In SI engines, the valve timing has prime importance and if we look at the injection of petrol in the manifold, it significantly has importance. By varying injection timing, the working of SI engine is directly affected.
In order to understand the behavior of injection timing in case of SI engines, consider a single cylinder petrol engine. During working, when piston is going to reach TDC in exhaust stroke before 60° crank angle, fuel injects in the inlet manifold where air fuel mixture is produced. This air fuel mixture is further improved if we produce some turbulence.
After completing exhaust stroke, intake valve opens and this air fuel mixture enters in the cylinder. It is assume that the pressure of air fuel mixture entering in the cylinder is equal to atmosphere but in actual practice, the pressure is less and the main reason is due to frictional losses and it even don’t inject when piston reaches TDC. In actual practice, the intake valve opens 5 – 30° before TDC.
The main reason behind such kind of intake valve opening in exhaust stroke is to enhance air fuel mixture quantity in one cycle and electronic control unit (ECU) makes sure it.
Similarly if we talk about 4-cylinder SI engine, the timing of individual air fuel mixture formation is controlled by ECU and it makes sure that sufficient air fuel mixture enters in the suction stroke.
If we talk about exhaust gases, the pressure is higher than atmospheric and the main reason is that the exhaust gases don’t expand completely till piston reaches BDC. Moreover exhaust valve in actual practice opens when piston is going to reach BDC. The main reason behind this behavior is due to enhance engine efficiency to exclude exhaust gases quickly.
During engine working, we will see that a time comes when both valves are overlapping. This stage has prime importance and it also increases the efficiency. This overlapping occurs between exhaust stroke and intake stroke. Using this overlapping, fresh air fuel mixture also helps engine to exclude exhaust gases.
The overall actual valve timing diagram of SI engine is shown