This paper summarizes a program to investigate the impact of a variety of oxygenates on diesel exhaust emissions, especially particulate matter (PM) and NOx emissions. Oxygenates have been studied at great length already and have been shown to be an effective method for reducing particulate emissions, although high cost remains a barrier to their widespread use. Our objectives were to assess whether some oxygenates could be more effective than others and why.
Fourteen different oxygenates were studied. Testing was carried out primarily in a single cylinder heavy duty Caterpillar engine under high and low load conditions. Complementary testing was performed in three vehicles spanning a range of vehicle technologies. Most of the testing used a single base fuel which served as the reference fuel although some tests were also done using a newly produced ultra low sulfur automotive diesel oil (ULSADO).
Larger particulate matter reductions were found at high load than at low load. Surprisingly, large differences were found between oxygenates at both high and low load conditions and some oxygenates were found to be much more effective than others in reducing particulate matter. The most effective oxygenates on an equal oxygen content basis were the C9 -C12 alcohols in both the engine and vehicle testing.
Despite the large number of oxygenates studied, the large number of fuel and engine variables meant multiple correlations could be found between PM levels and the variables measured. Although no simple causal explanation was found to explain all the differences seen, some very strong correlations were found and these are discussed.
Compositional analysis indicated little difference in particulate composition between reference fuel and oxygenated fuels in terms of dry soot and volatile organic fraction (VOF). No significant increase in NOx was seen at either high or low load conditions overall.