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Investigating the dynamic effects on flow structures generated during the intake stroke in heavy-duty diesel engines using Large Eddy Simulations
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. Scania CV, Sweden. (Laszlo Fuchs)ORCID iD: 0000-0003-1511-2235
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.ORCID iD: 0000-0001-9976-8316
KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The aim of this work is to investigate whether a quasi-steady assumption is applicable for the flow entering a heavy-duty diesel engine. That is, what errors can be expected when a steady swirl test rig is used to characterize the flow structures entering the cylinder. The main conclusion is that the dynamic effects have an effect on the flow generated during intake.In detail, we show that the swirl coefficient is higher during fluid deceleration as compared to during fluid acceleration. The swirl coefficient was also found to be around 40~\% higher during valve closing as compared to similar lifts during valve opening. The flow of angular momentum into the cylinder is found to be delayed by the time it takes to empty the volume above the valves. A new ratio, port delay ratio (Rp), is proposed to account for this phenomena. Minimizing the port delay ratio during engine design is likely to lead to a stronger and more homogeneously distributed swirling motion.Taken together, the dynamic effects on swirl and mass flow expected during the intake have been investigated. This knowledge is important when measured mean swirl numbers are used as initial condition in sector model simulations of combustion. Sector model simulations are very common during the optimization process of diesel combustion and a small difference between real and simulated swirl numbers may have a significant effect on engine performance.

National Category
Fluid Mechanics and Acoustics Vehicle Engineering
Identifiers
URN: urn:nbn:se:kth:diva-164888OAI: oai:DiVA.org:kth-164888DiVA: diva2:806423
Note

QS 2015

Available from: 2015-04-20 Created: 2015-04-20 Last updated: 2015-04-20Bibliographically approved
In thesis
1. Creation and destruction of in-cylinder flows: Large eddy simulations of the intake and the compression strokes
Open this publication in new window or tab >>Creation and destruction of in-cylinder flows: Large eddy simulations of the intake and the compression strokes
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis is to increase engine efficiency by studying the flow structures created in an engine cylinder during the intake phase and the effect of the subsequent compression.

The invention of the combustion engine has enabled three centuries of economic growth fueled by energy stored as hydrocarbons. However, during the latter part of the twentieth century negative consequences on health and environment of the combustion engine were observed. In order to reduce emissions without increasing fuel consumption, improved knowledge of all physical processes occurring in the engine are necessary. The aim of this thesis is to increase the understanding of the flow prior to combustion, which can lead to reduced engine emissions and fuel consumption.

Intake flow structures are studied using large eddy simulations and experiments on a steady swirl test rig. Flow acceleration was observed to reduce the swirl coefficient, and higher swirl coefficient was found during valve closing as compared to during valve opening. This implies that the rotation is stronger during the later part of the intake then what has been previously assumed. In addition, the computations show that the volume above the valves has a profound effect on the swirl created during the intake. To take this into account a novel way of calculating the swirl number was suggested. This approach gives a lower swirl number as compared to the commonly used Thien methodology. The effects of compression are studied using simulations of predefined flow structures undergoing compression. The peak turbulence levels were found to be increasing with tumble number and decreasing with swirl. It was noted that compression increased the turbulent fluctuations in the cylinder axis leading to anisotropic turbulence and that a small tilt angle was observed to have a significant effect on swirl homogeneity at top dead center.  In this thesis, a new methodology was proposed and validated for calculation of in-cylinder turbulence for a flat piston.

The results of the thesis enhance the understanding of the dynamic effects encountered during intake as well recognizing that a small tumble component has a strong effect on the flow structures prior to combustion. These results can be used to improve the simplified computational methods used to optimize the engine.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. ix, 109 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2015:03
Keyword
Swirl, Tumble, Compression, Engine, LES, CFD, engine turbulence, engine simulations, intake flow structures
National Category
Fluid Mechanics and Acoustics Vehicle Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-164889 (URN)978-91-7595-471-4 (ISBN)
Public defence
2015-05-08, Sal D3, Lindstedtsvägen 3, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Energy Agency
Note

QC 20150420

Available from: 2015-04-20 Created: 2015-04-20 Last updated: 2015-04-20Bibliographically approved

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Söder, MartinPrahl Wittberg, Lisa

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