Question

Explain why the brake mean effective pressure of a naturally-aspirated diesel engine is lower than that of a naturally-aspirated spark-ignition engine. Explain why the bmep is lower at the maximum rated power for a given engine than the bmep at the maximum torque.

Answer 1

Cl engines typically run on a leaner fuel-air mixture
even at full load operation, as compared to SI
engines. The fuel-air mixture cannot be enriched
beyond a limiting value in diesel engine due to the
tendency of increased smoke formation.
It should also be noted that the air-fuel mixture in a
CI engine is not homogenous like that in an SI
engine. There will be rich and lean pockets as the
liquid fuel is sprayed into the cylinder and there is
no much time to mix homogenously before the
combustion. In contrast, the fuel and air is mixed
prior to the intake in an SI engine.
But if you compare the global air-fuel ratio (i.e. total
air intake/total fuel intake of the engine), SI engine
air-fuel mixtures are richer than their CI engine
counterparts.