FUEL INJECTION SYSTEMS

• Introduction :

•         Fuel injection system is a well-designed mechanism for admitting fuel into an I.C. engine.

•         It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s.

•         A fuel injection system is designed and calibrated specifically for the type(s) of fuel it will handle. Most fuel injection systems are for gasoline or diesel applications.

•    The basic function of a fuel injection system is to inject fuel to the engine cylinders in proper quantity, at correct time and at a pre-determined rate.

 

Requirements  Of Fuel Injection System:

The injection system of the compression ignition

  engine should fulfill the following objective

s consistently and precisely:

1.Meter the appropriate quantity of fuel, as 

demanded by the speed of, and the load on, the 

engine at the given time.

 

2.Distribute the metered fuel equally among

 cylinders in a multi-cylinder engine.

 

3.Inject the fuel at the correct time (with respect to 

crank angle) in the cycle.

 

4.Inject the fuel at the correct rate (per unit time or

 crank angle degree).

 

5.Inject the fuel with the correct spray pattern and

 sufficient atomization as demanded by the design

the combustion chamber.

 

6.Begin and end injection sharply without dribbling.

There are two major divisions of Fuel Injection systems:

•       Solid Injection systems, which inject only the liquid fuel through the injectors.

•        Air Injection systems, which inject air along with the liquid fuel.

    However, the air injection systems have proved to be less efficient and costlier than other systems and hence, is now obsolete.

 

 Air Injection Systems 

Fig. Show the schematic diagram of an air injection system. 

The fuel is metered and pumped to the fuel valve by a camshaft driven fuel pump.
The fuel valve is opened by means of a mechanical linkage operated by the camshaft which controls the timing of injection.
The fuel valve is also connected to a high pressure air line fed by a multi-stage compressor which supplies air at a pressure of about 60 to 70 bar.

Advantages of air injection system:

 

The main advantage of the air injection system is the good atomization obtained.

1.        A high mep can be attained as rapid combustion results due to good mixing of fuel and air.
2.        Heavy and viscous fuels, which are cheaper, can also be injected.
3.        The fuel pump is required to develop only a small pressure.

 Disadvantages of air injection system:

1.It require a high pressure multi-stage compressor .

2.The large number of parts, the inter-cooler , etc.

3. make the system complicated and expensive.

4.A separate mechanical linkage is required to time the operation of the fuel valve.

5.Reduced bhp due to power loss in operating  the compressor and the linkage.

6.The fuel in the combustion chamber burns very near to the injection nozzle which many times leads sto overheating and burning of the valve and its seat.

 

Solid Injection System:

 

•      The commonly used types of Solid Injection system are:

•     Individual pump system: This consists of a separate metering and compression pump for each cylinder.

•   Common rail system: A single pump for compressing the fuel, plus a metering element for each cylinder.

Advantages of CRS(common rail system)

•     It increases fuel economy.

•     It lowers emission.

•     It delivers less noisy operation.
•     The system can be easily integrated in wide variety of  engines.
•     It is relatively simple in construction & needs less maintenance.