Abstract
Due to the more restrictive standards of pollution and to the necessity to combine the main advantages of the gasoline engines (the homogeneous charge) with the main advantages of the diesel engines (higher compression ratios which determinates higher efficiencies) the homogeneous charge compression ignition (HCCI) engines were developed. One of the major challenges is to control the start of the combustion. The exhaust gas recirculation is the most common method used to control the HCCI combustion.
A diesel HCCI engine model was developed to study the effects of the EGR on the combustion characteristics. The HCCI engine model is made from two system boundaries, one air cleaner, three intake pipes, two plenums, one cylinder, three exhaust pipes and the one catalyst. The mixture formation is external. The tests were made for four different operating points, keeping the engine speed constant and increasing the EGR from 0 to 60%.
This paper presents the influence of the EGR on the cylinder pressure, cylinder temperature, heat release rate and mass fraction burned. Due to the two stage combustion characteristic to the HCCI engines, two peaks appear on the heat release rate diagram, one corresponding to the low temperature oxidation and the other to the high temperature oxidation. With increasing EGR the ignition delay increases and the cold flame and main ignition can not be distinguished any more on the heat release rate. Another effect of the EGR is the reduction of the cylinder temperature. Using different EGR rates, the maximum temperature was reduced with more than 500 K. A lower temperature leads to lower nitrous oxides emissions.
Keywords: HCCI, EGR, heat release rate