Effect of the Cetane Number on Jet Fuel Spray Ignition at High-Temperature and -Pressure Conditions

Abstract

This study investigates the impact of the fuel cetane number on the ignition and combustion dynamics of a high-pressure jet fuel spray under realistic diesel engine conditions. Under the desirable single-fuel concept for U.S. military applications, jet fuels would be used in traditional diesel engines. However, there is no cetane specification for such fuels, and substantial variation has been observed. To better understand the ignition and combustion dynamics of jet fuels subjected to diesel conditions, a series of test fuels have been constructed with varying cetane numbers between 30 and 55. Schlieren and chemiluminescence imaging were performed on the ignition process of four cetane number fuels, 30, 40, 50, and 55, at three ambient pressure conditions 4, 6, and 8 MPa. Fuel breakdown prior to ignition is tracked, and the size of the ignition kernel is extracted from the Schlieren imaging. Hydroxyl radical (OH) chemiluminescence intensity is used to estimate the heat release rate. Ignition and initial fuel breakdown delay from ignition were found to typically decrease with the increasing cetane number. The decrease in ignition delay is smaller at higher ambient pressures. Early timescale kernel growth was found to be dominated by the cetane number with a higher cetane number leading to faster kernel growth. Growth at later timescales is determined by injection pressure. The effect of cetane number decreases with increasing ambient pressure. The estimated heat release rate from OH chemiluminescence shows increasing heat release with the increasing cetane number at 4 MPa ambient, but there is no clear relationship at higher pressures.