Comprehensive Study on Two-Stage Combustion for the Reduction of Soot and NOx Emissions from DI Diesel Engines
The paper presents comprehensive results of series of works on two-stage combustion which has been conducted by the author and his colleagues. The concept of the two-stage combustion process is that the primary combustion is made fuel-rich to reduce NOx and then the increased particulate is oxidized by strong turbulence generated during the secondary combustion. The strong turbulence was generated by jet of burned gas from an auxiliary chamber (termed CCD) installed in the cylinder head. Experimental result of simple turbulence introduction showed that turbulence induced during the main combustion reduced smoke remarkably without increasing NOx. With these characteristics in mind, it was attempted to reduce NOx, particulate, and fuel consumption simultaneously by the two-stage combustion concept, together with EGR, combustion chamber modification, and fumigation. The results showed that NOx was reduced to one third of the base engine while maintaining fuel consumption and particulate emissions. However the favorable result was limited to the range other than the full load due to increased smoke. As the combustion process is strongly dependent on quick mixing of fuel and air, analysis was made on the major parameters controlling the mixing strength, showing a dimensionless mixing parameter. An optical analysis of the combustion process was conducted with a constant volume combustion bomb, which equips a jet-generating cell. The result showed soot disappearance process and its difference by jet locations relative to the main nozzle.
Key Words: Diesel engine, Combustion, NOx, Smoke, Turbulence, Two-stage combustion, Jet
Reduction in smoke and NOx emitted from diesel engines is a major concern in engine research. The authors have been reported significant reductions of particulate emissions by generating strong turbulence during the combustion process, and also showed NOx reductions with a two-stage combustion concept; i.e. the primary combustion is made fuel-rich to reduce NOx and then the increased particulate is oxidized by strong turbulence generated during the secondary combustion. This paper presents comprehensive results of these series of works as a review of the research. The strong turbulence was generated by injecting a small amount of fuel into an auxiliary chamber installed in the cylinder head during the main combustion.
In 1986, when reduction of smoke became a major topic in automotive diesel engine research due to proposal of particulate emission regulation, the author and his colleagues started trials to examine turbulence effect on the reduction of smoke emitted from diesel engines. Most of the research at that time was increasing turbulence by intake process and combustion chamber configurations, and these trials generally had stronger turbulence in compression stroke rather than during combustion, accompanied by increasing NOx emissions. Thus the author and his colleagues examined to generate turbulence during combustion process by the system mentioned above.
Prior to this work, Kamimoto et al. attempted to adapt an air cell to a direct injection diesel engine in 1984 to activate the oxidation of soot . The result was insignificant for the reduction of smoke due to insufficient stringent of the jet from the air cell. The author's system is somewhat resemble to the air cell, but made it possible to control timing and strength of the turbulent jet compared to the air cell. The result was significant in smoke reduction, and the authors first presented the result at the Japan IC engine symposium, Tokyo in 1988 . This report was presented in English at the CIMAC congress in 1989  and at SAE in 1992 with additional results . Following this, Yamaura et al. conducted basic research to determine the effects of turbulence on smoke reduction, by injecting various gases into the combustion chamber . They showed that the kinetic energy of the gas jet and the kind of gas are important factors in the smoke reduction. As more practical system with single injection nozzle, Kawazoe et al. reported a plunger type air-jet generator in 1990 , where the plunger is in an air cell and compresses air with mechanical system to generate a turbulent jet. The same group at Toyota Central Research, Nagano et al., reported a spring-accumulated air-jet generator (SAAG) to counter the power loss with the plunger type air-jet generator in 1991 . Here a plunger with a spring is actuated by high pressure gas from the main chamber, and as the piston goes down, the plunger pushes air out into the main chamber. The author here and his colleagues continued research on the high turbulence system, and presented additional results with details of the smoke reduction mechanism in 1993 .
Then, using the system with a high degree of freedom in the control of turbulence, two stage combustion concept was proposed and examined to reduce NOx, particulate, and fuel consumption simultaneously by forming a fuel rich mixture at the initial combustion stage to prevent NOx formation, and by inducing strong turbulence at the later stage of combustion;  in 1993 and  in 1995. The result showed 50% NOx reduction without increasing smoke, indicating possibility of the two stage combustion. In order to realize further emission reduction, it was attempted to combine the two-stage combustion with EGR, combustion chamber modification, and fumigation. The results showed that NOx was reduced to one third of the base engine while maintaining fuel consumption and particulate emissions;  in 1996 and  in 1999. However the favorable result was limited to the range other than the full load due to increased smoke, and this is the limit of the emission improvement with this system at the present moment.
* Division of Mechanical Science, Hokkaido University
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