HCCI Combustion by Retaining Residuals. Development and Analyses of a Method to Reduce Fuel Consumption for Passenger Car Gasoline Engines

Doktorsavhandling, 2005

The demand for more powerful engines on one hand, and greater fuel-efficiency on the other, requires a revolution in engine combustion technology. Homogeneous charge compression ignition (HCCI) can be a fuel-saving, low emissions approach that seems to be very suitable for large displacement passenger car engines. HCCI is used as a generic name for auto-ignited combustion in an engine, initiated by a temperature rise due to the compression of a mixture of fuel, air and/or residuals. Fuel consumption and carbon dioxide (CO2) emissions are reduced, in a window within a relevant engine speed and load domain, but the emissions levels are not compromised compared to a spark-ignited (SI) engine with catalytic exhaust-gas after-treatment, mainly due to the ultra low nitrogen-oxides (NOx) emissions. Since HCCI combustion can only be valid for a specified speed/load window (due to a required dilution of the combustion mixture), the method should be applied as a mode that can be engaged or disengaged, instead of running in it all the time, thus avoiding compromising the power output of the engine in SI mode. A very high temperature is required to auto-ignite gasoline (about 1100K), but by trapping hot combustion products (residuals) of the previous combustion using a negative valve overlap (NVO) this can be accomplished. The overlap is created with valve timings, different than during conventional SI mode, where the exhaust valves are closed early and the intake valves late. Also direct fuel injection can have a positive influence: before piston top dead center of the negative valve overlap a small amount of fuel can be injected (the pilot fuel) which will auto-ignite in the NVO, thereby elevating the temperature of the captured mixture further. There are no direct means for controlling the start of combustion (as the spark timing is for SI combustion). The auto-ignition timing, however, can be controlled by varying the negative valve overlap; a large overlap advances the auto-ignition timing while a shorter NVO period delays it. It can also be controlled by the excess air ratio, effective compression ratio (varied by adjusting the timing of the intake valve closure), timing of exhaust valve opening, direct fuel injection timing and the amount of pilot fuel. The thesis describes the result of engine experiments in which it was possible to utilize advanced hardware and equipment in combination with engine simulations (chemical kinetics, gas exchange and CFD) focusing on discovering the mechanisms behind the method (HCCI NVO). In addition to the detailed investigations published in the papers, which are added in the back of the thesis, more information can be found on: HCCI in general, different methods of generating HCCI combustion, the history of the project, the methods of experimenting, and HCCI NVO including a comparison with SI combustion.