How fuel injection in a gasoline engine is different from direct injection in diesel engines?

Question

...I'm a mechanical engineering student...so technical terms are welcome..

If possible could you please tell me the difference between the injectors used ?

Answer 1

two main differences:

(1) in gasoline engines, the fuel gets injected into air to create the mix, BEFORE the mix reaches the cylinder. Whereas in direct injection diesel engines, the injection is directly into the combustion chamber

(2) as a result of the above, gasoline injectors need only to work at low pressure, whereas diesel direct injectors need to work at very high pressure

hope this helps

Answer 2

To amplify on both above excellent answers:
Gasoline burns well only when mixed with a proportional amount of air. If the fuel:air ratio is too high or too low, the engine will not run well and will create more emissions. Gasoline engines require the gasoline and air to be uniformly mixed to have proper combustion. This takes some time to accomplish, so the injection has to occur well before combustion begins. It is easiest to do this by injecting fuel into the intake manifold or intake port.

Diesel fuel burns much more quickly than gasoline. If a cylinder was "charged" with a mixture of air and diesel fuel, it would burn way too quickly once ignition occurred. The pressures in the cylinder would be too high and combustion would be uncontrolled leading to poor performance and overstressing of the mechanical components. Diesel engines therefore don't inject fuel until it is time for combustion to begin. The rate the fuel is injected is important for controlling how combustion happens and how the engine performs. This kind of injection has to happen right into the cylinder, and while the pressures and temperatures are very high. Also, since the injection is happening almost at the same time as combustion, the diesel fuel doesn't mix with all the air in the cylinder uniformly. This is OK because diesel fuel will burn over a very wide range of fuel:air ratios.

Gasoline injectors operate at low pressure -- about 50 psi. They usually have a single spray feature, usually a hole with a "pintle" that is moved with a solenoid to allow the fuel to spray through.

Diesel injectors operate at very high pressure -- well over 10,000 psi. They frequently have several spray holes, with an internal needle valve. The valve is usually operated hydraulically by the fuel itself, although the timing and injection rate may be controlled by solenoids. Injectors are sometimes actuated by the engine camshaft, but the cam is really pushing an internal piston that sends hydraulic fuel pressure to lift the needle valve off its seat.

Answer 3

To expand on the previous answer, with a gasoline engine the ignition is controlled by the spark plug. In a diesel engine the ignition is controlled by the timing of the injection of the fuel into the cylinder. Because of the compression ratio and the resulting heat build up in the cylinder, the diesel fuel will ignite when injected without the need for a spark plug.