Combustion of highly non-homogeneous fuel-air mixtures and the use of hydrodynamic lubrication of piston rings are identified as barriers to long-term progress in diesel engines. The characteristics of non-homogeneous mixtures are discussed and the effects of various injection system modifications are illustrated using a turbulent fuel jet model. The problems caused by the hydrodynamic oil film at the piston rings are identified, and the potential of boundary lubrication is discussed.
