Experimental and Numerical Investigations on HCCI- Combustion

Numerical and experimental investigations are presented with regard to homogeneous-charge-compression-ignition for two different fuels. N-heptane and n-butane are considered for covering an appropriate range of ignition behaviour typical for higher hydrocarbons. One fuel is closer to diesel (n-heptane), the other closer to gasoline ignition properties (n-butane). Butane in particular, being gaseous under atmospheric conditions, is used to also guarantee perfectly homogenous mixture composition in the combustion chamber. Starting from detailed chemical mechanisms for both fuels, reaction path analysis is used to derive reduced mechanisms, which are validated in homogeneous reactors. After reduction, reaction kinetics is coupled with multi zone modeling and 3D-CFD through the Conditional Moment Closure (CMC) approach in order to predict autoignition and heat release rates in an I.C. engine. Multi zone modeling is used to simulate port injection HCCI technology with n-butane. Comparison with experimental results for a passenger car engine – obtained at the University of Stuttgart – yield good agreement. 3D-CFD with Conditional Moment Closure is used to simulate direct injection HCCI technology with diesel. Comparison with experimental results for a common-rail-passenger car engine – again obtained at the University of Stuttgart – yield good agreement.

On the experimental part a rapid compression machine (RCM) with wide optical access is used to provide autoignition and combustion data under well defined conditions, thus covering a wide range of engine operating parameters in terms of pressure, temperature and mixture composition around TDC. So far passive optical techniques, in particular chemiluminescence is used to provide – in addition to heat release rates obtained by pressure measurements – information on ignition locations, cyclic variability of combustion and distribution of reaction zones. The experimental test rig allows both external and internal fuel injection and therefore different degrees of mixture (in)homogeneities. Variable heating of the cylinder wall temperatures can be set in the RCM.