How many governing modes does the electronic
speed governing system provide?
How far should the inlet and outlet lines be from
the bottom of a fuel tank?
METHODS OF INJECTION
LEARNING OBJECTIVE: Describe the
principles and operation of the different diesel
You have probably heard the statement that "the
fuel injection system is the actual heart of the diesel
engine." When you consider that indeed a diesel could
not be developed until an adequate fuel injection system
was designed and produced, this statement takes on a
much broader and stronger meaning.
In this section, various methods of mechanical
injections and metering control are described. There
have been many important developments in pumps,
nozzles, and unit injectors for diesel engines over the
years with the latest injection system today relying on
electronic controls and sensors.
FUEL INJECTION SYSTEMS
Diesel fuel injection systems must accomplish five
particular functions-meter, inject, time, atomize, and
create pressure. A description of these functions
METERAccurately measure the amount of
fuel to be injected.
INJECT Force and distribute the fuel into the
TIMEInjection of the fuel must start and stop
at the proper time.
ATOMIZEBreak the fuel up into a fine mist.
CREATE PRESSURECreate the necessary
high pressure for injection.
You can remember these functions by the initials,
MITAC. All five of these functions are necessary for
complete and efficient combustion
Accurate metering or measuring of the fuel means
that, for the same fuel control setting, the same quantity
of fuel must be delivered to each cylinder for each
power stroke of the engine. Only in this way can the
engine operate at uniform speed with uniform power
output. Smooth engine operation and an even
distribution of the load between the cylinders depend
upon the same volume of fuel being admitted to a
particular cylinder each time it fires and upon equal
volumes of fuel being
delivered to all cylinders of the
A fuel system must also control the rate of injection.
The rate at which fuel is injected determines the rate of
combustion. The rate of injection at the start should be
low enough that excessive fuel does not accumulate in
the cylinder during the initial ignition delay (before
combustion begins). Injection should proceed at such a
rate that the rise in combustion pressure is not to great,
yet the rate of injection must be such that fuel is
introduced as rapidly as possible to obtain complete
combustion. An incorrect rate of injection affects
engine operation in the same way as improper timing.
When the rate of injection is too high, the results are
similar to those caused by an injection that is too early;
when the rate is too low, the results are similar to those
caused by an injection that is too late.
In addition to measuring the amount of fuel
injected, the system must properly time injection to
ensure efficient combustion so that maximum energy
can be obtained from the fuel. When the fuel is injected
too early in the cycle, ignition may be delayed because
the temperature of the air, at this point, is not high
enough. An excessive delay, on the other hand, gives
rough and noisy operation of the engine. It also permits
some fuel to be lost due to the wetting of the cylinder
walls and piston head. This, in turn, results in poor fuel
economy, high exhaust gas temperature, and smoke in
the exhaust. When fuel is injected too late in the cycle,
all the fuel will not be burned until the piston has
traveled well past top center. When this happens, the
engine does not develop enough power, the exhaust is
smoky, and fuel consumption is high.
Atomization of Fuel
As used in connection with fuel injection,
atomization means the breaking up of the fuel, as it
enters the cylinder into small particles, which form a
mistlike spray. Atomization of the fuel must meet the
requirements of the type of combustion chamber in use.
Some chambers require very fine atomization; while