<div class="htmlview paragraph">The combustion of diesel fuel is discussed by considering each phase of the process and pointing out, in so far as possible, the effect of important fundamental variables.</div> <div class="htmlview paragraph">It has been shown that both physical and chemical processes are involved and that these processes operate under exceedingly heterogeneous conditions, making experimentation difficult and complicating interpretation of results. In spite of this, it is possible to explain most of the observed phenomena and to formulate a satisfactory qualitative description of the combustion process in the diesel engine.</div> <div class="htmlview paragraph">It is pointed out that the cetane number of fuel is related basically to chemical delay but does not necessarily measure the performance of the fuel during the inflammation stage.</div> <div class="htmlview paragraph">The origin of products of incomplete combustion is discussed and the importance of these products on combustion studies is emphasized.</div>