Premixed Burn Fraction: Its Relation to the Variation in NO x Emissions between Petro- and Biodiesel

It is commonly reported in the literature that NO x emissions from a diesel engine increase when fuelling with biodiesel. However, some studies report varying or opposite results. This work scrutinized operating conditions known to yield both increases and decreases in NO x emissions when running on biodiesel. This involved sweeping the injection timing of an instrumented 2 L diesel engine from 14 BTDC (before top-dead-center) to 3 ATDC (after top-dead-center), under loads of 40 Nm and 80 Nm (equating to BMEP (brake mean effective pressure) of 2.5 bar and 5 bar, respectively), using ultralow sulfur diesel (ULSD) and rapeseed methyl ester (RME). Under a 40 Nm load, RME consistently generated lower NO x emissions than ULSD, whereas, under an 80 Nm load, RME generated higher NO x emissions at all but the most advanced/retarded injection timings. This behavior was linked to differences in combustion duration, ignition delay (ID), and the relative size of the premixed burn fraction (PMBF). Combustion tended to progress more quickly overall for the fuel that generated highest NO x emissions at most operating conditions. ID was always reduced when fuelling with RME, and hence PMBF was also reduced. Thus, reduced ID exerted conflicting influences over relative RME NO x emissions; a tendency to increase NO x , due to advanced start of combustion (SOC), and a tendency to decrease NO x , due to reduced PMBF. Additionally, calculations indicated that for the same SOC and PMBF RME would normally be expected to generate higher NO x emissions than ULSD. However, as the level of premixing increased, the magnitude of the ceteris paribus RME NO x increase appeared to decline. That is, as PMBF increases, the impact of the inherent factors beyond advanced SOCthat lead to higher NO x emissions when fuelling with biodiesel appear to be reduced. This may be related to variations in soot radiative heat losses. Changes in operating PMBF may therefore explain some of the variety that exists in the literature relating to the effects of biodiesel fuelling on NO x emissions.