Premixed Burn Fraction: Its Relation to the Variation in NO_x Emissions between Petro- and Biodiesel

Abstract

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.

An overview of the effects of fuel molecular structure on the combustion and emissions characteristics of compression ignition engines

“…In several studies, a strong relationship has been observed between the duration of fuel ignition delay (which is affected by fuel molecular structure as discussed in the second section of this paper) and the level of exhaust NO x emissions. 4,10,31,36,37,69 Figure 9 shows the effect of the variation in ignition delay of n -alkanes and 1-alkenes on the exhaust emissions of NO x and the premixed burn fraction at constant injection timing and constant ignition timing. 31 In Figure 9(a), it can be seen that there is a linear increase in the exhaust level of NO x emissions whereas, in Figure 9(b), it is apparent that there is a concurrent increase in the extent of the premixed burn fraction with increasing ignition delay.…”

Ultralow Sulfur Diesel and Rapeseed Methyl Ester Fuel Impact on Performance, Emitted Regulated, Unregulated, and Nanoparticle Pollutants

“… 94 This enhancement increases the oxidation rate of the formed fine particles. 94 Also, the absence of aromatics in the RME formulation reduced the activated PM molecules during the accumulation mode. 82 The result obtained (reduction of PM formation during the accumulation mode when the engine is running on RME) is consistent with the findings of refs ( 79 , 103 − 105 ).…”

“…When working with ULSD, the ignition delay period for this fuel is longer and the pre-mixed combustion fraction is larger, which increases the combustion temperature, causing high NO X levels for the low-load condition. Pearce et al confirmed that RME combustion emits lower NO X levels compared to ULSD under the low-load conditions at the pre-mixed combustion fraction condition. However, the exact opposite happens at high loads.…”

“…Equations and were used to calculate the AHRR, and these two equations show a reasonable change in the energy applied to the piston. Previous studies explained in detail how temperature at certain limits clearly affects the formation of the peak heat release rate inside the combustion chamber. , Peirce et al indicated that the constant temperature is usually between 1.3 and 1.35. In the current study, two different types of fuel will be used under various engine operating conditions; therefore, the constant temperature = 1.35 was adopted according to what was directed by ref .…”

“…Previous studies explained in detail how temperature at certain limits clearly affects the formation of the peak heat release rate inside the combustion chamber. , Peirce et al indicated that the constant temperature is usually between 1.3 and 1.35. In the current study, two different types of fuel will be used under various engine operating conditions; therefore, the constant temperature = 1.35 was adopted according to what was directed by ref . The AHRR can be considered as the sum of the net heat of release rates at each crank angle (CAD), which was calculated from the start of the injection and is represented by the following relationship: …”