In this paper, we describe an optimization followed by a fuel-saving evaluation of a new concept of a hybrid pneumatic–combustion engine that can be obtained by modifying a conventional internal combustion engine without developing a new cylinder head. Until now, most studies on the pneumatic hybridization of internal combustion engines have dealt with a two-stroke pure pneumatic mode. The few concept studies that have dealt with a hybrid pneumatic–combustion four-stroke mode required a supplementary valve to be added to charge compressed air in the combustion chamber. This heavy modification cannot be carried out by simply adjusting an existing internal combustion engine because a new cylinder head should be developed. It is therefore not logical to suggest this concept as an option in vehicle powertrains to reduce fuel consumption. Moreover, those studies focus on spark-ignition engines; there are reasons to think that their concepts might not work adequately for diesel engines. Our concept is capable of making a diesel engine operate under two-stroke pneumatic motor modes, two-stroke pneumatic pump modes and four-stroke hybrid modes, without requiring an additional valve in the combustion chamber. This fact constitutes our study’s strength and innovation. The evaluation of our concept is based on ideal thermodynamic cycle modeling. The optimized valve actuation timings for all modes lead to generic maps that are independent of the engine size. The fuel economy is calculated based on the new European driving cycle and on the assessment and reliability of transport emission models and inventory system urban and rural cycles.