Effects of butanol isomers on the combustion and emission characteristics of a heavy-duty engine in RCCI mode 2020-01-0307
Butanol is one of the potential alternative biofuels due to its similarities with gasoline and low sooting tendency. In this paper, three butanol isomers (n-butanol, iso-butanol, and tert-butanol) are used as low reactive fuel while n-heptane is used as high reactive fuel in a heavy-duty engine running in RCCI mode. This work investigates the influences of butanol structure on combustion performance and emission characteristics of RCCI combustion concepts. The tested load ranges from 4 bar to 12 bar gIMEP in a step of 2 bar. Since the reactivity of the three isomers can be sequenced by n-butanol > tert-butanol> iso-butanol, different EGR rate and direct injection strategies are applied. N-butanol and tert-butanol require double direct injection and highest EGR rate to maintain a proper combustion phasing. However, iso-butanol yields in ultra-low NOx and soot emissions up to 8 bar gIMEP with single direct injection and 0% EGR due to the higher octane rating. All three butanol isomers display increased peak cylinder pressure and pressure rise rate as the load increases. Specifically, n-butanol results in 28.2 bar/oCA at 12 bar gIMEP due to the too early combustion phasing. This high pressure rise rate constrains the operating load. No fuel-related differences are observed with HC, CO and NOx emissions. However, tert-butanol shows the less soot mass and less particle number concentration compared to the other two isomers. Combustion efficiency increases at high load due to the decreased HC and CO emissions. All three butanol isomers achieve high gross indicated efficiency (>49%) in the whole operating load range.