The HCCI Engine – High Load Performance and Control Aspects
Sammanfattning: Current combustion engines provide excellent performance for vehicle propulsion and power generation. However, recent demands from society ask for higher efficiencies and lower emissions. These requirements are hard to meet with current technology and call for new solutions. Homogeneous Charge Compression Ignition, HCCI, offers a potential to combine high efficiency with very low emissions. It operates with high compression ratio and without throttling, thus avoiding some sources of efficiency penalties of the spark ignited engine. Due to the usage of compression ignition, premixed and very lean, or diluted, mixtures can be used. Highly diluted mixtures keep the combustion temperature low and minimize production of Oxides of Nitrogen, NOX. On the other hand, the diluted operation causes lower power density as well as higher emissions of Unburned Hydrocarbons, HC. The HC can be oxidized in a catalyst, but lower the combustion efficiency and thus also the brake thermal efficiency. Compression ignition of a premixed charge is also very challenging from a controls perspective: there is no direct means of controlling the onset of combustion.The results presented in this thesis are mainly based on experiments. These are performed on two different truck size Diesel engines and one passenger-car size engine, converted to HCCI. For the diesel engines the displacement volume is between 1.5 and 2 l/cylinder and the speed ranges from 1000 to 2000 rpm. Double fuel systems are used, providing one fuel that is easy to ignite and one fuel more reluctant to auto ignition. By adjusting the ratio between the two fuels, the onset of combustion is controlled. The fuels used are n-heptane, isooctane, ethanol, gasoline, natural gas and hydrogen; in all cases port injected. For the passenger-car size engine the displacement is only 0.3 l/cylinder and the tested speed range is 1000 to 5000 rpm. This engine uses fully blended gasoline RON92 and is controlled either by its variable compression ratio or by the mixing valves for heated and cool air.The thesis discusses operating domain, efficiency, emissions and control of HCCI engines. The HCCI process is compared to the existing processes for Spark Ignited, SI, engines and Direct Injected Compression Ignition, DICI, engines. It is shown that the HCCI process is somewhat closer to the ideal Otto cycle, compared to the other processes. The influences on efficiency of heat losses, friction, boost, compression ratio and combustion are discussed in comparison to SI and DICI engines. Emissions of NOX, HC and CO are also discussed. Different methods for characterizing and controlling the HCCI combustion are outlined and demonstrated. The fundamental aspects of controlling the HCCI combustion are outlined and discussed. The real-time requirements are reviewed with respect to a controller implementation in a PC. Process characteristics are identified and compared for different operating points. Qualitative characterization as well as state of the art System Identification are applied. Two strategies of controller design are discussed and compared. Both the manually tuned PID controller and the more systematic method of LQG controller design are shown to give acceptable controller performance.
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