Effects of n-butanol, 2-butanol, and methyl octynoate addition to diesel fuel on combustion and emissions over a wide range of exhaust gas recirculation (EGR) rates

• Effects of fuel and oxygenated structure on soot reduction depend on EGR rate.
• The fuel oxygen of n-butanol20 is the uppermost factor to reduce soot.
• For oxygenated structure, n-butanol20 has 12.6% lower soot than methyloctynoate20.
• The locations of OH between n-butanol and 2-butanol have small effect on soot.
• Fuel properties have little effects on the NOx, CO, THC emissions and ITE.

The effects of fuel properties and oxygenated structures on the combustion and emissions were investigated on a diesel engine. Five different fuels, n-heptane, iso-octane, n-butanol, 2-butanol and methyl octynoate, were added into diesel fuel with the same blending ratio (20% v/v), referred to as n-heptane20, iso-octane20, n-butanol20, 2-butanol20 and methyloctynoate20, respectively. A wide range of exhaust gas recirculation (EGR) rates from 0% to 62% were used to cover both the conventional diesel combustion and the low temperature combustion. Results demonstrate that there is little difference in combustion characteristics between n-heptane20 and the diesel fuel over a wide range of EGR rates. The ignition delay of iso-octane20 is retarded compared to that of n-heptane20 and the difference in ignition delay between iso-octane20 and n-heptane20 becomes larger with the increase of EGR rate. The combustion characteristics of n-butanol20 are similar to those of iso-octane20. The oxygen in n-butanol20 is the main factor for soot reduction in comparison with the diesel fuel. The secondary factor for soot reduction is the improvement in fuel mixing process due to the longer ignition delay caused by lower cetane number of n-butanol. Other changes in physical and chemical properties of n-butanol20 have very limited effect on soot reduction. Compared to methyloctynoate20, additional 12.6% reduction in the total soot emissions can be achieved by fueling n-butanol20 with the addition of 2-ethylhexyl nitrate (EHN), which can be attributed to the different molecular structures of the oxygenated fuels. The difference in locations of OH radical between n-butanol and 2-butanol has very small effect on soot emissions. The soot-reduction efficiency caused by fuel properties and oxygenated structures varies as the EGR rate changes. Fuel properties have little effects on NOx, CO, THC emissions and gross indicated thermal efficiency (ITE) under similar EGR conditions, while the EGR rates play a dominant role on the gaseous emissions and ITE.