ME - 413

INTERNAL COMBUSTION
ENGINES
Teacher In-charge
PROF. DR. ASAD NAEEM SHAH
anaeems@uet.edu.pk
CARBURATION

Arranged by Prof. Dr. Asad Naeem Shah

 CARBURETOR
▪ In SI engines, the liquid fuel and the air are generally mixed
prior to their arrival in the combustion chamber. This mixing
process is termed as carburation.
▪ Thus, a carburetor is a mechanical device designed to fulfill the
following functions:
1. Meter the liquid fuel in such quantities as to produce the 𝐴Τ𝐹
ratio required to meet engine operating conditions.
2. Atomize the fuel and mix it homogeneously with the air.
▪ Due to engine operating characteristics, the 𝐴Τ𝐹 ratio required
by engine will vary somewhat over the engine operating range.
▪ Thus, a carburetor must be designed so that it will provide, as
nearly as possible, the 𝐴Τ𝐹 ratios which the engine requires,
and this ratio must be within the combustible range.
Arranged by Prof. Dr. Asad Naeem Shah
 CARBURETOR Cont.
▪ The combustion, in a homogeneous mixture, in an SI engine
will occur for a limited range of 𝐴/𝐹 𝑜𝑟 𝐹 Τ𝐴 ratios.
▪ This range of useful 𝐴/𝐹 ratios runs from about 20/1 (lean) to
8/1 (rich) as shown in Fig. 1. Outside of this range, the ratio is
either too rich or too lean to sustain the flame propagation.

Fig. 1: Useful air-fuel mixture range.

Arranged by Prof. Dr. Asad Naeem Shah
 THE A/F−RATIO VARIATIONS
▪ From the standpoint of both design and operation,
theoretically, a carburetor must operate at a chemically
correct 𝐴/𝐹 ratio, as shown in Fig. 1. Engine operating
requirements imposed upon the carburetor, however, prohibit
such a simple solution.
For example, if an actual
engine is operating at full
throttle and constant
speed, the 𝐴/𝐹 ratio will
affect both the power
output and the brake
specific fuel consumption
(𝑏𝑠𝑓𝑐).

Fig. 1: Carburetor performance as per engine requirement.

Arranged by Prof. Dr. Asad Naeem Shah
 THE A/F−RATIO VARIATIONS Cont.
▪ The mixture corresponding to the maximum point on the
power curve is called the best power mixture. The mixture
corresponding to the minimum point on the bsfc-curve is
called the best economy mixture.
▪ Obviously, the best power mixture is richer than the
chemically correct mixture, and the best economy mixture is
leaner than the chemically correct (Fig. 1).
▪ Moreover, the 𝐴/𝐹 (or 𝐹 Τ𝐴) ratios for best bmep and best
economy at part throttle are not strictly the same as at full-
throttle. For example:
▪ If the maximum bmep occurs at full throttle & 𝐹 Τ𝐴 = 0.08 at
point A (Fig. 2), 80% of this maximum bmep will be obtained
by reducing the throttle with same 𝐹 Τ𝐴 = 0.08 at point B. This
will, however, result in an increase 𝑏𝑠𝑓𝑐 from A to B on the
𝑏𝑠𝑓𝑐 −curve.
Arranged by Prof. Dr. Asad Naeem Shah
 THE A/F−RATIO VARIATIONS Cont..
Alternatively, if point C
is approached (from
point A) by reducing the
𝐹 Τ𝐴 ratio and following
the full throttle-curve
(again to get 80% of
maximum bmep), the
𝑏𝑠𝑓𝑐 will be decreased
this time because of the
lean mixture developed
in this case.

Fig. 2: 𝑏𝑚𝑒𝑝 & 𝑏𝑠𝑓𝑐 VS. 𝐹 Τ𝐴 at different throttle positions. Arranged by Prof. Dr. Asad Naeem Shah
 SIMPLE FLOAT TYPE
Choke valve CARBURETOR

Figure 1 is a schematic diagram of

a simple, down draft, float type
carburetor with both an idling
Throttle valve enrichment jet and a choke valve.

Fig. 1: A float type carburetor.

