The Performance, Combustion and Emissions of Mechanical Supercharging Modifications on a High-Speed Spark Ignition Engine

Currently, high-speed gasoline engines are increasingly focusing on miniaturization and efficiency. Compared with turbocharging, mechanical supercharging is undoubtedly the more suitable technical approach for small, high-speed gasoline engines. To clarify the influence of the proposed supercharging approach on power, thermal efficiency and emissions within a broad range of engine speeds, this study designed two supercharging schemes (with different supercharger/crankshaft transmission ratios), and conducted bench tests comparing with the original engine. The results showed that the boost effect was more pronounced under medium load conditions. At full load, the high-speed supercharging scheme (94.5/86 ratio) on average improved torque by 10.8%, while the low-speed boost mode (86/61 ratio) only took effect after 5500 rpm. But at 60% load, 94.5/86 and 86/61, respectively, improved torque by 24.4% and 11.7%; thermal efficiencies of both supercharging schemes were almost the same and higher than that of the original operation by 0.8%; thus, the specific fuel consumption was reduced, on average, by ~9.5%. After boosting, the ignition phase was delayed due to the knock limit, but the high cylinder temperature promoted the recovery of the combustion rate in the later stage. In terms of emissions, NOx increased by 28% with the 94.5/86 scheme, while it decreased very slightly with the 86/61 scheme. CO rose by 3.7% under the 94.5/86 scheme, while it almost did not change under the 86/61 scheme operation, and HC increased by 13% and decreased by 21%, respectively, under the high and low boosting schemes. In conclusion, our proposed supercharging approach improved power and thermal efficiency and afforded a compromise emission effect. This study has revealed the performance trade-off rules of different boosting modes, which can provide important theoretical and technical support for the mechanical supercharging modification of high-speed gasoline engines.