Background
When octane ratings were first codified nearly 100 years ago, it represented engines, fuels and operating conditions of the time. Since then, both the fuels and the engines in the market have changed significantly. Octane numbers have steadily increased, and today, many market fuels include high octane blend components, typically alcohols and ethers, to comply with renewable fuels legislation. Engine technology has undergone an equally dramatic evolution: higher compression ratios, forced induction, fuel injection, better materials, addition of aftertreatment and huge advances in engine controls, to name a few. The industry at large, realizing the disconnect between the knock-limited operation in a Cooperative Fuel Research (CFR) octane rating engine and the engines in the market, is tackling the problem by one of two broad approaches. The first is to adapt the octane rating to be more representative of modern engines. The second approach is simply to use the octane numbers as measured and attempt to correlate Research and Motor Octane Numbers (RON and MON respectively) to the knock-limited performance of modern engines.
Approach
This project reconciled knock in different engines and the underlying autoignition by studying fuels in both the CFR engine and the SwRI-developed Single Cylinder Research Engine (SCRE), representing a modern direct injection, turbocharged spark ignited engine. Thirteen different fuels were selected for this study, which included primary reference and toluene standardization fuels, as well as three full boiling range gasolines with a selection of 0%, 10% and 20% ethanol content. Both engines utilized high-speed data acquisition to capture cylinder pressure, exhaust oxygen and a variety of temperatures. Data was used to develop a unifying autoignition model that was able to describe and predict knock in both modern engines and the CFR engine, and parameterized the model for individual engine cycles, rather than the traditional approach of utilizing a single median or representative pressure trace.
Accomplishments
All test fuels were prepared, and relevant properties determined, including laminar burning velocity. CFR engine tests were completed at standard knock intensity under both RON and MON test conditions, as well as with a stoichiometric mixture and under borderline knock and knock-free conditions. SCRE test points covered the full range of speeds and loads and included both knock-free and knocking conditions. For knocking conditions, additional tests were completed to assess each fuel’s sensitivity to varying ignition timing and air-fuel ratios. Fuel consumption measurements for the different fuels were used to predict fuel economy values over various standard emission drive cycles using a vehicle model. The analyses revealed that lower ethanol content and higher fuel density correlated with improved predicted fuel economy and that the impact of higher RON depended on the drive cycle, providing some benefit for the higher loaded US06 and Worldwide harmonized Light-duty vehicles Test Cycles drive cycles. These fuel economy results were presented at the 2019 SAE World Congress (WCX19). It was further revealed that the knock severity in the CFR engines — using conventional metrics of knock-peak values — were lower for certain fuel compositions, even when tested at standard knock intensity as defined by the ASTM during octane rating. This implication that the real-world knock performance of certain fuels may be underestimated by the octane numbers were presented at the JSAE/SAE 2019 International Powertrains, Fuels and Lubricants Meeting.