Supercritical combustion is a new concept of combustion technology aiming at reducing the fuel consumption and pollutant emission levels of a diesel engine. Supercritical combustion is a controlled detonation technique in which the fuel will be introduced into the combustion chamber in its supercritical state, resulting in substantial reduction in ignition delay which aid in achieving shortest combustion duration thereby higher fuel economy and less NOx emissions can be achieved. Any substance at a temperature and pressure above its critical point is called supercritical state. Fuels at supercritical state show novel properties like low viscosity, high diffusivity, low surface tension and these properties will result in high combustion efficiency, high power output and less pollutant formation. Factors such as low ignition delay, shorter combustion duration and high reaction rate can be achieved by making the fuel to undergo supercritical combustion.
Super critical combustion in diesel engine is achieved by heating diesel above 300°C and pressurizing it above 100 bar which is the critical temperature and pressure of diesel fuel respectively. In this process physical delay is eliminated but a chemical delay is inevitable. In order to reduce the chemical delay, fuel is partially oxidized by coating the fuel lines with catalytic materials like nickel & molybdenum. Oxygen will be supplied for this oxidation process by adding ethanol (2%) with diesel. This fuel is injected in to combustion chamber at high pressure which would combust at high rate of combustion. Since super critical combustion does not have an ignition delay, fuel can be injected after TDC which results in reduction of compression work and injection should be phased in a manner such that peak pressure occurs 10-15° after TDC as desired which yields maximum expansion work from the engine.