Influence of a non-equilibrium discharge impact on the low temperature combustion stage in the HCCI engine

Abstract

A method for the combustion organisation in the cylinder of an HCCI engine is proposed. This method is based on the selective impact of electrical discharge in the form of a high frequency corona. The discharge is switched on at a certain crank angle and affects the mixture for a short time (a few rotation degrees of the crankshaft). The aim of the given study was to clarify how the discharge affects the low temperature combustion (LTC) stage to control the inflammation in an HCCI engine. It was shown that the best discharge impact timing is the beginning of the low temperature heat release (LTHR) or intermediate temperature heat release (ITHR) stages. The discharge does not directly ignite the mixture but only “pushes” the beginning of these stages and promotes their more rapid passing, controlling the heat release. It is mostly due to the dissociation of fuel and oxidiser, instead of heating the area of impact. Consequently, a non-equilibrium discharge can control the combustion timing, which is especially important in the case of a cold start or use of a lean mixture that is characterised by misfiring. It has been shown that the combustion timing after the top dead centre depends on the energy deposited into the injected mass of the fuel–air mixture treated by the discharge and the discharge switching on time. The dependence of the concentrations of NOx, CO, and unburned hydrocarbons on a specific deposited energy and the initial conditions is presented. The chemical kinetic mechanism of a selective discharge effect is revealed. The numerical simulation has been performed for the equivalence ratio ϕ = 0.33–0.5 and the intake temperature Tin = 360–400 K in a propane-air mixture.