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Isolating the Effects of EGR on HCCI Heat-Release Rates and NOX Emissions

SAE International Journal of Engines

Sandia National Laboratories-John E. Dec, Magnus Sjöberg, Wontae Hwang
  • Journal Article
  • 2009-01-2665
Published 2009-11-02 by SAE International in United States
High-load HCCI operation is typically limited by rapid pressure-rise rates (PRR) and engine knock caused by an overly rapid heat-release rate (HRR). Exhaust gas recirculation (EGR) is commonly used in HCCI engines, and it is often stated in the literature that charge dilution with EGR (or high levels of retained residuals) is beneficial for reducing the PRR to allow higher loads without knock. However, EGR/retained-residuals affect other operating parameters such as combustion phasing, which can in turn influence the PRR independently from any effect of the EGR gases themselves. Because of the multiple effects of EGR, its direct benefit for reducing the PRR is not well understood.In this work, the effects of EGR on the PRR were isolated by controlling the combustion phasing independently from the EGR addition by adjusting the intake temperature. The experiments were conducted using gasoline as the fuel at a 1200 rpm operating condition. EGR levels were increased from zero up to the amount required to replace all excess air with combustion products (i.e. stoichiometric intake) at a relatively high-load for…
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Characterizing the Development of Thermal Stratification in an HCCI Engine Using Planar-Imaging Thermometry

SAE International Journal of Engines

Sandia National Laboratories-John E. Dec, Wontae Hwang
  • Journal Article
  • 2009-01-0650
Published 2009-04-20 by SAE International in United States
A planar temperature imaging diagnostic has been developed and applied to an investigation of naturally occurring thermal stratification in an HCCI engine. Natural thermal stratification is critical for high-load HCCI operation because it slows the combustion heat release; however, little is known about its development or distribution. A tracer-based single-line PLIF imaging technique was selected for its good precision and simplicity. Temperature-map images were derived from the PLIF images, based on the temperature sensitivity of the fluorescence signal of the toluene tracer added to the fuel. A well premixed intake charge assured that variations in fuel/air mixture did not affect the signal. Measurements were made in a single-cylinder optically accessible HCCI research engine (displacement = 0.98 liters) at a typical 1200 rpm operating condition. Since natural thermal stratification develops prior to autoignition, all measurements were made for motored operation. Calibrations were performed in-situ, by varying the intake temperature and pressure over a wide range. Although the absolute accuracy is limited by the pressure-derived temperatures used for calibration, an uncertainty analysis shows that the precision of…
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Detailed HCCI Exhaust Speciation and the Sources of Hydrocarbon and Oxygenated Hydrocarbon Emissions

SAE International Journal of Fuels and Lubricants

Lawrence Livermore National Laboratory-M. Lee Davisson, Roald N. Leif
Sandia National Laboratories-John E. Dec, Magnus Sjöberg, Wontae Hwang
  • Journal Article
  • 2008-01-0053
Published 2008-04-14 by SAE International in United States
Detailed exhaust speciation measurements were made on an HCCI engine fueled with iso-octane over a range of fueling rates, and over a range of fuel-stratification levels. Fully premixed fueling was used for the fueling sweep. This sweep extended from a fuel/air equivalence ratio (ϕ) of 0.28, which is sufficiently high to achieve a combustion efficiency of 96%, down to a below-idle fueling rate of ϕ = 0.08, with a combustion efficiency of only 55%. The stratification sweep was conducted at an idle fueling rate, using an 8-hole GDI injector to vary stratification from well-mixed conditions for an early start of injection (SOI) (40°CA) to highly stratified conditions for an SOI well up the compression stroke (325°CA, 35°bTDC-compression). The engine speed was 1200 rpm.At each operating condition, exhaust samples were collected and analyzed by GC-FID for the C1 and C2 hydrocarbon (HC) species and by GC-MS for all other species except formaldehyde and acetaldehyde. These two species were analyzed using high-performance liquid chromatography. In addition, standard emissions-bench exhaust analysis equipment was used to measure total HC,…
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Fuel Stratification for Low-Load HCCI Combustion: Performance & Fuel-PLIF Measurements

Sandia National Laboratories-Wontae Hwang, John E. Dec, Magnus Sjöberg
Published 2007-10-29 by SAE International in United States
Fuel stratification has been investigated as a means of improving the low-load combustion efficiency in an HCCI engine. Several stratification techniques were examined: different GDI injectors, increased swirl, and changes in injection pressure, to determine which parameters are effective for improving the combustion efficiency while maintaining NOx emissions below U.S. 2010 limits. Performance and emission measurements were obtained in an all-metal engine. Corresponding fuel distribution measurements were made with fuel PLIF imaging in a matching optically accessible engine. The fuel used was iso-octane, which is a good surrogate for gasoline.For an idle fueling rate (ϕ = 0.12), combustion efficiency was improved substantially, from 64% to 89% at the NOx limit, using delayed fuel injection with a hollow-cone injector at an injection pressure of 120 bar. Relative to this base case, changing to an 8-hole injector provided the single largest improvement, increasing combustion efficiency to 92%. The effects of swirl varied with injector type, but increased injection pressure was beneficial for both injectors. The highest combustion efficiency of 92.5% at the NOx limit was achieved with…
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Thermodynamic and Chemical Effects of EGR and Its Constituents on HCCI Autoignition

Sandia National Laboratories-Magnus Sjöberg, John E. Dec, Wontae Hwang
Published 2007-04-16 by SAE International in United States
EGR can be used beneficially to control combustion phasing in HCCI engines. To better understand the function of EGR, this study experimentally investigates the thermodynamic and chemical effects of real EGR, simulated EGR, dry EGR, and individual EGR constituents (N2, CO2, and H2O) on the autoignition processes. This was done for gasoline and various PRF blends. The data show that addition of real EGR retards the autoignition timing for all fuels. However, the amount of retard is dependent on the specific fuel type. This can be explained by identifying and quantifying the various underlying mechanisms, which are: 1) Thermodynamic cooling effect due to increased specific-heat capacity, 2) [O2] reduction effect, 3) Enhancement of autoignition due to the presence of H2O, 4) Enhancement or suppression of autoignition due to the presence of trace species such as unburned or partially-oxidized hydrocarbons.The results show that the single-stage ignition fuels iso-octane and gasoline are more sensitive to the cooling effect of EGR, compared to the two-stage ignition fuels PRF80 and PRF60. On the other hand, the two-stage ignition fuels…
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An Investigation of Thermal Stratification in HCCI Engines Using Chemiluminescence Imaging

Sandia National Laboratories-John E. Dec, Wontae Hwang, Magnus Sjöberg
Published 2006-04-03 by SAE International in United States
Chemiluminescence imaging has been applied to investigate the naturally occurring charge stratification in an HCCI engine. This stratification slows the pressure-rise rate (PRR) during combustion, making it critical to the high-load operating limit of these engines. Experiments were conducted in a single-cylinder HCCI engine modified with windows in the combustion chamber for optical access. Using this engine, chemiluminescence images were obtained from three different view angles. These included both single-shot images with intensified CCD cameras and high-speed (20kHz) sequences with an intensified CMOS video camera. The engine was fueled with iso-octane, which has been shown to be a reasonable surrogate for gasoline and exhibits only single-stage ignition at these naturally aspirated conditions.The chemiluminescence images show that the HCCI combustion is not homogeneous but has a strong turbulent structure even when the fuel and air are fully premixed prior to intake. Images were acquired with three fueling strategies to change the possible sources of charge stratification. The results indicate that the inhomogeneities are caused primarily by thermal stratification due to heat transfer during compression, combined with…
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