Review and Benchmarking of Alternative Fuels in Conventional and Advanced Engine Concepts with Emphasis on Efficiency, CO 2 , and Regulated Emissions

Alternative fuels have been proposed as a means for future energy-secure and environmentally sustainable transportation. This review and benchmarking show that several of the alternative fuels (e.g. methanol, ethanol, higher alcohols, RME, HVO, DME, and biogas/CNG) work well with several different engine concepts such as conventional SI, DICI, and dual fuel, and with the emerging concepts HCCI, RCCI, and PPC. Energy consumption is in most cases similar to that of diesel or gasoline, with the exception of methanol and ethanol that use less energy, especially in SI engines. Tailpipe emissions of CO2 with respect to engine work output (tank-to-output shaft) can be reduced by more than 15% compared to a highly efficient gasoline SI engine, and are the lowest with CNG / lean-burn SI and with alcohols in several engine concepts. Alternative fuels are considered safe and in most cases are associated with reduced risk with respect to cancer and other health and environmental issues. Apart from differences in handling depending on whether the fuel is gaseous or liquid, engine-out emissions of soot, NOx, HC, and CO vary between the fuels, although the levels typically are lower than for gasoline or diesel. The comparably small differences during engine operation indicate that production and distribution will be more important when it comes to the environmental performance and operating costs of the different alternative fuels. RME and ethanol are already established and work well in engines. So do biogas/CNG and RME. Diesel and gasoline already co-exist, and so there is good reason to use several alternative fuels in parallel. For example, increased amounts of RME in diesel and ethanol + methanol in gasoline (compatible with E85 vehicles) are relevant steps forward that essentially rely on current engine technology. New combustion engine concepts can be co-developed with new fuels and will lead to further reductions in energy consumption. Increased hybridization and integration with the electricity grid will provide better energy utilization as well as potential for further reductions in fuel consumption from new engine operation strategies. This enables realistic opportunities for sustainable alternative fuel production as well as energy-secure and environmentally sustainable transportation. (Less)