| 1. |
- Andrae, Johan C. G., et al.
(författare)
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Autoignition of toluene reference fuels at high pressures modeled with detailed chemical kinetics
- 2007
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 149:02-jan, s. 2-24
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Tidskriftsartikel (refereegranskat)abstract
- A detailed chemical kinetic model for the autoignition of toluene reference fuels (TRF) is presented. The toluene submechanism added to the Lawrence Livermore Primary Reference Fuel (PRF) mechanism was developed using recent shock tube autoignition delay time data under conditions relevant to HCCI combustion. For two-component fuels the model was validated against recent high-pressure shock tube autoignition delay time data for a mixture consisting of 35% n-heptane and 65% toluene by liquid volume. Important features of the autoignition of the mixture proved to be cross-acceleration effects, where hydroperoxy radicals produced during n-heptane oxidation dramatically increased the oxidation rate of toluene compared to the case when toluene alone was oxidized. Rate constants for the reaction of benzyl and hydroperoxyl radicals previously used in the modeling of the oxidation of toluene alone were untenably high for modeling of the mixture. To model both systems it was found necessary to use a lower rate and introduce an additional branching route in the reaction between benzyl radicals and O-2. Good agreement between experiments and predictions was found when the model was validated against shock tube autoignition delay data for gasoline surrogate fuels consisting of mixtures of 63-69% isooctane, 14-20% toluene, and 17% n-heptane by liquid volume. Cross reactions such as hydrogen abstractions between toluene and alkyl and alkylperoxy radicals and between the PRF were introduced for completion of chemical description. They were only of small importance for modeling autoignition delays from shock tube experiments, even at low temperatures. A single-zone engine model was used to evaluate how well the validated mechanism could capture autoignition behavior of toluene reference fuels in a homogeneous charge compression ignition (HCCI) engine. The model could qualitatively predict the experiments, except in the case with boosted intake pressure, where the initial temperature had to be increased significantly in order to predict the point of autoignition.
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| 2. |
- Andrae, Johan C. G., et al.
(författare)
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HCCI experiments with toluene reference fuels modeled by a semidetailed chemical kinetic model
- 2008
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 155:4, s. 696-712
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Tidskriftsartikel (refereegranskat)abstract
- A semidetailed mechanism (137 species and 633 reactions) and new experiments in a homogeneous charge conic pression ignition (HCCI) engine on the autoignition of toluene reference fuels are presented. Skeletal mechanisms for isooctane and n-heptane were added to a detailed toluene submechanism. The model shows generally good agreement with ignition delay times measured in a shock tube and a rapid compression machine and is sensitive to changes in temperature, pressure, and mixture strength. The addition of reactions involving the formation and destruction of benzylperoxide radical was crucial to modeling toluene shock tube data. Laminar burning velocities for benzene and toluene were well predicted by the model after some revision of the high-temperature chemistry. Moreover, laminar burning velocities of a real gasoline at 353 and 500 K Could be predicted by the model using a toluene reference fuel as a surrogate. The model also captures the experimentally observed differences in combustion phasing of toluene/n-heptane mixtures, compared to a primary reference fuel of the same research octane number, in HCCI engines as the intake pressure and temperature are changed. For high intake pressures and low intake temperatures, a sensitivity analysis at the moment of maximum heat release rate shows that the consumption of phenoxy radicals is rate-limiting when a toluene/n-heptane fuel is used, which makes this fuel more resistant to autoignition than the primary reference fuel. Typical CPU times encountered in zero-dimensional calculations were on the order of seconds and minutes in laminar flame speed calculations. Cross reactions between benzylperoxy radicals and n-heptane improved the model prediction,,; of shock tube experiments for phi = 1.0 and temperatures lower than 800 K for an n-heptane/toluene fuel mixture, but cross reactions had no influence on HCCI Simulations.
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| 3. |
- Kalghatgi, G. T., et al.
