Air-Fuel mixture

1. Mixture Requirements

2. Air- Fuel Mixtures

3. Air- Fuel Mixtures
Air – Fuel Mixtures
An engine is generally operated at different loads and speeds.
For this, proper air-fuel mixture should be supplied to the
engine cylinder. Fuel and air are mixed to form three different
types of mixtures.
• Chemically correct mixture
• Rich mixture and
• Lean mixture

4. Air- Fuel Mixtures
• Chemically correct or stoichiometric mixture is one in which
there is just enough air for complete combustion of the fuel. For
example, to burn one kg of octane (C8H18) completely 15.12 kg of
air is required. Hence chemically correct A/F ratio for C8H18 is
15.12:1 usually approximated to 15:1. This chemically correct
mixture will vary only slightly in numerical value between different
hydrocarbon fuels. Completely combustion means carbon in the
fuel is converted to CO2 and all hydrogen to H2O.
• Rich mixture is one which contains less air than the stoichiometric
mixture (example, A/F ratio of 12:1. 10:1 etc).
• Lean mixture is one which contains more air than the
stoichiometric mixture (example, A/F ratio of 17:1. 20:1 etc).

5. Air- Fuel Mixtures
• There is, however a limited range of A/F ratios in a
homogeneous mixture only within which combustion in
an SI engine will occur. Outside this range the ratio is
either too rich or too lean to sustain flame propagation.
This range of useful A/F ratio runs from approximately
0:1 (rich) to 19:1 (lean) as indicated in Fig.1.
• The carburettor should provide A/F ratio in accordance
with engine operating requirements and this ratio must be
within the combustible range.

6. Fig. 1 Useful air-fuel mixture range of
gasoline

7. Mixture Requirements at Different
Loads and Speeds

8. Mixture Requirements at Different Loads and
Speeds
The air-fuel ratio at which an engine operates has a
considerable influence on its performance. Consider
an engine operating at full throttle and constant speed
with varying A/F ratio. Under these conditions, the
A/F ratio will affect both the power output and brake
specific fuel consumption as shown in Fig.2.

9. Fig. 2 A/F Ratio (kg of air/kg of fuel)

10. Mixture Requirements at Different Loads and
Speeds
The mixture corresponding to the maximum
output on the curve is called the best power
mixture with an A/F ratio of approximately 12:1.
The mixture corresponding to the minimum point
on the BSFC curve is called the best economy
mixture. The A/F ratio is approximately 16:1. It
may be noted that the best power mixture is much
richer than the stoichiometric mixture and the best
economy mixture is slightly leaner than
stoichiometric mixture.

11. Mixture Requirements at Different Loads and
Speeds
• Fig. 2 is based on full throttle operation. The A/F ratios for the best
power and best economy at part throttle are not strictly the same as
at full load. If the A/F ratios for best power and best economy are
constant over the full range of throttle operation and if the influence
of other factors is disregarded, the ideal fuel metering device would
be merely a two position carburettor. Such a carburettor could be
set for the best economy power mixture when maximum
performance is desired and for the best economy.
• In Fig. 3 these two settings are indicated by the solid horizontal lines
X-X’ and Z-Z’, respectively. Under normal conditions it is desirable
to run the engine on the maximum economy mixture viz., around
16:1 air fuel ratio. For quick acceleration for maximum power, rich
mixture viz., 12:1 air fuel ratio is required.

12. Actual Automotive Engine Air-Fuel Mixture
Requirements
Actual air-fuel mixture requirements in an
automobile engine vary considerably from the
ideal conditions discussed in the previous
section. For successful operation of the
engine, the carburettor has to provide mixtures
which follow the general shape of the curve
ABCD (single cylinder) and A’B’C’D’
(multicylinder) in Fig. 3 which represents a
typical automotive engine requirements.

