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Honda has developed a new hybrid system targeting the C and D segments that aims for the latest environmental performance, high fuel economy, and enhanced acceleration feeling in driving. The new engine to be applied to this new hybrid system has been developed with the goal of expanding the high thermal efficiency range, realizing the latest environmental performance, and high quietness. The new engine has adopted the Atkinson cycle and cooled exhaust gas recirculation (EGR) carried over from the previous model [1], and employed an in-cylinder direct fuel injection system with fuel injection pressure of 35 MPa. The combustion chamber and ports have been newly designed to match the fuel system changes. By realizing high-speed combustion, the engine realized a high compression ratio with the mechanical compression ratio of 13.9.

The goal of this research was to improve thermal efficiency under conditions of stoichiometric air-fuel ratio and 91 RON (Research Octane Number) gasoline fuel. Increasing compression ratio and dilution are effective means to increase the thermal efficiency of gasoline engines. Increased compression ratio is associated with issues such as slow combustion, increased cooling loss, and engine knocking. Against these challenges, a higher stroke-bore ratio (S/B ratio) and a lower effective compression ratio were tried as countermeasures. With respect to increased dilution, combustion of a high-EGR (Exhaust Gas Recirculation) was tried. High-energy ignition and optimized combustion chamber shape with high tumble port were tried as countermeasures against slow combustion and reduced ignitability due to a higher EGR rate.

This study examined a high-speed, high-powered diesel engine featuring a pent-roof combustion chamber and straight ports, with the objective of improving the specific power of the engine while minimizing any increase in the maximum cylinder pressure (Pmax). The market and contemporary society expect improvements in the driving performance of diesel-powered automobiles, and increased specific power so that engine displacement can be reduced, which will lessen CO2 emissions. When specific power is increased through conventional methods accompanied with a considerable increase in Pmax, the engine weight is increased and friction worsens. Therefore, the authors examined new technologies that would allow to minimize any increase in Pmax by raising the rated speed from the 4000 rpm of the baseline engine to 5000 rpm, while maintaining the BMEP of the baseline engine.