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Optical study of swirl during combustion in a CI engine with different injection pressures and swirl ratios compared with calculations
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.
2012 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Spray and mixture formation in a compression-ignition engine is of paramount importance in the diesel combustion process. In an ngine transient, when the load increases rapidly, the combustion system needs to handle low operation without producing high NO x emissions and large amounts of particulate matter. By changing the in-cylinder flow, the emissions and engine efficiency are affected.

Optical engine studies were therefore performed on a heavy-duty engine geometry at different fuel injection pressures and inlet airflow characteristics. By applying different inlet port designs and valve seat masking, swirl and tumble were varied. In the engine tests, swirl number was varied from 2.3 to 6.3 and the injection pressure from 500 to 2500 bar. To measure the in-cylinder flow around TDC, particle image velocimetry software was used to evaluate combustion pictures. The pictures were taken in an optical engine using a digital high-speed camera. Clouds of glowing soot particles were captured by the camera and traced with particle image velocimetry software. The velocity-vector field from the pictures was thereby extracted and a mean swirl number was calculated. The swirl number was then compared with 1D simulation program GT-POWER and CFD based correlations. The GT-POWER simulations and CFD based correlation calculations were initiated from steady-state flow bench data on tested cylinder heads.

The main conclusions from this study were that the mean swirl numbers, evaluated with the PIV software from combustion pictures around TDC, agreed with CFD based correlations and the low swirl numbers also correlated with the 1D-simulation program. Most of the induced swirl motion survives the compression and combustion, while the induced tumble does not survive to the late combustion phase. The tumble however, disturbs the swirl motion and offsets the swirl centre. This offset survives the compression and combustion. The diesel sprays that are injected symmetrically in the combustion chamber are thereby exposed to the swirl asymmetrically. This study also shows that the angular velocity at different piston bowl radii deviates from solid body rotation. The angular velocity is higher closer to the centre and decreases to be at the lowest value at the outer piston bowl edge. When the injection pressure is increased, the deviation from solid body rotation increases due to spray effects.

Place, publisher, year, edition, pages
Detroit: Society of Automotive Engineers, 2012.
National Category
Energy Engineering
Identifiers
URN: urn:nbn:se:kth:diva-89095Scopus ID: 2-s2.0-84877175283OAI: oai:DiVA.org:kth-89095DiVA: diva2:502685
Conference
SAE 2012 World Congress, Detroit, USA, April 24-26, 2012
Note
QC 20120308Available from: 2012-04-24 Created: 2012-02-14 Last updated: 2013-12-10Bibliographically approved
In thesis
1. Flow measurements using combustion image velocimetry in diesel engines
Open this publication in new window or tab >>Flow measurements using combustion image velocimetry in diesel engines
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This work shows the in-cylinder airflow, and its effects on combustion and emissions, in a modern, heavy-duty diesel engine. The in-cylinder airflow is examined experimentally in an optical engine and the flow field inside the cylinder is quantified and shown during combustion, crank angle resolved. Cross-correlation on combustion pictures, with its natural light from black body radiation, has been done to calculate the vector field during the injection and after-oxidation period. In this work, this technique is called combustion image velocimetry (CIV). The quantified in-cylinder flow is compared with simulated data, calculated using the GT-POWER 1-D simulation tool, and combined with single-cylinder emission measurements at various in-cylinder airflows. The airflow in the single cylinder, characterised by swirl, tumble and turbulent intensity, was varied by using an active valve train (AVT), which allowed change in airflow during the engine’s operation. The same operation points were examined in the single-cylinder engine, optical engine and simulated in GT-POWER.

This work has shown that the in-cylinder airflow has a great impact on emissions and combustion in diesel engines, even at injection pressures up to 2,500 bar, with or without EGR and load up to 20-bar IMEP. Swirl is the strongest player to reduce soot emissions. Tumble has been shown to affect soot emissions negatively in combination with swirl. Tumble seems to offset the swirl centre and the offset is observed also after combustion in the optical engine tests. Injection pressure affects the swirl at late crank angle degrees during the after-oxidation part of the combustion. Higher injection pressure gives a higher measured swirl. This increase is thought to be created by the fuel spray flow interaction. The angular velocity in the centre of the piston bowl is significantly higher compared with the velocity in the outer region of the bowl. Higher injection pressure gives larger difference of the angular velocity.

Calculated swirl number from the CIV technique has also been compared with other calculation methods, GT-POWER and CFD-based method. The result from the CIV technique are in line with the other methods. CFD-based calculations, according to [62], has the best fit to the CIV method. The GT-POWER calculations shows the same trend at low swirl number, but at high swirl number the two methods differs significantly.

Place, publisher, year, edition, pages
Stockholm: Department of Machine design, Royal Institute of Technology, 2012. 80 p.
Series
Trita-MMK, ISSN 1400-1179 ; 2012:03
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-90817 (URN)
Presentation
2012-03-01, B319 Gladan, Brinellvägen in 83, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20120302

Available from: 2012-03-02 Created: 2012-02-29 Last updated: 2013-11-12Bibliographically approved
2. In-cylinder Flow Characterisation of Heavy Duty Diesel Engines Using Combustion Image Velocimetry
Open this publication in new window or tab >>In-cylinder Flow Characterisation of Heavy Duty Diesel Engines Using Combustion Image Velocimetry
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In-cylinder flow in diesel engines has a large impact on combustion and emission formation. In this work, the flow is characterised with a new measurement method called combustion image velocimetry (CIV). This technique is used to explain how airflow introduced during induction affects soot emissions and interacts with injection pressures up to 2500 bar. The CIV measurements enable flow analysis during the combustion and post-oxidation phases. The flow velocities inside the cylinder of a heavy duty optical engine, was measured with a crank angle (CA) resolution of 0.17° at injection pressures of 200–2500 bar and up to nearly full load (20 bar indicated mean effective pressure (IMEP)), were investigated with this method. The flow field results were combined with optical flame temperature and soot measurements, calculated according to Planck’s black body radiation theory.

At the high injection pressures typical of today’s production standard engines and with rotational in-cylinder flow about the cylinder axis, large deviations from solid-body rotational flow were observed during combustion and post-oxidation. The rotational flow, called swirl, was varied between swirl number (SN) 0.4 and 6.7. The deviation from solid-body rotational flow, which normally is an assumption made in swirling combustion systems, formed much higher angular rotational velocities of the air in the central region of the piston bowl than in the outer part of the bowl. This deviation has been shown to be a source for turbulent kinetic energy production, which has the possibility to influence soot burn-out during the post-oxidation period.

The measured CIV data was compared to Reynolds-averaged Navier–Stokes (RANS) CFD simulations, and the two methods produced similar results for the flow behaviour. This thesis describes the CIV method, which is closely related to particle image velocimetry (PIV). It was found in this work that the spatial plane in the cylinder evaluated with CIV corresponds to a mean depth of 3 mm from the piston bowl surface into the combustion chamber during combustion. During the post-oxidation phase of combustion, the measured spatial plane corresponds to a mean value of the total depth of the cylinder. The large bulk flow that contributes to the soot oxidation is thereby captured with the method and can successfully be analysed. The link between changes in in-cylinder flow and emissions is examined in this work.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. vi, 97 p.
Series
TRITA-MMK, ISSN 1400-1179 ; 2013:17
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-136978 (URN)978-91-7501-963-5 (ISBN)
Public defence
2014-01-15, Q1, Osquldasväg 4, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131210

Available from: 2013-12-10 Created: 2013-12-10 Last updated: 2014-01-20Bibliographically approved

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