Arranged by Prof. Dr. Asad Naeem Shah
 FLOAT TYPE CARBURETOR Cont.
▪ The fuel supply to the float chamber is controlled by the action of
the float and the attached fuel supply valve. If the amount of fuel
in the float chamber falls below the designed level, the float
lowers, thereby opening the fuel supply valve. However, when the
designed level has been reached, the float closes the fuel supply
valve, thus stopping additional fuel flow from the supply system.
▪ Air from the atmosphere is drawn through the venturi by the
action of the pistons on the intake stroke. As the air passes
through the venturi, its velocity is increased and the pressure in
the venturi throat is decreased. Because of this fact, and since
the float chamber is vented to the atmosphere, a pressure
differential exists between the float chamber and the tip of the fuel
discharge nozzle. This differential causes the fuel to discharge
into the air stream in an amount dependent upon the magnitude
of this pressure difference.
Arranged by Prof. Dr. Asad Naeem Shah
 FLOAT TYPE CARBURETOR Cont.
▪ Since the pressure drop in the venturi is dependent upon the
rate of air flow, and since the fuel flow is dependent upon the
pressure drop in the venturi, the A/F ratio provided by the
carburetor is theoretically constant.
➢ IDLING JET: During idling, nearly closed throttle causes
reduction in the mass of air flowing through the venturi. At such
low rates of air flow, the pressure differential between the float
chamber and the fuel discharge nozzle becomes very small. It
is insufficient in fact to cause fuel to flow over the restraining
lips of the fuel discharge nozzle.
▪ To compensate for this fact, and to provide the engine with the
required rich 𝐴Τ𝐹, an idling jet is added (Fig. 1). The idling jet is
located in the wall of the air system, on the downstream side,
and adjacent to the edge of the nearly closed throttle valve.
Arranged by Prof. Dr. Asad Naeem Shah
 FLOAT TYPE CARBURETOR Cont.
▪ Thus, the piston descending on the intake stroke, causes a
reduction in pressure at the idling jet leading to pressure
reduction of the idling air bleed at idling air restriction, and
providing the air-fuel mixture to the engine. The desired air-fuel
mixture is regulated by the idling adjustment valve.
➢ CHOKE VALVE: During starting and warmup in cold weather,
it is necessary to provide an extra rich mixture to ensure that
enough fuel is available, in vaporized form, for combustion. This
is accomplished by inserting a choke valve in the air intake
system on the upstream side of the venturi.
▪ When the choke valve is nearly closed, a vacuum is created in
the area around the fuel discharge nozzle. The pressure
differential between the float chamber and venturi area forces
additional fuel into the air stream, thereby fulfilling the extra rich
engine requirement.
Arranged by Prof. Dr. Asad Naeem Shah
 CALCULATION OF THE AIR/FUEL
RATIO FOR A SIMPLE CARBURETOR
▪ Consider a simple version of a carburetor as shown in Fig.1.
The section AA (plane 1) is taken at the entry to the carburetor
and the section BB (plane 2) at the venturi throat.

Arranged by Prof. Dr. Asad Naeem Shah

 CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
➢ AIR:
▪ Applying the steady-flow energy equation (SFEE) between
the sections AA and BB and considering unit mass of air flow:
𝑢12 𝑢22
𝑞 + ℎ1 + = ℎ2 + +𝑤 → (𝟏)
2 2
Where 𝑞 and 𝑤 are the heat and work transfers per unit
mass of air flow between planes 1 and 2 and ℎ and 𝑢
denote the enthalpy and velocity of air, respectively.
▪ The flow is assumed to be isentropic and there is no work
transfer between planes 1 and 2. Therefore, 𝑞 = 0 & 𝑤 = 0.
▪ Also, velocity of air 𝑢1 is negligible compared to velocity 𝑢2
∴ 𝑢1 = 0 ∵ 𝑢2 ≫ 𝑢1
Arranged by Prof. Dr. Asad Naeem Shah
 CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
⇒ 𝑢2 = 2(ℎ1 − ℎ2 ) → 𝟐
▪ Assuming air to be an ideal gas, therefore, taking ℎ = 𝑐𝑝 𝑇 leads to:

𝑇2
⇒ 𝑢2 = 2𝑐𝑝 (𝑇1 − 𝑇2 ) = 2𝑐𝑝 𝑇1 1− → (𝟑)
𝑇1

▪ For an isentropic process:

𝛾−1
𝑇2 𝑃2 𝛾
= → (𝟒)
𝑇1 𝑃1

𝛾−1
𝑃2 𝛾
⇒ 𝑢2 = 2𝑐𝑝 𝑇1 1 − → (𝟓)
𝑃1

where 𝑃 = 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 & 𝑇 = 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒.

Arranged by Prof. Dr. Asad Naeem Shah
CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
▪ From the continuity equation, the mass flow rate of air is:
𝑚ሶ𝑎 = 𝜌1 𝐴1 𝑢1 = 𝜌2 𝐴2 𝑢2 → 𝟔
where 𝐴 are 𝜌 are the cross-sectional area & density of
air, respectively.
For an isentropic process, ∵ 𝑃1 𝑉1𝛾 = 𝑃2 𝑉2𝛾
𝛾 𝛾
𝑃2 𝑉1 𝜌2
= =
𝑃1 𝑉2 𝜌1
1/𝛾
𝑃2
⇒ 𝜌2 = 𝜌1 → (𝟕)
𝑃1
▪ Now, putting Eqn. (7) into Eqn. (6):
1/𝛾
𝑃2
𝑚ሶ𝑎 = 𝜌1 𝐴 2 𝑢2
𝑃1
Arranged by Prof. Dr. Asad Naeem Shah
 CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
1 𝛾−1
𝑃2 𝛾 𝑃2 𝛾
⇒ 𝑚ሶ𝑎 = 𝜌1 𝐴2 2𝑐𝑝 𝑇1 1 − 𝑃𝑢𝑡𝑡𝑖𝑛𝑔 𝐸𝑞𝑛. (5)
𝑃1 𝑃1

2 𝛾+1
𝑃2 𝛾 𝑃2 𝛾
⇒ 𝑚ሶ𝑎 = 𝜌1 𝐴2 2𝑐𝑝 𝑇1 − → (𝟖)
𝑃1 𝑃1

▪ From the equation of state, ∵ 𝜌1 = 𝑃1 /𝑅𝑇1

2 𝛾+1
𝑃1 𝑃2 𝛾 𝑃2 𝛾
⇒ 𝑚ሶ𝑎 = 𝐴2 2𝑐𝑝 − → (𝟗)
𝑅 𝑇1 𝑃1 𝑃1

▪ However, the actual mass flow rate of air is:

