The Performance of a Multi Cylinder HCCI Engine using Variable Compression Ratio and Fast Thermal Management

Abstract: The prime mover in the world today is the Internal Combustion (IC) engine. The development and improvement of the internal combustion engines since Nicolaus August Otto and Rudolf Diesel has continued until today and will continue long into the future. No major competitor to the IC engine has yet emerged. There is a long list of potential competitors but they still have to prove their superiority in practice before they can make it to the market. New technologies, as hybrids, improving the use of IC engines in vehicles have during the last few years become common. These technologies suppress the drawbacks of the IC engines, i.e. where it has poor efficiency, and take advantage of its benefits. The environmental impact of the IC engine, due to its large numbers, is unacceptable. The advanced engine control and exhaust after treatment of the conventional IC engines have decreased the regulated emissions, NOx, CO, HC, and particulates, to very low levels and the coming legislations will decrease them further. However, the main greenhouse gas, CO2, from IC engines is and will continue to be a problem in the future. The global heating of the world is directly connected to the increasing in CO2 emissions emitted to the atmosphere by human activities. To decrease CO2 emission from IC engines running on fossil fuel we need more fuel efficient IC engines. Homogeneous Charge Compression Ignition (HCCI) combustion is a further improvement potential for the IC engine. HCCI combustion decreases the fuel consumption of gasoline engines to the level of diesel engines whereas HCCI combustion in a diesel engine eliminates almost all particulates and NOx emissions. Before HCCI combustion technology will come widely in use some issues have to be resolved, e.g. how shall the auto-ignition governed HCCI combustion be controlled and in what operating range, in terms of speed and load, can it be used. In this thesis the potential of a Variable Compression Ratio engine to control the HCCI combustion and its performance, in terms of operating range and efficiency, are investigated. The HCCI combustion is achieved using high compression ratio, inlet air heating, and air as dilution. The combustion phasing is controlled by Variable Compression Ratio and Fast Thermal Management of the inlet air temperature. Combustion phasing, calculated from cylinder individual pressure measurement, is used as feedback. The inlet air temperature is heated with an exhaust-to-inlet heat exchanger and controlled by mixing a hot and cold air flow using throttle valves. Both conventional PID controllers and state feedback controllers are tested to improve the performance of combustion phasing control. Transient control in the European drive cycle, EC2000, is done to investigate controller- and engine performance. Compression ratios between 9:1 and 30:1 are tested and the trade-off between inlet air temperature and compression ratio is optimized. The operating range with HCCI combustion and the affecting parameters are also investigated. Engine loads between -1 and 6.1 bar BMEP and engine speeds between 650 and 5000rpm are run. Minimum load performance can be improved using throttling, early combustion phasing, low temperature oxidizing catalysts, low temperature reactions, and exhaust residuals. Maximum load can be increased by turbocharging, pre-turbine catalyst, cylinder balancing, low octane number fuels, and spark assisted HCCI combustion. To cover the entire drive cycle combustion mode transfer and mixed HCCI and SI combustion are investigated. The performance of three multi-cylinder HCCI engines with different displacements and combustion chamber area-to-volume ratios are compared and the differences in brake efficiency explained.