Primary & Secondary Reference Fuel Effects as a Function of Compression Ratio in a CFR Diesel Engine

Abstract Primary Reference Fuels (PRFs) and Secondary Reference Fuels (SRFs) in the range of cetane from 30 to 60 were operated in a Waukesha Diesel Cooperative Fuels Research (CFR) engine under operating conditions that emulate the cetane rating test. Due to the large number of test points in this study, the exact ASTM cetane rating protocol was not followed precisely, however these results are representative of cetane characterization testing with very similar equivalence ratio and combustion phasing across a broad range of Ignition Delays (IGDs) that varied as a result of Compression Ratio (CR) changes in the eleven to twenty-two range. Intake air temperature was operated both heated, as in the cetane rating test, as well as at ambient laboratory conditions. Additional research instrumentation was added beyond the standard CFR equipment for advanced combustion analysis. Combustion analysis shows that engine torque and efficiency increase significantly with increases in CR. At longer IGDs representative of the cetane rating test (13 deg IGD), the increase in IGD with reduced cetane number is relatively linear. For all of the fuels tested, IGD steadily monotonically decreases with increased CR significantly by more than a factor of two. Shorter IGDs lead to longer burn durations; fuel effect differences become less important at very high CRs. Associated companion analysis shows that at the time of fuel injection (Start Of Injection – SOI), cylinder pressure roughly doubles over the CRs studied, however, cylinder charge temperature only moderately increases. This effect leads to a doubling in cylinder air charge concentration at the highest CRs showing an important effect on the fast kinetics at high CRs. A common IGD correlation was evaluated showing good agreement except for the high CN fuel. New IGD correlations are also presented.