Comparison of Heat Release and NOx Formation in a DI Diesel Engine Running on DME and Diesel Fuel 2001-01-0651
Although there seems to be a consensus regarding the low emission potential of DME, there are still different opinions about why the low NOx emissions can be obtained without negative effects on thermal efficiency. Possible explanations are:
The physical properties of DME affecting the spray and the mixture formation
Different shape and duration of the heat release in combination with reduced heat losses
In this paper an attempt is made to increase the knowledge of DME in relation to diesel fuel with respect to heat release and NOx formation. The emphasis has been to create injection conditions as similar as possible for both fuels. For that purpose the same injection system (CR), injection pressure (270 bar), injection timing and duration have been used for the two fuels. The only differences were the diameters of the nozzle holes, which were chosen to give the same fuel energy supply, and the physical properties of the fuels.
During the experiments, the start of injection was kept constant, and changing the time for the end of injection varied the duration. This was done in order to study the contribution of the total NOx and other emission at full load from the different stages of the combustion. The corresponding heat release rates were used as input to a zero-dimensional multizone combustion model where the local equivalence ratios were adapted so that the calculated NOx emissions corresponded with those measured.
It was found that the general shape of the heat release rates was about the same for the two fuels. Emissions of CO and HC were higher and soot considerably higher with diesel fuel than with DME. Emissions of NOx were about the same, but the relation between the fuels varied with injection duration. The results from the calculations with the multizone combustion model indicate that the average local equivalence ratio was quite high (ϕ>1) and about the same for the two fuels. As the injection pressure used was considerably lower than normal for diesel fuel operation with a CR injection system, it is assumed that the mixing between the diesel fuel and air was worse than in the normal case. This is also manifested in the higher soot emissions. Thus one explanation why DME yields lower NOx emissions, as compared with diesel fuel injected at considerably higher pressures, is that the local equivalence ratio is higher, giving lower local oxygen concentration and NOx formation.
Some differences between the two fuels were observed regarding the fuel injection. The needle opening speed was somewhat slower with DME and the amount of fuel injected at a given duration varied between the fuels. One reason for that was found to be throttling in the needle valve seat in the DME case.