Sanghoon Kook

The influence of charge dilution and injection timing on low-temperature diesel combustion and emissions

The effects of charge dilution on low-temperature diesel combustion and emissions were investigated in a small-bore single-cylinder diesel engine over a wide range of injection timing. The fresh air was diluted with additional N 2 and CO 2 , simulating 0 to 65% exhaust gas recirculation in an engine. Diluting the intake charge lowers the flame temperature T due to the reactant being replaced by inert gases with increased heat capacity. In addition, charge dilution is anticipated to influence the local charge equivalence ratio Φ prior to ignition due to the lower O 2 concentration and longer ignition delay periods. By influencing both Φ and T, charge dilution impacts the path representing the progress of the combustion process in the Φ-T plane, and offers the potential of avoiding both soot and NO x formation. In-cylinder pressure measurements, exhaust-gas emissions, and imaging of combustion luminosity were performed to clarify the path of the combustion process and the effects of charge dilution and injection timing on combustion and fuel conversion efficiency. Based on the findings, a postulated combustion process in the Φ-T plane is presented for different dilution levels and injection timings. Although the ignition delay increased with high dilution and early injection, the heat release analysis indicated that a large portion of the combustion and emissions formation processes was still dominated by the mixing-controlled phase rather than the premixed phase. Because of the incomplete premixing, and the need to mix a greater volume of charge with unbumed or partially-burned fuel to complete combustion, the diluted mixtures increased CO emissions. Injecting the fuel at earlier timings to extend the ignition delay helped alleviate this problem, but did not eliminate it. Fuel conversion efficiencies calculated for each dilution level and start of injection provide guidance as to the appropriate combustion phasing and practical levels of charge dilution for this low-temperature diesel combustion regime.

The effect of swirl ratio and fuel injection parameters on CO emission and fuel conversion efficiency for high-dilution, low-temperature combustion in an automotive diesel engine.

Support for this research was provided by the U.S. Department of Energy, Office of FreedomCAR and Vehicle Technologies. The research was performed at the Combustion Research Facility, Sandia National Laboratories, Livermore, California. Sandia is a multiprogram laboratory operated by Sandia Corporation,a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. The BK21 and Future Vehicle Technology Development Corps. of Korea supported Sanghoon Kooks visiting research. The authors express their appreciation to Mark Musculus and Lyle Pickett for providing the high speed camera and the Matlab source code to calculate the adiabatic flame temperature.

Combustion Control Using Two-Stage Diesel Fuel Injection in a Single-Cylinder PCCI Engine

The authors would like to appreciate the support by the national research laboratory scheme, Korea and in part by the Brain Korea 21 Project.

Effects of Multiple Injections in a HSDI Diesel Engine Equipped with Common Rail Injection System

The authors would like to appreciate the support of the National Research Laboratory scheme of Korea.

Fundamental spray and combustion measurements of soy methyl-ester biodiesel

Although biodiesel has begun to penetrate the fuel market, its effect on injection processes, combustion and emission formation under diesel engine conditions remains somewhat unclear. Typical exhaust measurements from engines running biodiesel indicate that particulate matter, carbon monoxide and unburnt hydrocarbons are decreased, whereas nitrogen oxide emissions tend to be increased. However, these observations are the result of complex interactions between physical and chemical processes occurring in the combustion chamber, for which understanding is still needed. To characterize and decouple the physical and chemical influences of biodiesel on spray mixing, ignition, combustion and soot formation, a soy methyl-ester (SME) biodiesel is injected into a constant-volume combustion facility under diesel-like operating conditions. A range of optical diagnostics is performed, comparing biodiesel to a conventional #2 diesel at the same injection and ambient conditions. Schlieren high-speed imaging shows virtually the same vapour-phase penetration for the two fuels, while simultaneous Mie-scatter imaging shows that the maximum liquid-phase penetration of biodiesel is higher than diesel. Differences in the liquid-phase penetration are expected because of the different boiling-point temperatures of the two fuels. However, the different liquid-phase penetration does not affect overall mixing rate and downstream vapour-phase penetration because each fuel spray has similar momentum and spreading angle. For the biodiesel and diesel samples used in this study, the ignition delay and lift-off length are only slightly less for biodiesel compared to diesel, consistent with the fuel cetane number (51 for biodiesel, 46 for diesel). Because of the similarity in lift-off length, the differences in equivalence ratio distribution at the lift-off length are mainly affected by the oxygen content of the fuels. For biodiesel, the equivalence ratio is reduced, which, along with the fuel molecular structure and oxygen content, significantly affects soot formation downstream. Spatially resolved soot volume fraction measurements obtained by combining line-of-sight laser extinction measurements with planar laser-induced incandescence imaging show that the soot concentration can be reduced by an order of magnitude for biodiesel. These integrated measurements of spray mixing, combustion and quantitative soot concentration provide new validation data for the development of computational fluid dynamics spray, combustion and soot formation models suitable for the latest biofuels.

