Mechanical Engineering - Thermodynamics - Discussion

This range is used because if more compression is used there is the possibility of occurrence of knocking which can affect the system adversely.

Sir, what is the comp, ratio of diesel engine?

For petrol engine the compression ratio range from 7:1 to 10:1 and diesel engine its range from 14:1 to 20:1.

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.

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.

Why the compression ratio of petrol is low?

Petrol have low ignition temperature. Because of that if compressed beyond that, it will automatically ignite and combust. This is detonation.

I didn't understand sir please explain clear.

Compression ratio of petrol is 9 to 11 & diesel is 15 to 20.

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.