An Experimental Study Of Methanol HCCI Engine Based On External Exhaust Gas Recirculation

Energy lacking and environment pollution have become the main issues that all countries in the world focus on at present. Internal combustion engines, as the prime mover of most power plants, are the main customer of petroleum and environment. Therefore, energy saving and exhaust emission reducing for internal combustion engines become more and more vital. Homogeneous charge compression ignition (HCCI) combustion technology integrates the advantages of traditional engines with high thermal efficiency and ultra-low NOX and PM emissions, thus receiving increased attentions in the automotive field. How to accurately control the combustion process and extend its operation range are the most important challenges that HCCI engines are faced with now.Exhaust gas recirculation (EGR), including internal EGR and external EGR, is a valid and important method to achieve and control HCCI combustion. In this paper, the impacts of external EGR on methanol HCCI combustion and emission characteristics are investigated on a modified diesel engine. The impacts of external EGR on engine performance and operation-range extension of the methanol HCCI engine are also analyzed by experiments in this paper. The main conclusions were listed as follows.External EGR has a remarkable influence on HCCI combustion phase. As the EGR ratio increases, CA50 gradually delays and the combustion duration prolongs. However, the changing level varies in different conditions of mixture concentration. There is an optimal EGR ratio to promote the indicated mean effective pressure (pi) and thermal efficiency in each condition of mixture concentration. Moreover, the optimal EGR ratio gradually increases with the increase of the mixture concentration. According to the experimental data, exhaust heating just has a weaker influence on HCCI combustion than dilution and thermal-capacity increasing.As EGR ratio increases, the emissions of unburned hydrocarbon (HC) and monoxide carbon (CO) quickly increase. Nitrogen oxide (NOX) is only produced in the condition of relative rich mixture that excessive air coefficient is equal to 1.6 or 1.5. In addition, its quantity quickly decreases with EGR ratio goes up. Therefore, the level of NOx emission can be taken as one of the accordance to adjust knock burning. External EGR can extend mixture concentration towards to the much richer, thus increasing pi from 0.37MPa without EGR to 0.50MPa by around 35%. The pi range gradually extends with EGR ratio goes up. When EGR ratio keeps between 12% and 15%, the maximum range is obtained around 0.22MPa. However, as the mixture concentration becomes leaner, the ability of extending the maximum pi for external EGR gradually decreases. When excessive air coefficient is equal to 2.0, external EGR loses the ability of extending high load.