFO, nat gas

2013

12 x 18V50DF + 12 x 18V50DF

Wrtsil

21 x SG

Wrtsil

17 x 20V46F

Wrtsil

Boyuk Shor

Azerbaijan

Suape II

Brazil

Sammarah

Iraq

Geramar I+II

Brazil

384

Nat gas

Unknown

382.5

HFO

2011

341

HFO

331.8

HFO

2010

20 x 18V46

Wrtsil

38 x 20V32

Wrtsil

18 x 16.6 MW 50DF

Wrtsil

Sangachal

Baku, Azerbaijan

306.8

FO, nat gas

2012

Coloane, Macau

China

271.4, CC

HFO, diesel

1978-97

Aliaga Alosbi-II2

Izmir, Turkey

270.6, CC

HFO, nat gas

2007

4 x 18V46, 28 x 20V34SG,
2 x 13.5 MW steam

Wrtsil

Pavana III

Honduras

267.2

Oil

2004

16 x 18V46

Wrtsil

27 x W20V34DF

Wrtsil

14 x 18V48/60

MAN Diesel & Turbo

2x24, 2x38.6, 2x53.1, +2x20 ST MAN DT, Peter Brotherhood

Kiisa ERPP 1 & II

Estonia

250

Nat gas, LFO

2014

Choloma I, II, III

Honduras

250

HFO

2003-5

IPP4

Jordan

240

HFO, DFO, gas

2014

16 x 50DF

Wrtsil

241.5

HFO

1994

21 x 16ZA40S

Sulzer, Alstom

Bauang La Union

Philippines
3

Plains End, Colorado

USA

231

Nat gas

2002, 2006

STEC Red Gate, Texas

USA

225

Nat gas

2014

12 x 50SG

20xW18V34SG, 14xW20V34SG Wrtsil

Wrtsil

Atlas

Pakistan

225, CC

Furnace oil

2009

11 x 18V48/60

MAN Diesel & Turbo

Port-Est

Reunion

222

HFO

2010

12 x 18V48/60

MAN Diesel & Turbo

Port Westward Unit 2

Oregon, USA

220

Nat gas

2015

12 x 50SG

Wartsila

Kribi

Cameroon

216

Nat gas/LFO

2013

Pearsall, Texas

USA

202.5

Nat gas

2010

24 x 20V34SG

Wrtsil

Linhares

Brazil

204

Nat gas

2010

24 x 20V34SG

Wrtsil

Pesangarran, Bali

Indonesia

205

Nat gas, HFO

2014-5

Nishat

Pakistan

201, CC

HFO

2010

Pernambuco III

Brazil

200.8

HFO

Nishat Chunian

Pakistan

200

HFO

2010

11 x 18V46

Wrtsil

Vasavi

India

200

HFO

1998

4 x 12K90MC-S

MAN D&T, Hyundai, ABB

Abidjan

Cte dIvoire

200

Nat gas

2010-13

Garabito

Costa Rica

200

HFO

2010

11 x 18V48/60

MAN Diesel & Turbo

Wrtsil

12 x 50DF

Wrtsil

11 x 18V46C, 1x14 MW ST

Wrtsil, Peter Brotherhood

23 x W20W32

Wrtsil

Aggreko, rental

United Ashuganj

Bangladesh

200

Gas

Liberty Power Tech

Pakistan

200, CC

HFO

Arun

Indonesia

184

LNG

2015

19 x 34SG

Wrtsil

Ressano Garcia

Mozambique

176

Nat gas

2014

18 x V34SG

Wrtsil

Viana

Brazil

175

HFO

2009

20 x 20V32

Wrtsil

La Paz (Baja California Sur)

Mexico

173

Diesel oil

2005-13

4 units, CC plant

Man Diesel & Turbo

Eklutna, Alaska

USA

171

Nat gas, LFO

2014

10 x W18V50DF

Wrtsil

Cear

Brazil

168

HFO

2010

8 x 20V46F

Wrtsil

Clifton Pier

Bahamas

165

HFO

1963

1 x 6 MW, 4 x 10 MW, 2 x 26.5

MW, 2 x 33 MW

Sulzer, MAN Diesel & Turbo

Rio Negro

Brazil

164.9

HFO

10 x 18V46

Wrtsil

Campina Grande

Brazil

164

HFO

2010

20 x 20V32

Wrtsil

King Salmon

California

163

Nat gas, diesel

2010

10 x 18V50DF

Wrtsil

Planta Arizona

Guatemala

160

HFO, LFO

2003

10 x 18V46

Wrtsil

Sapugaskanda

Sri Lanka

160

HFO

1984

16 x 10 MW

SEMT, MAN D&T, Siemens

Attock

Pakistan

160

HFO

2008

9 x 18V46

Wrtsil

Bangkanai

Indonesia

155

Nat gas

2015

16 x 34SG

Wrtsil

Samsun I

Turkey

150

HFO, nat gas

2003

18V46s + 1x18V50SG, CC, CHP Wrtsil

2010

20 x W20V34SG

Wrtsil

11 x 18V46, CC plant (1xST)

Wrtsil

Source: extract from The largest recip-based power plants worldwide, Modern Power Systems, February 2015, pp. 12-15.

