CHAPTER – 8

ELECTRONIC FUEL INJECTION AND

IGNITION SYSTEM
 Introduction

In trying to keep up with emissions and fuel efficiency laws, the fuel
system used in modern cars has changed a lot over the years. Fuel
injection is not a new development. Fuel injection has been around,
since 1950s and electronic fuel injection was used widely on European
cars starting around 1980. Now, almost all the cars sold in all over
world have fuel injection systems.

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 Feedback Carburetor systems (FBC):

• There are practical difficulties faced in the carburetors for switching

over to electronic fuel injection system. A carburetor is a mechanical
device. It is neither totally accurate nor particularly fast at
responding to changing engine needs. The intake manifold is also a
mechanical device and usually the following problems occur.
 Intake manifold distribution problems. Intake manifold supplies
different mixture strength to the engine cylinders for the same
operating condition called ‘mal- distribution’.
 Intake manifold is to be designed for the smooth flow of both fuel
and air, which is very difficult.
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 Cont…

The result, when these two devices are used together, is an engine that
does not run efficiently at low and high speeds.
• If the air-fuel mixture can be precisely controlled and the
distribution to the cylinders can be made more efficient, then today’s
strict emissions standards can be met, while at the same time the
engine will run better and consume less fuel. Fuel injection is one of
the best ways to get this precise control.

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 Types of Electronic Fuel Injection Systems:

• There are two basic types of gasoline fuel-injection systems,

throttle body injection and multi-point injection. At first, carburetors
were replaced with throttle body injection (TBI) systems. Gradually as
new engines were designed, throttle body fuel injection was replaced
by multi-point fuel injection (MPFI) system.
• The difference is in how many places fuel (under pressure) is
injected into the engine. In single-point system; fuel enters the intake
air at only one place. This is usually the throttle body on the intake
manifold. In multi-point fuel injection system; fuel is injected into the
intake air for each cylinder.
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 Throttle body injection system (TBI):

• It is also known as single point fuel injection system, central fuel

injection (CFI) or digital fuel injection (DFI). In 1980, this system was
introduced. This system has one or two small injection valves
positioned just above the throttle valve in the throttle body. The
throttle body is similar to the carburetor throttle body. There they
spray fuel into the air stream passing through.
• The drawback of this system is mal-distribution is not eliminated
totally.

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Cont…

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 Multi-port or Multi-point fuel injection system (MPFI):

It is also known as port injection or sequential injection. Multi-point

injection systems have a fuel injector for each cylinder. Each injector
is placed so that it points at the intake valve for the cylinder. The
injectors are connected to fuel rails that are connected through a fuel
filter to an electric fuel pump. Only air enters the intake manifold.
Then as the air approaches the intake valves, injection valves open to
spray fuel into the air stream. The mal-distribution problem is
completely eliminated with MPFI systems. These systems provide
more accurate fuel metering and quicker response.

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Cont…

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Working principle of MPFI:

In MPFI system, a high – pressure electric fuel pump is located in or
near the fuel tank. The pump sends fuel from the tank through the fuel
– pressure regulator to the injectors at constant high pressure all the
time the engine runs. Injectors are operated by a small electric
solenoid. When the solenoid is connected to the battery, it pulls back
on the nozzle needle, opening the valve. Now fuel can spray out of the
valve.

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 Cont…

Various sensors located on the engine detect the operating mode of the
engine and signal this condition to the electronic control unit (ECU). It
processes the information. Then the ECU computes when and for
how long the injectors should be opened. The ECU sends a voltage
pulse to the injectors.
The voltage pulse causes the injectors to remain open for the required
time. To increase engine speed, the pulse width increases so that
injectors stay open longer. If less fuel is needed, the pulse width
decreases. Therefore the injectors close earlier and engine speed
decreases.
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 Gasoline Direct Injection (GDI):

Gasoline direct-injection engines generate the air-fuel mixture in the

combustion chamber. During the suction stroke, only the air flows
through the intake valve. The fuel is injected directly into the
combustion chamber at high pressure by special fuel injectors.
As in a diesel engine, air-fuel mixture formation takes place inside the
combustion chamber.
The electric fuel pump delivers fuel to the high-pressure pump at a
pre-supply pressure of 3 to 5 bar. The highly pressurized fuel flows
into and is stored in the fuel rail.