Diesel-fuelled homogeneous charge compression ignition engine with optimized premixing strategies

Abstract The operation of a diesel-fuelled homogeneous charge compression ignition (HCCI) engine was studied in a single-cylinder, direct-injection diesel engine with regard to three key parameters: Spray penetration, time for premixing, and dilution of the premixed charge. The relationships between these parameters were clarified through spray measurements, flame imaging, and combustion analysis. The spray penetration was optimized by a small included angle to avoid wall impingement at low pressure and temperature in the cylinder. However, the hole diameter did not affect spray penetration. Sufficient time for premixing was realized by advanced injections earlier than 100 crank angle degrees (CAD) before top dead centre (BTDC) at 800 r/min. Dilution of the premixed charge to control ignition timing was investigated by adopting exhaust gas recirculation (EGR). The optimized premixing strategies of two-stage injection, with a small amount of the ignition-promoting fuel (1.5 mm3) - which was injected near TDC to assist the combustion of a premixed charge (10 mm3) - resulted in an indicated mean effective pressure of up to 250 kPa within 3 per cent fluctuation, along with a significant reduction in particle matter and nitrogen oxides emissions, while 46 per cent EGR rate was applied to the premixed charge with preheated intake air at 433 K.

Influence of fuel injection timing and pressure on in-flame soot particles in an automotive-size diesel engine.

The current understanding of soot particle morphology in diesel engines and their dependency on the fuel injection timing and pressure is limited to those sampled from the exhaust. In this study, a thermophoretic sampling and subsequent transmission electron microscope imaging were applied to the in-flame soot particles inside the cylinder of a working diesel engine for various fuel injection timings and pressures. The results show that the number count of soot particles per image decreases by more than 80% when the injection timing is retarded from -12 to -2 crank angle degrees after the top dead center. The late injection also results in over 90% reduction of the projection area of soot particles on the TEM image and the size of soot aggregates also become smaller. The primary particle size, however, is found to be insensitive to the variations in fuel injection timing. For injection pressure variations, both the size of primary particles and soot aggregates are found to decrease with increasing injection pressure, demonstrating the benefits of high injection velocity and momentum. Detailed analysis shows that the number count of soot particles per image increases with increasing injection pressure up to 130 MPa, primarily due to the increased small particle aggregates that are less than 40 nm in the radius of gyration. The fractal dimension shows an overall decrease with the increasing injection pressure. However, there is a case that the fractal dimension shows an unexpected increase between 100 and 130 MPa injection pressure. It is because the small aggregates with more compact and agglomerated structures outnumber the large aggregates with more stretched chain-like structures.

Two-stage Fuel Direct Injection in a Diesel Fuelled HCCI Engine

Two-stage fuel direct injection (DI) has the potential to expand the operating region and control the autoignition timing in a Diesel fuelled homogeneous charge compression ignition (HCCI) engine. In this work, to investigate the dual-injection HCCI combustion, a stochastic reactor model, based on a probability density function (PDF) approach, is utilized. A new wall-impingement sub-model is incorporated into the stochastic spray model for direct injection. The model is then validated against measurements for combustion parameters and emissions carried out on a four stroke HCCI engine. The initial results of our numerical simulation reveal that the two-stage injection is capable of triggering the charge ignition on account of locally rich fuel parcels under certain operating conditions, and consequently extending the HCCI operating range. Furthermore, both simulated and experimental results on the effect of second injection timing on combustion indicate that there exists an optimal second injection timing to gain maximum engine output work for a given fuel split ratio.

EFFECT OF MULTIPLE INJECTIONS ON FUEL-AIR MIXING AND SOOT FORMATION IN DIESEL COMBUSTION USING DIRECT FLAME VISUALIZATION AND CFD TECHNIQUES

The main objective of this study is to investigate the effect of pilot-, post- and multiple-fuel injection strategies on fuel-air mixing and emissions formation in diesel combustion, using a combination of experimental observations and Computational Fluid Dynamics (CFD) analysis. The experimental study was carried out on a single-cylinder optical direct-injection diesel engine equipped with a high pressure common rail fuel injection system. The experimental work was supported by CFD simulations on the single-cylinder engine in order to investigate the effect of multiple injections on mixture formation. The limitations of the soot formation model were identified through direct comparisons with experimental flame visualization.Copyright

Studying the Influence of Direct Injection on PCCI Combustion and Emissions at Engine Idle Condition Using Two dimensional CFD and Stochastic Reactor Model

A detailed chemical model was implemented in the KIVA3V two dimensional CFD code to investigate the effects of the spray cone angle and injection timing on the PCCI combustion process and emissions in an optical research diesel engine. A detailed chemical model for Primary Reference Fuel (PRF) consisting of 157 species and 1552 reactions was used to simulate diesel fuel chemistry. The model validation shows good agreement between the predicted and measured pressure and emissions data in the selected cases with various spray angles and injection timings. If the injection is retarded to -50° ATDC, the spray impingement at the edge of the piston corner with 100° injection angle was shown to enhance the mixing of air and fuel. The minimum fuel loss and more widely distributed fuel vapor contribute to improving combustion efficiency and lowering uHC and CO emissions in the engine idle condition. Finally, the coupling of CFD and multi-zone Stochastic Reactor Model (SRM) was demonstrated to show improvement in CO and uHC emissions prediction.