For the compression ignition engine, it is very important to promote a means of injecting fuel into the cylinder at the proper time in the cycle. This is so because the injection system starts and controls the combustion process.
The injection system of the compression ignition engine should fulfil the following objectives consistently and precisely:
Meter the appropriate quantity of fuel, as demanded by the speed of, and the load on, the engine at the given time.
Distribute the metered fuel equally among cylinders in a multi-cylinder engine.
Inject the fuel at the correct time (with respect to crank angle) in the cycle.
Inject the fuel at the correct rate (per unit time or crank angle degree).
Inject the fuel with the correct spray pattern and sufficient atomization as demanded by the design of the combustion chamber, to provide proper penetration also. Begin and end injection sharply without dribbling or after injection
To accomplish these objectives, a number of functional elements are required. These constitute together, the fuel injection system of the engine. These elements are as follows.
Pumping elements to transfer the fuel from the tank to the cylinder, along with the associate piping and hardware.
Metering elements to measure and supply the fuel at the rate as desired by the speed and load conditions prevailing.
Metering controls to adjust the rate of the metering elements for changes in load and speed of the engine.
Distributing elements to divide the metered fuel equally among the cylinders in a multi cylinder engine.
Timing controls to adjust the start and stop of injection.
Mixing elements to atomize and distribute the fuel within the combustion chamber
The function of fuel injection equipment is to supply the engine with fuel in qualities exactly metered in proportion to the power required and timed with utmost accuracy , so that the engine will deliver that power within the limits prescribed for fuel consumption, exhaust smoke, noise and exhaust emissions. The fuel must be injected through suitable nozzles at pressures high enough to cause the required degree of atomization in the combustion chamber and to ensure that it mixes with sufficient air for complete combustion in the cycle time available. In multi cylinder engines the periods of injection, the timing and the delivered quantity must be accurately metered to ensure an even balance between the cylinders.
There are two main classifications for fuel-injection systems, namely
Air injection which had become obsolete but now some interest has been shown by researchers (however very high pressure is required for air) and
solid (or airless) injection systems.