An investigation of the effect of post-injection schemes on soot reduction potential using optical diagnostics in a single-cylinder optical diesel engine

Abstract

This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine was operated under a multiple injection scheme of two pilot and one main injection, typical of a partially premixed combustion mode, at the lower end of the load and engine speed range (ca 2.0 bar IMEP at 1200 r/min). Experiments considering the influence of the post-injection fuel amount (up to 15% of the total fuel quantity per cycle) and the post-injection timing within the expansion stroke (5, 10 and 15 CAD aTDC), under a constant total fuel mass per cycle, have been conducted. Findings were analysed via means of pressure trace and apparent rate of heat transfer analyses, as well as a series of optical diagnostic techniques, namely, high-speed flame natural luminosity imaging, CH * , C Ã 2 and OH * line-of-sight chemiluminescence, as well as planar laser-induced incandescence measurements at 31 and 50 CAD aTDC. The combination of post-injection fuel amount and timing has substantial effects on charge reactivity and soot oxidation potential. The analysis reveals that an amount of fuel (7% of the total fuel mass per cycle) injected more than 10 CAD after the main combustion event leads to higher levels of soot emissions, while a larger amount of fuel (15% of the total fuel mass) injected 5 CAD after the main combustion event appears to have a beneficial effect on the soot oxidation processes. Overall, results indicate that a post-injection scheme close to the main combustion phasing could reduce soot levels and improve engine performance, that is, higher IMEP levels at the same fuel consumption rates, although it could increase engine noise.

Review of morphological and chemical characteristics of particulates from compression ignition engines

“…102 In Figure 6, an early post-injection approach showed lower soot than a late post-injection approach. 103,104 Premixed fuel conditions (solid line) showed lower soot than without premixed fuel conditions (dashed line). However, in another study, 105 pilot-main injection conditions showed higher soot than a single injection.…”

Investigation of injection parameters coupled with fuel reactivity on combustion and emissions in dual direct-injection GCI engine

“…15,16 Nonetheless, the majority of literature concerning post-injection is centered around diesel fuels. 15,17–23 Little is relevant to gasoline-like fuels. 7,24,25 The potential of applying post-injection to GCI engines is intriguing as it could mitigate challenges faced by the GCI mode at high loads, expanding the high load limit.…”

Mechanism of the reduction in afterburning and thermal efficiency improvement with highly oxygenated fuels in diesel combustion

“…There are several explanations for the mechanism of increases in afterburning at high load operation in diesel engines, and visualizations of the spray flame with constant volume vessels, 20–26 the in-cylinder visualization engines, 27–30 and rapid compression and expansion machines (RCEM) 31,32 have been attempted to elucidate the mechanism. Kondo et al suggests that the increase in afterburning is mainly due to the accumulation of rich air-fuel mixture around the tip of the fuel spray, 33 and it is necessary to make the inside of the fuel spray leaner to reduce the afterburning.…”