A combustion model has been developed for a 4 cylinder, direct injection, naturally aspirated, diesel engine. The model is based on the vibration transmission characteristics of various components of the engine structure which relate internal forces to the vibration of the external, radiating surfaces. The model is constructed from measurements of the vibration characteristics of the individual engine components when disassembled. The assumption that this model is valid for the running engine is investigated particularly for the moving parts of the engine. The vibration transmission has been measured during the operation of the engine and compared to that predicted by the model.