Octane Response of a Highly Boosted Direct Injection Spark Ignition Engine at Different Compression Ratios

Stringent regulations on fuel economy have driven major innovative changes in the internal combustion engine design. (E.g. CAFE fuel economy standards of 54.5 mpg by 2025 in the U.S) Vehicle manufacturers have implemented engine infrastructure changes such as downsizing, direct injection, higher compression ratios and turbo-charging/super-charging to achieve higher engine efficiencies. Fuel properties therefore, have to align with these engine changes in order to fully exploit the possible benefits. Fuel octane number is a key metric that enables high fuel efficiency in an engine. Greater resistance to auto-ignition (knock) of the fuel/air mixture allows engines to be operated at a higher compression ratio for a given quantity of intake charge without severely retarding the spark timing resulting in a greater torque per mass of fuel burnt. This attribute makes a high octane fuel a favorable hydrocarbon choice for modern high efficiency engines that aim for higher fuel economy. Prototype engine builds with novel design concepts allow for studying the impact of fuel octane number on engine performance: In this regard, a 2.0 L engine based on the previously published Ultraboost concept (utilizing absolute air-intake pressures of 2 bar or higher) was tested at two different compression ratios of 9:1 and 11:1 using a fuel matrix of varying octane grades. As expected the octane number of the fuels had a higher impact at 11:1 compression ratio than 9:1 in terms of the spark advance achieved and engine efficiency. The combustion analysis highlighted the importance of octane number, not only in permitting a more advanced spark but also on the burn duration of the compressed charge, with both factors contributing to enhanced efficiency. This study demonstrates the continued importance of fuel octane number on SI engine designs in the foreseeable future.