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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.

This paper investigates the performance, exhaust emissions, and combustion characteristics of a dual fuel diesel engine fueled by CNG (compressed natural gas) as the main fuel. The experiments used a small single cylinder DI diesel engine and two kinds of fuels for the ignition: FAME (fatty acid methyl ester) fuels such as Methyl Oleate (OME) and OME-Methyl Palmitate (PME) blends, major components of biodiesel, and ordinary gas oil. The rate of the CNG supply was defined as the proportion of the heat energy of the supplied CNG to the total heat energy available in the cylinder. Compared with gas oil ignition, the FAME fuels had shorter ignition delays and significantly reduced smoke densities regardless of the PME contents. The PME contained in the FAME fuels gave rise to slight improvements in ignitability. The results also showed that the conditions where operation with CNG/FAME fuels is possible are very similar to those of the CNG/gas oil.

In order to reduce the smoke emission of PME/1-butanol blend by increasing the 1-butanol content, PME/1-butanol blend is tested using a DI diesel engine with jerk-type fuel injection pump. With PME/1-butanol blend, there is no problem on the start-ability and stability of the engine operation up to 60 mass% of 1-butanol. On the other hand, with gas oil/1-butanol blend, there is no problem on those up to 40 mass% of 1-butanol. The PME/1-butanol blend has longer ignition delay compared with PME due to the low cetane number of 1-butanol. With increasing 1-butanol content, the smoke emissions of PME/1-butanol blend decrease although the HC and CO emissions increase due to the longer ignition delay.

This paper describes the influence of different kinds of FAME (fatty acid methyl ester) on the smoke emissions of a small single cylinder DI diesel engine and the soot formation characteristics in suspended single droplet combustion. The study used eight kinds of commercial FAME and diesel fuel blends. The tested FAMEs are saturated fatty acids with 8 to 18 carbon molecule chains, and with three different double bonds with C18. The results show that with all the FAME mixtures here, the brake thermal efficiencies with the FAME-diesel fuel blends were similar to neat diesel fuel operation while the smoke emissions with all of the tested FAME-diesel fuel blends were lower. To examine the differences in the soot formation characteristics, measurements of the formed soot mass were also performed with a basic experimental technique with suspended single droplet combustion. The soot was trapped on a glass fiber filter, and the mass of the filter was measured with an electronic microbalance.