Analysis of combustion in a small homogeneous charge compression assisted ignition engine

Combustion analysis has been conducted on a small two-stroke glow ignition engine, which has similar combustion characteristics to homogeneous charge compression ignition (HCCI) engines. Difficulties such as unknown ignition timing and the polytropic index have been addressed by combining both heat release and mass fraction burn analyses. Results for all operating conditions have shown good correlations between the two methods. The engine has been fuelled with a mixture of methanol, nitromethane, and lubrication oil. The effect of nitromethane on combustion is difficult to determine, since altering nitromethane content also changes the air–fuel ratio under the current experiment set-up. However, it is still possible to show that nitromethane shortens the combustion periods beyond the uncertainty created by the mixture strength and cycle-by-cycle variations. The results further show that a faster combustion does not necessarily give a higher indicative mean effective pressure (i.m.e.p.) in this engine. This is because the start of combustion can shift away from its optimum value when nitromethane is added. The initial combustion period is found to be between 10 and 30° CA (crank angle); the main combustion period is between 25 and 50° CA. These combustion periods are comparable to a traditional spark ignition engine. In a very rich mixture, the ‘hot’ glow plug has been found to change significantly the combustion characteristics. Further study would be recommended to elucidate the effect of glow plugs. Lastly, in the case of poor combustion, cycle-by-cycle analysis shows that a misfire or partial burn cycles are always followed by high i.m.e.p. and fast burn cycles.