Internal Combustion Engines

85

Load matching
Diesel Combined Cycle (DCC)
&%%4GHGTGPEG#VNCU2QYGT

*GCVDCNCPEGQHC&%%RTQEGUU

Chimney

'PIKPGRTQEGUU
HWGNGPGTI[

Air cooled condenser

%CDNGVTCHQNQUUGU
#WZKNKCT[RQYGT

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'NGEVTKECNRQYGT

'NGEVTKECNRQYGT

'NGEVTKECNRQYGTVQITKF

6TCPUHQTOGT
UVCVKQP

Exhaust steam duct

4CFKCVKQPKPEN
IGPGTCVQTNQUUGU
Steam turbine

Gearbox

Condensate tank
Condensate pump

.6EQQNKPIYCVGT

LP steam
LP steam drum
HP steam drum

*6EQQNKPIYCVGT

HP steam

Alternator

*6EQQNKPIYCVGT
Silencer

5VGCO
RTQEGUU

Exhaust gas boiler

Boiler bypass
LP feed water pump
HP feed water pump

.QUUGU

HT cooling water

ed water tank

'ZJCWUVICUJGCV

To radiator

#WZKNKCT[UVGCO

Engine

From radiator

Condensate pre-heater

4GEQQNKPI

4GHGTGPEG2TQLGEV
'ZCORNGHQTC&%%RQYGTRNCPV

CP[QVJGTGPIKPGEQPIWTCVKQPKURQUUKDNG
0QCPFGPIKPGV[RG
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&%%PGVRNCPVQWVRWV
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 8
M9

M9
M9


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Source: MAN Diesel & Turbo Power Plant Programme

Internal Combustion Engines

86

Load matching
Combined Heat and Power generation (CHP)

*QVYCVGTIGPGTCVKQPHQTFKHHGTGPVCRRNKECVKQPU

'PGTI[QYFKCITCOHQTJQVYCVGTCRRNKECVKQPU
Fuel Input 100%

%#%
*6*GCV
4GEQXGT[

%#%

&KUVTKEV
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0GVYQTM

Electricity to
Grid 45.5%

Low temperature
heat 5%
Losses 9.2%

9*4$

High temperature
heat 39.5%

$[RCUU

Plant auxiliaries,
trafo losses
0.8%

'NGEVTKEKV[

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output 46.3%

%JKOPG[

Heat to heat
consumers 44.5%

$CEMWR
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Total CHP efciency 90%

 $CUGFQP8)+51EQPFKVKQPUCPFGHEKGPEKGUXCNKFHQT
 4GVWTPNKPGVGORGTCVWTG%
 5WRRN[NKPGVGORGTCVWTG%

Source: MAN Diesel & Turbo Power Plant Programme

Internal Combustion Engines

87

Load matching
CHP systems: example of state-of-art engine performance
Key Performance Data

Ins

Performance Data

J920
(50Hz / 1,000 rpm)

J920
(60Hz / 900 rpm)

Electrical Output

9,500 kW

8,550 kW

En
Ge

Electrical Efficiency 48.7%

48.7%

Heat Rate

7,392 kJ/kWh

7,392 kJ/kWh

Thermal Output

8,100 kWth

7,300 kWth

Total Efficiency

90%

90%

TC

Output and efficiency at generator terminals, ISO 3046,

Nat. Gas MN >80, Power Factor 1.0, 500 mg/Nm3 (@ 5% O2) NOx, Efficiency at LHV

Installed Dimensions
Engine

Length

Width

Height

Weight

8.4 m

2.9 m

3.3 m

87 t

Generator

5.2 m

2.5 m

2.9 m

54 t

TCA Module

3m

6.4 m

3.4 m

36 t

GE Power & Water, J920 FleXtra Jenbacher

Internal Combustion Engines

88

Load matching
CHP systems: example of state-of-art engine performance

technical data
Configuration
Bore (mm)
Stroke (mm)
Displacement/cylinder (lit)
Speed (rpm)
Mean piston speed (m/s)

