Experimental Effects of Low Octane Primary Reference Fuels on Burn Rates and Phasing Limits in a Homogeneous Charge Compression Ignition Engine with Negative Valve Overlap.

Abstract

Homogeneous charge compression ignition (HCCI) remains an active area of engine research, promising to deliver high thermal efficiency while producing low levels of nitrogen oxides (NOx) and particulate emissions. The pre-mixed auto-ignition nature of HCCI allows the use of a wide variety of fuels, including fuels with lower octane number (ON) than traditional gasoline spark-ignited engines. A method to achieve HCCI traps high levels of internal exhaust gas residuals (iEGR) which introduce thermal and compositional gradients. The contributions in this work compare fuel effects on burn rates and phasing limits of low ON primary reference fuels (PRFs) to gasoline, separating the effects of fuel from iEGR effects. Specifically, in this study, increased load limits were demonstrated for a low ON fuel, but changes in composition obfuscated fuel effects. A new experimental method was therefore developed which isolated composition effects across wide levels of iEGR. Using this method, gasoline at fixed combustion phasing was shown to exhibit sensitivity to increasing iEGR with burn rates decreasing by 15% compared to the lowest iEGR case. Examining the effect of iEGR on stability limits demonstrated iEGR increased cyclic variability due to cyclic feedback. Further experiments showed burn rates of the primary reference fuel PRF40 were 35% faster than gasoline at equal iEGR, but PRF40 showed no dependence on iEGR. PRF40 required a reduced IVC temperature compared to gasoline, which could reduce thermal gradients and increase burn rates. Increased phasing limits were consistently demonstrated for PRF60 compared to gasoline as iEGR was reduced. Both PRF40 and PRF60 demonstrated increasing levels of low temperature heat release (LTHR) as engine speed was reduced, and at 1000 rpm PRF60 showed no phasing dependence on iEGR. Compared to gasoline, observed differences in behavior for the low ON PRFs are attributed to enhanced non-Arrhenius ignition delay behavior which is understood to reduce sensitivity to thermal gradients (or iEGR) and cyclic variations in temperature. The results of this study are the first to isolate charge composition effects during HCCI operation, and the results provide important quantitative insight into the relative importance of thermal stratification and chemical effects of fuels and iEGR.