𝑚ሶ 𝑎 𝑎𝑐𝑡. = 𝐶𝑑𝑎 𝑚ሶ 𝑎
where 𝐶𝑑𝑎 is the coefficient of discharge for the venturi.
Arranged by Prof. Dr. Asad Naeem Shah
 CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
2 𝛾+1
𝑃1 𝑃2 𝛾 𝑃2 𝛾
⇒ 𝑚ሶ 𝑎 𝑎𝑐𝑡. = 𝐶𝑑𝑎 𝐴2 2𝑐𝑝 − → (𝟏𝟎)
𝑅 𝑇1 𝑃1 𝑃1

➢ FUEL:
For the calculation of the mass flow rate of fuel, __________
1
the Bernoulli’s theorem can be used as the fuel is
considered to be incompressible. Therefore,
applying the Bernoulli’s theorem between sections
CC (plane 3) and BB (plane 2):
𝑃3 𝑢32 𝑃2 𝑢22
+ = + + 𝑔𝑍 → 𝟏𝟏
𝜌𝑓 2 𝜌𝑓 2
where 𝜌𝑓 is the density of fuel and Z is the height
of the nozzle exit above the level of fuel in the float
chamber.
Arranged by Prof. Dr. Asad Naeem Shah
 CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
▪ The fuel velocity 𝑢3 at section CC is negligible, as the level of
fuel does not drop in the reservoir.
▪ Thus, the fuel velocity at the nozzle exit i.e., 𝑢2 can be
obtained from Eqn. (11) as:

(𝑃3 − 𝑃2 )
𝑢2 = 2 − 𝑔𝑍 → (𝟏𝟐)
𝜌𝑓

▪ Pressures at plane 1 and plane 3 are both atmospheric,

therefore, 𝑃3 = 𝑃1 .

(𝑃1 − 𝑃2 )
𝑢2 = 2 − 𝑔𝑍 → (𝟏𝟑)
𝜌𝑓

2
⇒ 𝑢2 = (Δ𝑃 − 𝜌𝑓 𝑔𝑍)
𝜌𝑓
Arranged by Prof. Dr. Asad Naeem Shah
 CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
▪ From the continuity equation, the mass flow rate of fuel is
given by:

𝑚ሶ𝑓 = 𝐴𝑗 𝑢2 𝜌𝑓 = 𝐴𝑗 2𝜌𝑓 (Δ𝑃 − 𝜌𝑓 𝑔𝑍) → (𝟏𝟒)

where 𝐴𝑗 is the area of cross-section of the fuel jet at the

exit from the nozzle.
▪ The actual rate of mass flow of fuel is:

𝑚ሶ 𝑓 = 𝐶𝑑𝑓 𝐴𝑗 2𝜌𝑓 (Δ𝑃 − 𝜌𝑓 𝑔𝑍) → (𝟏𝟓)

𝑎𝑐𝑡.

where 𝐶𝑑𝑓 is the coefficient of discharge for fuel nozzle.

➢ AIR-FUEL RATIO:
𝐴 𝑚ሶ 𝑎 𝑎𝑐𝑡.
=
𝐹 𝑚ሶ 𝑓
𝑎𝑐𝑡.
Arranged by Prof. Dr. Asad Naeem Shah
 CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
2 𝛾+1
𝑃2 𝛾 𝑃2 𝛾
𝐴 𝐶𝑑𝑎 𝑃1 𝐴2 𝑐𝑝 −
𝑃1 𝑃1
⇒ = → (𝟏𝟔)
𝐹 𝐶𝑑𝑓 𝑅 𝑇1 𝐴𝑗 𝜌𝑓 Δ𝑃 − 𝜌𝑓 𝑔𝑍

➢ A/F-RATIO NEGLECTING THE COMPRESSIBILITY OF AIR:

▪ In this case, the Bernoulli’s theorem can be applied on air flow
as well. Therefore, considering the section AA and section BB
and neglecting the change in potential energy:

𝑃1 𝑢12 𝑃2 𝑢22
+ = + → (𝟏𝟕)
𝜌𝑎 2 𝜌𝑎 2

2 𝑃1 − 𝑃2 2Δ𝑃
⇒ 𝑢2 = = → 𝟏𝟖 ∵ 𝑢2 ≫ 𝑢1
𝜌𝑎 𝜌𝑎

Arranged by Prof. Dr. Asad Naeem Shah

 CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
∵ 𝑚ሶ𝑎 = 𝐴2 𝑢2 𝜌𝑎 → (𝟏𝟗)
▪ Putting Eqn. (18) in Eqn. (19):

