Injection Systems

Principles of Gasoline And Diesel Fuel Systems
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a. Fuel Injection Principles.

(1) Methods.

There are two methods of injecting fuel into a

compression-ignition engine. One method is air injection. This method uses

a blast of air to force a measured charge of fuel into the combustion

chamber.

The other method is solid injection, where direct mechanical

pressure is placed on the fuel itself to force it into the combustion

chamber.

Only the solid injection system will be discussed in this task

because air injection is virtually unused in automotive applications.

(2) Fuel Atomization and Penetration.

The fuel spray entering the

combustion chamber must conform to the chamber's shape so that the fuel

particles will be well distributed and thoroughly mixed with the air. The

shape of the spray is determined by the degree of atomization and

penetration produced by the orifice through which the fuel enters the

chamber. Atomization is the term used to indicate the size of the droplets

into which the fuel is broken down.

Penetration is the distance from the

orifice that the fuel droplets attain at a given phase of the injection

period. The dominant factors that control penetration are the length of the

nozzle orifice, the diameter of the orifice outlet, the viscosity of the

fuel, and the injection pressure of the fuel. Increasing the ratio of the

length of the orifice to its diameter will increase penetration and decrease

atomization. Decreasing this ratio will have an opposite effect. Because

penetration and atomization are opposed mutually and both are important, a

compromise is necessary if uniform fuel distribution is to be obtained. The

amount of fuel pressure for injection is dependent on the pressure of the

air in the combustion chamber, and the amount of turbulence in the

combustion space.

(3) Function of the Injection System. It is impossible to cover the

operation and construction of the many types of modern injection systems in

this lesson.

However, the operation of the more common systems will be

discussed. If the three basic functions of diesel fuel injection are kept

in mind while studying the operation of the systems, it will be easier to

understand how they work. The three basic functions are: