Viollet, Y., Abdullah, M., Alhajhouje, A., and Chang, J., "Characterization of High Efficiency Octane-On-Demand Fuels Requirement in a Modern Spark Ignition Engine with Dual Injection System," SAE Technical Paper 2015-01-1265, 2015, doi:10.4271/2015-01-1265.
In a regulatory environment for spark ignition (SI) engines where the focus is continuously looking into improvements in fuel economy and reduction in noxious emissions, the challenges to achieve future requirements are utmost. To effectively reduce CO2 emissions on a well-to-wheel basis, future fuels enabling high efficiency SI engines will have to not only satisfy advanced engine requirements, i.e. high knock resistance, but also produce less CO2 emissions in the refinery. This paper describes how to characterize SI combustion's on-demand octane requirement with three different dual fuel configurations. Refinery naphtha was used for low octane component, and three oxygenates were used for high octane knock inhibiting component, such as, Methanol and Methyl tert-butyl ether (MTBE) and Ethyl tert-butyl ether (ETBE). Each low and high octane fuel was introduced via production gasoline direct injector (DI) and port fuel injector (PFI) in both configurations. It was found that benefits of the high RON component was amplified when it was introduced through DI while the low RON naphtha was injected in PFI. Methanol had been shown to be most effective through DI due to its high heat of evaporation and charge cooling. Consequently, the minimum methanol requirement to maintain MBT is less than MTBE or ETBE by volume. An optimum oxygenate map was found within a range of 1500 to 3500 rpm in speed and 1bar to 13bar Brake Mean Effective Pressure (BMEP) in load range conditions. As a result, the engine could operate solely with low octane naphtha, up to loads of 4-bar BMEP. At a high load condition, such as 1500 rpm, 13bar BMEP, minimum requirement of methanol was 43% of total dual fuel, while MTBE requirement was 74%. This was because methanol's charge cooling in the chamber had a dominant effect on knock suppression, although a RON of MTBE was higher than methanol. On the contrary, the total fuel consumption with naphtha-methanol was higher than one with naphtha-MTBE. This was due to the fact that methanol's lower energy value required more naphtha as the primary fuel source, while MTBE could contribute 1.5 times higher energy than methanol.Three fuels with both high and low octane properties could provide the optimum octane on an as-required basis. This can enable engine manufacturers to further downsize engines by means of an additional parameter to dynamically control the knock boundary.