FUEL AND COMBUSTION 2

CHAPTER IN A NUTSHELL
Combustion 30T
Combustion may be defined as a relatively
chemical combination of hydrogen and carbon in therapid
fuel
with the oxygen in the air resulting in liberation of
energy
in the form of heat.
The conditions necessary for combustion chamber
20
1 The presence of a combustible mixture.
2. Some means of initiation combustion.
3. Stabilization and propagation of flame in the
combustion chamber.
Ignition Limits
10
Spark;
Motoring (Non fir ng)
Ignition ofthe charge is only possible within certain limits ***....n---
offuel-air ratio. It is called ignition limits.
The fuel air must be between about 1:30 and 1:7 80 60 40 20 TDC 20 40 60 80
Stages of Combustion in SI Engine Fig.
and Requirements of Combustion
In a spark-ignition engine a sufficiently homogeneous Types
mixture of vapourized fuel, air and residual gases is ignited Chamber (SI engine)

by a single intense and high temperature spark

between Types of Combustion Chamber: The most
the electrodes, leaving
spark plug behind a thin thread of commonly accepted combustion chamber designs are
flame. asfollows:
the (1) T-head combustion chamber: This design has
According to Recardo's theory of combustion,
combustion can be imagined as ifdeveloping in three stages. the following disadvantages: (a) Having two cam shafts.
(6) Being very prone to detonation.
and development of a semi propagating
(1) The growth (2) L-head or side valve combustion chamber
nucleus of flame called ignition lag of preparation A side valve engine has an advantage both from a
manufacturing and maintenance point of view. It is easy
phase. the combustion
(2) The spread of the flame throughout to enclose and lubricate the valve mechanism. But it
gave
a poor performance because of the following main defects:
chamber.
(3) After burning. () Lack of turbulence as the air had to take two right
angle turns to enter the cylinder and in doing so lost
Abaormal Combustion in SI Engine much of its initial velocity.
travels across the
If the flame started by the spark (ii) Extremely prone to detonation due to large flame
it is called nomal
combustion chamber in a fairly even way, length.
conditions the
combustion. But under certain operating (i) Extremely sensitive to ignition timing due to slow
combustion deviates from its
normal course which result combustion procesSS.
in loss of performance and possible
damage to the engine. (3) Overhead valve or 1 head combustion
The possible abnormal
combustion are chamber: These engines are most commonly used. In
Detonation or Knocking these engines overhead valve combustion chamber both
(1) the valves are in the head. These
(2) Pre-ignition side valve or L head
engines are superior to
3) Run out
engine at high compression ratio.
 B.Tech. (VII em) ME. Solved Papers
ICE.20 had time to spread thernelves over a wide area and they
(4)F-head combustion chamber : Combustion
chamber in which one valve is in head and other in block have fresh aír all around then.
chamber. This design is
are knowm as F-head combustion
The of rapid or uncontrolled combustion is
perid
compromise between side valve (L-head) combustion to the poínt off
chamber and overhead valve (I-head) combustion counted from the end of delay period
indicator diagram
chamber. maximurn pressure on the
Combustion: At
(3) Third Stage - Controlled and pressure are
the end of vecond stage the temperature
so high that the fuel droplets injected
during the last stage
further pressure rise
bur almost as they enter and any
mechanical means i.e. by the
c a n be controlled by purely

injection rate.
Fig
first proposed by
To these stages of combustion,
added-late burning or after
Ricardo, a fourth stage can be
in all cases.
burning This stage may not be presented
(4) Fourth Stage Aster Burning: Theoretically
shall end after the
it is expected that combustion process
Fig third stage. However, because of poor
distribution of the
fuel particles, combustion continues during part of the
remainder of the expansion stroke. This afer burning can
be called the fourth stage of combustion.
Diesel Knock: If the delay period or ignition lag is
long in the diesel engine, a large amount of fuel will be
injected and accumulated in the chamber. The auto ignítion
ofthis large amount of fuel may caused knocking in diesel
engine.
A long delay period not only increase the amount of
fuel injected by moment of ignition, but also improves the
homogeneity of the fuel-air mixture and its chemical
Fig.
Fig.: Overhead valve combustion chamber design preparedness for explosion type self ignition similar to
detonation in SI engine.
Combustion in CI engine : In the CI engine, air
alone is compressed through a large compression ratio (12:1 Fuels: Fuel must have certain Physical, Chemical
to 22:1) during the compression stroke raising its pressure and Combustion properties such as :
and temperature. In this highly compressed and highly (1) High energy density.
heated air in the combustion chamber one or more jets of (2) High thermal stability
fuel are injected in the liquid state, compressed to a high
(3) Good combustion qualities.
pressure of 110 to 200 bar by
means
of a fuel pump. (4) Low deposit forming tendencies.
Stages of Combustion in CI Engine Ricardo :
considered CI engine combustion as taking place in hree
(5) Compatibility with the engine hardware.
distinct stages. (6) Good fire safety.
(1) First Stage - Ignition Delay Period: During (7) Low toxicity.
this period some fuel has been admitted but has not been (8) Lowpollution.
ignited. The ignition delay period is counted from the
start
9) Easy transferability and on board vehicle storage
of injection to the point where P-6 curve separates from Structure of Petroleum : Petroleum is basically a

the pure air compression curve. It is termed as mixture of hydrocarbons, compounds which are made up
preparatory phase. exclusively of carbon and hydrogen atoms. In addition
it
Rapid or Uncontrolled other compounds having
(2) Second Stage may contain small quantities of
-

Combustion : In this second stage the pressure rise is sulphur, oxygen and nitrogen.
rapidbecause during the delay period the fuel droplet have
I.C. Engines -ICE.21
Basic families of a
of Cetane number
hydrocarbons Rating of CI Engine
Fuels :
volune of
normal

Family of the percentage by

General fuel is defined as c e t a n e and
a-methyl
hydrocarbons Formula Molecular
Structure Saturated/Stability c e t a n e in a
mixture of normal
has the same ignition
characteristics
Paraffin C H2n- 2 Chain Unsaturated
Saturated Stable
naphthalene which
combustion is camiad out in a
standand
Olefin CHn Chain as the test fuel when conditions
Naphthalene C,H2n Ring
Unsaturatcd Unstable engine under specified
operating
Saturated Stable
Aromatic C H2n-6 Alternative Fuels
Ring Highly Most fuei because
Coal in an altermative
unsaturated unstable | (1) Solid Fuels : available.
SI
Engine Fuels : Gasoline which is mostly used in of large supply which is
the present day. The (2) Liquid Fuels
requirement ofan ideal gasoline
(1) It should mix readily with air and afford uniform Alcobol
manifold distribution i.e. it should ) Methanol
(2) It should not pre-ignite easily. easily vaporize. ) Ethanol
(3) It should be easy to handle. Water-Gasoline mixture for Sl Engine
iv)
(4) It should be cheap and should be available (3) Gaseous Fuels
everywhere. Hydrogen
(5) It must have a high calorific value. mixture of
Natural gas is a
(6) It should not form gum and varnish.
Natural Gas : with
components, consisting mainly of
methane (60-95%)
CI Engine Fuels: Diesel fuels are petroleum small amounts hydrocarbon
of other
fuel components.
fraction that lie between kerosene and the lubricating oils.
Compressed Natural Gas (CNG)
The properties important for a good diesel fuel Petroleum Gas (LPG)
Liquified
(1) Satisfactory handling and storage. Other Possible Fuels
(2) Smooth and efficient burning.
Continued cleanliness during user. )Biogas
(3)
Rating of SI Engine Fuels Rating of SI engine i) Producer gas
fuels describe by of octane number. ii) Blast furnace gas
The octane number fuel is defined as the percentage, (iv) Coke oven gas
exactly
by volume, of iso-octane and normal heptane, whichstandard (v) Benzol
matches the knocking intensity of the fuel in a (vi) Acetone
conditions. (vi) Diethyl Ether
engine under a set standard operating