∴ 𝑚ሶ𝑎 = 𝐴2 2𝜌𝑎 Δ𝑝 → 𝟐𝟎

𝑚ሶ 𝑎 𝑎𝑐𝑡. = 𝐶𝑑𝑎 𝐴2 2𝜌𝑎 Δ𝑃

𝐴 𝑚ሶ𝑎 𝑎𝑐𝑡𝑢𝑎𝑙
∵ =
𝐹 𝑚𝑓ሶ
𝑎𝑐𝑡𝑢𝑎𝑙

𝐴 𝐶𝑑𝑎 𝐴2 𝜌𝑎 Δ𝑃
= → (𝟐𝟏)
𝐹 𝐶𝑑𝑓 𝐴𝑗 𝜌𝑓 Δ𝑃 − 𝜌𝑓 𝑔𝑍

Arranged by Prof. Dr. Asad Naeem Shah

 CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
➢KEY FINDINGS:
▪ From Eqn. (14), the fuel flow will take place only when:
Δ𝑃 > 𝜌𝑓 𝑔𝑍
▪ When Δ𝑃 ≤ 𝜌𝑓 𝑔𝑍, there will be no flow of fuel. As the pressure
difference Δ𝑃 increases, the rate of mass flow of fuel increases,
and the mixture becomes progressively richer.
▪ Minimum air velocity at the throat which may cause fuel flow
can be estimated from Eqn. (18) as follows:

2Δ𝑃 2𝜌𝑓 𝑔𝑍
𝑢2 = =
𝜌𝑎 𝜌𝑎
Arranged by Prof. Dr. Asad Naeem Shah
CALCULATION OF THE AIR/FUEL RATIO
FOR A SIMPLE CARBURETOR Cont.
▪ If 𝑧 ≅ 0; or when ΔP is very large compared to 𝜌𝑓 𝑔𝑍 (i.e., at
high air flow rate), the term 𝝆𝒇 𝒈𝒁 can be neglected in Eqn.
(21). Therefore,

𝐴 𝐶𝑑𝑎 𝐴2 𝜌𝑎
⇒ =
𝐹 𝐶𝑑𝑓 𝐴𝑗 𝜌𝑓

▪ Moreover, the Eqn. (21) also reveals that as the density of air
reduces, the A/F ratio also decreases, i.e., the mixture
becomes richer. It will happen at:
o The high altitudes where the density of air is low.
o The throat where velocity increases (i.e., pressure drops), and
thereby density decreases.
Arranged by Prof. Dr. Asad Naeem Shah
 PROBLEM
A 4-stroke gasoline engine of 1.71𝑙 is to develop maximum
power at 5400 rpm. The volumetric efficiency at this speed is
assumed to be 70% and the 𝐴Τ𝐹 is 13:1. Two carburetors are to
be fitted and it is expected that at peak power, the air speed at
the throat (critical section of choke) will be 105 m/s. The
coefficient of discharge for the venturi is assumed to be 0.85 and
the main petrol jet is 0.66. An allowance should be made for the
emulsion tube, the diameter of which can be taken as 1/2.5 of
the throat diameter. The petrol surface is 6.5 mm below the
throat at this engine condition. Calculate the pressure at throat,
size of a suitable throat, emulsion tube and main jet. The specific
gravity of gasoline is 0.75. Atmospheric pressure and
temperature are 1.013 bar and 15°𝐶, respectively.

Arranged by Prof. Dr. Asad Naeem Shah

 SOLUTION
(a)

𝛾−1
𝑃𝑡 𝛾
∵ 𝑢𝑡 = 2𝑐𝑝 𝑇𝑎 1 −
𝑃𝑎

0.4
𝑃𝑡 1.4
⇒ 105 = 2 × 1.005 × 103 × 288 1 −
𝑃𝑎
𝑃𝑡
⇒ = 0.935 ⇒ 𝑷𝒕 = 0.935 × 1.013 = 𝟎. 𝟗𝟒𝟕 𝒃𝒂𝒓
𝑃𝑎
∵ 𝑉ሶ𝑡 = 𝑢𝑡 𝐴𝑡 𝐶𝑑𝑎

1/𝛾
𝑉𝑡 𝑃𝑎
⇒ 𝐴𝑡 = 317 𝑚𝑚2 ∵ = ⇒ 𝑉ሶ𝑡 = 0.0283 𝑚3 Τ𝑠
𝑉𝑎 𝑃𝑡
Arranged by Prof. Dr. Asad Naeem Shah
 2
𝜋 2 2
𝜋 2 1
∵ 𝐴𝑡 = 𝐷 − 𝑑 = 𝐷 1 −
4 4 2.5
⇒ 𝐷 = 𝟐𝟏. 𝟗𝟐 𝒎𝒎
⇒ 𝑑 = 𝟖. 𝟕𝟔𝟖 𝒎𝒎

(b)
∵ 𝑚𝑓ሶ = 𝑐𝑑𝑗 𝐴𝑗 𝑢𝑗 𝜌𝑓 → (1)

𝐴 𝑚ሶ 𝑎 13
∵ = = → (2)
𝐹 𝑚ሶ 𝑓 1
𝑉ሶ𝑑 = 𝑉𝑑 𝑁Τ𝑛𝑅 = 1.71 × 10−3 × 5400Τ2 × 60 = 0.07695 𝑚3 Τ𝑠
0.07695
⇒ 𝑉ሶ𝑑 = = 0.038475 𝑚3 Τ𝑠
2
𝑉𝑎ሶ = 𝜂𝑣 × 𝑉ሶ𝑑 = 0.7 × 0.038475 = 0.02693 𝑚3 Τ𝑠

