Substantial efforts have been made for the last ten years with a view to reducing the noise level of private cars equipped with high speed Diesel engines. Among others, such efforts resulted in a substantial reduction of the characteristic diesel engine noise at idling speed as well as in a substantial improvement of acoustical comfort within the vehicle passenger compartment.More recently, systematic studies were conducted with a view to reducing the external noise level of such vehicles under urban traffic conditions. The main criterion retained was the noise level as measured under conditions specified by ISO Standard R 362.Significant progress has already been made to reduce noise connected with fuel injection and combustion. With current engines, engine high speed noise is primarily mechanical. It is not contemplated, at least in the short term, to put in production an engine designed on a fundamental new basis, on account of the large investments involved, and of the required time for resolving technical and industrial problems.Under such conditions, possibilities for reducing noise directly at its source remain very limited, since they mainly concern modifications to housings, especially the timing cover and oil pan. Also studied was the possibility of reducing noise by means of insulating screens and absorbing materials in the engine compartment.The impact of a reduction in the noise from the engine itself is substantial only provided that the noise from other sources is also limited, such as from the exhaust system, intake system, cooling fan and clutch-gearbox-transmission assembly.As from now, the noise level of private cars equipped with high speed Diesel engines under legislated conditions currently in force in Europe, is no higher than for other vehicles of the same class.In spite of difficulties inherent to the Diesel engine, the fact is stressed that most solutions retained or contemplated with a view to reducing outer noise levels are applicable to private cars equipped with spark ignition engines.