PREVIOus YEARS QUESTIONS

5. Low pollution.
6. Easy transportation/transferability and storage.
PART-AA 7. Compatibility with the engine hardware.
8. Good fire safety.
9. Low deposit forming tendency.
LC 10. Economically viable in very large quantities.
desirable propertjes of good 11. Easy mixing with air and low latent heat of
What are the RT.U. 2017, 2015]
Q.1
engines Jfuels? evaporation (h.
12. No chemical reaction with engine components
are designed to satisfy
used in .C. engines system in
through which it flows.
Ans. The fuels of the engine
requirements the
the performance should p o s s e s s
Thus fuel Explain the effect of engine variables on ignition
which they a r e used. Q2
lag in S.l. Engine IR.T.U. 2015
foliowing properties:
density (kJ/kg8)
g h energy
H i Good combustion qualities. Ans. Ignition lag is not a period of inactivity but is a
2 stability chemical process. The ignition lag in terms ofcrank angles
3 High thermal
Low toxicity.
 ICE.22 B.Tech. (VI Sem.) M.E. Solued Papers
is 10 to 20° and in terms of seconds 0.0015 seconds or is also responsible for a gasoline engine when it
"diesels"
so. or continues to run after the ignition is turned off.
Effects of engine variables on ignition lag: Detonation : Detonation occurs when low octane
) Fuel. Ignition lag depends on chemical nature of fuel is used in a high compression engine. Similar to
fuel. The higher the self ignition temperature of fuel, pre-
ignition. But detonation is not caused by hot, glowing
longer the ignition lag. carbon. Detonation is also caused by a lean mixture
or
Gi) Mixture ratio. Ignition lag is smallest for the mixture when the engine is running at a lower RPM than
what is
ratio which gives the maximum temperature. This
mixture ratio is somewhat richer than the
required for the engine load (downshift into a lower gear)
stoichiometric ratio.
Q.5 DiscCuss the suitability of vegetable oil
(ii) Initial temperature and pressure. Ignition lag is in CI. engine. fuel
as a

reduced ifthe initial temperature and pressure are R.T.U. 2012

increased and the initial temperature and pressure Ans. Vegetable Oils
can be increased by increasing the compression Vegetable oils have better ignition qualities for diesel
ratio. engines than light alcohols. There are many vegetable oils
iv) Turbulence. Ignition lag when expressed in degrees which can be used in diesel engines like peanut oil, linseed
of crank rotation increases linearly with engine oil, but most important oil is sunflower oil.
speed. Increasing the engine speed means The basic difference in sunflower oil and diesel oil
leads to the following problem in use of
increasing the turbulence. vegetable oils in
diesel engine.
Q3 Differentiate between petrol and diesel injection. () The viscosity of the vegetable oils is much higher
than that of diesel. It can cause problem in
R.T.U. 2014]
() Vegetable oils are slower combustible.
handling.
Ans. Petrol and Diesel injection (ii) The indirect injections are most suitable than direct
injection.
S. Diesel injection Petrol injection
No.
1. Fuel is directly injected Fuel is injected in the
into the cylinder. inlet manifold. PART-B
2. Injection pressure is |Injection pressure is
high. low.
The moment at which Q.6 Describe the Phenomenon of detonation in C.I.
Timing is by no means
fuel injection critical and the fuel may Engine. R.T.U. 2017
commences has a very be injected during the
Ans. Detonation Phenomenon in CI
important influence. suction stroke. Engine :
4 Fuel is injected at the Fuel is injected during Detonation is the sound produced by the very
rate of pressure rise
rapid
end of the compression the induction stroke. during the early part of the
uncontrolled second phase of
stroke. combustion. The
primary cause of an excessively high pressure rise
is due to aprolonged delay period.
Q.4 Differentiate between detonation and knocking.
An
[R.T.U. 2014 exten_ive delay period can be due to the
followingfactors
Ans.Knocking: Knocking occurs when there are two (a) A low design compression ratio permitting only a
ignition the spark plug and a hot spot (usually
sources of -

marginal self-ignition temperature to be reached.

glowing carbon chip in a high mileage engine). That (b) A low combustion pressure due to worm
creates two flame fronts in the combustion chamber. or badly seating valves.
piston rings
When the two flame fronts meet,
you hear the knocking (c) Poor-fuel ignition quality; that is a low cetane
sound. That hot carbon that starts the
is called pre-ignition. Pre-ignition is
burning of the fuet number fuel.
when the two flame fronts meet. That het
followetby the knock (d) A poorly atomized fuel spray preventing early
carbon deposit ignition.
 ICE.23
I.C. Engines diesel engine
is
() An inadequate delay in a
injector need is delay : The ignition the start of
injection
producing coarso droplet formation. spring load Ignition
defined as the time
interval
between
consists of
This delay period
80F and the start ofcombustion.
wherein atomisation,
vaporization
and
Diescl (a) physical delay, delay
attributed
70 knock and (h)
ofchemical

mixing ofair fuel

occur

reactions. Physical
and chemical delays
the method adapted
pre-combustion
Normal to
To reduce NOx,
5o combustion occur simultaneously.
delay. For
is to reduce the ignition
in modern engines therefore, the
Injoction
Point heat release in modern engines,
predicting

Compres ion presure

However,
Shpr
estimation ofignition delay
is no more important.
dolay level that
period to such a low
the coiling on NOx is dipping
1,ong
delay accurate prediction of ignition
delay has become important
periód sprays is a strong
cven ifit is small. Ignition delay ofdiesel
60 40 20 T.D.C. 20 40 600 and pressure. However,
function of ambient temperature in
Before ANer been modeled satisfactorily
the physical delay has not
Fig the literature.
A very long ignition lag after
injection causes a large Turbulence is necessary to break
proportion of the fuel discharge to enter the cylinder Ans.(ii) Turbulence:
and to atomize before ignition and the each and every part of
propagation the flame front into pieces so that
of burning actually occurs. Accordingly, when combustion chamber gets flame to ignite the homogeneous
combustion does commence a relative amount of air fuel mixture. Ifthere is flame distribution then
uneven

heat energy will be released almost immediately, there is incomplete combustion which gives
less torque
this and also causes pollution. In case of CI engines
fuel is
correspondingly produces the abnormally high
rate of pressure rise, which is mainly responsible injected in the highly compressed air. There is always
for rough and noisy combustion process under these near the injector will get
conditions.
possibility that the portion of air
more fuel and thus again farthest point from injector will
In order to decrease the tendency of knock it is
get less fuel supply. This again gives rise to unequal fuel
necessary to start the actual burning as early as distribution and incomplete combustion.
possible after the injection begins. In other words, Swirl: Swirl is the rotational flow ofcharge within
it is necessary to decrease the amount of fuel
of the first few the cylinder about its axis. Swirl is generated by constructing
present when the actual burning
the intake system to give a tangential component to the
droplets start.
intake flow as it enters the cylinder. This is done by shaping
and contouring the intake manifold, value ports and even
Q.7 Diferentiate between: the piston face. Swirl greatly enhances the mixing of air
() Delay period and gnition delay and fuel to give a homogenous mixture in the very short
Turbulence & swirl R.T.U. 2014] time available for this in modern high speed engines. It is
(i)
also a main mechanism for spreading of the flame front
during the combustion Process.
: In C.I (Compression ignition)
Ans.(i) Delay Period
is in atomised from is considerably There is difference between turbulence and swirl.
engine, the fuel which
air in the cylinder. Although Turbulence is random movement but swirl is orderly
colder than the hot compressed
instantaneous, an appreciable movement which is particularly used for CI engines.
the actual ignition is almost This
combustion is in full progress.
time elapses before the It
the delay period or ignition lag. Q.8 What is meant by knock rating? Explain how
time occupied is called fuel during
following injecton ofthe cetane no. ofa diesel fuel sample is decided
is the time immediately and the pressure
is being initiated
which the ignition process would have due to RT.U. 2014)
the value it
does not rise beyond
compression of air. Ans. Knock rating : It determine whether or not a fuel
about 13,
movement
for will known in a given
The delay period
extends
decreases with engine under
the given operating
time for which
it occurs
conditions.
of the crank. The
increase in the engine speed.
 ICE.24- B.Tech. (Vn Sem.) M.E. Solued Papers
Since knock is an abnormal from
that
of combustion in a
special engine with primary reference fuels. The
strongly limits the engine
performance, the fuel reference fuels are normal cetane, C,H which is
resistance to knock is a
very important
fuel has to be used in property, if the arbitrarily assigned a cetane number of 100 and
alpha
spark ignition engines. methyl naphthalene, C,,H with an assigned cetane
Really knock is a complex phenomenon strongly number of 0. Cetane number of a fuel is
defined as the
influenced by engine design and almost every
operating
variable, but it primarily depends on the fuel anti-knock
percentage by volume of normal cetane in a mixture of
normal cetane and a-methyl
naphthalene
which has the
property. In particular hydrocarbons show very different same ignition characteristics (ignition delay) as the test
abilities in resisting to knock, depending on their molecular
size and structure. Moreover it has been clearly
fuel when combustion is carried out in a standard
engine
proven under specified operating conditions. Since
that even the relative knocking tendency of any two fuels ignition
is the primary factor in controlling the initial auto delay
significantly depends on the method used to compare them. in the CI engine, it is reasonable to conclude that
ignition
knock
Thus it was recognized that a comparative scale of the
fuel knocking resistance must be based on the use of
should be directly related to the ignition delay of fuel.
the
Knock resistance property of diesel oil can be
standardized test engine, operation conditions, improved
by adding small quantities of compounds like amyI nitrate,
measurement of knock intensity and comparison with ethyl nitrate or ether.
reference fuels. Cetane number : The cetane rating of diesel fuel
Rating of SI Engine Fuels is a measure of its ability to autoignite quickly when it is
Resistance to knocking is an extremely important injected into the compressed and heated air in the engine.
characteristic of fuel for spark-ignition engines. These Though ignition delay is affected by several engine design
fuels differwidely in their ability to resist knock depending parameters such as compression ratio, injection rate,
on their chemical
composition. A satisfactory rating injection time, inlet air temperature etc., it is also dependent
method for comparing to antiknock qualities of the various on
hydrocarbon composition of the fuel and to some extent
fuels has been established. In addition to the chemical on its volatility characteristic.
characteristics of hydrocarbons in the fuel, other operating The cetane number is a numerical measure of the
parameters such as fuel-air ratio, ignition timing, dilution, influence the diesel fuel has in determining the ignition
engine speed, shape of the combustion chamber, ambient delay. Higher the cetane rating of the fuel lesser is the
conditions, compression ratio etc. Affect the tendency to propensity for diesel knock.
knock in the engine cylinder. Therefore, in order to
determine the knock resistance characteristic of the fuel, 16 High volat itity fuel
the engine and its operating variables must be fixed at Low volatility fuel
standard values.
13-4
According to a standard practice, the antiknock
value of an SI engine fuel is determined by comparing its
antiknock property with a mixture oftwo reference fuels, 10-7