V 60
190
220
6.24
1,500 (50 Hz); 1,500 with gearbox (60 Hz)
11 (1,500 1/min)
Generator set, cogeneration system,
containerized package

Scope of supply
Applicable gas types

Natural gas, flare gas, biogas, landfill gas, sewage

gas. Special gases (e.g., coal mine gas, coke gas,
wood gas, pyrolysis gas)

Engine type
No. of cylinders
Total displacement (lit)

J612 GS
12
74.9

J616 GS
16
99.8

J620 GS
20
124.8

J624 GS*
24
149.7

Dimensions l x w x h (mm)1
Containerized package
Generator set

J612 - J620 15,000 x 6,000 x 7,300

J612 GS 7,600 x 2,200 x 2,800
J616 GS 8,300 x 2,200 x 2,800
J620 GS 8,900 x 2,200 x 2,800
J624 GS* 12,100 x 2,450 x 2,900
J612 GS 7,600 x 2,200 x 2,800
J616 GS 8,300 x 2,200 x 2,800
J620 GS 8,900 x 2,200 x 2,800
J624 GS* 12,100 x 2,450 x 2,900

Cogeneration system

Weights empty (kg)1

J612 GS
20,600
21,100

Generator set
Cogeneration system

J616 GS
26,000
26,500

J620 GS
30,700
31,300

J624 GS*
49,900
49,500

1) Dimensions and weights are valid for 50 Hz applications.

*J624 with 2-stage turbocharging

outputs and efficiencies

Natural Gas
NOx <

500 mg/m

3
N

250 mg/m3N

1,500 rpm | 50 Hz

1,500 rpm | 60 Hz

Type

Pel (kW)1

el (%)

Pth (kW)

th (%)

tot (%)

Pel (kW)1

el (%)

Pth (kW)

th (%)

tot (%)

612
616
620
624*
612
616
620
624*

2,004
2,679
3,352
4,313
2,004
2,679
3,352
4,313

44.8
44.9
44.9
46.1
43.5
43.6
43.7
44.3

1,883
2,510
3,110
3,931
1,932
2,575
3,211
4,101

42.0
42.0
41.7
41.6
42.0
41.9
41.8
42.1

86.8
86.9
86.6
87.7
85.5
85.6
85.5
86.4

1,984
2,652
3,319

44.3
44.4
44.5

1.902
2.535
3.141

42.5
42.5
42.1

86.8
86.9
86.6

1,984
2,652
3,319

43.1
43.2
43.2

1.952
2.601
3.244

42.4
42.4
42.3

85.5
85.6
85.5

Type

Pel (kW)1

el (%)

Pth (kW)

th (%)

tot (%)

Pel (kW)1

el (%)

Pth (kW)

th (%)

tot (%)

612
616
620
612
616
620

1,818
2,433
3,044
1,818
2,433
3,044

42.8
42.9
43.0
42.3
42.4
42.5

1,787
2,385
2,982
1,805
2,405
3,008

42.1
42.1
42.1
42.0
42.0
42.0

84.9
85.0
85.1
84.3
84.4
84.5

1,800
2,408
3,013
1,800
2,408
3,013

42.4
42.5
42.6
41.9
42.0
42.1

1,805
2,409
3,012
1,823
2,429
3,038

42.5
42.5
42.5
42.4
42.4
42.4

84.8
85.0
85.1
84.3
84.4
84.5

*J624 with 2-stage turbocharging

Biogas
NOx <

500 mg/m3N

250mg/m3N

1,500 rpm | 50 Hz

1,500 rpm | 60 Hz

1) Electrical output based on ISO standard output and standard reference conditions according to ISO 3046/I-1991 and p.f. = 1.0 according to VDE 0530 REM with respective tolerance;
minimum methane number 80 for natural gas
All data according to full load and subject to technical development and modification.

GE Power & Water, Jenbacher Type 6 Gas Engines

Internal Combustion Engines

89

Load matching
Ground propulsion: ideal performance characteristics
In order to make the best use of the power available on the
whole operating range, the ideal performance map requires:
o constant power output
o torque decreasing (with a hyperbolic law) with the
rotational speed
At low speeds, before reaching maximum power output, the
performance map should have the following characteristics:
o constant torque (limited by tire-ground adhesion conditions)
o power linearly increasing with rotational speed
In order to approximate the ideal characteristic as much as
possible, a gear shift (and a clutch) is needed in order to
change the speed ratio between engine shaft and wheels:
the plot on the right shows the use of a discontinuous fourspeed gearbox
Force transmitted to the wheels is proportional to torque

Image source: M. Ehsani, Y. Gao, S.E. Gay, A. Emadi, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles, CRC Press LLC, New York, 2005

