Combustion Characteristics of a Dual Fuel Diesel Engine with Natural Gas (Lower limit of Cetane Number for Ignition of the Fuel)

Dual fuel diesel engines using compressed natural gas (CNG) are an attractive low polluting application, because natural gas is a clean low CO₂-emitting fuel with superior resource availability. In dual fuel diesel engines with natural gas as the main fuel the natural gas is supplied from the intake pipe and the pre-mixture formed in the cylinder is spontaneously ignited by an injected spray of ordinary gas oil. Dual fuel engines of this type have the advantages that only limited engine modifications are needed and that low calorie gas fuels such as biogas can be used. To clarify the influence of the cetane number (C.N.) of the ignition fuel on the ignition performance, combustion characteristics, and emissions of the dual fuel operation, the present study used standard ignition fuels prepared by n-hexadecane and heptamethylnonane which define the ignitability of diesel combustion. The experiments focused on determining the lower C.N. limit of the ignition fuel and used standard ignition fuels of different C.N., 30 to 55, in five C.N increments. It was found that at high loads the dual fuel operation needs ignition fuels with C.N. higher than 45, while normal diesel operation is possible with C.N. 35 to 40 fuels. It was confirmed that for ordinary gas oil and for fuels with C.N. higher than 45, there is a strong negative correlation between the coefficient of variance of IMEP and the brake thermal efficiency.