Effect of post injections on mixture preparation and unburned hydrocarbon emissions in a heavy-duty diesel engine

We present large-eddy simulation (LES) of a high-pressure gas jet that is injecting into a quiescent inert environment. The injection is through a nozzle with a diameter of 1.35 mm. Four injection strategies are considered in which the results of a single continuous injection case are compared with those of double injection cases with different injection splitting timing. In all double injection cases, the injection pulsing interval is kept the same, and the total injected mass is equal to that of the single injection case. On the other hand, the splitting timing is varied to investigate the effects of various injection splitting strategies on the mixture formation and the penetration length of the jet. Results show that the jet penetration length is not so sensitive to the splitting timing whereas the mixing quality can significantly change as a result of shifting the onset of injection splitting toward the end of injection. Especially, it is found that by adopting a post-injection strategy where a single injection splits into the main injection and late small injection near the end of injection period the mixing between the injected gas and ambient air is significantly improved. This trend is not as obvious when the injection splitting timing shifts toward the beginning or even in the middle of injection period. The increase of entrainment in the tail of each injection is one of the underlying physics in the mixing improvement in double injection cases. In addition to that, splitting a single injection into two smaller injections increases the surrounding area of the jet and also stretches it along the axial direction. It can potentially increase the mixing of injected gas with the ambient air.

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“…To define realistic injection strategies, real engine experiments in Ref. [ 4 , 26 ] are considered. In Ref.…”

Section: Resultsmentioning

confidence: 99%

The Effect of Splitting Timing on Mixing in a Jet with Double Injections

Hadadpour

Jangi

Bai

2018

Flow Turbulence Combust

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“…With increased entrainment of ambient gas after end of injection (EOI), vapor-fuel mixtures near the injector transition from fuel-rich to fuel-lean and the flow decelerates pretty fast, which causes it to stagnate near the injector, contributing to incomplete combustion and UHC [9][10][11] . Post injection can push the residual lean mixture near the nozzle downstream and reach second-stage combustion with higher equivalence ratio and higher temperature environment 12 . In addition, it has been proved that the engine-out soot can be reduced by post injection with a proper dwell time and quantity [12][13][14] .…”

Section: Introductionmentioning

confidence: 99%

“…12 In addition, it has been proved that the engine-out soot can be reduced by post-injection with a proper dwell time and quantity. 12–14…”

Section: Introductionmentioning

confidence: 99%

Optical study on characteristics of non-reacting and reacting diesel spray with different strategies of split injection

Desantes

García-Oliver

García

et al. 2018

International Journal of Engine Research

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Even though studies on split-injection strategies have been published in recent years, there are still many remaining questions about how the first injection affects the mixing and combustion processes of the second one by changing the dwell time between both injection events or by the first injection quantity. In this article, split-injection diesel sprays with different injection strategies are investigated. Visualization of n-dodecane sprays was carried out under both non-reacting and reacting operating conditions in an optically accessible two-stroke engine equipped with a single-hole diesel injector. High-speed Schlieren imaging was applied to visualize the spray geometry development, while diffused background-illumination extinction imaging was applied to quantify the instantaneous soot production (net result of soot formation and oxidation). For non-reacting conditions, it was found that the vapor phase of second injection penetrates faster with a shorter dwell time and independently of the duration of the first injection. This could be explained in terms of one-dimensional spray model results, which provided information on the local mixing and momentum state within the flow. Under reacting conditions, interaction between the second injection and combustion recession of the first injection is observed, resulting in shorter ignition delay and lift-off compared to the first injection. However, soot production behaves differently with different injection strategies. The maximum instantaneous soot mass produced by the second injection increases with a shorter dwell time and with longer first injection duration.

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“…With an optimized configuration of injection and dwell timing, injection of small quantities prior to the principal fuel delivery has been proven capable of reducing fuel consumption, particulate matter, and CO and NOx emissions in different load regimes [1][2][3][4][5]. Supplementary injections after the main delivery, known as post-injections, may significantly reduce engine-out soot and unburnt hydrocarbon (UHC) emissions [3,6], but the reported efficacies of this differ significantly [7].…”

Section: Introductionmentioning

confidence: 99%

Transient Flame Development in a Constant-Volume Vessel Using a Split-Scheme Injection Strategy

Maes

Bakker

Dam

et al. 2017

SAE Int. J. Fuels Lubr.

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Multiple-injection strategies are characterized by a complex and transient interplay between high-and low-temperature reactions. Tracking low-temperature reaction products such as formaldehyde (CH2O) is particularly important to understand ignition phenomena and the so-called "combustion recession" that is observed in experiments. Experimentally, it is often difficult to discriminate between formaldehyde and other species such as poly-aromatic hydrocarbons, which is why a selective excitation approach is used in this work. Simultaneous high-speed imaging of the chemicallyexcited hydroxyl radical (OH*) is used to improve indication of flame location and second stage ignition. During experiments in a constant-volume vessel, two 0.5-ms injections of n-dodecane, separated by 0.5-ms dwell time, are injected into a 900-K ambient. The global flame development is characterized based on high-speed diagnostics, followed by an investigation into the spatial distribution of formaldehyde at four different times after start-of-injection (aSOI). Results show significant influence of the first injection on characteristics of the second. Ignition delay and lift-off location of the second injection are prominently reduced, while flame penetration is greatly enhanced by the wake of the first injection. Formaldehyde structure is observed during both end-of-injection transients, reaching as far upstream as 6 mm from the nozzle. Even after the second injection, the flame structure still appears to be influenced by the first, with a shorter lift-off length and compressed formaldehyde structure. Based on the selective excitation procedure, it becomes clear that the interpretation of laser-induced fluorescence (LIF) images obtained by 355-nm excitation alone is prone to ambiguity.

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Effect of post injections on mixture preparation and unburned hydrocarbon emissions in a heavy-duty diesel engine

Cited by 48 publications

References 45 publications

The Effect of Splitting Timing on Mixing in a Jet with Double Injections

The Effect of Splitting Timing on Mixing in a Jet with Double Injections

Optical study on characteristics of non-reacting and reacting diesel spray with different strategies of split injection

Transient Flame Development in a Constant-Volume Vessel Using a Split-Scheme Injection Strategy

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