Fuel flexibility potential for isobaric combustion in a compression ignition engine: A computational study

Abstract

This work numerically explored the potential of using various kinds of fuels for the isobaric combustion concept in a compression ignition engine. Primary reference fuels (PRFs) including PRF0, PRF20, PRF40, PRF60, PRF80, and PRF100 were employed at the same middle engine load. Different injection strategies ranging from single to four injections were studied. The results demonstrated that the tested PRFs showed significant differences when using a single injection method, due to the different fuel auto-ignition characteristics: the lower fuel reactivity led to the longer ignition delay and thus more premixed combustion heat release. Fuel flexibility was achieved by utilizing at least two injection events, under which condition various fuels shared similar engine combustion performance and emissions. Predicted in-cylinder distribution of temperature revealed that the first injection event generated initial high-temperature pockets downstream of the nozzle, which helped to ignite the upstream air–fuel mixture from the following injection events and resulted in a relatively stable spray-controlled heat release process. To reduce soot emissions, various amounts of three shorter-chain alcohols (methanol, ethanol, and n-butanol) were blended with the baseline fuel (n-heptane). The methanol-blended fuels yielded the lowest soot emissions, but the fuel economy suffered due to the highest heat transfer losses. By increasing the nozzle number and introducing an adequate amount of isochoric combustion, the fuel economy for pure methanol combustion was effectively improved.