Internal Combustion Engines

90

Load matching
Ground propulsion: vehicle resistance
Vehicle resistance is caused fundamentally by
4 phenomena:
1. tire rolling resistance (
2. grading resistance (

3. aerodynamic drag (

4. acceleration (

cos )
sin )
)

d /d )

Taking into account constant cruising speed

conditions ( = 0), rolling and grading
resistance do not depend on vehicle speed
(at least if the influence of on is neglected),
while aerodynamic drag increases with the
square of the vehicle speed:
=

Engine power output thus depends on vehicle

speed according to the following equation:
+
=
Image source: M. Ehsani, Y. Gao, S.E. Gay, A. Emadi, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles, CRC Press LLC, New York, 2005

Internal Combustion Engines

91

Load matching
Ground propulsion: vehicle resistance
Aerodynamic resistance (drag)
=
High pressure

Rolling resistance
=

+
Low pressure

P
Moving direction

Moving direction

Moving direction

r
rd

Vehicle Type

Coefficient of Aerodymanic Resistance

Px

z
Open convertible

0.50.7

Van body

0.50.7

(a)

Ponton body

0.40.55

FIGURE 2.2
Tire deflection and rolling resistance on a (a) hard and (b) soft road surface

Wedge-shaped body; headlamps

and bumpers are integrated into
the body, covered underbody,
optimized cooling air flow

0.30.4

Headlamp and all wheels in

body, covered underbody

0.20.25

K-shaped (small breakway

section)

0.23

Optimum streamlined design

0.150.20

Trucks, road trains

Buses
Streamlined buses
Motorcycles

(b)

Rolling Resistance Coefficients

0.81.5
0.60.7
0.30.4
0.60.7

Conditions

Rolling resistance coefficient

Car tires on concrete or asphalt

Car tires on rolled gravel
Tar macadam
Unpaved road
Field
Truck tires on concrete or asphalt
Wheels on rail

0.013
0.02
0.025
0.05
0.10.35
0.0060.01
0.0010.002

Source: M. Ehsani, Y. Gao, S.E. Gay, A. Emadi, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles, CRC Press LLC, New York, 2005

Internal Combustion Engines

92

Load matching
Ground propulsion: specific consumption at constant speed
Top chart shows that with a given power
output (corresponding to a given vehicle
speed), the fuel consumption is usually
lower at low engine speed than at high
speed.
The bottom chart shows the operating
points of an engine at constant vehicle
speed, with the highest gear and the second
highest gear.
The engine has a much lower operating
efficiency in low gear than in high gear.
Therefore, the fuel economy of a vehicle
can be improved with more gear
transmission or continuous variable
transmission.

Image source: M. Ehsani, Y. Gao, S.E. Gay, A. Emadi, Modern Electric, Hybrid Electric, and Fuel Cell Vehicles, CRC Press LLC, New York, 2005

Internal Combustion Engines

93

Pollutant formation and control

SI engines: formation mechanisms

Oil layers absorb HC

NO forms in high-temperature
burned gas

CO present at high T
or with fuel-rich mixtures

As burned gases cool,

first NO chemistry, then CO chemistry
freezes

Outflow of HC
from crevices;
some HC
burns

Deposits
absorb HC

Oil layers
desorb HC
Piston
scrapes HC
off walls

Entrainment of
HC from wall into
bulk gas

Unburned mixture
forced into
crevices

COMPRESSION

Deposits desorb HC

COMBUSTION

EXPANSION

EXHAUST

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

94

Pollutant formation and control

SI engines: influence of air/fuel ratio
Carbon monoxide (CO) increases rapidly
as the excess air decreases (rich mixtures);
it is very low for lean mixtures.

Nitrogen
oxides

Fuel
consumption

Fuel consumption [g/MJ]

Lean
mixture

Carbon monoxide CO [%]

Unburned hydrocarbons [ppm as C1]

The formation of nitrogen oxides (NOx)

is facilitated by high temperatures and high
oxygen content: maximum emissions are
found for slightly lean mixtures ( 0,9).

Rich
mixture

Nitrogen oxides [ppm as NO]

Unburned hydrocarbons (HC) too are high

for rich mixtures, and decrease as the
air/fuel ratio increases even beyond the
stoichiometric ratio, up to a threshold level
beyond which a fraction of the HC are not
oxidized during the final part of the working
cycle due to the decrease in temperature.

