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Combustion of premixed lean mixture which arises from multi-point ignition is very promising and necessary for achieving both higher efficiency and lower nitrogen oxide (NOx) emission. A Premixed-Charge Compression Ignition (PCCI) engine has been manufactured experimentally and evaluated in terms of fuel economy and NOx reduction. The PCCI engine manufactured is a single cylinder engine with inlet port injection of gasoline, and has a compression ratio of 17.4. The PCCI engine operates stably in the air-fuel ratio range of 33-44. In the PCCI engine, spontaneous ignition occurs at unspecified points as it does in diesel engines. The flame then develops rapidly throughout the combustion chamber. Under conditions of stable combustion, the PCCI engine achieves equivalent fuel economy and much lower NOx emission compared with diesel engines. Furthermore, the effects of intake air heating and supercharging on extending the range of stable combustion have been examined.

The reduction of NOx without the degradation of soot emission and/or fuel economy necessitates the grasp of the in-cylinder temperature behavior, since NOx is mainly generated within localized high-temperature regions in the cylinder of diesel engines. Therefore, the in-cylinder temperature distributions were measured by the two-color method. This temperature distribution measurement enabled the characterization of the NOx reduction mechanisms for two methods by injection control; injection timing retard and pilot injection. Injection timing retard lowers the temperature throughout the combustion chamber, where NOx is reduced by controlling the spatial development of localized high-temperature regions. On the other hand, pilot injection delays the development of high-temperature regions. NOx is reduced by controlling the temporal development of the localized high-temperature regions. Furthermore, a novel method to reduce NOx has been proposed.

A new combustion system for a small direct injection diesel engine has been developed, with a spheroidal cavity and swirl nozzle characterized by weak spray penetration and wide spray angle. This system is intended to realize airborne mixture formation and good combustion processes over wide operating ranges. In-cylinder observations of the system reveal that droplets are easily bent in the direction of air movement, ignition occurs near the spray tip, and the flame is hard to envelop the spray. In a single cylinder engine of 460 cm3 swept volume, the system realizes active diffusion burning and mild premixed burning despite of long ignition delay, and provides low fuel consumption and low smoke emission, especially at low speeds. The above results show a promise to construct a new combustion system, different from conventional combustion systems.