Advanced Mechanisms of Combustion for IC Engines

The adoption of Euro 6 Diesel engine emission standards for soot and NOx, coming approximately by 2014, is imposing engine makers to develop> new technological solutions as well as advanced concepts of in-cylinder combustion. The reason why it is so difficult to control NOx and soot emissions in Diesel engines is related to the basic mechanism of the conventional Diesel combustion. The air-fuel mixture is overall lean but strongly non-homogeneous, with local conditions in the combustion bowl of very high temperature and oxygen concentration that is ideal for NOx generation. Moreover, local rich mixture conditions promote soot formation. In order to overcome these kind of difficulties, Homogeneous Charge Compression Ignition (HCCI) has been widely studied as a premixed combustion strategy in which the combustion occurs simultaneously and homogeneously throughout the combustion chamber volume with shorter combustion duration. HCCI is controlled primarily by chemical kinetics, it can allow a simultaneous reduction in NOx and soot emissions by premixing the fuel with air to overall lean conditions. However, it is a stable combustion mechanism at only engine part load. In fact, a mixture of fuel and air will ignite when the fuel concentration is within the ignition limits and temperature of the reactants is sufficiently high. Once ignited, combustion occurs very quickly causing high in-cylinder pressure rise and consequently high noise and vibration emission. For this reason, HCCI can be typically operated at lean overall fuel mixtures and thus low-load conditions. In order to overcome this limitation, different new combustion concepts have to be explored. In the present lecture a new combustion approach and new fuels for IC engines will be discussed.