iso-octane (CH) and normal heptane (C,H). Iso-

octane chemically being a very good antiknock fuel, is
arbitrarily assigned a rating of 100 octane number. Normal
heptane (C,H,), on the other hand, has very poor
antiknock qualities and is given a rating of0 octane number.
The Octane number is
fuel defined as the percentage, by
volume, of iso-octane in a mixture of iso-octane and normal
heptane, which exactly matches the knocking intensity of 27
the fuel in a standard engine under a set of standard
operating conditions.
Rating of CI Engine Fuels C D
In compression-ignition engines, the knock Engine
resistance depends on chemical characteristics as well Fig. 1: Effect of fuel volatility on NO, levels measured over
as on the operating and design conditions of the engine. 13-mode. Federal test cycle for 4-stroke DI (diesel engine). A
turbocharged vee, is naturally aspirated vee, C naturally
Therefore, the knock rating of a diesel fuel is found by
aspirated ignite and D turbocharged low emission engine).
comparing the fuel under prescribed conditions of operation
(1Rnylne
ICL 23)
ta Mite ttee,
cctane nmber is
ctually measured in detonalion

uile eylinder (TR test engine. ln the stndard test. IncTease

the
whleh

l iutee tion ia begundegrees belore lop dead centre

9.9 Wrlhe flve factors where as they
wll

l e n y In S.I engine
an
Give detall
the compONwion rato of the engine in ndjusted till the tn a dlesel englne,
Control knncking TU.2914
ition
ignin bopinsntoxaotly lopdend centro, i.o., nn ignition explanaton.
delay of 3 degre. iu nchiovod. 'Tho nctunl numerical
alues ane net by noN Ol roloronco fuol, The cotanG deonation 15 9
Ans, The factr that tend o increase This is
nber of a lel ia the percenlnge by volume ofcetane in ín C.I engine.
a mixtue of cetanean u iethylnaptlulono (CM,CH) S18ine tond to reduce knoxking
justified by the following facvs: ratio
that has the same performanee in the ntandard test ongine
Compression ratio : If the
umpression
) the
As that ofthe fuel. Cetane (C,)is arbitrarily givena and pressure at
Increaed then air temperature
nunber 100 and ortgialy d-nethylnapthalono was given íncreases, thereby
end of compression also
a mmber 0 but now releronce in lheptnncthylnonane which So the tendency tor
valuo 15,
decreasing the delay period. the compression.
is iven the detonation increases by increasing
Inble: lgntion qunlity test dlosel fuels. increase in the inlet
i) Inlet temperature : Any
at the end
ngine Specd 900 rmp emperaturc increases the temperature
100C the terdency
Jacket nir tenperaturo of the compresion thecreby increasing
Inlet nir temperature 65.6°C constant at 13° bofore to knocking case of S. engines.
TDC The self-ignition
(i)Self-gnition temperature: of auto-ignition of
Injection advanco 13 fixod injection point temperature is the temperature
S.I engines.
gnition delay |Pressure rising at TDC charge. This causes detonation in
ato
For a gasoline engine wo are concernced with However, in case of C.I engines, the early
resistanee to spontaneous ignition as measured by octane ignition is necessary to avoid knocking
nmumber. For the diesel, case of spontancous ignition or (iv) Time-lag of ignition of fuel: The time-lag of
cetane number becomes the critoria. Octan and cctanc ignition of fuel should be sort to avoid knocking in
therefore are inverse measurements of the samo property. C.I engines. However short ignition time causes
This relationship for some typical fuol is shown in Fig. 2. knocking in S.I engines.
A simple, though not rigorous, relationship between octane (v) Combustion chamber wall temperature : If the
number and cetane number is temperature of combustion chamber wall is high
104-ON then auto-ignition of the charge takes place, causing
CN detonation early in the S.I engine. On the other hand
2.75
quick auto-ignition due to high wall temperature
Ho.1Diesel
t A

o.2Desd
helps in reducing knocking in C.I engine.
AV

Q.10Diferentiate between an IDI & DI engine

OiscFuel
RT.U. 2014
40 Ans. IDI -Indirect Injection Diesel: IDI diesel engines
G a s e l i ns e

utilize a pre-combustion chamber, generally referred to

as a pre-chamber. Fuel is injected into the
20 pre-chamber,
vhere it rapidly mixes with oxygen and ignition occurs.
As the flame front expands in the pre-chamber, it forces
fuel to enter the combustion chamber rapidly, effectively
20 60
Research octane No. mixing the fuel with air in the cylinder and atomization is
Fig. 2: Fuel octance cetane relationship achieved. The glowplug is also located in the pre-chamber,
It is clear that diesel engine fuel is a bad
a good and the shape of the pistons in an IDI resemble those of a
cetance
gasoline engine fuel. Diesel fuels have typical
gasoline engine.
alng of 401o 60 while high octane fuels as gasoline whicn DI Direct Injection Diesel : DI engines inject fuel
numbers of about directly into the combustion chamber, right into the top of
are difficult to autoignite have cetane as a diesel fucl.
0to 20 indicating their poor suitability
 (ICE.26 B.Tech. (VI Sem.) M.E. Solved Papers
the piston. The pistons on a DI engine have a bowl or
cup greater formation of chemical
machined
into them engines
that the fuel is directed into. DI
knocking. Hence the tendency tospecies
knock
responsible fur
operate at higher injection pressures and therefore more
(ii) Spark advance: When the increases.
complete atomization occurs, meaning these engines do is
gas compressed by the
spark is advanccd,
burnin
not require a pre-chamber to ensure proper diffusion of rising piston and thercfore both
the fuel into the air.
temperature and pressure are increased. Thus
to knock increase
with advanced tendency
IDI Vs. DI spark
pressures are reached further down on timings and peak
IDI engines are known for their and the power stroke
of maintenance since the
simplicity and ease thus of lower
are
magnitude.
injection systems are (iii) Increasing flame travel distance: The
mechanical and the technology more primitive (no of knock is increased by increasing the distancepossibility
computer controls or sensor to malfunction). has to travel in order to the flame
traverse the combustion
IDI and DI engines have distinct differences in (iv) Location of spark plug and exhaust chamber.
sound to a well trained ear. valve:
plug which is centrally located in the combustion A spark
IDI Engines, in general, are
quieter than DI engines,
though many newer DI engines utilize intake baffles
has minimum
tendency to knock as the flame chamber
minimum. The flame travel can be reduced travel is
and advanced injection controls to or more
spark plugs. by using two
suppress sound,
making them extremely quiet. (v)Octane rating of fuel: The
Direct injection is much more efficient than knock is very much affected the tendency of an engine to
indirect by
injection due to the level of atomization that is used. In general, lower the self properties of the fuel
ignition
achieved. the fuel or
greater its pre-flame reactivity, temperature of
the greater the
Naturally aspirated IDI engines have much tendency to knock. Higher the octane
compression ratios than DI turbodiesels. higher the tendency to number,
knock, Paraffin series have the lesser is
TDI engines can be
naturally and aromatic series the
minimum maximum
turbocharged, where as DI enginesaspirated tendency knock.
or to

turbocharged.
are
always Q.12 Discuss the
IDI engines operate at much requirements of a
chamber used in SI good combustion
lower fuel injection engine.
pressures than DI engines. R.T.U. 2012
OR
Injectors for IDI engines are much cheaper than Discuss the measures
DI engines. Lower injection pressure also mean S.I. engine adopted in the design of
I
combustion chambers to reduce the
injectors will outlast DI possibility of knocking
Direct injection diesels are injectors. R.T.U. 2011
efficiency, performance potential, favored due to their
and cleaner exhaust
Ans. The
design of combustion chamber has an
emissions. IDI diesels, influence upon the important
however, have built a reputation engine performance
of reliability and their ease of service/ properties. The design of combustion and its knock
nonetheless. repair is alluring the shape of the combustion chamber, the chamber involves
sparking plug and the disposition of inletlocation of the
Q.11 Discuss the effect valves. Because of and exhaust
of following variables upon the importance combustion chamber
of
detonation in S.I.
engine design, it has been a subject of considerable
() research and amount of
(iii) Compression
ratio
(ii) Spark advance development
resulted in raising the
in the last
fifty years. It has
Tncreasing flame
(iv) Location travel distance the first world war compression ratio
I1om4: from 4:11 before
of spark plug and exhaust period
period 8:1 to
to 8:1 to 11:1 in
(v) Octane rating valve withspecial
specialco
combustion chamber designs andpresent times
of fuel IR.T.U. 2012 knock fuels. suitable anti-
Ans. Effect of following variables
upon detonation in SI Basic requirements of a good Combustion
engine. Chamber: A good SI combustion
() Compression Ratio:
Increasing the compression ratio
increases both the temperature influence the knock property of the chamber design will
and pressure. reduce the engine and will try to
temperature reduces the delay period Increase in tendençy of knock. Thedesign of combustion
of the chamber will ínclude the combustion
Increase in temperature as well as end gas. chamber design,
increase in pressure location of the spark plugs and the
both lead to
greater collisions of molecules exhaust valves. deposition of inlet and
resulting in
(I.C. Engines ICE.27)
combustion engine,
The special measures adopted in design to reduce Ans. Flame speed : In
an internal

