| 1. |
- Berntsson, Andreas, 1978, et al.
(författare)
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A LIF-study of OH in the Negative Valve Overlap of a Spark-assisted HCCI Combustion Engine
- 2008
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Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627.
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Tidskriftsartikel (refereegranskat)abstract
- Future requirements for emission reduction from combustionengines in ground vehicles might be met byusing the HCCI combustion concept. In this study,negative valve overlap (NVO) and low lift, short duration,camshaft profiles, were used to initiate HCCIcombustion by increasing the internal exhaust gas recirculation(EGR) and thus retaining sufficient thermalenergy for chemical reactions to occur when a pilot injectionwas introduced prior to TDC, during the NVO.One of the crucial parameters to control in HCCI combustionis the combustion phasing and one way of doingthis is to vary the relative ratio of fuel injected inpilot and main injections. The combustion phasing isalso influenced by the total amount of fuel supplied tothe engine, the combustion phasing is thus affectedwhen the load is changed. This study focuses on thereactions that occur in the highly diluted environmentduring the NVO when load and pilot to main ratio arechanged.To monitor these reactions, planar laser-induced fluorescence(PLIF) from OH radicals was analyzed ina series of experiments with an optical single-cylinderengine, since these radicals are known to be associatedwith high temperature reactions. A series of experimentswas also performed using a multi-cylinderengine with varied NVO timings, which showed thatthe combustion phasing was influenced by both theratio between the pilot and main injection amountsand the total amount of fuel. Data acquired from correspondingoptical analysis showed the occurrenceof OH radicals (and thus high temperature reactions)during the NVO in all tested operating conditions. Theresults also indicate that the extent of the high temperaturereactions was influenced by both varied parameters,since decreasing the relative amount of the pilotinjection and/or increasing the total amount of fuel ledto larger amounts of OH radicals.
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| 2. |
- Berntsson, Andreas, 1978, et al.
(författare)
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LIF imaging of OH during the Negative Valve Overlap of a HCCI Combustion Engine
- 2007
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Ingår i: The Spark Ignition Engine of the Future Conference, Strasbourg - INSA 28/11 - 29/11 2007. ; :Code: R-2007-01-22, s. (9)-
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Konferensbidrag (refereegranskat)abstract
- Future requirements for emission reduction from combustion engines in ground vehicles might be met by using the HCCI combustion concept. In this concept a more or less homogenous air fuel mixture is compressed to auto-ignition. This gives a good fuel economy compared to a normal SI engine and it has a positive impact on exhaust emissions. In the study presented here negative valve overlap (NVO) was used to initiate HCCI combustion by increasing the exhaust gas recirculation (EGR) and thus retaining sufficient thermal energy to reach auto-ignition temperatures, and raising the temperature during the NVO sufficiently to allow reactions to occur when a pilot injection was made prior to the NVO. The focus of the investigation was on the resulting high temperature reactions.Since OH radicals are associated with high temperature reactions, the reactions were followed by monitoring planar laser-induced fluorescence (PLIF) of OH, using a dye laser and an intensified LaVision Dynamight camera. The presence of OH radicals detected during the negative valve overlap indicates that high temperature reactions do occur in the highly diluted environment of the trapped exhaust gases during the NVO. Reactions were identified from 20 CAD prior to TDC (during the NVO) and to around 60 CAD after TDC, with the intensity peak at about TDC.
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| 3. |
- Dahl, Daniel, 1982, et al.
