Effects of engine operating parameters on diesel low-temperature combustion with split fuel injection

Abstract

This paper shows that a split-fuel-injection strategy can achieve robust, near-zero smoke and nitrogen oxide emissions at reduced exhaust gas recirculation levels under low-temperature combustion conditions. The overall objective of the work was to investigate the sensitivity (in terms of the engine emissions and the fuel economy) of a 50:50 (by mass) split-injection strategy to variations in the key engine operating parameters. Experiments were performed at operating conditions corresponding to a gross indicated mean effective pressure of 500 kPa at an engine speed of 1500 r/min in a 0.51 l single-cylinder high-speed direct-injection diesel engine. The paper presents the effects of different relative fuel injection timings at a variable intake oxygen mass fraction (10.5% and 12%) at a constant intake pressure (120 kPa, absolute) on the smoke, total hydrocarbon and carbon monoxide emissions with the split-main-injection strategy. The effects of a variable fuel injection pressure (90 MPa and 110 MPa) on diesel low-temperature combustion with split injection are also reported, as are the effect of an increased intake pressure (150 kPa, absolute). The combined effects of the operating parameters and the fuel injection timing on the smoke, nitrogen oxide, total hydrocarbon and carbon monoxide emissions and the gross indicated specific fuel consumption are described. For selected operating conditions, the cycle-resolved spray and combustion processes are visualized together with the flame temperature measurement using two-colour optical pyrometry to understand the combustion phenomena occurring in the split-injection strategy. The results of the optical studies show that different low-temperature combustion operating conditions producing similarly low levels of ‘engine-out’ smoke emissions have substantially different histories of soot formation and soot oxidation. An increase in the intake oxygen mass fraction reduced the total hydrocarbon emissions and the gross indicated specific fuel consumption at a given intake pressure, while a higher intake pressure reduced them further. Although significant soot formation took place from the second injection event, the majority of the soot was subsequently oxidized because of a slightly higher flame temperature and slightly higher oxygen concentration than in single-injection high-exhaust-gas-recirculation low-temperature combustion. A higher injection pressure did not have any significant effect on the emissions and the gross indicated specific fuel consumption.

Effects of intake-port throttling on combustion behaviour in diesel low-temperature combustion

“…In contrast to the fast FID data that were presented for the lower-load, partially premixed, Condition I, the temporal evolution of the HC signal differs substantially for different swirl ratios. A previous work on this engine 9 has shown that for a -30°ATDC SOI injection timing, there is the strong likelihood that the injected fuel will not be fully contained within the bowl, and that some fraction of the fuel will enter the piston squish region. In agreement with Colban et al, 13 the initial spike that is seen in the low swirl, Rs = 0.76, data are thought to be associated with HC emissions from fuel trapped in the squish volume and in quench layers along the head and valve surfaces.…”

“…The robustness of the operating strategy in this regard is attributed to the relatively late injection (SOI = –7.5° ATDC (after top dead centre)) timing that ensures that the injected fuel is fully contained within the combustion bowl. 9 With regard to the cycle average HC emissions behaviour, for a fixed oxygen concentration and fuelling, throttling the intake ports in order to increase swirl will reduce the trapped mass and hence increase the global equivalence ratio in-cylinder. It is possible that the increase in HC and CO that is observed when moving from the Rs = 0.76 swirl ratio to the Rs = 1.33 swirl ratio is due to this effect—note that the calculated in-cylinder temperatures are very similar for these two cases.…”

“…The research engine facility, which has been fully described and illustrated schematically in a number of previous works from the authors' research group, [7][8][9][10][11] was instrumented with the provision to measure boosted intake air temperature and pressure, fuel and air flow rates, and in-cylinder pressure. Intake pressure was measured using Kistler 4045A5 piezo-resistive pressure transducer while the in-cylinder pressure was measured with a piezoelectric AVL QC34C flushmounted water-cooled transducer.…”

The impact of intake pressure on high exhaust gas recirculation low-temperature compression ignition engine combustion using borescopic imaging

“…The imaging borescope was connected to a Prosilica GC1380 monochrome charge coupled device (CCD) camera to provide spray and combustion imaging within the borescope's field of view (FOV). The specifications of the various optical components used in the experimental set up are given in Table 2; more information is available elsewhere [22,24,25]. The region inside the piston bowl imaged through the borescope at different crank angles is shown in Fig.…”

Load transient between conventional diesel operation and low-temperature combustion

“…23,24 This splitinjection strategy was also implemented in the intermediate cycles during a combustion mode transition from high-EGR LTC to conventional diesel operation.…”