A four-stroke cycle main propulsion diesel engine fails to start at normal cranking speed with normal fuel pressure and ambient temperature. The reason for the failure could be __________.

Correct Answer: Option D — excessive air leakage by rings and valves

Diesel combustion relies heavily on high compression pressure to achieve auto-ignition temperature. Excessive air leakage past worn piston rings and unseated valves drops peak cylinder pressure and temperature below the threshold required for fuel ignition.