Dependence of premixed low-temperature diesel combustion on fuel ignitability and volatility

Abstract

A comprehensive study of fuel property effects in internal combustion engines is required to enable fuel diversification as well as the development of applications to advanced engines 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 of premixed low-temperature combustion (LTC) modes in diesel engines. A total of 23 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 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, O2in, only when PCN < 40, the load range can be expanded significantly to higher loads. By holding the combustion phasing at top dead centre and varying intake oxygen concentration, the nitrogen oxides 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.