the possibility of knock can be discussed accordingly. with the help ofa spark or high
the fuel mixture is burned
(a) To prevent detonation the length a thin film of flame is generated
or flame travel pressure, in both ways
from the combustion chamber to burn all
sparking plug tothe farthest in which travels across the
combustion chamber should be as short as point the possible fuel mixture available,
the rate at which the
This consideration involves the location possible.
of spark flame front travels is called the
flame speed.
plug, the position of valves and the shape of the The compression of the
(1) Air fuel ratio:
combustion chamber. the rate of combustion and
working mixture influences fuels the
(b) The consideration of the length of the flame also the amount of heat evolved with hydrocarbon
dictates that bore of the cylinder should be small. mixture strength
maximum flame velocities occur when
SI engines are, therefore, When the mixture is made
generally limited up to is 110% of stoichiometric.
100 mm bore of cylinder. of flame
leaner or is enriched and still more the velocity
(c) To reduce the possibility of detonation there
should
diminishes. Lean mixture release less thermal energy
e no hot surfaces in the end flame speed.
gas region. Exhaust resulting in lower flame temperature and
valve being a very hot surface should not be in the which results
Very rich mixture has incomplete combustion
end gas region. It means that the exhaust in production of less thermal energy and hence flame speed
valve
should be near the sparking plug. It would also avoid is again low.
surface ignition. (2) Turbulence: The effects of turbulence can be
(d) The compression ratio is limited by the
phenomenon summarized as follows
of detonation. Detonation depends on the design of () Turbulence accelerates chemical action by intimate
combustion chamber. Any change in design that mixing of fuel and oxygen. Hence turbulence allows
improves the anti-knock characteristics of a the ignition advance to be reduced and therefore
combustion chamber permits the use of a higher weak mixture can be burnt.
compressive ratio which should result in higher (i) The increase of flame speed due to turbulence
output and efficiency. reduces the combustion time and hence minimizes
(e) Since turbulence increases the rate of combustion the tendency to detonate.
and hence decreases the tendency to knock. (ii) Turbulence increases the heat flow to the cylinder
Therefore, combustion chamber design must wall and in the limit excessive turbulence may
provide an optimum degree of turbulence. It can extinguish the flame.
be done by providing 'squish' a method of rapid (iv) Excessive turbulence results in the more rapid
pressure rise and the high rate of pressure rise
injectionof gas trapped between the piston and
causes the crank shaft to spring and rest of the
some flat or corresponding surface in the cylinder
engine to vibrate with high periodicity resulting in
head. rough and noisy running of the engine.
to avoid
()Satisfactory cooling ofthe spark plug (points Correct mixture
head which is the
pre-ignition) and exhaust valve
chamber. 006
hottest region ofthe combustion
combustion chamber will
T h e more compact the 005-
knock, since
be the lesser the tendency to produce
time will be shorter. 004
flame travel and combustion
criterions while
These are the various design
chamber to avoid the chances
.003
designing the combustion
other criterions which are 002
of knocking, while there are
and efficiencies.
targeted to increase the output power .001

How does turbulence and .000

Q.13 Hhat is flame speed? alr mixture affect the
equivalence ratio of fuel- IR.T.U. 201
60 80 100 120 140 160 180 200 220 240
Jlame speed? Weak RichStrength
Mixture
Fig
 ICE.28 B.Tech. (VI Sem.) M.E. Solved
Papers
2. Ethanol ers
It is used as automobile fuel for
many years. As
the methanol the development of
PART-C system using mixture of
gasoline and ethanol continues. There are two
which are important-E85 and El0. Ethanol canmixturçs
also be
made from ethylene. Much of it is made of
note on alternative fuel for
1C corn,
.14 Write a short beets, surgarcane. Ethanol has less HC emission sugar
engines R.T.U. 2017
gasoline but more than methanol. than
The methods of production of ethanol are
Ans. Alcohol as an alternative fuel
1. Methanol (a) Ethanol from grains
Methanol is one of the most promising and major
is (b) Ethanol from sugarcane
research. Pure methanol and mixtures of
methanol and Table: Comparison with methanol and Ethanol
and by Gasoline
gasoline in various percentage has tested in engines S.
vehicles. The most common mixtures are M85 and MI10, No.
Property Gasoline Methanol Ethanol
M85 means 85% methanol and 15% gasoline. Some 1. (Chemical formula
of using a random mixture oof
capable 2. Molecular weight
MC,H2n CHOH CHOH
flexible engines
are 112 32.0 46.0
methanol and gasoline. In this an Electronic Monitoring 3. Specific gravity at
Systems (EMS) are used. By using sensors, the 15.5°C
0.7 to 0.75| 0.796 0.794
information is gathered from intake and exhaust and then
4. Boiling point or range 30°C 65°C 78°C
for a fuel
proper fuel ratio, ignition timing and valve timing 5. Vapor pressure at 0.8 0.32 0.21
mixture are used. Methanol can be obtained from many 58°C
sources, both fossil and renewable. These include coal, 6. Octane number
petroleum, natural gas, biomass, wood and even the ocean. a) research 91 114
(a) Methanol from Lignite or Coal 6) motor 82
Synthesis 94
gas 7. Cetane number 8-14 8
Oxygen T o sulphur 8. Lower calorific value 10,500 4700.000 6400.0
HS recovern
Coal or Preparation (Kcal/ke)
lignite Acid gas 9. Composition by
dncd & Gasifier removal
pulvenized coal weight
Ash
CO+H+CO+H$ a)carbon 84.0 37.5 52.0
6)b) hydrogen 16.0 13.0 12.5

Amine
Ko) oxygen NIL 35.050.0
solution Hydrogen : A number of companies have built
automobiles with prototype or modified
CO shift co2 Methanol
Methanol
purification operate on hydrogen.
engines which
Conversion removal synthesis to remove
water AH2 powered car being developed in Melbourne
To adjust CuZn'Cr catalyst
H COCO ratio (Low pressure process) Methanol University Department of Mechanical Engineering
Fig. 1:Schematic Diagram for Methanol Production from Coal is achieving 40 percent energy saving
over
or Lignite conventional petrol engine. The car is a converted
b) Methanol from Municipal Solid Wastes Ford Cortina Wagon which carries enough fuel in
Oxygen 4-cylinders to travel upto 50 km at a speed of 130
Iron Gas
km/h.
Gasifier Acid gas
Sbredded removal
scrubbing removal German cars are so developed that they canbe
*asies
converted for driving either gasoline or H, by making
Slag Waste water to
a few simple adjustments. Presently a few vehicles
treatmen Sulphur
recovery are running on road as there are very few public
filling stations for liquid H2. Many more are planned
COsht CO
converuona removal
Methanol
Methanol for future.
synthesis
punfication Methanol Advantages of hydrogen as 1.C. engine fuel :
Fig. 2: Schematic Diagram for
Methanol Production from . Low emissions. Essentially no CO or HC in the
Municipal Solid Wastes exhaust as there is no carbon in the fuel. Most
exhaust would be H,0 and N2.
LC. Engines) (ICE.29
of
combination
2 Fuel availability. There are chemical
number of different
a defined as relatively rapid
a in the air,
ways of making hydrogen, in the fuel with the oxygen
water. including electrolysis of hydrogen and carbon
of energy in the form of heat.
3 Fuel resulting in liberation for combustion
leakage to environment is not a pollutant. Following conditions a r e necessary
4 High energy content per volume when totake place
liquid. This would give a large vehicle storedforasSa 1.
A combustible mixture.
given fuel tank. range combustion.
Some means to initiate
2. in the
5. Hydrogen-air mixture burns ten-times faster Stablization and propagation of flame
3.
compared to gasoline-air mixture. Since the burning combustion chamber.
a carburettor
rate is considerably high, it is more
preferred in In spark ignition (S.I.) engines,
mixture and the electric
high speed engines. generally supplies a
combustible
6 Hydrogen-ignition limits are much wider than a spark-plug
initiates the combustion.
spark from
Combustion phenomenon is of two types:
gasolines. So it can bum easily and give considerably
higher efficiency. 1. Normal Combustion
:

In a S.I. engine a single intensely high

7. Hydrogen has high self-ignition temperature (S.I.T.) temperature
but very little energy(1/50 th leaving behind a thin

to ignite it
of gasoline) is required spark passes a c r o s s the electrodes, combustion spreads
thread of flame. From this thin thread,
it at a
8. The exhaust heat can be used to extract
H2 fromn to the envelope of mixture immediately surroundingof the
the hybride reducing the load
engine. rate which depends primarily upon the temperature
both
9. Besides being a relatively clean burning renewable flame front itself and to a secondary degree, upon
source, H2 as I.C. engine fuel is very efficient as the temperature and the density of the surrounding
the mixture is not
there are no losses associated with throttling. envelope. In the actual engine cylinder,
at rest but is in highly turbulent condition. The turbulence
Disadvantages
. The handling of H, is
more difficult and storage breaks the filament of a flame into a ragged front, thus
from which heat
requires high capital and running cost particularly presenting a far greater area of surface
for liquid H. is being radiated; hence its advance is speeded up
2. Difficult to refuel. enormously.
3. Poor engine volumetric efficiency. Any time a 2. Abnormal Combustion:
gaseous fuel is used in an engine, the fuel wil Due to excessively weak mixtures combustion may
be slow or may be mis-timed. This is however obvious.
displace some of the inlet air and poorer volumetric
efficiency will result. There are two combustion abnormalities, which are
4. Fuel cost would be high at present day technology lessobvious:
and availability. The first ofthese is pre or post ignition ofthe mixture
Can detonate. by incandescent carbon particles in the chamber.
emissions because of high flame This will have the effect of reducing the work
6. High No, transfer.
temperature.
7. In hydrogen engines there is a danger
of back fire (i) The second abnormality is generally known as
which can melt the knock and is a complex condition with many facets.
and induction ignition
carburettor. Therefore in H2
fuel system, flame A simple explanation shows that knock occurs when
arresters are necessary. the unburnt portion of the gas in the combustion
traps, flask back chamber is heated by combustion and radiation so
must be vented to prevent
Additionally, crankcases that its temperature becomes greater than the self
mixtures.
accumulation of explosive
ignition temperature. If normal progressive
in S.l. engine. Draw combustion is not completed before the end of the
Q.15 Explain the combustion the three sta es of induction period then a simultaneous explosion of
P-Q diagram and explain RT.U. 2016] the unburnt gas will This
combustion.
occur. explosion is
OR accompanied by a detonation (pressure) wave
in S.I. which will be repeatedly reflected from the
Explain briefly the process
of combustion cylinder
the stages of combustion walls setting up a high
engine and also explain
frequency resonance which
IR.T.U. 2017] gives an audible noise. The detonation wave causes
with the help of P-Q diagram.
as P-0 diagram/ excessive stress and also destroys the thermal
INote: Read P-Q diagram
Combustion may be
boundary layer at the cylinder walls causing
Aus. Combustion in
S.I. Engine: overheating.
 B.Tech. (VI Sem.) M.E. Solved
Paper
ICE.30Three stages of combustion in S.I. Engine:
combustion in the SI engine Q.16 Explain the four stages of combustion in
C.1.
According to Ricardo, the engine and draw P-Q diagram for
can be imagined as if developing in three stages: combustion.
INote: Read Q as 8 in this questlion. IR.T.U. 2016
) The growth and development of a semi propagating8 OR
nucleus of flame called ignition lag or preparalion Discuss the stages of combustion in a C.I
with the engine
phase. help of a neat
pressure-crank angle
the combustion diagram.
(1) The spread of the flame throughout R.T.U. 2011
chamber. OR
(ii) After burning.
In the first
Explain in briefly the stages of combustion in CI
preparation phase :
engine. What is the basic difference in the
1gnition lag or

ofself propagating nucleus combustion processes of SI and

stage the growth and development
chemical process depending8
CI engine?
of flame takes places. It is a
and the R.T.U. 2015
upon the natureof the fuel, temperature, pressure, OR
proportion of the exhaust gas
and also upon the temperature Draw a P- diagram, showing the diferent stages
coefficient of the fuel. of normal combustion of a C.I. Engine. What are
Pressure the basic differences in the combustion process
of
SI and CI engines?

b
Expansion IR.T.U. 2009, Raj. Univ. 2007, 2005, 2003|
CompresSion Ans. Four Stages of Combustion in C.I. Engine
TDC The combustion in a CI engine is considered to be taking
place in four stages. The details of these stages are
180 360
Crank angle explained as follows:
Fig.: Theoretical p-0 diagram (1) Ignition Delay Period: It is also called the
(ii) Propagation of Flame: The second stage is preparatory phase during which some fuel has already
mechanical one and hence pure and simple. It is concerned been admitted but has not yet been ignited.
with the spread of flame throughout the combustion It is counted from the start of injection to the
point
The
chamber. starting point of the second stage is where where the pressure time curve separates from the motoring
first
the measurable riseofpressure is seen on the indicator
diagram i.e. the point where the line ofcombustion departs
(non firing) curve indicated as start of combustion in the
given fig.
from the compression line (point B in the P-0 diagram).
During this stage the flame propagates practically at a
90
80
constant velocity.
(ii) After Burning: The starting point of the third 70
2i3
stage is usually taken as the instant at which the maximum 60
pressure reached on the indicator diagram. (see point C Start of combusion Compression pressure
50
on the P-0 diagram). The flame velocity decreases
during
this stage. The rate of combustion becomes low due to 40
clon stan
Motoring
lower flame velocity and reduced flame front surface. 0.001 Sec. (nonfiring)
30

20
Injection
Flgnition lag
l=Propagation of
20lame
lll-Ater buming 120 100 80 60 40 20 TDC 20 40 60 80 100 120
Time, degrees of crankshaft rotation
Fig. Stages of combustion in the CI engine
Motoring (Non- Fino The delay period in the CI engine exerts a
very great
influence on both engine design and performance. It is of
extreme importance because of its effects on both the
40 60 80 combustion rate and knocking and also its influence on
Crank angle ()
engine starting ability and the presence of smoke in the
Fig.: Stages of combustion in SI engine exhaust.
 (1.C. Engines (ICE.31
The fuel does not
ignite
into the eombustion
chamber. immediately upon injection
"There is a definite
inactivity between the time when the first period of
hits the net air in the droplet
combustio chamber and the oftimefuelit
starts through the actual lgnition Bsart od
gnition delay
known as burning phase. This period is delay combustion (b)
ignition delay Stat of-
shown on pressurc erank period. fig. the delay period is
In
injection(a)
angle (or time) diagram between Area A
points à and b.
Point 'a'
represents the time Area A- Motoring
represents the time at which theof injection and point "b'
pressure curve (caused Time Mixdng period
by combustion) first separates from the motoring curve.
Interaction period
The ignition delay period can be Pressure time diagram illustrating ignition
delay
divided into two parts, the Fig. Combustion: The rapid
physical delay and the chemical delay. (3) Period of Controlled
Physical Delay: The physical delay is the time the third stage, the
combustion period is followed by
between the beginning of and pressure in
injection and the attainment of controlled combustion. The temperature
chemical reaction conditions.
During this period, the fuel the second stage is high. Hence the fuel
already quite
is atdmized, the second stage burn faster with
vaporized, mixed with air and raised to its self droplets injected during find the necessary
ignition temperature. The physical delay depends on the reduced ignition delay as soon as they
type of fuel i.e. for light fuel the physical further pressure rise is controlled by the
delay is small oxygen and any
controlled combustion is
while for heavy viscous fuels the
physical delay is high. injection rate. The period of
The physical delay is greatly reduced by assumed to end at maximum cycle temperature.
using high injection not cease with
pressures, high combustion temperatures and high (4) After Burning: Combustion does
turbulence to facilitate break up of the jet and improving the completion of the injection process. The unburnt and
chamber
evaporation. partially burnt fuel particles left in the combustion
start burning as as they come into contact with the
Chemical Delay: During the chemical delay, soon
called
reactions start slowly and then accelerate until oxygen. This process continues for a certain duration
the 'after burning period'. Usually this period starts from
inflammation or ignition takes place. Generally, the
chemical delay is longer than the physical delay. However, the point of maximum cycle temperature and continues
cver a part of the expansion stroke rate. The duration of
it depends on the temperatures of surroundings and at higBh
the after burning depends on the velocity of diffusion and
temperatures the chemical reactions are faster
physical delay becomes longer than the chemical delays.
and turbulent mixing of unburnt and partially burnt fuel with
It is clear that, the ignition lag in the SI engine is essentially the air. The duration of the after burning phase may
In most correspond to 70-80 degrees of crank travel from TDC.
equivalent to the chemical delay for the CI engine. duration of The sequence of events in the entire combustion
is shorter than the
CI engine the ignition lag process in a CI engine including the delay period is shown
injection. Combustion: The period of
in the following figure.
(2) Period of Rapid Mixdure lgnition Final combustion Products
the uncontrolled combustion, formaton

rapid combustion also called rise is taken place.