Arranged by Prof. Dr. Asad Naeem Shah

▪ But mass flow rate of air is :
1.013 × 105
𝑚ሶ 𝑎 = 𝜌𝑎 × 𝑉𝑎ሶ = × 0.02695 = 0.033 kg/s
287 × 288
▪ From Eqn. (2), we have:
⇒ 𝑚ሶ 𝑓 = 0.00254 𝑘𝑔/𝑠
▪ Now

2Δ𝑃
∵ 𝑢𝑗 = → (3)
𝜌𝑓

▪ But the pressure differential across the main jet of gasoline is:
∆𝑃 = 𝑃𝑎 − 𝑃𝑡 − 𝑍 𝑔 𝜌𝑓
0.0065 × 9.81 × 0.75 × 103
∆𝑃 = 1.03 − 0.947 − 5
= 0.0655 bar
10
Arranged by Prof. Dr. Asad Naeem Shah
▪ Thus, from Eqn. (3), we have 𝑢𝑗 as:

2Δ𝑃 2 × 0.0655 × 105

𝑢𝑗 = = 3
= 4.179 𝑚/𝑠
𝜌𝑓 0.75 × 10

▪ Finally, from Eqn. (1), 𝐴𝑗 is obtained as:

𝑚𝑓ሶ 0.00254
𝐴𝑗 = =
𝑐𝑑𝑗 𝑢𝑗 𝜌𝑓 0.66 × 4.179 × 0.75 × 103
⇒ 𝐴𝑗 = 1.228 𝑚𝑚2
𝜋 2
⇒ 𝑑𝑗 = 1.228
4
⇒ 𝑑𝑗 = 𝟏. 𝟐𝟓 𝒎𝒎
▪ As jet dia. is designated by jet number, thus:
𝑱𝒆𝒕 𝑵𝒐. = 𝟏𝟐𝟓
Arranged by Prof. Dr. Asad Naeem Shah
FUEL INJECTION
SYSTEMS

Arranged by Prof. Dr. Asad Naeem Shah

 ELECTRONIC FUEL INJECTION
(EFI) SYSTEMS IN SI ENGINES
▪ In automotive engines, a continuous metered quantity of the
gasoline-air mixture must be ensured to make the engine run
smoothly.
▪ In SI injection system, the fuel is injected into the intake
manifold or near the intake port through an injector.
▪ The gasoline is received by the injector from the pump and is
sprayed into the air stream in a finely atomized form. Thus,
compared to carburetion, the mixing of gasoline with the air
stream is better in fuel injection (FI).
▪ Moreover, uniformity of mixture strength is difficult to realize in
each cylinder of a multi-cylinder carburetor engine. Figure 1
shows a typical pattern of mixture distribution in an intake
manifold of carburated multicylinder engine.
Arranged by Prof. Dr. Asad Naeem Shah
 ELECTRONIC FUEL INJECTION
SYSTEMS Cont.
▪ It may be noted that the intake valve is open in cylinder 2, and
that gasoline moves to the end of the manifold and
accumulates there. This enriches the mixture going to the end
cylinders.

Fig.1: A pattern of mixture distribution in a multi-cylinder engine.

Arranged by Prof. Dr. Asad Naeem Shah
 ELECTRONIC FUEL INJECTION
SYSTEMS Cont.
▪ However, the central cylinders, which are very close to the
carburetor, get the leanest mixture. Thus, the various
cylinders receive the air-gasoline mixture in varying quantities
and richness.
▪ This problem is called the maldistribution and can be solved
by the FI system by having the same amount of gasoline
injected at each intake manifold and thereby, each cylinder
can get the same richness of the air-gasoline mixture.
▪ Thus, recent automotive engines are equipped with gasoline
injection system, instead of a carburetion for one or more of
the following reasons:
o To have uniform distribution of fuel in a multicylinder engine.
o To improve the breathing capacity i.e., volumetric efficiency.
Arranged by Prof. Dr. Asad Naeem Shah
 ELECTRONIC FUEL INJECTION
SYSTEMS Cont.
o To reduce or eliminate detonation.
o To prevent fuel loss during scavenging in case of two-stroke
engines.
➢ TYPES OF INJECTION SYSTEMS:
▪ The FI systems can be classified as follows:
1) Gasoline direct injection (GDI) into the cylinder
2) Port injection (Timed and Continuous)
3) Manifold injection (MI)
▪ The above FI systems can be grouped under two heads, i.e.,
single-point injection (SPI) system and multi-point injection
(MPI) system.
▪ In the SPI system, one or two injectors are mounted inside the
throttle body assembly.
Arranged by Prof. Dr. Asad Naeem Shah
 ELECTRONIC FUEL INJECTION
SYSTEMS Cont.
▪ In such a case, fuel sprays are directed at one point or at the
center of the intake manifold. Another name of the SPI is the
throttle body injection (Fig. 2).
▪ In throttle body injection, an injector is placed slightly above
the throat of the throttle body. This throttle body is similar to
the carburetor throttle body, with the throttle valve controlling
the amount of air entering the intake manifold.

Fig. 2: Throttle body injection

(Singe point injection) system.

