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A numerical study was carried out to investigate combustion characteristics of a dual-fuel gas diesel engine, using a multi-dimensional model combined with detailed chemical kinetics, including 43 chemical species and 173 elementary reactions. In calculations, the effects of initial temperature, EGR ratios on ignition, and combustion were examined. The results indicated EGR combined with intake preheating can favorably reduced NOx and THC emissions simultaneously. This can be explained by the fact that combustion mechanism is changed from flame propagation to HCCl like combustion.

An experimental study has been made of a single cylinder, direct-injection diesel engine having a re-entrant combustion chamber designed to enhance combustion so as to reduce exhaust emissions. Special emphasis has been placed on controlling the inert gas concentration in the localized fuel-air mixture to lower combustion gas temperatures, thereby reduce exhaust NOx emission. For this specific purpose, an exhaust gas recirculation (EGR) system, which has been widely used in gasoline engines, was applied to the DI diesel engine to control the intake inert gas concentration. In addition, supercharging and increasing fuel injection pressure prevent the deterioration of smoke and unburned hydrocarbons and improve fuel economy, as well.

Dual-fuel operation of a direct-injection diesel engine with natural gas fuel can yield a high thermal efficiency almost comparable to the diesel operation at higher loads. The dual-fuel operation, however, at lower loads inevitably suffers from lower thermal efficiency and higher unburned fuel. To improve this problem, engine tests were carried out on a variety of engine parameters including diesel fuel injection timing advance, intake throttling and hot and cooled exhaust gas recirculation (EGR). It was found that diesel injection timing advance gave little improvement in thermal efficiency and increased NOx. Intake throttling promoted better combustion and shortened its duration with a consequent improvement in efficiency at higher natural gas fractions. Hot EGR raised thermal efficiency, reduced smoke levels, and maintained low NOx levels. Cooled EGR reduced NOx emissions but lowered thermal efficiency.

An experimental study was conducted to determine combustion and exhaust emissions characteristics in an automotive direct-injection diesel engine dual-fueled with natural gas with the objective of improving exhaust emissions and thermal efficiency. Dual-fuel operation can yield a high thermal efficiency almost comparable to the diesel operation and very low smoke at higher loads. However, NOx cannot be reduced by dual-fueling. On the other hand, at lower loads, a dual-fueled engine inevitably suffers from lower thermal efficiency and higher unburned fuel. To resolve these problems, the effects of exhaust gas recirculation (EGR) were investigated. The results show that in dual-fuel operation, hot EGR can improve thermal efficiency and reduce unburned fuel emission at lower loads, While cooled EGR can considerably reduce NOx at higher loads. A Pt oxidation catalyst can be used for additional reduction in unburned fuel emitted due to dual-fueling.

An experimental and numerical study has been conducted on the emission and reduction of HCHO (formaldehyde) and other pollutants formed in the cylinder of a direct-injection diesel engine fueled by methanol. Engine tests were performed under a variety of intake conditions including throttling, heating, and EGR (exhaust gas recirculation) for the purpose of improving these emissions by changing gas compositions and combustion temperatures in the cylinder. Moreover, a detailed kinetics model was developed and applied to methanol combustion to investigate HCHO formation and the reduction mechanism influenced by associated elementary reactions and in-cylinder mixing.

The diesel smoke-depressant effects of 26 fuel additives marketed in Japan were tested with the cooperation of the Fuel and Lubricants Committee of the Japan Automobile Research Institute (JARI) and the Tokyo Metropolitan Office. These bench tests were carried out with a compound on several types of engines under the conditions of overload with heavy smoke; the four effective barium-containing additives were tested under further conditions which included improper engine maintenance, transient operation, and exhaust gas recirculation (EGR). The exhaust gas compositions and emitted barium solids were analyzed. Also over-the-road tests were performed with 12 city buses for two years.