13. Actual Automotive Engine Air-Fuel Mixture
Requirements
As indicated in Fig. 3 there are three general
ranges of throttle operation. In each one of these,
the automotive engine requirements differ. As a
result, the carburettor must be able to supply the
required air-fuel ratio to satisfy demands. These
ranges are:
• Idling (mixture must be enriched)
• Cruising (mixture must be leaned)
• High Power (mixture must be enriched)

14. Fig. 3 Anticipated carburettor performance to
fulfil engine requirements

15. Actual Automotive Engine Air-Fuel Mixture
Requirements
As indicated in Fig. 3 there are three general
ranges of throttle operation. In each one of these,
the automotive engine requirements differ. As a
result, the carburettor must be able to supply the
required air-fuel ratio to satisfy demands. These
ranges are:
• Idling (mixture must be enriched)
• Cruising (mixture must be leaned)
• High Power (mixture must be enriched)

16. Actual Automotive Engine Air-Fuel Mixture
Requirements
Idling Range
• An idling range is one which operates at no load
and with nearly closed throttle. Under idling
conditions, the engine requires a rich mixture, as
indicated by point A in Fig. 3. This is due to the
existing pressure conditions within the
combustion chamber and the intake manifold
which cause exhaust gas dilution of the fresh
charge. The pressures indicated in Fig. 4 are
representative values which exist during idling.

17. Actual Automotive Engine Air-Fuel Mixture
Requirements
• The exhaust gas pressure at the end of the exhaust does not vary
greatly from the value indicated in Fig. 4, regardless of the throttle
position. Since, the clearance volume is constant; the mass of
exhaust gas in the cylinder at the end of the exhaust stroke tends to
remain fairly constant throughout the idling range. The amount of
fresh charge brought in during idling, however, is much less than
that during full throttle operation, due to very small opening of the
throttle (Fig. 4). This results in a much larger proportion of exhaust
gas being mixed with the fresh charge under idling conditions.
• The presence of exhaust gas tends to obstruct the contact of fuel and
air particles, a requirement necessary for combustion. It is
therefore, necessary to provide more fuel particles by richening the
air-fuel mixture. This richening increases the probability of contact
between fuel and air particles and thus improves combustion.

18. Actual Automotive Engine Air-Fuel Mixture
Requirements
As the throttle is gradually opened from A to B
(Fig.3), the pressure differential between the
inlet manifold and the cylinder becomes
smaller and the exhaust gas dilution of the
fresh charge diminishes. Mixture requirement
then proceed along line AB (Fig. 3) to a leaner
A/F ratio required for the cruising operation.

19. Fig. 4 Schematic diagram of combustion
chamber and induction system at the start of
intake stroke

20. Actual Automotive Engine Air-Fuel Mixture
Requirements
Cruising Range
• In the cruising range from B to C (Fig. 3), the
exhaust gas dilution problem is relatively
insignificant. The primary interest lies in
obtaining the maximum fuel economy.
Consequently, in this range, it is desirable that
the carburettor provides the engine with the
best economy mixture.

21. Actual Automotive Engine Air-Fuel Mixture
Requirements
Power Range
• During peak power operation the engine requires a richer mixture, as indicated by
the line CD (Fig. 3) for the following reasons:
• To provide best power: Since high power is desired, it is logical to transfer the
economy setting of the cruising range to that mixture which will produce the
maximum power, or a setting in the vicinity of the best power mixture, usually in
the range of 12:1.
• To prevent overheating of exhaust valve and the area near it: At high power, the
increased mass of gas at higher temperatures passing through the cylinder results in
the necessity of transferring greater quantities of heat away from critical areas such
as those around the exhaust valve. Enriching the mixture reduces the flame
temperature and the cylinder temperature. This reduces the cooling problem and
also reduces the tendency to damage exhaust valves at high power. In the cruising
range the mass of charge is smaller and the tendency to burn the exhaust valve is
not as high.

22. Actual Automotive Engine Air-Fuel Mixture
Requirements
Fig. 3 is better representative of typical engine
requirements for the carburettor. Automobile
engine requirements are similar in the idling and
cruising range but tend to be relatively lower or
less rich in the power range (C to D in Fig. 5).
A more representative engine requirement curve
for automobile is shown in Fig. 5. The portion of
the curve from D to E indicates the requirements
after the throttle is wide open and the load is
further increased.

23. Fig. 5 Performance curve of an Automobile
carburetor

24. Fuel injection systems – Monopoint,
Multipoint & Direct injection

25. Introduction
• In a carburettor engine, uniformity of mixture strength is difficult to realize
in each cylinder of a multicylinder engine. 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 port injection system
by having the same amount of gasoline injected at each intake manifold.
Therefore there is an urgent need to develop injection systems for gasoline
engines. By adopting gasoline injection each cylinder can get the same
richness of the air-gasoline mixture and the maldistribution can be avoided
to a greater extent.
• Recent automotive engines are equipped with gasoline injection system,
instead of a carburetion for one or more of the following reasons:
• To have uniform distribution of fuel in a multicylinder engine.
• To improve volumetric efficiency.
• To prevent fuel loss during scavenging in case of two-stroke engines.
Fuel injection systems – Monopoint,
Multipoint & Direct injection

