Comprehensive assessment of methanol as an alternative fuel for spark-ignition engines

Abstract

A promising pathway to defossilize the operation of spark-ignition engines is to use methanol as a fuel. This clean alternative liquid energy carrier offers major efficiency and emission benefits compared to gasoline. Its high knock-resistance and high heat of vaporization allow knock-free operation at high compression ratios while maintaining low emissions of nitrogen oxides. In addition, it is the simplest alcohol to synthesize and the distribution infrastructure for existing liquid fuels can be used. Previous research on mixture formation and combustion of methanol in passenger car sized spark-ignition engines is limited, reported on various experimental configurations, and mainly focused on one particular aspect. This study sets out to provide a coherent evaluation of the mixture formation and performance of methanol in a spark-ignition engine in comparison to gasoline by successively addressing and combining fundamental and optical spray measurements, engine experiments and complementing numerical analyses all with matching boundary conditions for these aspects. First, a theoretical analysis regarding the primary spray break-up behavior was performed using methanol and the gasoline surrogate iso-octane, which showed a comparable atomization performance between the two fuels. This theoretical result was then experimentally validated under realistic engine operating conditions on a low-pressure chamber by means of shadowgraph imaging. These investigations showed no notable differences in spray penetration length and spray angle between methanol and gasoline under the same boundary conditions. However, under realistic engine operation, the spray penetration length of methanol increased by up to 64.5% due to the increased injection duration required because of its lower energy content. In a next step, methanol was experimentally studied on a thermodynamic single-cylinder research engine with a compression ratio of 10.8. Using the same fuel injector, the combustion and emission behavior was investigated in comparison with gasoline. A load variation at an engine speed of 2500 1/min and a variation of the relative air/fuel ratio at the same engine speed and a net indicated mean effective pressure of 16 bar were performed. Depending on the engine load, methanol achieved 7%–24% higher net indicated efficiencies and a 9 bar higher maximum load compared to gasoline. At lean-burn operation, methanol not only enabled higher net indicated efficiencies exceeding 45%, however, also an extended lean-burn limit of 0.1 units. The experimental engine investigations were complemented with 0D/1D simulations, which showed that especially the wall heat transfer and the burn duration are significantly lower in the case of methanol. Finally, 3D computational fluid dynamics simulations were performed using a newly developed methanol spray model which showed that an increased fuel mass impinges the combustion chamber walls with methanol. However, toward the end of compression, the methanol wall film dropped to the level of gasoline.