The diesel engines have low rpm as compared to gasoline or petrol engines since the diesel engine has a larger stroke length for the high compression ratio. Hence piston has to travel more distance than in petrol engines which causes the low rpm in diesel engines.
Diesel engines have lower rpm than petrol or gasoline engines due to the following three reasons:-
1] Large stroke length:
The compression ratio of the diesel engine is high hence the diesel engine has more stroke length and short bore. Hence due to the longer stroke length, more torque with low speed obtains at the output.
2] Heavy construction:
The diesel engine is heavier in construction because of the high compression ratio hence this heavy construction reduces the speed of the diesel engine.
3] Longer combustion:
The combustion cycle of the diesel engine is longer than the petrol engines therefore it reduces the speed of the diesel engine.
Therefore, due to the following reasons, diesel engines have low rpm.