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The bore scope system can visualize not only diesel combustion in the research engine but also in actual diesel combustion in a multi-cylinder engine. The experimental engine has 2.0-liter displacement and has an external supercharger with up to two times the boost pressure of the naturally aspirated engine condition. The video camera used for this study is nac GX-1. From observed data under the several experimental conditions such as increased injection pressures, swirl ratios, and EGR rates, the flame temperature and KL factor were obtained by two-color method analysis. The diesel combustion processes are understood well by analyzing high-speed movies of the diesel flame motion and its temperature. NOx and smoke are mutually related to maximum flame temperature even in an engine with low NOx emission and lean smoke. It is reconfirmed as necessary to maintain a maximum flame temperature of 2,000–2,200K to reduce emissions of NOx and smoke simultaneously.

Thermodynamic characteristics of premixed compression ignition combustions were clarified quantitatively by heat balance estimation. Heat balance was calculated from temperature, mole fractions of intake and exhaust gases, mass and properties of fuels. Heat balance estimation was conducted for three types of combustion; a conventional diesel combustion, a homogeneous charge compression ignition (HCCI) combustion; fuel is provided and mixed with air in an intake pipe in this case, and an extremely early injection type PREmixed lean DIesel Combustion (PREDIC). The results show that EGR should be applied for premixed compression ignition combustion to complete combustion at lower load conditions and to control ignition timing at higher load conditions. With an application of EGR, both HCCI and PREDIC showed low heat loss characteristics at lower load conditions up to 1/2 load.

Premixed lean Diesel Combustion (PREDIC) provides low NOx, however, it has some challenges associated with high HC emission, high fuel consumption and difficult timing control of compression ignition. To solve these problems, an impingement spray system with two injectors was tested to obtain positional controllability and larger volume of the air-fuel mixture formation in our previous study. The positional controllability means mainly the fuel mixture formation is the center of the cylinder with some space from the cylinder wall. The larger volume means the fuel mixture formation is leaner air-fuel mixture than that of free spray, which results in a possibility of higher thermal efficiency. Thus, the impingement spray system has a possibility of HC reduction and fuel consumption improvement in PREDIC.

HCCI (Homogeneous Charge Compression Ignition) combustion results in very low NOx emissions, however, it is not without problems. One of them is that the heavy load operation range is limited by knock, due to an exceptionally high heat release rate. Knock increases the heat loss to the cylinder walls and piston, reducing thermal efficiency. To help solve these problems, direct (in-cylinder) water injection has been suggested to lower the local temperatures that seem to cause knock in HCCI. Water injection was adapted in an HCCI engine fueled with DME and Propane. Results showed that the indicated thermal efficiency was improved by about 2% (λ = 3.0, NA), and the operation range was expanded from 460kPa to 700kPa (NA) maintaining a low NOx level.

As a technology required for future commercial heavy-duty diesel engines, this study reexamines the potential of the multiple injection strategy for improving the thermal efficiency while maintaining low engine-out exhaust emissions with a high EGR rate of more than 50% and high boost pressure of 276.3 kPa abs under medium load conditions. The experiments were conducted with a single cylinder research engine. The engine was operated at BMEP of 0.8 MPa at a medium speed. Using multiple injections, the temporal and spatial in-cylinder temperature distribution was changed to investigate the effect on fuel consumption and exhaust emissions. The results showed that the multiple injection strategy combined with higher EGR rate could improve fuel consumption by about 3% due to the reduction of heat loss from the wall.

For reducing exhaust emissions of heavy-duty diesel engines, the authors made an experimental study of diesel combustion using a single cylinder engine. The engine performance and exhaust emissions have been measured using a wide range and high EGR rate under the conditions of high boost intake pressure. The engine test cell has been equipped the external supercharger that is able to raise the boost pressure to 500 kPa, and also equipped the EGR system to increase the EGR rate until 50% under the 500 kPa boost condition. In various test conditions of load and engine speeds the authors have obtained the results, that is, NOx has been reduced drastically without increasing Particulate Matter (PM).

The purpose of this study is to improve low-temperature flow-properties of biodiesel fuels (BDF) by blending with ethanol and to analyze the combustion characteristics in a diesel engine fueled with BDF/ethanol blended fuel. Because ethanol has a lower solidifying temperature, higher oxygen content, lower cetane number, and higher volatility than BDF, ethanol blending would have a large effect on cold flow performance, mixture formation, ignition, combustion, and exhaust emissions. The engine experiments in the study were performed with a diesel engine and blends of BDF and ethanol at different blending ratios. The cold flow performance of the blended fuels was evaluated by determining the fuel cloud point. The experimental results show that the ethanol blending lowers the cloud point of the blended fuel and significantly reduces smoke emissions from the engine without deteriorating other emissions or thermal efficiency.

To find more effective lean mixture preparation methods for smokeless and low NOx combustion, a numerical study of the effects of in-cylinder flow field before injection on mixture formation in a premixed compression ignition engine was conducted. Premixed compression ignition combustion is a very attractive method to reduce both NOx and soot emissions, but it still has some problems, such as high HC and CO emissions. In case of early direct injection, it is important to avoid wall wetting by spray impingement, which can cause higher HC and CO emissions. Since it is not easy to examine the effects of initial flow and injection parameters on mixture formation over the wide range by practical engine tests, a computer program named “GTT (Generalized Tank and Tube)” code was used to simulate the in-cylinder phenomena before autoignition.