(författare)
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Reducing pressure fluctuations at high loads by means of charge stratification in HCCI combustion with negative valve overlap
- 2009
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Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; 2009
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Tidskriftsartikel (refereegranskat)abstract
- Future demands for improvements in the fuel economy of gasoline passenger car engines will require the development and implementation of advanced combustion strategies, to replace, or combine with the conventional spark ignition strategy. One possible strategy is homogeneous charge compression ignition (HCCI) achieved using negative valve overlap (NVO). However, several issues need to be addressed before this combustion strategy can be fully implemented in a production vehicle, one being to increase the upper load limit. One constraint at high loads is the combustion becoming too rapid, leading to excessive pressure-rise rates and large pressure fluctuations (ringing), causing noise. In this work, efforts were made to reduce these pressure fluctuations by using a late injection during the later part of the compression. A more appropriate acronym than HCCI for such combustion is SCCI (Stratified Charge Compression Ignition). The approach was evaluated in tests with a single-cylinder metal research engine and a single-cylinder optical engine. The latter was used to characterize the combustion in laser-based analyses including laser-induced florescence (LIF) determinations of fuel tracer, OH and CH 2O (formaldehyde) distributions. A high speed camera was also used for direct imaging of chemiluminescence. The effects of two main parameters were studied: the proportion of fuel injected late to create a stratified charge and the timing of the late injection. In addition, two fuels were used: a certification gasoline fuel and a blend of n-heptane, iso-octane and 3-pentanone. Both fuels were used in the metal engine for comparison. Use of a stratified charge allowed the maximum pressure-rise rates and ringing intensity to be reduced at the expense of increases in NOx and CO emissions, regardless of fuel type. Optical results indicated that both the fuel distribution and combustion were not homogenous.
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| 4. |
- Dahl, Daniel, 1982, et al.
(författare)
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The Origin of Pressure Waves in High Load HCCI Combustion: A High-Speed Video Analysis
- 2011
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 183:11, s. 1266-1281
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Tidskriftsartikel (refereegranskat)abstract
- Homogeneous charge compression ignition is an alternative combustion strategy for spark-ignited gasoline engines that improves engine efficiency and thus reduces CO2 emissions, which is crucial to meet targets set by legislation. However, this combustion strategy is limited to low loads, mainly due to pressure oscillations that arise if combustion is too rapid. The aim of the work described in this article was to record and identify these pressure oscillations and correlate them with the preceding combustion. This was performed using an image-intensified high-speed video camera with a sampling rate of 111 kHz filming inside the combustion chamber of an engine with optical access through a quartz piston. A strategy is described whereby video analysis is used to extract the acoustic modes resulting from combustion. In this work it was possible to detect four different acoustic modes. It is shown that the type and magnitude of these modes can be correlated to the size and position of combustion chamber zones with rapid combustion and the light intensity development (combustion speed) in these zones. It is also shown that the highest combustion rates occur in regions where combustion starts late.
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| 5. |
- Dahl, Daniel, 1982, et al.
(författare)
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The role of charge stratification for reducing ringing in gasoline engine homogeneous charge compression ignition combustion investigated by optical imaging
- 2013
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Ingår i: International Journal of Engine Research. - : SAGE Publications. - 1468-0874 .- 2041-3149. ; 14:5, s. 525-536
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Tidskriftsartikel (refereegranskat)abstract
- Homogeneous charge compression ignition offers the possibility to reduce the fuel consumption of gasoline passenger car engines. However, the combustion strategy is limited to low loads due to pressure oscillations at higher loads. A strategy for extending the homogeneous charge compression ignition load range is charge stratification, using, for example, late direct injection to prolong the combustion duration and reduce the rate of pressure rises and pressure oscillations. In this study, local temperatures and fuel concentrations near top dead centre in a gasoline engine operating in homogeneous charge compression ignition mode were measured using two-wavelength planar laser-induced fluorescence, and the following combustion was analysed using high-speed video to investigate the effects of fuel and temperature stratification on combustion in order to explain the ringing inhibiting effect of charge stratification for fuels displaying single-stage ignition. The extent of spatial distribution of combustion timing correlated well with the extent of fuel and temperature stratification. Furthermore, the gas was leaner and hotter in early igniting regions, while it was richer and colder in late igniting regions. The dampening effects of charge stratification on the combustion speed and pressure oscillations are probably due to rich conditions in the latest burning regions (where combustion is usually most intense) slowing down combustion, which explains why the strategy only works when the global air-to-fuel ratio is not excessively lean.
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