Abstract
In the last years a higher attention was given to the homogeneous charge compression ignition (HCCI) engines, due to their low NOx emissions. The HCCI engines combine the homogeneous mixtures from the gasoline engines with the compression ignition from the diesel engines. Unlike the conventional gasoline engines, where the start of the combustion can be controlled using the spark from the spark plug, or the conventional diesel engines were the start of the combustion depends on the injection timing, the HCCI engines don’t have a direct mechanism to initiate the combustion. Indirect mechanisms, which affect the time-temperature history of the intake charge or the chemical properties of the fuel, have to be used. The HCCI engines can be obtained using the gasoline engines or the diesel engines as a starting platform. A series of adjustments have to be made in order to obtain the homogeneous mixture and the compression ignition. If the gasoline engine represents the starting platform, the compression ratio and the temperature of the fresh charge have to be raised in order to obtain the autoignition. The burned gas trapping is a method which can be used to rise the temperature of the intake gases. This method can be used also to control the autoignition timing if a variable valve system is used. To maintain a part of the burned gases the exhaust valves have to be closed (EVC) before the piston riches the top dead center (TDC). A part of the burned gases remain trapped in the cylinder and compressed by the piston during the end of the exhaust stroke. The quantity of the burned gases which are maintained in the cylinder depends on the moment when the exhaust valves are closing. If the valves are closing earlier, a higher quantity of burned gases is trapped in the cylinder and trapped during the end of the exhaust stroke. This leads to higher cylinder pressures during the end of the exhaust stroke and during the beginning of the intake stroke. The burned gases are heating the fresh charge, which leads to an earlier autoignition of the mixture. In the HCCI engines the combustion process takes place in the same time in all the cylinder volume. This leads to a very high rate of the heat release. Due to this, when modeling the combustion process, a new approach has to be used to estimate the rate of the heat release for the HCCI engines. This paper presents a method to estimate the heat release rate in a gasoline HCCI engine model depending on the quantity of burned gases maintained in the cylinder. A combination of three Vibe functions was used to estimate the rate of the heat release. The influence of the trapped gases was studied, especially on the start, on the duration and on the shape parameters of the Vibe functions.
Keywords: HCCI, Combustion, Vibe, Heat Release Rate, Gas Trapping