PhyslcalS
delay
>Chemical
delay
is that in which the rapid pressure
the droplets have had time
Delay period Inflammation
During the delay period, fresh air is always available Disintegration of Thermal Oxidation of
to spread o v e r a wide area
and stream of lInjected decomposition of fuel-air
would have
around the droplets. Most
of the fuel admitted fuel mixture

mixture with air. By Mixing of Preflame

a combustile Bquid fuel
evaporated and formed would have also been
oxidation of
TUe
Mixture of
products of partial
reactions Wiln air
this time, the preflame
combustion is counted from
0xdation or of
thermal decompositon
completed. The period of rapid to the Vaportzation Local with air

end of delay period or the

beginning of combustion The of fuel lgnitton
on the indiçator
diagram.
points of maximum pressure this period. Temperature and Temperature
is maximum duringreached during the
and oxygen
Mixing of fuel Oxygen
rate of heat release vapour wlth concentration
Concentration
Products
that the pressure unfavourable favourable for
It may be noted on the
duration of ror compkste complete of
complete
combustion will depend
Combustion
ofrapid rapid and combustion Combuston
period
the delay the Decreased by
m o r e (Chiling.
the dela period (the long m o r e fuel
would have over lean and
Products of
rise since Over rich mixture) Incomplete
higher is the pressure
the delay period). Combustion
a c c u m u l a t e d in the
cylinder during Fig.: Combustion in the CI engine
 ICE.32 B.Tech. vn Sem-) ME. Soloed Papers
P-6 diagram for Combustion in C.I. Engine (b) Wha are the requirements of combustion
90r chamber for CI engines? Describe the various
80 opes of combustion chambers. RT.U. 2013
Starn of
Comprersin Ans. (a) Detonation Phenomenon in CI Engine: Refer
combustion pressure
to Q.6.
Conditions which encourage detonation in S.I.
injection .
engine reduces knocking in C.I. engine : Refer to
0.001 ec. 0.9.
Comparison of Knock in SI and CI Engines
1+2i34- We know that knocking in SI engines and CI
0
engines is due to the auto-ignition of the fuel air mixture.
Inection
10 Atmoepherc
But careful examination of the knocking phenomenon in
********°**

SIengines and CI engines reveals the followin

120 100 0 TOC 2 00 120 differences
Tne, degrees of crankshat rotalion (e)-
Fig.: P- diagram for Combustion in C.I. Engine
A comparison of knocking process in SI and C
engines is shown on the pressure-time figures. (see fig. l)
Differences between the Combustion process of SI (1) In SI engines, the auto-ignition ofthe end gas away
and CI engine from the spark plug, most likely near the end of
In Cl engine, only air is composed during the combustion causes knocking.
compression stroke and the ignition can take place In CI engines, the auto-ignition of the charge
only after fuel is injected just before the top dead causing knocking is at the start of combustion.
center. While in SI engine the mixture of air and fuel (2) For CI engines, the explosive auto-ignition is more
(petrol) is compressed in the combustion chamber or less over before the peak pressure.
during the compression stroke. For SI engines, the condition for explosive auto-
In spark ignition engine, the charge that ignition of the end charge is more favorable after
auto-ignites
is homogeneous and therefore, intensity
of knocking the peak pressure.
or the rate of
pressure rise at explosive auto ignition (3) In order to avoid knocking in SI engines, it is
is likely to be more than that in necessary to prevent auto-ignition of the end gas
compression ignition to take place at all.
engines where the mixture is heterogeneous.
In spark ignition engines the auto-ignition of end In CI engine, the earliest possible
auto-ignition is
charge, away from the spark plug, occurs, most likely necessary to avoid knocking.
(4) In SI engine, the charge that auto-ignites is
at
the end ofcombustion. But in compression ignition
engine the auto ignition of the charge takes place at homogeneous and therefore intensity of knocking
or the rate of pressure rise at
the start of combustion process. explosive
is likely to be more than that in CI auto-ignition
Since in the compression ignition engine the fuel is
the fuel and air are not
engines where
injected into the cylinder only at the end of the homogeneously
when explosive auto-ignition ofthe
mixed even
compression stroke there is no question ofpre ignition charge
Therefore, it is often called detonation in Sl
occurs.
or "premature ignition' as in the SI
engine. engines.
(5) In CI engines, only air is compressed during the
Q.17 State the advantages and disadvantages of compression stroke and the ignition can take place
hydrogen as 1.C. engine fuel. R.T.U. 2015] only after fuel is injected just before the top dead
centre. Therefore there can be no pre-ignition in
Ans. Hydrogen: Refer to Q.14. CI engines as in spark ignition
engines.
(6) In the SI engines, it is relatively easy to
distinguish
Q.18(a) Explain the knocking phenomenon in C.L. between knocking and non-knocking operation as
engine and compare it with S.1. engines. Discuss the human ear easily finds the distinction.
the effects of operating variables on delay period In the case of CI engines, the normal ignition is
and diesel knock. Is it true that the condition itself by auto-ignition and hence most Cl engines have a
which encourages knocking in Sl engine reduces sufficiently high rate of pressure rise per degree of crank
angle to cause audible noise. When such noise becomes
knocking in CI engines?
cxcessive or there is excessive vibration in engine
 ICE.33
G period
th engue is
sAnd to knock. Thus in the Cl engine Delay Period Delay
b no deinte distinction berween Variables Affecting factors.
normal and is affected by the following of fuel as far
Fuel: The most important property
(1) temperature.
as is concemed is the selfignition
delay period means a wider marg1n
A lower self ignition temperature air and
of the compressed
between it and the temperature
hence lower delay period. scale
Cetane number is a
Cetane Number (CN): diesel
for comparing the ignition delay
angle of various
m e a n s a lower delay period
fuels. A higher cetane number
The cetane number
and, smoother engine operation.
of fuel. The more
depends on the chemical composition
contained in the fuel, the higher
paraffinic hydrocarbons are
will be its cetane number.
which affects the delay period
Other properties fuel
of and surface tension.
are volatility, latent heat, viscosity
affect the time taken too
The volatility and latent heat
surface
form an envelope of vapour. The viscosity and
tension ifluence the fineness of atomization.
pressure orsize of droplet: It seems
2) Injection combustion
Time that in CI engine in order to achieve complete
fluid shouldbe
in the very short time available, the liquid
injected in droplet of snnallest size
to obtain largest surface
volume ratio. But it should be bome in mind
that the rate
which the
of burning depends primarily upon the rate at
from the surface
product of combustion can be removed
and replaced by fresh oxygen i.e. it depends upon the rate
to the
at which the burning droplet can move relative
surrounding air. A smaller droplet will have lesser
momentum and hence lesser relative velocity and once its
initial velocity is lost it will travel in air with little relative
Time
the pressure rise velocity resulting in its partial suffocation by its won products
Fig. 1: Illustrating effect of ignition delay on

Period: The
Effect of Operating Variable on Delay
of combustion. Secondly, as the pressure rise after ignition
i.e. the delay
first stage of combustion in the Cl engine depends on the area of inflammation, the smaller the size
influence on both engine design and greater the number of droplets the larger will be the
period, exerts a very great
needs a detailed study. aggregate area of inflammation and therefore the greater
and performance and therefore classified as
can be broadly the uncontrolled pressure rise. The disadvantage of larger
The ignition delay period
droplet is of course that subsequent rate of burning is too
follows delay is
The time of physical slow and hence a compromise is to be struck. As the size
a)Physical delay :
and attainment
the time between the beginningof injection of droplet depends on the injection pressure, it can be said
conditions. In the physical delay
period that lower the injection pressure the lower the rate of
ofchemical reaction mixed with air and raised
the fuel is atomized, vaporized, pressure rise during the uncontrolled phase and smoother
in temperature. the running.
second part of the delay
(b) delay : The
Chemical (3) Injection advance angle: The delay period
reactions start
is called chemical delay in
which preflame increases with increase in injection advance angle. The
inflammation or
accelerate until local reason for increase in delay period with increase in injection
slowly and then chemical delay is longer
gnition takes place. Generally, on
advance angle is that the pressure and temperature are
However, it depends
than the physical delay. reaction is
chemical
lower when the injection begins. When the injection
temperature. At high temperatures advance angles are small, the delay period reduces and
than chemical delay.
and physical delay is longer operation of the engine is smoother but the power is
quicker and chemical
period refers to the sum of physical reduced because amount of fuel burns during
The delay the ignition delay is shorter than larger
delay. In most Cl engines expansion (after burning). The optimum angle of injection
the duration of injection.
 (ICE.34 B.Tech. (VI Sem.) M.E. Solved Papers
but generally it varies large amount of fuel in cylinder so there is more chances
advance depends on many factors, of diesel knock.
between 12° to 20° btdc. This would cause peak pressure
to occur 10° to 15° after top dead centre. Engine Output > Increase
Note: btdc: before top dead centre] Quality of Fuel> Lower Self Ignition Temperature
(4) Compression ratio: Increase
in compression Intake Temperature Increase
ratio reduces the delay period as it raises both temperature Intake Pressure> lIncrease
the
and density. As the compression ratio increases, Fuel Temperature > Increase
At the same time the A/F Ratio> Decrease
temperature of the air increases. decreases due to
minimum auto-ignition temperature in closer Ignition delay period a knocking in CI engine.
increased density of the compressed air, resulting It is true that condition which
reduces the time
contact of the molecules which, thereby,
difference knocking in SI engine decrease knocking inencourages
CI engine.
of reaction when fuel is injected. As the
between compressed temperature and the minimum auto Factors which increase detonation in SI engines, tend to
decreases. reduce knocking in CI engine due to following
ignition temperature increases, the delay period The detonation in the SI engine is due to
reasons
From the analysis, we may conclude, in diesel engines simultaneous
auto ignition of the lost part of
the highest possible compression ratio should be used to the charge. To eliminate
achieve the lowest delay period. detonation in the SI engine we want to prevent
altogether the auto ignition of the last part of the
Intake temperature: Increasing the intake
(5) charge and therefore desire a long delay period and
temperatures would result in the compressed air high self ignition temperature of the fuel.
temperatures, which would reduce the delay period.
To eliminate knock in the CI
However, increasing the intake temperature, say by engine we want to
achieve auto ignition as early as
pre heating, would be most undesirable because it possible and
would reduce the density of air and hence volumetric therefore desire a short delay period and low self
efficiency and power output. This loss would be much ignition temperature of the fuel.
greater than the small gain in reduction of delay period. Following table gives the factors which reduce
Pre-heating the air by 100°C reduces the delay angle detonation in the SI engine and knocking in the CI engine.
by barely 2. Table
(6) Jacket water temperature: Increase in jacket S. Factors CI Engine|SI Engine
water temperature also increases compressed air No.
temperatures and hence delay period is reduced. 1. Self-ignition temperature of fuel Low High
(7) Fuel temperature: Increase in fuel temperature 2. Time lag or delay period for fuel Short Large
would reduce both physical and chemical delay
period. 3. Speed Low High
(8) Intake pressure and super charging: Increase 4. Inlet temperature High Low
in intake pressure or
supercharging reduces the auto- 5. Inlet pressure High Low
ignition temperature and hence reduces delay period. 6. |Combustion chamber wall High Low