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 Cont…

The high-pressure fuel injectors are mounted on the fuel rail, also
known as the “common rail”. These injectors are actuated by the
engine ECU and spray the fuel into the cylinder combustion chambers.
In developing the gasoline direct injection engine, it is aimed to cool
the interior of the cylinder as much as possible by promoting fuel
vaporization and uniform mixing of atomized fuel and air. This
produces a high charging efficiency of the air-fuel mixture and a high
compression ratio, which results in significant improvements in both
torque and fuel efficiency.

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 Cont…

• Characteristics of GDI engine:

• Fuel is injected from a tiny nozzle into a relatively large cylinder,
so it has a high latent heat of vaporization, which efficiently cools the
air within (in cylinder cooling effect).
 The air temperature in the cylinder decreases, which means:
• 1. More air may be charged into the combustion chambers,
which produce increased torque.
• 2. The engine is less prone to knocking. This contributes to
increased torque and enables a higher compression ratio that
also contributes to good fuel efficiency.
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 Advantages of Electronic fuel injection (EFI):

1. Equal air-fuel ratio to all the cylinders.

2. Accurate air-fuel ratio can be obtained throughout all engine rpm
changes.
3. Better cold starting
4. Reduces fuel consumption with no loss of engine performance
5. 10% advance of fuel economy over carbureted systems
6. Quick response
7. Reduce pollution levels
8. Precise control of engine operation
9. Earlier default detection.

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 Electronic fuel injection system controls:

• The figure shows a typical control layout for a fuel injection

system. Depending on the sophistication of the system, idle speed and
idle mixture adjustment can be either mechanically or electronically
controlled.
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 Electronic ignition systems:

• Electronic ignition is now fitted to almost all spark ignition

vehicles. This is because the conventional mechanical system has
some major disadvantages.
 Mechanical problems with the contact breakers, not the least of
which is the limited lifetime.
 Current flow in the primary circuit is limited to about 4 A or
damage will occur to the contacts – or at least the lifetime will be
seriously reduced.
 Legislation requires stringent emission limits, which means the
ignition timing must stay in tune for a long period of time.
 Weaker mixtures require more energy from the spark to ensure
successful ignition, even at very high engine speed.

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 Cont…

• These problems can be overcome by using a power transistor to

carry out the switching function and a pulse generator to provide the
timing signal. Very early forms of electronic ignition used the existing
contact breakers as the signal provider. This was a step in the right
direction but did not overcome all the mechanical limitations, such as
contact bounce and timing slip. Most of the systems nowadays are
constant energy, ensuring high performance ignition even at high
engine speed. Figure shows the circuit of a standard electronic ignition
system.

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• Advantages of electronic ignition systems:

• Electronic ignition system uses electronic switching devices
instead of mechanical contact breaker points to send a high voltage to
the spark plug.
 It is small and light weight
 It gives higher ignition voltage
 It gives longer duration of spark
 It has very accurate control of timing
 It improves cold weather starting
 Less maintenance
 Longer service life
 It helps vehicle meet exhaust emission regulations and reduces fuel
consumption.
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 Electronic spark timing control (or) Electronic spark advance
(ESA):
In the electronic spark advance (ESA) system, the engine is provided
with nearly ideal ignition timing characteristics. The engine control
module (ECM) determines ignition timing based on sensor inputs and
on its internal memory contains the optimal ignition timing data for
each engine running condition. After determining the ignition timing,
the ECM sends the ignition timing signal (IGT) to the igniter. When
the IGT signal goes off, the igniters will turn ON, shut off primary
current flow in the ignition coil producing a high voltage spark in the
cylinder.

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 Cont…

• Since the ESA always ensures optimal ignition timing, emissions

are lowered and both fuel efficiency and engine power output are
maintained at optimal level.

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