Arranged by Prof. Dr. Asad Naeem Shah

 ELECTRONIC FUEL INJECTION
SYSTEMS Cont.
▪ The MPI system has one injector for each engine cylinder. In
this system, fuel is injected in more than one locations. This is
more common and is often called port injection system.
▪ In both systems, the amount of gasoline injected depends
upon the engine speed and power demands.
▪ Typical fuel injection methods used in 4-stroke SI engine are
shown in Fig. 3.

Fig. 3: Four-stroke SI injection systems.

Arranged by Prof. Dr. Asad Naeem Shah
 COMPONENTS OF AN
INJECTION SYSTEM
▪ As the objectives of the fuel injection system are to meter,
atomize and uniformly distribute the fuel throughout the air
mass in the cylinder. At the same time, it must maintain the
required air-fuel ratio as per the load and speed requirement
of the engine.
▪ To achieve all the above tasks, a number of components are
required in the fuel injection system, the functions of which
are as follows:
o Pumping element – moves the fuel from the fuel tank to the
injector. This includes necessary piping, filter etc.
o Metering element – measures and supplies the fuel at the
rate demanded by load and speed conditions of the engine.
o Mixing element – atomizes the fuel and mixes it with air to
form a homogeneous mixture.
Arranged by Prof. Dr. Asad Naeem Shah
 COMPONENTS OF AN INJECTION
SYSTEM Cont.
o Metering control – adjusts the rate of metering in
accordance with load and speed of the engine.
o Mixture control – adjusts fuel-air ratio as demanded by the
load and speed.
o Distributing element – divides the metered fuel equally
among the cylinders.
o Timing control – fixes the start and stop of the fuel-air
mixing process.
o Ambient control – compensates for changes in temperature
and pressure of either air or fuel that may affect the various
elements of the system.
▪ Modern gasoline injection systems are known as EFI
systems, as they use electrical and electronic devices to
monitor and control engine-operation.
Arranged by Prof. Dr. Asad Naeem Shah
 SENSORS FOR AN EFI SYSTEM
▪ The EFI systems use engine sensors, an electronic control
unit (ECU) or computer, and solenoid operated fuel injectors
to meter and inject the right amount of fuel into the engine
cylinders.
▪ The ECU receives electrical signals in the form of current or
voltage from various sensors. It then uses the stored data to
operate the injectors, ignition system and other engine related
devices.
▪ Consequently, less unburned fuel leaves the engine as
emissions, and the vehicle gives better mileage. Typical
sensors for an electronic fuel injection system includes the
following:
o Exhaust gas or oxygen sensor – senses the amount of
oxygen in the engine exhaust and calculates air-fuel ratio.
Sensor output voltage changes in proportion to air-fuel ratio.
Arranged by Prof. Dr. Asad Naeem Shah
SENSORS FOR AN EFI SYSTEM Cont.
o Engine temperature sensor – senses the temperature of the engine
coolant, and from this data the computer adjusts the mixture strength
to rich side for cold starting.
o Air flow sensor – monitors mass or volume of air flowing into the
intake manifold for adjusting the quantity of fuel.
o Air inlet temperature sensor – checks the temperature of the
ambient air entering the engine for fine tuning the mixture strength.
o Throttle position sensor – senses the movement of the throttle plate
so that the mixture flow can be adjusted for engine speed and
acceleration.
o Manifold pressure sensor – monitors vacuum in the engine intake
manifold so that the mixture strength can be adjusted with changes in
engine load.
o Camshaft position sensor – senses rotation of engine camshaft/
crankshaft for speed and timing of injection.
o Knock sensor – microphone type sensor that detects ping or
preignition noise so that the ignition timing can be retarded.
Arranged by Prof. Dr. Asad Naeem Shah
 MERITS OF AN EFI SYSTEM
▪ Following are the advantages of an EFI operated SI engine
compared with a carburetor unit:
o Improvement in the volumetric efficiency due to relatively less
pressure losses.
o Manifold wetting is eliminated due to the fuel being injected
into or close to the cylinder
o Atomization of fuel is independent of cranking speed and
therefore starting will be easier.
o Better atomization and vaporization will make the engine less
knock prone.
o Formation of ice on the throttle plate is eliminated.
o Distribution of fuel being independent of vaporization, less
volatile fuel can be used.
Arranged by Prof. Dr. Asad Naeem Shah
 MERITS OF AN EFI SYSTEM Cont.
o Variation of air-fuel ratio is almost negligible even when the
vehicle takes different positions like turning, moving on
gradients, uneven roads etc.
o Position of the injection unit is not so critical and thereby the
height of the engine (and hood) can be less.
▪ However, some demerits associated with the EFI system are
also given here:
• High maintenance cost as the components of the injection
system are of high precision workmanship and finish.
• Difficulty in servicing
• Possibility of malfunction of some sensors
• The fuel injection pump continuously runs at half the speed of
crankshaft and thus causes wear of plungers, barrels, and
valves and their seatings.
Arranged by Prof. Dr. Asad Naeem Shah
 THE MPFI SYSTEM
▪ The main purpose of the Multi-Point Fuel Injection (MPFI) or
simply port injection (PI) system is to supply a proper ratio of
gasoline and air to the cylinders.
▪ In the port injection arrangement, the injector is placed on the
side of the intake manifold near the intake port (Fig. 1). The
injector sprays gasoline into the air, inside the intake manifold.
▪ The gasoline mixes with the air in a reasonably uniform
manner, and the mixture then passes through the intake valve
and enters the cylinder.