(9) Speed: As the engine speed increases the loss temperatures

ofheat during compression decreases with the result that 7.JCompression ratio High Low
both the temperature and pressure of the 8. Cylinder size Large Small
tend to rise, thus reducing the compressed air
delay period in milli seconds. It is also clear from the table that à
good CI engine
(10) Air fuel ratio (load): With increase in air fuel fuel is a bad SI engine fuel and a
good SI engine fuel is a
ratio (leaner mixture) the combustion bad CI engine fuel. In other words, diesel oil has low self-
and temperature lowered
cylinder wall temperatures are reduced and hence ignition temperatures and short time lag whereas petrol
the delay period increases.
has high selfignition temperature and long ignition lag. In
Other factors: terms of full rating, diesel oil has high cetane number (40-
(11) Engine size 60) and low octane number (about 30), and petrol has high
octane number (80-90) and low cetane number (18).
(12) Type of combustion chamber
Following fig.2 indicates typical diagram of a diesel
Effect of Operating Variable on Diesel Knock engine with sharp pressure oscillating caused by shock
Ifthe delay period is excessively long: there is waves when using petroi.
 ICE.353
1.C. Engines
2. The turbulent type
(a) Turbulent chamber
(b) Pre-combustion chamber
(c) Energy cell
1. Open Direct Combustion Chamber
or
(Non-
turbulent Type)
The usual design of open combustion chamber is
shown in the fig.l which is representative of non-turbulent
40 60 80 100 type.
80 60 40 20 TDC 20 The fuel is injected directly into the
Crank angk upper portion
engine when using petrol of the cylinder, which acts as a combustion
Fig. 2:
Indicator diagram of diesel
Table chamber. To perform mixing, this depends little on
Condition SIengine CI engine turbulence.
Quality of High self ignition Lower self ignition Consequently, heat loss to the chamber walls is
tempy relatively low and starting results are easy. For
fuel tempy proper penetration and dispersal of the fuel, high
Intake tempy Decrease Increase
Decrease Increase injection pressures and multi-orifice nozzles are
Intake required. This necessitates small nozzle openings
pressuree
Increasee Decrease and results in more frequent clogging of fuel spray
A/F ratio by accumulated carbon particles, with consequent
of combustion chamber for
Ans. (b) Requirement
higher maintenance costs.
CI engine
InCI engines fuel is injected into the combustion Basically this chamber is used on low speed engines,
where injection is spread through a greater period
chamber at about 15°C before T.D.C. during the
of time and thus ignition delay is a relatively less
compression stroke. For the best efficiency the combustion important factor. So less costly fuel with longer
must complete within 15° to 20° of crank rotation after
T.D.C. in the working stroke. It is clear that injection and ignition delay may be used.
Few attempts were made to improve the air motion
combustion both must complete in the short time. For best
in open chambers, important are:
combustion mixing should be completed in the short time.
(a) by shrouding the inlet valve.
In SI engine mixing takes place in carburettor,
however in CI engine this has to be done in
(b) by providing squish.
combustion chamber. To achieve this requirement
in a short period is an extremely difficult job
particularly in high speed CI engines.
To achieve high efficiency and power the .
combustion must be completed when the piston is
nearer to T.D.C. it is necessary to have rapid mixing
offuel and air during the third stage ofcombustion.
The design of combustion chamber for CI engines (
must also take consideration offuel injection system
and nozzles to be used. Fig.1: (a) Air motion by shrouding the inlet valve
Various Types of Combustion Chamber (b) squish air motion inside Cylinder
motion is given
In CI engines, several types of combustion By shrouding the inlet valve, swirl
to the air entering the cylinder which is believed to
Cnambers are used. Each of these has its own peculiarities
stroke and the time of
and desirable as, well as undesirable features. No persist during compression
Ombustion chamber design has yet been developed which injection. Due to
at low speeds.
will produced the best result in all types of engines. Four It gives better performance
shroud, on account of reduction in inlet area,
Specific designs which find wide use in CI engines are as
volumetric efficiency reduces.
follows: the air at the end of
1. Non-turbulent type Squish is provided by pushing whose diameter
(a) Open or direct combustion chamber the compression stroke in the space
 (B.Tech. (VI Sem.) M.E. Solved Pape
(ICE.36 shortening the delay period of fuel which is high
is smaller than the cylinder bore. Because of the
small clearance between the head and piston topP desirable.
-Atomiser
when at T.D.C., air is pushed into combustion space
providing air movement known as squish. This helps
Pre-combustion
in mixing of fuel and air. chamber
2. Turbulent Chamber (Turbulent type)
In the turbulent chamber, the upward moving piston forces
all the air (70-80%. of all air) at a greater velocity into a Main
combustioon
small antechamber, thus imparting a rotary motion to the
space
air passing the pintle type nozzle. As the fuel is injected O
into rotating air, it is partially mixed with the air and
commences to burn. The pressure so built up in
the Cylinder
antechamber by these expanding burning gases force the
burnt, unburned fuel and air mixture back into the main Fig. 3
chamber, again imparting high turbulence, further assisting The product from this chamber rushes into main
combustion. combustion space through restricted passages, creating
violent air motion. This violent air motion helps in rapid
mixing and burning in the main combustion space. The
fuel present in the main combustion chamber has no delay
period as the temperature is already high due to combustion
O in pre-combustion chamber.
Advantages
) Due to short or practically no delay period for
the
fuel entering the main combustion space,
Fig. 2: Turbulent Chamber
to knock is minimum, and
tendency
Advantages running is smooth.
(1) Minimum heat loss during compression because of
i) The combustion in the third stage is rapid.
lower surface area to volume ratio and hence better (i) As the mixing of fuel and air is thorough, the fuel
efficiency. injection system design need not be critical.
(2) Fire atomization because of multi hole. Disadvantages
(3) The insulated and hot running combustion chamber ) The velocity of burning mixture is too high
the passage from during
shortens the delay period and limits the rate of pre-chambers, so the heat loss is
pressure rise, resulting in smooth running. very high. As a result of which thermal
reduces and this can be offset efficiency
(4) The turbulence is responsible for rapid mixing and by increasing the
burning of fuel during the third stage of combustion. compression ratio.
(5) The demands on the fuel (i) Cold starting is difficult as the air losses heat to
severe as it is not to be
injection system are not chamber walls during
distribution etc.
depended upon for mixing, 4. Energy Cell compression.
Disadvantage The 'energy cell' is more
Cold starting is diff+cult since air combustion chamber. As the complex than the pre
compression stroke, some of the piston
losses heat to moves up on the
combustion chamber walls during the air is forced into the
The combustion chamber is compression stroke. and minor chambers of the major
3. Pre-combustion Chamber relatively cool at the starting. energy cell. When the fuel is
injected through the pintle type nozzle,
part of fuel
This combustion chamber is
separated into two
across the main
combustion and enters the minorpasses cell,
chambers. The smaller one of where it is mixed with the
the chambers occupy about entering air. Combustion
combustion chamber wherefirst
30% of the total combustion takes place in the main
The communication
between the two chambers is aspace.
narrow restricted temperature higher, but rate of burning is slower in this
is
the
or a number of small
holes. The air is forced into passage location, due to insufficient
burning in the minor cell is mixing
the pre- of the fuel and air. The
combustion chamber by piston during the slower at the start, but due to
stroke. Fuel is injected into the compression bettet mixing, progresses at a more rapid
The chamber is pre-combustion chamber. rate. The
designed to run hot and this results in pressure built up in the minor cell forces the
burning gases
out into the main combustion chamber, hence creating
 (ICE.37
1.C. Engines
added turbulence and producing better combustion in this (2) Octane Number: This method has been devised
chamber. In the mean time, pressure is built up in the major to determine the anti-knock measure of the fuel. In this
method the octane number test is conducted by which the
cell, which then prolongs the action of jet stream entering
performance of unknown gasoline is compared with that
the main chamber, thus continuing to induce turbulence in of a series of reference fuels consisting of mixtures of
the main chamber.
iso-octane and n-heptane. Iso-octane is a low boiling point
5. M-Combustion Chamber
This combustion chamber
branched chain compound and has a very slight tendency
to knock and hence, arbitrarily assigned an octane number
is developed for small high speed
of 100. n-heptane detonates rapidly and hence, has been
engines. It differs from the otheer assigned an octane number of zero. These fuels are known
open combustion chamber
as primary reference fuel (PRF) and the octane number
engines in the respect that fuel of unknown fuel is defined as the percentage of iso-octane
spray impinges tangentially
on,
in the PRF that give the same knock intensity. Thus, a
and spread over, the surface of gasoline that gives the same knock intensity as 90 PRF
spherical space in the piston. (90% vol. iso-octane and 10% vol. n-heptane).
There is always Some mtmImINTA (3) Research and Motor Octane Number
impingement of spray on the Research test is carried out under relatively mild operating
combustion chamber walls in all
conditions (low speed and low mixture temperature) and
successful diesel engine designs. Fig. 4 the motor test is carried out under more severe operating
Advantages conditions (High engine speed and high mixture
() Low peak pressure. research test
temperature). Octane number determined by
() Low rate of pressure rise. is termed as Research Octane Number (RON) that
(i) Lowsmoke level. determined by motor method is termed as Motor Octane
(iv) Ability to operate on a wide range of liquid fuels. Number (MON).
Disadvantages Research Octane Rating can be related to antiknock
) Lowvolumetric efficiency. quality of the fuel when used in a engine which is highly
() Since fuel vaporization depends upon the surface loaded at low speed.
cold
temperature of the combustion chamber, Sensitivity: The difference between research octane
starting requires certain aid. number and motor octane number reflects the difference
(i) At starting and idling conditions hydrocarbon between results in the laboratory and on load test. This
emissions may occuur. difference is called sensitivity.
Sensitivity RON MON
=

