An Experimental Comparison of Methanol Combustion Strategies: Spark Ignition Versus Compression Ignition

Abstract

Methanol has garnered significant attention over the past few years due to its potential as a hydrogen carrier in a future green economy. As a result, it is a leading candidate to displace fossil fuels in the transportation sector. Although recent research efforts have been directed toward adopting the fuel in compression ignition engines, methanol is an ideal fuel for light-duty high compression ratio spark ignition engines due to its fast laminar flame speed, high auto-ignition resistance, and high cooling potential. In this work, methanol was combusted in a single-cylinder spark ignition engine with a compression ratio of 14.8 and compared to E10 (regular grade gasoline – 10% ethanol, 90% ethanol by volume), E75 (75% ethanol, 25% gasoline by volume), and hydrous ethanol (92% ethanol, 8% water by mass) at 6 bar net indicated mean effective pressure (IMEPn). Methanol achieved a net fuel conversion efficiency of 42.5% compared to 41.6% with hydrous ethanol, 39.5% with E75, and 36.2% with E10. Next, the performance of the high compression ratio spark ignition engine was then compared to a methanol-fueled light-duty single-cylinder compression ignition engine. At loads of 6 bar and 10 bar IMEPn, the net fuel conversion efficiency of stoichiometric spark ignition was higher than lean mixing-controlled compression ignition by 2.6 and 3.3 percentage points, respectively. The net fuel conversion efficiency of mixing-controlled compression ignition was higher than spark ignition by 0.2 percentage points at a load of 16 bar IMEPn. The competitiveness of spark ignition with mixing-controlled compression ignition was due to the high thermodynamic penalty associated with injecting a high heat of vaporization fuel like methanol close to top dead center where heat from the working fluid is absorbed to evaporate the fuel rather than being converted to thermodynamic work. To remedy this, an advanced compression ignition strategy using premixed and partially premixed injections was demonstrated to provide the highest net fuel conversion efficiency across the tested combustion strategies by avoiding this thermodynamic penalty and taking full advantage of the lean and unthrottled nature of compression ignition.