Fig. 1: Port injection.

Arranged by Prof. Dr. Asad Naeem Shah

 THE MPFI SYSTEM Cont.
▪ In MPFI or PI system, every cylinder is provided with an
injector in its intake manifold e.g., if there are six cylinders,
there will be six injectors (Fig. 2).

Fig. 2: A port injection or

MPFI near port.

▪ The MPFI systems are classified further into two categories:

(1) The D-MPFI system and, (2) The L-MPFI system.
Arranged by Prof. Dr. Asad Naeem Shah
 1−THE D-MPFI SYSTEM
▪ The D-MPFI system is also known as D-Jetronic EFI system,
as the first generation of EFI at BOSCH was called D-
Jetronic; where D stands for ‘Druck’, which means pressure.
▪ This name is derived from the fact that one of the main input
signals is the intake manifold pressure. Figure 1 gives the
block diagram of the D-MPFI system.

Fig. 1: D-MPFI gasoline injection system.

Arranged by Prof. Dr. Asad Naeem Shah
 THE D-MPFI SYSTEM Cont.
▪ In this type, the vacuum in the intake manifold is first sensed.
As air enters the intake manifold, the manifold pressure
sensor detects the intake manifold vacuum and sends the
information to the ECU.
▪ The speed sensor also sends information about the rpm of the
engine to the ECU.
▪ The ECU in turn sends commands to the injector to regulate
the amount of gasoline supply for injection.
▪ When the injector sprays fuel in the intake manifold, the
gasoline mixes with the air and the mixture enters the
cylinder.

Arranged by Prof. Dr. Asad Naeem Shah

 2−THE L-MPFI SYSTEM
▪ The L-MPFI system is the second generation EFI system
developed at BOSCH, and is also known as L-Jetronic EFI
system, where L stands for ‘Luftmengenmessung’, which
means air flow measurement.
▪ The block diagram of the L-MPFI system is shown Fig.1.

Fig. 1: The L-MPFI gasoline

injection system.

Arranged by Prof. Dr. Asad Naeem Shah

 THE L-MPFI SYSTEM Cont.
▪ In this type, the fuel metering is regulated by the engine
speed and the amount of air that actually enters the engine.
This is called air-mass metering or air-flow metering.
▪ As air enters the intake manifold, the air flow sensor
measures the amount of air and sends information to the
ECU. Similarly, the speed sensor sends information about the
speed of the engine to the ECU.
▪ The ECU processes the information received and sends
appropriate commands to the injector, in order to regulate the
amount of gasoline supply for injection. When injection takes
place, the gasoline mixes with the air and the mixture enters
the cylinder.

Arranged by Prof. Dr. Asad Naeem Shah

FUEL-INJECTION IN CI ENGINES
▪ In diesel engines, a fuel-injection system is required to inject
a definite quantity of fuel at the desired time and at a definite
rate into the combustion chamber (CC).
▪ The system must also atomize the fuel and distribute it to the
various parts of the CC.
▪ The injection system is responsible for initiating and
controlling the combustion process. Consequently, the
performance of CI engines largely depends upon the good
design of the fuel-injection system.
▪ Air drawn into the engine cylinder during the suction stroke of
a CI engine, is compressed during the compression stroke to
a high pressure, and thereby raising the temperature of the
compressed air higher than the self-ignition temperature of
the injected fuel.
Arranged by Prof. Dr. Asad Naeem Shah
FUEL-INJECTION IN CI ENGINES Cont.
▪ Fuel is injected into the CC near the end of the compression
stroke, and at that time the pressure in the cylinder is usually
between 20 bar and 35 bar.
▪ As the combustion of the injected fuel proceeds, the
maximum cylinder pressure reaches up to 70 bar.
▪ The fuel is thus injected into a high-pressure CC. It requires
the injection pressure of the fuel to be much higher than this
pressure.
▪ Also, the good atomization of the fuel depends upon the high
injection velocities which require a high-pressure difference.
Thus, high pressures, usually between 100 and 200 bar, are
required for the fuel-injection system.
▪ In order to meet the requirements of the injection system, the
following components are required in a fuel-injection system:
Arranged by Prof. Dr. Asad Naeem Shah
FUEL-INJECTION IN CI ENGINES Cont.
o Pumping elements: Pumping elements pump the fuel from
the fuel tank to the cylinder through pipelines and injectors.
o Metering elements: Metering elements measure and supply
the fuel at the rate required by the engine speed and load.
o Metering controls: Metering controls adjust the rate of the
metering elements for changes in engine speed and load.
o Distributing elements: Distributing elements distribute the
metered fuel equally among the cylinders.
o Timing controls: Timing controls adjust the start and the
stop of injection.
o Mixing elements: Mixing elements atomize and distribute the
fuel in the combustion chamber.
Arranged by Prof. Dr. Asad Naeem Shah
 COMMON RAIL FI SYSTEM
▪ In a common rail system, the fuel from the fuel storage is
drawn through the primary fuel filters by a low-pressure fuel-
feed pump.
▪ The discharge from this pump
enters the high-pressure fuel
injection pump through filters.
This pump serves only to deliver
fuel, under high pressure, to a
common rail, called the header
or reservoir or accumulator with
the pressure held constant by a
pressure regulating valve.Thus,
the maximum pressure is under
direct control.

