Mechanical Engineering - IC Engines and Nuclear Power Plants - Discussion

Since the higher the compression ratio of an engine the higher the thermal efficiency, so diesel engines generally have a better fuel efficiency than petrol engines. To power a similar-sized vehicle, a diesel engine will be about 15% - 25 % better on fuel efficiency than a petrol engine. The petrol engine has usually efficiency of around 30-35%, hence diesel cycle efficiency due to better compression ration is 40-45% We can take 50%, not 70% FOR SURE.

Gasoline engines are typically 30% efficient while diesel engines can convert over 45% of the fuel energy into mechanical energy.

But Question asked about themal efficiency not mechanical efficiency. So answer is correct.

Diesel engine has the highest thermal efficiency of any standard internal or external combustion engine due to its very high compression ratio. Low-speed diesel engines can have a thermal efficiency that exceeds 50%.

A typical diesel automotive engine operates at around 30% to 35% of thermal efficiency. About 65-70% is rejected as waste heat without being converted into useful work, i.e. work delivered to wheels.

since maximum compression ratio for a diesel engine is 22:1; by considering the thermal efficiency of an Otto cycle.

Efficiency = 1- (1/ CR^(1.4-1)). CR=22 so therefore efficency is 70%.

The thermal efficiency of diesel engines is about 70% while the thermal efficiency of petrol and gas engines is about 30%.

The thermal efficiency of the diesel engine varies from 40 to 50 per cent and in petrol 25 to 30 percentage.

Max is 40% for diesel engine and thermal efficiency of ic engine is higher than diesel.

Ref: R S Khurmi.

If the compression ratio of diesel engine is high then thermal efficiency of diesel engine is also high.

30% is maximum thermal effect in C.I. engine but 25% in case of S.I. engine.