An experimental study of the load and heat influence from combustion on engine friction

The friction power losses of a turbo-charged heavy-duty diesel engine of the 13 litre class are investigated both by fired engine tests as well as by pressurized motoring tests. During pressurized motoring compressed air is applied to the engine intake which creates loads comparable with fired operation but without the strong and changing thermal influence of combustion. By using this combined approach the influence of the load and the thermal influence of the combustion can be studied separately for the first time. It is found that pressurized motoring yields comparable but generally a bit higher friction power losses as in fired operation. In particular, for full load operation, the agreement between the two methods is very good which supports the reasoning that for full load operation the mechanical load is the dominant factor for the friction power losses. However, for part load operation significant differences arise. Without the thermal influence from combustion, increasing the load on the engine leads to a rather linear increase in the friction power losses as is seen from pressurized motoring. This is in contrast to the fired engine tests, where the friction power losses stay almost constant over a rather large range of part loads and only increase for full load operation. It is argued that the reason for this different behaviour is the thermal impact from combustion.