INVESTIGATION OF COMBUSTION EMISSIONS IN A HCCI ENGINE – MEASUREMENTS AND A NEW COMPUTATIONAL MODEL

The CO and hydrocarbon emissions of a HCCI engine have been explained by inhomogeneities in temperature induced by the boundary layer and crevices according to a stochastic reactor model. The boundary layer is assumed to consist of a thin film (laminar sublayer) and a turbulent buffer layer. The heat loss through the cylinder wall leads to a significant temperature gradient in the boundary layer. The partially stirred plug flow reactor model PaSPFR, a stochastic reactor model (SRM), is used to model turbulent mixing between boundary layer, crevices and the turbulent core and to account for the chemical reactions within the combustion chamber. The combustion of natural gas in the engine is described by a detailed chemical mechanism that is incorporated in the SRM. Molecular diffusion induced by turbulent mixing is described by the simple IEM mixing model. The turbulent mixing intensity that describes the decay of the species and temperature fluctuations is estimated from measurements of the velocity fluctuations and the integral length scale of the turbulent flow in the engine. Pressure, CO emissions and unburned hydrocarbons are also measured. Comparison between the mean quantities obtained from the SRM and these measurements show very good agreement. It is demonstrated that the SRM clearly outperforms a previous PFR based one-zone model. The PaSPFR-IEM model captures the pressure rise that could not be described exactly using a simple one-zone model very well. The emissions of CO and hydrocarbons are also predicted well. Scatter plots of the marginal PDF of CO 2 and temperature reveal that the emissions of hydrocarbons and CO can be explained by stochastic particles that undergo incomplete combustion because they are trapped in the colder boundary layer or in the crevices.