26. Fig. 6 Types of fuel injection system
The fuel injection system can be classified as:
Throttle Body Injection (TBI)
Port Injection
Gasoline Direct Injection (GDI)

27. Throttle-Body Injection (TBI) or Single-Point
Injection System
• Throttle body injection (TBI) system is also named as
Single point injection system. An injector is placed
slightly above the throat of the throttle body. The
injector sprays gasoline into the air in the intake
manifold where the gasoline mixes with air. This
mixture then passes through the throttle valve and
enters into the intake manifold.
• One or two injectors inject the fuel into the air flow
directly above the throttle body. Fuel sprays are
directed at one point or at the centre of the intake
manifold.

28. Fig. 7 Throttle Body Injection System (Single
Point)

29. Port injection (or) Multipoint injection system
• In the port injection arrangement, the injector is placed
on the side of the intake manifold near the intake port
(Fig.8). The injector sprays gasoline into the air, inside
the intake manifold. The gasoline mixes with the air in
a reasonably uniform manner. This mixture of gasoline
and air then passes through the intake valve and enters
into the cylinder.
• Every cylinder is provided with an injector in its intake
manifold. If there are six cylinders, there will be six
injectors. Figure shows a simplified view of a port or
multipoint fuel injection (MPFI) system.

30. Fig. 8 Port Injection System (MPFI)

31. 3.2.3 Gasoline direct injection (GDI) systems
In Gasoline Direct Injection (GDI) systems,
gasoline is injected directly into the combustion
chambers. Directly injecting fuel into the
combustion chamber requires high-pressure
injection. The gasoline is highly pressurized, and
injected via a common rail fuel line directly into
the combustion chamber of each cylinder, as
opposed to conventional multipoint fuel
injection that injects fuel into the intake tract or
cylinder port.

32. Fig. 9 Gasoline Direct Injection System (GDI)

33. 3.3 METHODS OF GASOLINE FUEL
INJECTION
Gasoline fuel injection system used in a SI engine can be
either of continuous injection or timed injection.
• Continuous injection systems: In continuous injection
system, gasoline is sprayed continuously from the injectors.
• Timed fuel injection systems: In the timed injection
systems, gasoline is sprayed from the injectors in pulses.
The port injection system and the throttle body injection
system may be either pulsed systems or continuous systems.
In both systems, the amount of gasoline injected depends
upon the speed and power demands.

34. 3.4 CLASSIFICATIONS OF MPFI SYSTEMS
MPFI systems are classified into two systems:
1. D-MPFI
2. L-MPFI.

35. D-MPFI System
• The D-MPFI system is the manifold fuel injection system.
In this type, the vacuum in the intake manifold is first
sensed. In addition, it senses the volume of air by its
density. Fig. 10 gives the block diagram regarding the
functioning of the D-MPFI system.
• As enters into 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.

36. Fig. 10 D-MPFI gasoline injection system

37. L-MPFI System
The L-MPFI system is a port fuel-injection system. 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. The block
diagram of L-MPFI system is shown in Fig.11.
As air enters into 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 send 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.

38. Fig. 11 L-MPFI gasoline injection system

39. ELECTRONIC FUEL INJECTION SYSTEM
• Modern gasoline injection systems use engine sensors,
a computer, and solenoids operated fuel injectors to
meter and inject the right amount of fuel into the engine
cylinders. These systems called electronic fuel
injection (EFI) use electrical and electronic devices to
monitor and control engine operations.
• An electronic control unit (ECU) or the computer
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 deices. As a result, less unburned fuel
leaves the engine as emissions and the vehicle gives
better mileage.

40. ELECTRONIC FUEL INJECTION SYSTEM
Typical sensors for an electronic fuel system include the following:
• Exhaust gas or oxygen sensor
• Engine Temperature sensor
• Air flow sensor
• Air inlet temperature sensor
• Throttle position sensor
• Manifold pressure sensor
• Camshaft position sensor
• Knock sensor
The fuel injector in an EFI is nothing but a fuel valve. When it is not
energized, spring pressure makes the injector to remain closed and no fuel
will enter the engine. When the computer sends the signal through the
injector coil, the magnetic field attracts the injector armature. Fuel then
spurts into the intake manifold.