Mechanical Engineering - Thermodynamics - Discussion

Diesel engines come in all sizes. Some smaller and some quite larger than gasoline engines. A diesel engine makes no more H.P. than a gasoline engine but makes lots more torque and it's H.P. at low RPMs. Diesels typically have a compression ratio of around from 16 to 1 to as high as 23 to 1. Most gasoline engine will have around a 9 to 1 ratio. The higher compression ratio of a diesel allows it to make much more torque at lower RPMs. Torque is pulling power which is what a truck needs.

Diesel fuel is much dense then the petrol or gesoline it is required high pressure and temp to burn so by using high compression ratio burning temp of the fuel is minimised. With the use of injector air and fuel mixture get inside in engine and burn with high compression in the range of 16-20 but in case of petrol engine which is much smaller size then diesel high compression is not possible so spark plug is used which is ignite the air fuel mixture comes from carburettor.

Both knocking and detonation are the same term. In the case of petrol engine, it occurs due to pre-ignition. Due to some reason temperature of fuel become self-ignite temperature, so it generates flame before spark plug ignites. Now, when spark plug ignites, there will be 2 flames inside the cylinder and when they collide knocking sound occurs.

Both knocking and detonation are not same because the term knocking is define has when the piston is moves from TDC-BDC or BDC-TDC then the piston will deflects or it will get collides to cylinder due to this we feel some sound then we call its knocking.

Detonation is nothing but incomplete combustion.

In petrol engines if we compress more (beyond comp pr), due to compression temp increases this temp is sufficient to ignite the fuel automatically without any sparking. Which damages the cylinder. This phenomenon we called as detonation.

Self-ignition temp of petrol is about 450°c. That is why a spark plug is needed in si engines. Whereas in CI engines diesel self-ignites at about 180°c soo as soon as the compression starts the diesel self-ignites.

Spark plug use in petrol engine because we can not burn petrol by compression or pressure in a petrol engine is much lower than diesel engine it's about to 8 to 10 bar but in diesel engine, it becomes up to 18 to 20 bar.

But nowaday, Most of the petrol engine comes with injector. And spark plug doesn't use in it. Why?

And I know petrol have low ignition temperature so how can we ignite it by compression in this type of petrol engine?

Compression ratio is the ratio between the volume of the cylinder from its largest capacity to smallest capacity.

(OR).

It is the ratio of the volume of the cylinder to vol of the combustion unit.

Compression ratio (r) = it is the ratio of total volume of cylinder to the clearance volume of cylinder.methematically,

r = total volume/clearance volume.

Vt = Vs+Vc.
Then r = Vt/Vc.