Section III. 5-13.
Fuel Injection Principles.
(3) To control the start, rate, and duration of the
a. 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
Multiple Unit Injection.
measured charge of fuel into the combustion chamber.
The other method is solid injection, where direct
a. General System Operation (Fig. 5-6). The basic
mechanical pressure is placed on the fuel itself to force it
system consists of a fuel supply pump, fuel filter, multiple
into the combustion chamber. This chapter only will
unit injection pump, and one injector for each cylinder.
cover solid injection systems because air injection
The operation of the system is as follows:
virtually is unused in automotive applications.
b. Fuel Atomization and Penetration. The fuel
(1) The fuel supply pump and the fuel filter
provide a low-pressure supply of fuel to the multiple unit
spray entering the combustion chamber must conform to
Pressure usually is regulated to
the chamber's shape so that the fuel particles will be well
approximately 15 psi (103.4 kPa).
distributed and thoroughly mixed with the air. The shape
of the spray is determined by the degree of atomization
(2) The multiple unit injection pump contains
and penetration produced by the orifice through which
an individual injection pump for each engine cylinder.
the fuel enters the chamber. Atomization (para 4-9e) is
Fuel is delivered to the injectors at each cylinder from the
the term used to indicate the size of the droplets the fuel
multiple unit injection pump in a timed sequence and a
is broken down into. Penetration is the distance from the
regulated amount based on accelerator pedal position
orifice that the fuel droplets attain at a given phase of the
and engine speed.
injection period. The dominant factors that control
penetration are the length of the nozzle orifice, the
(3) The injectors receive fuel charges from
diameter of the orifice outlet, the viscosity of the fuel, and
their respective injection pumps and spray it into the
the injection pressure of the fuel. Increasing the ratio of
combustion chambers in a spray pattern that is tailored
the length of the orifice to its diameter will increase
to provide the best overall performance for their
penetration and decrease atomization. Decreasing this
ratio will have an opposite effect. Because penetration
and atomization are opposed mutually and both are
b. The Multiple Unit Injection Pump (Fig.
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
(1) The multiple unit injection pump contains
turbulence in the combustion space.
an individual plunger-type injector pump for each
cylinder. These pumps are arranged in a line so that
c. Function of the Injection System. It is Impossible
they may be driven by a common camshaft. The lobes
to cover the operation and construction of the many
of the camshaft are arranged so that they operate the
types of modern injection systems in this text. However,
injection pumps in a sequence that coincides with the
the operation of the more common systems will be
firing order of the engine. This camshaft is driven by the
discussed. If the three basic functions of diesel fuel
engine through gears at a speed of exactly one-half that
injection are kept in mind while studying the operation of
of the crankshaft. This exact speed is maintained so that
the systems, it will be easier to understand how they
the injectors will each deliver their fuel charge at the
work. The three basic functions are:
beginning of their respective cylinder's power stroke.
Power strokes occur during every other crankshaft
(1) To meter the fuel accurately.
revolution in a four-stroke cycle diesel engine.
(2) To distribute the fuel equally to all of the
cylinders at a high enough pressure to ensure