The Influence of EGR on Auto-ignition Quality of Gasoline-like Fuels in HCCI Engines

In previous studies it has been shown that the auto-ignition quality of a fuel at a given engine condition can be described by an octane index defined as, OI=(1-K) RON + K MON, where RON and MON characterize the fuel and the K-value depends only on the engine design and operating conditions. It has been shown that the K-value is highly dependent on the pressure and temperature history. Another important parameter is OI₀, the OI of the fuel which gives heat release centred at top dead center; OI₀ can be considered to be the requirement of the engine. In previous work, empirical relations for both K and OI₀ in terms of in-cylinder pressure and temperature and engine speed and mixture strength were found but the influence of EGR was not considered. Therefore experiments have been done in a single cylinder engine, working in HCCI mode, at different operating conditions including various internal and external EGR rates, using fuels of different chemistries and different known RON and MON values.

Very high negative valve overlaps (NVO) produce high temperatures in the engine and fuel auto-ignition quality cannot be properly defined because different fuels release heat at roughly the same point in the engine cycle at a given operating condition. The same is found when the intake pressure is boosted to high levels. Away from these limits the previous empirical correlations for both K and OI₀ hold good even in the presence of EGR. Thus the primary effect of NVO is the increase of temperature in the engine. Thus there is no evidence that there are additional effects of EGR on auto-ignition over and above those due to changes in the thermodynamic parameters caused by EGR.