FUEL INJECTION SYSTEMS
injection systems must accomplish five particular functions-meter, inject, time, atomize,
and create pressure. A description of these functions follows:
measure the amount of fuel to be injected.
and distribute the fuel into the combustion chamber.
of the fuel must start and stop at the proper time.
the fuel up into a fine mist.
PRESSURECreate the necessary high pressure for injection.
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 engine.
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.
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 others function with coarser atomization.
Properly atomization makes it easier to start the burning process and ensures that each
minute particle of fuel is surrounded by particles of oxygen with which it can combine.
generally obtained when liquid fuel, under high pressure, passes through the small opening
(or openings) in the injector or nozzle. As the fuel enters the combustion space, high
velocity is developed because the pressure in the cylinder is lower than the fuel
pressure. The created friction, resulting from the fuel passing through the air at high
velocity, causes the fuel to break up into small particles.
A fuel injection system must increase the pressure of the fuel to overcome compression
pressure and to ensure proper dispersion of the fuel injected into the combustion space.
Proper dispersion is essential if the fuel is to mix thoroughly with the air and burn
efficiently. While pressure is a chief contributing factor, the dispersion of the fuel is
influenced, in part, by atomization and penetration of the fuel. (Penetration is the
distance through which the fuel particles are carried by the motion given them, as they
leave the injector or nozzle .)
atomization process reduces the size of the fuel particles too much, they will lack
penetration. Too little penetration results in the small particles of fuel igniting before
they have been properly distributed or dispersed in the combustion space. Since
penetration and atomization tend to oppose each other, a compromise in the degree of each
is necessary in the design of the fuel injection equipment, particularly if uniform
distribution of fuel within the combustion chamber is to be obtained.