Fig. 1: A common rail system.

Arranged by Prof. Dr. Asad Naeem Shah
 COMMON RAIL FI SYSTEM Cont.
▪ The high pressure in the header forces the fuel to each of the
nozzles located in the cylinders i.e., fuel is led from common
rail to the injector mounted on the cylinder head.
▪ At the proper time, a mechanically-operated valve by means
of a push rod and a rocker arm sends the fuel to the cylinder.
▪ The pressure in the fuel header must be sufficient to penetrate
and disperse the fuel in the combustion chamber and must be
in accordance with the injector system design.
▪ The common-rail system tends to be self-governing. As the
fuel pressure is maintained constant, with increased time, the
fuel supply will also be increased.
▪ Very accurate design and workmanship are required in this
type of fuel-injection mechanism.
Arranged by Prof. Dr. Asad Naeem Shah
COMMON RAIL FI SYSTEM Cont.
▪ The common-rail system were, once, quite popular for large,
slow-speed engines, but over the years, have been replaced
by the jerk-pump injection e.g., Jerk type BOSCH FI pump.
▪ The technology for electronically controlled unit fuel injection
system is now available. In this system, the injection timing
and metering functions are performed by a solenoid operated
control valve with increased flexibility.
▪ The injection occurs when the solenoid is energized. The
quantity of fuel injected is directly dependent on the duration
of the pulse when the injection pressure is constant. Sensors
on the crankshaft indicate its position and speed, and so the
timing of injection and its frequency can be controlled.
▪ In these electronically controlled units, very high injection
pressures of the order of 1500 bar can easily be achieved.
Arranged by Prof. Dr. Asad Naeem Shah
IC ENGINE
FUELS

Arranged by Prof. Dr. Asad Naeem Shah

 FUELS
▪ Fuels used in internal combustion engines come from all three
groups – gaseous, liquid, and solid.
▪ Gaseous fuels – natural gas, liquified petroleum gas (LPG),
producer gas, liquified natural gas (LNG), etc.
▪ Liquid fuels – derived mostly from petroleum e.g., gasoline and
fuel oil. Also, kerosene, alcohol, vegetable oils, etc.
▪ All liquid fuels can be divided into two main groups: vaporized
fuels – handled similarly to gas fuels (gasoline and alcohol) and
injected fuels – injected into the combustion space (fuel oils of
different characteristics).
▪ Solid fuels – coal, chiefly anthracite and coke – mostly used in
gas producers.
▪ Pulverized coal – used in CI engines but cause some problems
e.g., excessive wear of cylinder liner and piston rings.
Arranged by Prof. Dr. Asad Naeem Shah
 FUELS Cont.
➢ COMPOSITION: Two basic combustible elements are carbon
and hydrogen – are called hydrocarbons.
▪ At atmospheric pressure and temperature some of the
hydrocarbons are gases, while some are liquids.
▪ Crude oil is made up almost entirely of carbon and hydrogen
with some traces of other species like sulphur, oxygen,
nitrogen, water (humid). It varies from 83% to 87% carbon and
11% to 14% hydrogen by mass.
▪ There are four significant sources of crude oil: (i) petroleum; (ii)
coal liquefaction; (iii) shale oil, and (iv) tar sands.
▪ Most of the crude oil used to date has been petroleum derived
– requires little processing before delivery to a refinery.
▪ Coal, on the other hand, is treated to increase its hydrogen
content and remove undesirable elements such as nitrogen,
sulphur, etc. Arranged by Prof. Dr. Asad Naeem Shah
 FUELS Cont.
▪ Shale oil − difficult to get out of the ground since it is soaked
up in rocks.
▪ Tar sands contain hydrocarbons mixed with sand – difficult to
remove from the ground than petroleum.
▪ Oils from tar sands also require hydrogenation and removal of
undesirable chemicals from the crude before delivery to the
refinery.
▪ As petroleum supplies dwindle, more and more crude oil will
be from alternative sources. Regardless of the source, crude
oil contains a large number of different hydrocarbons, 25,000
different compounds have been found in one sample of
petroleum derived crude oil. The compounds range from
gases to viscous liquids and waxes.
Arranged by Prof. Dr. Asad Naeem Shah
 REFINING
▪ The crude oil mixture from the ground is separated into
component products by cracking and/or distillation using
thermal or catalytic methods at oil refinery (Fig.1).
▪ The purpose of refinery – to physically separate crude oil into
various fractions for the subsequent chemical processing.
▪ The physical properties of any fraction are controlled by the
distillation temperatures.
▪ Generally, the larger the molecular mass of a component, the
higher is its boiling temperature.
▪ The refinery produces fuels for engines (gasoline, diesel, jet),
fuels for heating (coke, kerosene, residual), chemical feed
stock (aromatics, propylene), and asphalt.
Arranged by Prof. Dr. Asad Naeem Shah
REFINING Cont.

Fig.1: Crude oil refining.

Arranged by Prof. Dr. Asad Naeem Shah