Abstract:A comprehensive study of fuel property effects in internal combustion engines is required to enable fuel diversification as well as applications to advanced engines being developed for operation with a variety of combustion modes. The objective of this paper is to investigate the effects of fuel ignitability and volatility over a wide range on premixed low-temperature combustion (LTC) modes in diesel engines. Twenty three fuels were prepared from commercial gasoline, kerosene, and diesel as baseline fuels and with the addition of additives, to generate a cetane number (CN) range from 11 to 75. Experiments with a single cylinder diesel engine operated in moderately-advanced-injection LTC modes were conducted to evaluate these fuels. The combustion phasing is demonstrated to be a good indicator to estimate the in-cylinder peak pressure, exhaust gas emissions, and thermal efficiency in the LTC mode. Fuel ignitability affects the combustion phasing by changing the ignition delay. The predicted cetane number (PCN) based on fuel molecular structure analysis can be fitted to the ignition delays with a higher coefficient of determination than CN, suggesting a good potential as a fuel ignitability measure over a wide range. The stable operating load range in the smokeless LTC mode depends more on the actual ignition delay or PCN rather than CN. With fixed injection timing and intake oxygen concentration, O 2in , only when PCN < 40, the load range can be expanded significantly to higher loads. With holding the combustion phasing at TDC and varying O 2in , the NOx and smoke emissions become limitations of the load expansion for some fuels. The effects of fuel volatility on the characteristics of LTC are small compared to ignitability. Finally, the operational injection timing range and robustness of the LTC to fuel ignitability are examined, showing that the advantageous ignitability range becomes narrower with fuel ignitability decreasing.

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The effects of fuel ignitability on the stable operating range of premixed low temperature diesel combustion (LTC) were investigated. With the intake oxygen concentration set at 14% and the injection timing at 30 °CA BTDC, the operating load range is almost unchanged for fuels with cetane number above 40; while with cetane number below 40 both the high and low load limits increase significantly with decreasing cetane numbers, the low load limit increases more steeply, resulting in a narrowing of the load range. When keeping the combustion phasing constant at TDC by controlling the injection timing, smoke emission limits at the higher load operation for high ignitability fuels and abrupt combustion for low ignitability fuels become problems, but a decrease in the intake oxygen concentration is effective to expand the high load limit. The low load limit can be expanded with the combustion phasing held constant at TDC, and it can be further expanded when increasing the intake oxygen concentration, at the expense of increases in NOx emissions however. Finally, the effects of fuel ignitability on the range of injection timings for stable LTC operation are also examined.

Characteristics of diesel combustion with low cetane number fuels with similar distillation temperatures to ordinary diesel fuel, including fuels with cetane number 32 and 39 (LC32, LC39), and a blend of normal cetane and heptamethylnonane with cetane number 32 (CN32), were investigated. The effects of cooled EGR and pilot injection on combustion and exhaust gas emissions with these fuels were examined in a single cylinder diesel engine equipped with a common-rail type fuel injection system. Even with the low cetane number fuels, quiet combustion with low levels of exhaust gas emissions comparable to ordinary diesel fuel was established by suitable control of intake oxygen levels and pilot injections. At light and medium loads, low smoke, low NOx, and quiet combustion is possible by decreasing intake oxygen concentration with EGR. At high loads, pilot injection can suppress NOx emissions and the maximum rate of pressure rise to levels similar to ordinary diesel fuel. Smoke emissions at low intake oxygen and NOx emissions at high intake oxygen with the low H/C ratio fuel, LC32, are higher than with CN32, the high H/C ratio fuel at the same cetane number.