An Experimental Study on the Variation of COVIMEP and Ringing Intensity at Different Air Excess Coefficients in a HCCI Engine
Increasing depletion of petroleum based fuels and consequently increasing cost have led researchers to develop new engines with low fuel consumption, high thermal efficiency and high performance. In addition, researchers are working intensively to reduce the pollutant effect of exhaust gases from internal combustion engines on the environment and to meet emission regulations imposed by governments [1 3] The homogeneous charge compression ignition (HCCI) engines are more advantageous compared to conventional spark ignition (SI) and compression ignition (CI) engines due to their high thermal efficiency, low NOx emissions and low heat transfer losses. In SI engines, when the air/fuel mixture is ignited by spark plug, high temperatures are generated in the regions where the flame front spreads, which causes high NOx emissions to occur. In order to meet emission limitations in SI engines, three way catalytic converter or EGR applications are being tried to reduce NOx emissions. In CI engines, fuel is injected into compressed high pressure air to start combustion. For this reason, soot emissions are obtained to result of the ignition of the fuel in the rich mixing zone. Moreover, high temperature combustion zones cause to NOx emissions in CI engines. In HCCI engines, the air fuel mixture is formed outside of the cylinder or fuel injected into the cylinder too early crank angle. The homogeneous mixture in the cylinder is then compressed and simultaneous self ignition occurs throughout the entire cylinder. As a result of, HCCI combustion mode is able to reduce both NOx and soot emissions simultaneously and effectively. HCCI engine can operate in lean mixtures without flame propagation. Therefore, low temperature combustion can be achieved [3 4] In addition to the current benefits of HCCI engines, problems such as ignition timing and control of combustion rate must be solved before commercial use is practically applied. It is rather difficult to come up with these two problems. First, there is no mechanism in the HCCI engine that controls ignition as a spark plug or controls the diffusion combustion phase by injecting fuel after combustion, such as direct injection. Secondly, chemical reactions due to fuel properties are more dominant in HCCI combustion. This means that low loads can cause misfiring and high loads can lead to knocking. Hence, the HCCI engine has a limited operating range. A number of Abstract