Q.19(a) What are the important qualities of S.I. and Sensitivity is thus a measure of the extent to which a
CI fuel? Also explain in detail the
knock rating gasoline is downgraded under severe conditions, the higher
the sensitivity the poorer its performance under severe
of S.I. and C.I. engine fuels.
(b) Explain alcohol as an
alternative for 1.C
fuel conditions.
knock
engine and also state its merits and demerits. 4) Performance Number: It is the ratio of the
[R.T.U. 2013] limited indicated mean effective pressure (klimep) of test
fuel to knock limited indicated mean effective pressure of
It should iso-octane.
Ans. (a) Qualities of S.I. & C.I. Fuel :

have: PN Klimep of testfuel

High energy density Klimep of iso octane
-

Good combustion quality Knock Rating of CI Engine Fuel

Good fire safety In Cl engines, the knock resistance depends upon
and
Low toxicity chemical characteristics as well as on the operating
are normal cetane,
Lowpollution design conditions. The reference fuels
Knock Rating of SI Engine Fuel which has 100 cetane number and a-methyl nepthalene
ratio (HUCR): The which has cetane number of 0.
(1) Highest useful compression compression
highest useful compression ratio is the highest ina
Cetane Number Of Fuel
detonation Cetane number of fuel is defined as the percentage
ratio at which a fuel can be used without
conditions mixture of normal
specificd test engine under specified operating by volume of normal cetane in thewhich has the same
and the ignition and mixture strength being adjusted give
to
cetane and a-methyl nepthalene
best efficiency. ignitioncharacteristic as the test fuel.
 ICE.30 B.Tech. (VI Sem.) M.E. Solved Pape
The knock resistant property of diesel oil can be If the unburned charge does not reach its critiça
improved by adding small quantities ofcompound like amyl temperature for auto ignition, it will not auto ignite
nitrate. and flame front BB' will travel across the
Ans.(b) Alcohol as an alternative fuel: Refer to Q.14.
unbumed
charge to D, normally.
The graph will, therefore be a smooth curve.
Q.20 What is delay period in C.l. engines & what are In knocking the end
charge auto ignites before the
the factors that alfect the delay period? flame fronts reaches it.
IR.T.U. 2012] According to the figure of knocking, when flame front
OR reach BB' the unburned charge BB'D reached the
What is meant by delay period'? Discuss the critical condition for auto ignition.
variables afecting the delay period. This causes extreme pressure fluctuation as shown
Raj. Univ. 2007, 2005, 20031 in the graph.
Ans. Delay Period : Refer to Q.18(a). This causes instantaneous burning of fuel and this
rapid energy release causes rise in pressure, 3 to 4
Q.21 Discuss the phenomenon of knocking and the times the normal pressure.
various engine factors which affect it in a SI This pressure rise give rise to immense pressure wave
engine. R.T.U. 2011] which strikes the cylinder wall and sets it vibrating
Ans. Knocking: In simple words, giving rise to a high pitched metallic pinking' or ringing
knocking is a type sound.
of abnormal combustion which is detrimental to engine
This phenomenon is therefore known as 'knocking'
life and its performance knocking or detonation puts a
limit on the compression ratio at which engine can be Effects of various engine factors on knock: It is
operated which in turm controls the efficiency and to some very well known that to prevent knock in SI engine
end gas should have
extent, power output.
The Knocking phenomenon can be fairly understood A low temperature
the following points: (i) A low density
(i) A long ignition delay
Consider combustion in SI engines, both normally and (iv) A non-relative composition
with knocking as shown bythe figure.
In normal combustion the flame front travels from Therefore, variables affecting knock can be classified
point Ato D in the combustion chamber. as, temperature factors, density factors, time factors and
Flame front compresses the unburned charge BB'D composition factors.
and increases its temperature, the temperature of (A) Temperature Factors : Increasing the
unburned mixture also increases due to reactions temperature of the unburned mixture by any factor in
taking places within itself. design or operation will increase the possibility of
Wwh The temperature can be increased by
knock.
following factors
Spark
) Raising the Compression Ratio : Increasing the
Spark compression ratio will increase both temperature and
pressure which lead to greater collisions of molecules
resulting in greater formation of chemical species
responsible for knocking.
30 20 1010 20 30 40 50 30 20 1010 20 30 40 50 60 70
(i) Supercharging: It also increases both temperature
Crank angle Crank angle and pressure.
B
(ii) Raising the inlet temperature : Delay period
decreases, velocity of flame travel increases.
(iv) Raising coolant temperature : Delay period
decreases.
(v) Increasing load (opening the throttle): An
increase in load increases the temperature of cylinder
B B and combustion chamber walls. Also, pressure is
Normal combustion Knocking increased.
Fig
 I.C. Engines ICE.30
(B) Density Factors:
unburned mixture by any ofIncreasing
the density
of the (ii) Decreasing the speed of engine : Decreasing the
the
increase the possibility of knock. foilowing methods will speed decreases the turbulence of the mixture
) Increasing the compression ratio resulting in reduced flame speed. Hence, knocking
tendency increases.
i) Opening of throttle
i) Supercharging the engine (D) Composition: The properties of the fuel and
fuel-air ratio are the primary means for controlling knock,
(iv) Increasing inlet pressure-its increase will increase once the compression ratio and engine dimensions are
the overall
pressure, thus increasing the tendency to selected.
knock.
(v) Advancing the spark timings. ( Octane rating of fuel : Octane number, is the
measure of resistance to knock. Higher the octane
(C) Time Factor : Increasing the time of number lesser is the tendency to
of the unburned mixture of exposure knock.
auto-ignition
of the following factor will increase
condition by any ) Fuel-air ratio: The most important effect of fuel
tendency to knock. air ratio is on the reaction time or ignition delay. By
) Increasing flame travel distance: Knock is making the mixture leaner or richer (F/A = 0.08) the
increased by increasing the distance the flame has tendency of knock is decreased.
to travel in order to traverse the combustion
chamber. ii) Humidity ofair: Increasing atmospheric humidity
(i) Decreasing the turbulence of mixture .: decreases the tendency to knock by decreasing the
Decreasing the turbulence of mixture decreases the reaction time.
flame speed and increases knock.
OO0