What is the need of high compression ratio?
Today every machine is expected to give higher efficiency. When we talk about petrol engine and diesel engine whose thermal efficiency is directly related to 'compression ratio'. That means if we want to achieve higher thermal performance in terms of efficiency, we should provide high compression ratio for both the engine.
What is compression ratio?
The compression ratio is the ratio of total volume of engine cylinder to the clearance volume of the same cylinder. As the name suggest, it relates compression of the air fuel mixture during compression stroke. When there is compression, there always be heat associated with and it increases the overall temperature of fuel air mixture.
Why compression ratio of petrol engine is kept low compared to diesel engine?
In petrol engine, petrol is used as fuel and when it is subjected to high compression ratio, the heat generated may increase the temperature of petrol+air mixture above the auto ignition temperature of petrol ( 280 °C) and the fuel mixture will automatically ignite without getting sparked.
This auto ignition produces very high pressure which may burst piston. This phenomenon is called 'detonation'. So, to reduce the detonation effect and to allow the combustion in exactly after compression stroke, high compression ratio cannot be used in petrol engine. In case of otto engines, the compression ratio is of the order of 4 to 8.