The First Law of Thermodynamics has been applied to the analysis of the dynamometer performance of a 2.0 litre,115 PS, common rail, turbocharged, automotive diesel engine operating under steady state conditions. Validation of the method is presented with correlation between the input fuel power and summed loss terms shown to be better than 3%. The study was conducted over a matrix of engine speed-load sites and maps of the underlying trends and magnitudes are presented. Detailed analysis of the relative heat balance contributions at a range of loads at fixed engine, water pump, and oil pump speeds is also presented. The proportions of heat rejected to the different primary paths (i.e. brake, coolant, oil, charge cooler, exhaust, and external) were found to vary with engine speed and load. Also, friction power was found to vary principally as a function of engine speed with some small dependency on engine load.