In the pressure-volume diagram shown in the illustration, curve 'A-d' indicates __________. Illustration MO-0035

Correct Answer: Option A — combustion at approximately constant pressure

Curve segment A-d on the indicator diagram illustrates the combustion phase occurring near top dead center where fuel burns at approximately constant pressure before expansion. This behavior characterizes the mixed or Sabathé cycle typical of modern medium-speed and slow-speed marine diesel engines.