(författare)
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Autoignition quality of gasoline fuels in partially premixed combustion in diesel engines
- 2011
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 33, s. 3015-3021
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Tidskriftsartikel (refereegranskat)abstract
- A single-cylinder diesel engine has been run on gasolines of different octane numbers and on model fuels, mixtures of iso-octane, n-heptane and toluene, at different operating conditions. The autoignition quality of the fuel is best described by an Octane Index, OI = (1 - K) . RON + K . MON for fuels in the gasoline autoignition range where RON and MON are, respectively, the Research and Motor Octane numbers and K is an empirical constant which is measured to be negative. Hence for a given RON, a non-paraffinic fuel, of lower MON, will have higher OI and more resistance to autoignition. For a given operating condition, ignition delay increases non-linearly with OI and changes little over the autoignition range of practical diesel fuels. Heat release following the autoignition is influenced by the stratification which will increase as the time between the end of injection and start of combustion decreases and combustion phasing parameters such as Combustion Delay, the difference between the 50% burn time and the start of injection, become less correlated with fuel autoignition quality. Higher ignition delays facilitate premixed combustion in the diesel engine. If two fuels have similar combustion phasing at the same injection timing, their emissions performance is also similar. Hence a good surrogate for gasoline in partially premixed compression ignition engines is a mixture of toluene, iso-octane and n-heptane with the same RON and MON. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 4. |
- Kalghatgi, G. T., et al.
(författare)
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Surrogate fuels for premixed combustion in compression ignition engines
- 2011
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Ingår i: International Journal of Engine Research. - : SAGE Publications. - 1468-0874 .- 2041-3149. ; 12:5, s. 452-465
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Tidskriftsartikel (refereegranskat)abstract
- Simple surrogate fuels are needed to model practical fuels, which are complex mixtures of hydrocarbons. The surrogate fuel should match the combustion and emissions behaviour of the target fuel as much as possible. This paper presents experimental results using a wide range of fuels in both the gasoline and diesel auto-ignition range, but of different volatilities and compositions, in a single cylinder diesel engine. Premixed combustion in a compression ignition engine is defined, in this paper, to occur when the injection event is clearly separated from the combustion and the engine-out smoke is very low - below 0.05 FSN (filter smoke number). Under such circumstances, if the combustion phasing is matched for two fuels at a given operating condition and injection timing, the emissions are also comparable regardless of the differences in composition and volatility. For the experimental conditions considered, combustion phasing at a given operating condition and injection timing depends only on the octane index (OI), OI = (1-K)RON + KMON, where RON and MON are research and motor octane numbers and K is an empirical constant that depends on operating conditions. A mixture of iso-octane, n-heptane and toluene can be found to match the RON and MON of any practical gasoline and will be a very good surrogate for the gasoline since it will have the same OI. If the compression ratio is greater than 14, practical diesel fuels, with DCN (derived cetane number) between 40 and 60, will have comparable ignition delays to n-heptane, which is an adequate surrogate for such fuels. However, premixed combustion can be attained only at much lower loads at a given speed with diesel fuels compared to gasolines.
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| 5. |
- Kalghatgi, G. T., et al.
(författare)
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The nature of "superknock" and its origins in SI engines
- 2009
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Ingår i: Internal combustion engines. - : Institution of Mechanical Engineers. - 9781843346074 ; , s. 259-269
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Konferensbidrag (refereegranskat)abstract
- Extremely high knock intensities are observed occasionally in turbo charged spark ignition (SI) engines. Such events have been informally described as "Super knock" and are often associated with pre-ignition. Knock is initiated by auto ignition at one or more "hot spots". The mode of propagation of the resulting pressure wave depends on the propagation velocity of the auto ignitive front. When this becomes coupled with the acoustic wave, a localised detonation begins to develop, resulting in a very high rate of pressure rise. It is shown, through semi quantitative analysis including chemical kinetic calculations, that developing detonation becomes more likely when end-gas pressures and temperatures increase and might be the reason for "Super knock".
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| 6. |
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