Mixing-limited combustion of Alcohol fuels in a Diesel engine. 2019-01-0552
Modern Diesel engines are highly optimized for the combustion characteristics of Diesel fuel. However, alternative fuels in Diesel engines are increasingly being considered as a solution to the emissions challenges faced by Diesel. These emissions challenges are not tied to autoignition or mixing-limited combustion, but rather to the challenges of the fuel itself. The long hydrocarbon chains that give Diesel fuel a high cetane number also favor soot formation, and the overall lean equivalence ratio imposed by soot formation makes NOx aftertreatment challenging. This paper explores modifications to the Diesel engine that enable it to utilize any fuel, regardless of cetane number. Thin thermal barrier coatings added to the piston and cylinder head, and uncooled turbocharging, create the elevated pre-injection temperatures needed to ignite low-cetane fuels. Careful choice of fuel – such as small-molecule alcohols – can mitigate soot formation, even in a mixing-limited Diesel-style combustion mode. Combustion of ethanol and methanol fuels in a single-cylinder oil test engine reveal equivalent efficiency and torque to Diesel-fueled operation. Soot emissions are more than 100x lower for the alcohols than for Diesel, with engine-out levels falling below the 2010 tailpipe regulation. Experimental results have demonstrated stoichiometric fuel loading, as well as exhaust-gas dilution for load variation – allowing the sootless exhaust to be compatible with three-way catalysis for aftertreatment. NOx emissions remained low enough to be easily handled with a TWC. Further, since the requirements of alternative-fuel combustion differ from those of Diesel (most notably soot formation is no longer a limiting phenomenon), the combustion chamber geometry can be re-optimized for high efficiency, stoichiometric alcohol-fueled combustion. A combination of CFD modeling and experimentation was used to explore the effects of piston geometry and injector spray angle on alcohol-fueled combustion compared to a Diesel-fueled baseline.
Julie Blumreiter, Bernard Johnson, Apeng Zhou, Gina Magnotti, Douglas Longman, Sibendu Som
ClearFlame Engines Inc., Argonne National Laboratory