Choongsik Bae

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.

Detailed Characterization of Morphology and Dimensions of Diesel Particulates via Thermophoretic Sampling

This project is supported by the Office of Heavy Vehicle Technologies of the U.S. Department of Energy. The constant support of Dr. Sidney Diamond is greatly appreciated. Authors also thank Mr. Gregory Hillman for his dedication to engine dynamometer operations and Dr. Russell Cook for his valuable advice on microscopy.

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.

Frictional modes of barrel shaped piston rings under flooded lubrication

A friction force measurement system using the floating liner method was developed to study the frictional behavior of piston rings. The measurement system was designed to control the effect of the secondary piston motion and to control temperatures of the cylinder wall and oil. The friction force between the barrel shaped piston ring and the cylinder liner was measured under flooded oil supply conditions. The measured friction forces were classified into five frictional modes with regard to the combination of predominant lubrication regimes (boundary, mixed and hydrodynamic lubrication) and stroke regions (mid-stroke and dead centers). The modes were identified on a Stribeck diagram, where the friction coefficients were evaluated both at mid-stroke and at the dead centers.

Effect of Nozzle Geometry on the Common-Rail Diesel Spray

The authors would like to acknowledge the support of National Research Laboratory scheme, Korean government.

Initial development of non-evaporating diesel sprays in common-rail injection systems

Abstract Transient nature of common-rail diesel sprays was characterized at the initial stage of fuel injection, the effects of injection rate being taken into account. Direct photography and the shadowgraph technique were used to obtain macroscopic spray images at several different experimental conditions. Highly versatile image processing software was developed to analyse the spray images. In the analysis, spray penetration, dispersion and spray angles were defined and accurately measured by the software with consistency. The effects of rail pressure and ambient gas density upon spray behaviours were discussed in detail. Based on the measured injection rates, the asymptotic relations of spray penetration were found at early and late stages of injection. The changes in spray volume were correlated with the measured injection rates. It was found that the initial spray penetration correlates well with the amount of fuel injected. The increase in injection rate during the injection period was observed to induce a smaller spray angle.

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.

The structure of a break-up zone in the transient diesel spray of a valve-covered orifice nozzle

Abstract A break-up zone in the diesel spray of a valve-covered orifice nozzle was investigated to observe the effect of the injection rate on the spray structure and to obtain physical insight into the development of a transient diesel spray. The surface shape and internal structure of the diesel spray from a common-rail injection system were visualized with high spatial and temporal resolution under atmospheric ambient conditions. During the injection period, common-rail pressures of 39.5 MPa and 112 MPa were used to obtain highly magnified spray images from the nozzle exit to about 260 nozzle diameters downstream. Before the atomization break-up regime appeared, a short transition period was observed, while ligaments were formed on the disturbed liquid column surface. During atomization, the spray was surrounded by these short fine ligaments, which were arranged and bent towards the direction of the spray penetration. The internal structure of the break-up zone consisted of complex entangled ligaments and dispersed liquid drops. The break-up process occurred simultaneously at the spray surface and the core. The entrained ambient air that penetrated through the crevices of the densely packed ligaments seemed to stretch the coherent structure and to carry the small droplets from the surface of the spray. The collapse of a cavitation bubble might have caused the ligaments to form near the nozzle exit at the core of the spray, although details of the process remain to be identified. Using image-processing techniques, the sizes of the liquid drops downstream from the spray were measured. The average size of the drops decreased as the injection rate decreased and, as expected, a higher common-rail pressure produced smaller droplets.