Direct-EGR tillämpat på HCCI

Sammanfattning: The Homogeneous Charge Compression Ignition (HCCI) engine is promising in terms of low NO x, low particulates and high efficiency. Due to its homogeneous charge, where the combustion starts almost simultaneously from a number of points in the combustion chamber, only lean mixtures are possible to burn because of the resulting rapid combustion, which is much faster than the combustion in an SI-engine or a diesel engine. When richer mixtures are used the combustion will start earlier and also be more violent. To phase the combustion later and lower the combustion speed, external cooled EGR can be used. The EGR-system used in this thesis uses a divided exhaust channel where one channel and its corresponding valve are used only for transporting EGR back to the intake system. Consequently the EGR-valve is connected to the intake pressure. This system is called Direct- EGR (D-EGR). Engine tests verified that the CA50 indeed is phased later when cooled EGR is used. The more cooled EGR that is available in the combustion chamber, the later the CA50 is phased. Hence power output of the HCCI-engine can be improved quite a bit when large EGR-rates are used, because of the possibility of increasing the injected fuel amount. Simulations in GT-Power were made to determine how the EGR-valve lift and exhaust valve lift should be made to transfer a certain amount of EGR with as good total efficiency as possible. The simulations showed that two different valve timing strategies should be used. Which one of the two strategies that should be used depends on the pressure difference between the intake and the exhaust as well as the engine speed. These valve timing strategies are called the Major lift strategy and the Minor lift strategy. To determine the efficiency of the D-EGR-system, a comparison with an ordinary external EGR system was carried out. The comparison showed that the D-EGR-system performs well, especially at high load and low engine speeds. At high engine speeds the increase in flow losses affects the performance in a negative way for the D-EGR-system. At low load the intake pressure may exceed the exhaust pressure, which makes it unbeneficial to do a part of the exhaust stroke with the EGR-valve open towards the higher intake pressure. In terms of transient response the D-EGR-system was proven to be fast, with a possibility to change the EGR-rate in just a couple of cycles.