Air equivalence ratio

Unburned
hydrocarbons

Carbon monoxide

Air/fuel ratio

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

95

Pollutant formation and control

SI engines: typical emissions with no control system

Operating mode ->

Idle

Acceleration

Constant speed

Deceleration

CO2 [%]

9,5

10,5

12,5

9,5

CO [%]

2,0

2,0

0,4

2,0

4000

2500

2000

20000

100

1500

1000

100

HC [ppm as C1]
NOx [ppm as NO]

Values given as volume fractions on a dry basis.

All main pollutants are emitted in significant quantities in all operating modes.
Therefore, a pollutant control system that can operate at the same time on CO, HC, NOx is needed.

Source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

96

Pollutant formation and control

SI engines: pollutant emission control
Exhaust gas recirculation (EGR)
It allows to control NOx formation by
diluting the mixture with inert gases,
thus achieving the same result given
by an increase in air/fuel ratio
without the corresponding increase
in oxygen available.

Air intake

Recirculation
valve switch

Control unit

Three-way catalytic converter

It performs the oxidization of CO, HC and
the reduction of NOx at the same time,
thanks to catalysts (noble metals such as
platinum for oxidization or rhodium for
reduction) capable to promote chemical
reactions even at relatively low
temperature.

EGR

Recirculation
valve

Oxygen
sensor

Fuel
pump

Fresh mixture
Exhaust gas
Three-way
catalytic converter

Exhaust

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

97

Pollutant formation and control

SI engines: pollutant emission control

1. air intake; 2. electronic control unit; 3. oxygen sensor; 4. three-way catalytic converter; 5. silencer;
6.pollutant inlet; 7. honeycomb ceramic monolith; 8. reactor casing; 9. exhaust gas outlet; 10. schematic of the catalysis process
Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

98

Pollutant formation and control

Conversion efficiency for all three main
pollutants depends heavily on the
equivalence ratio.
The effect of air/fuel ratio on efficiency in
NOx reduction and in CO and HC
oxidization is opposite.

Conversion efficiency [%]

SI engines: three-way catalytic converter effectiveness

Operating
range

Rich mixture

Therefore, it is necessary to control the

air/fuel ratio within a very small range
of the stoichiometric ratio.
In order to control the air/fuel ratio an
oxygen sensor in the exhaust gas flow is
required (lambda sensor).

Lean mixture

Air/fuel ratio

Air

Air

Platinum
electrodes

Ceramic

Exh.
gas
Cathode

Anode
Zirconia

Exhaust
gas

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

99

Pollutant formation and control

CI engines: formation mechanisms
Zone A: partial oxidation products (especially HC)
Zone B: complete oxidation products

Injection tail
Injector
hole

Zone C: at part-load, complete oxidation products

and NOx; at full load, partial oxidation products and
particulates
Zone D: partial oxidation products and particulates

Image source:
G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996 (top)
J.B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, New York, 1988 (bottom)

Internal Combustion Engines

100

Pollutant formation and control

CI engines: typical emissions with no control system

Operating mode ->

Idle

Acceleration

Constant speed

Deceleration

CO2 [%]

1,0*

11,0

7,0

--

CO [%]

0,4

0,2

0,04

--

1000

600

400

1000

100

2500

1000

100

HC [ppm as C1]
NOx [ppm as NO]

Values given as volume fractions on a dry basis.

CO and HC emissions in CI engines are particularly low (comparable to those of a large SI engine
equipped with a three-way catalytic converter), thanks to the high air/fuel ratio.
On the other hand, particulate emissions are very high, so that specific particulate traps are needed,
and also NOx emissions are on the same level of SI engines; NOx emissions can be controlled in CI
engines by means of specific catalytic converters (SCR, Selective Catalytic Reduction).

Source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

101

Pollutant formation and control

CI engines: particulate filter
A particulate filter is usually based on ceramic
honeycomb monolith traps: a ceramic monolith is
divided into a high number of parallel channels (cells),
alternately blocked and separated by a thin porous
wall (~0,3 mm), with pores having average size of 12
35 m. Exhaust gas is therefore forced to flow through
the porous walls, which carry out the filtering function.
Characteristics of this filter:
high efficiency (> 90%)
low pressure loss
(from 2 kPa when clean up to 20 kPa when dirty)
high mechanical and thermal resistance
It is necessary to regenerate periodically the filter: it
cleans the filter by means of an ad hoc, local
combustion process, which can be activated by
different means, such as an auxiliary burner, or a late
injection of fuel (post-injection in multijet systems).

Exhaust gas
inlet

Exhaust gas
outlet

Burner

Exhaust
gas

1st step:
soot
deposits

2nd step:
regeneration
starts

3rd step:
flame
propagation

Clean filter

Image source: G. Ferrari, Motori a combustione interna, Il Capitello, Torino, 1996

Internal Combustion Engines

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