Abstract
The homogeneous charge compression ignition (HCCI) engines combines the homogeneous charge from the gasoline engines with the compression ignition from the diesel engines. Unlike the conventional engines, there is not a direct mechanism to initiate the combustion like the spark from the gasoline engines or the fuel injection from the diesel engines. For this reason different strategies where developed for changing the mixture temperature or the mixture chemical properties. The usages of the variable compression ratio, variable valve timing or exhaust gas recirculation are common methods used to modify the auto ignition delay. The internal exhaust gas recirculation (EGR) is used to control the temperature of the mixture which has a big influence on the start of the combustion in the HCCI engines. The EGR is also a method used to reduce the nitrous oxide (NOx) emissions and has different effects on the combustion and on the emissions behavior of the engine.
This paper presents the influence of the internal EGR on the combustion characteristics in a gasoline HCCI engine. The tests were made on a gasoline HCCI single cylinder engine. The burned gases are trapped inside the cylinder by closing the exhaust valve before the piston reaches the top dead center. The amount of trapped gases is controlled varying the exhaust valve timings using cam phasers.
The results are showing the effects of the internal EGR on the mixture formation and on the combustion. At the beginning, the trapped burned gases due to their higher temperature are heating the fresh mixture of air and fuel. This effect is used to control the auto ignition timing. During compression and combustion, the residual gases have to be heated with the rest of the charge from the cylinder. The residual gases have a higher heating capacity than the fresh mixture. This is lowering the temperatures at the end of compression and during the combustion when very high internal EGR rates are used. The burned gases replace a quantity of air, so the oxygen (O2) concentration is lower. This is also lowering the temperature during the combustion.
The combustion analysis was made using the pressure traces obtained using different internal EGR rates. The mean temperature and the heat release rate were calculated and compared. The use of the internal EGR has a big influence on the temperature traces. When the internal EGR rate is higher, the temperatures during the beginning of the compression stroke are higher and the combustion is starting earlier. After the ignition, the maximum temperatures obtained are higher if the internal EGR rate is increasing, until a point when the internal EGR rate is too high, the mixture is too diluted and the maximum temperatures are decreasing. The NOx emissions depend on the temperature evolution during the combustion, so the very high internal EGR rates can be used to lower the NOx emissions.
Keywords: HCCI, EGR, heat release rate