Fuel injection is a system for mixing fuel with air in an internal combustion engine.
Functional objectives for fuel injection systems vary but all of them share the central task of supplying fuel to the combustion process. There are several competing objectives such as:
Power output, fuel emission, emissions performance, reliability, smooth operation, initial cost, maintenance cost.
Requirements of fuel injection system are:
i) Accurate metering of the fuel injected per cycle: The quantity of the fuel metered should vary to met changing speed and load requirements of the engine
ii) Timing the injection of the fuel correctly in the cycle: to obtain maximum power ensuring fuel economy and clean burning.
iii) Proper control of rate of injection: The desired heat - release pattern is achieved during combustion.
iv) Proper atomization of fuel into very fine droplets.
v) Proper spray pattern to ensure rapid mixing of fuel and air.
vi) Uniform distribution of fuel droplets throughout the combustion chamber.
vii) To supply equal quantities of metered fuel to all cylinders case of multi cylinder engines
viii) No lag during beginning and end of injection i.e., to eliminate dribbling of fuel droplets into the cylinder.
Air injection system: It was first developed by Rudolf Diesel. The arrangement of the system is shown in fig 10.5. In this system, air and fuel both are injected into the cylinder during the supply of fuel. The required pressure of the air for injecting the fuel is about 70 bar or higher.
A fuel pump is driven by the engine itself. (A cam shaft operates the fuel pump through a cam and power required to rotate the cam shaft is taken from main shaft of the engine with the help of gears and discharge a definite quantity of fuel into the injection valve as shown in fig. 3A.1 the injection valve is mechanically opened and high pressure air drives the fuel charge and some air into the combustion chamber. The amount of fuel delivered is under the control of oil pump suction valve, which is operated by a governor. The air pressure is raised to about 70 bar by a three stage compressor (as shown in fig 3A.1) providing intercooling. The compressor is also operated by the engine. The high pressure air projects the fuel into the combustion chamber and atomizes it.
This type of system rarely used now days in diesel engines.
The advantages and disadvantages of this system listed below:
It provides better atomization and distribution of fuel.
As the combustion is more complete, the BMEP is higher than with other types of injection systems.
It allows to use inferior fuels.
It requires complicated mechanism to run the compressor
The weight of the engine increases.
Part of the power is used to drive the compressor and so the B.H.P. of the engine is reduced.