Comparison between gasoline direct injection and compressed natural gas port fuel injection under maximum load condition

Gasoline spark ignition (SI) engines equipped with turbocharger systems have recently become popular owing to their high operating range compared to naturally aspirated SI engines. Under the maximum load condition, spark advance timing is retarded from the maximum brake torque timing to avoid knocking problems. Moreover, the fuel-enrichment method is adjusted to lower the exhaust gas temperature. These two operating strategies tend to worsen the engine's thermal efficiency. In this study, compressed natural gas (CNG) was supplied to a turbocharged SI engine instead of gasoline under the maximum load condition at various engine speeds. The degree of thermal efficiency improvement with the application of CNG was investigated. Further, the limitations of supplying CNG to turbocharged SI engines for better performance were discussed. The results indicate that CNG can improve thermal efficiency under the maximum load condition by advancing spark timing, because it has a higher octane number, although the maximum loads were 4–23% lower than those of gasoline direct injection (GDI) engines due to the port injection system. Furthermore, although engine-out nitrogen oxide (NO_x) emissions from CNG port fuel injection (PFI) combustion were higher than those from GDI combustion, lower CO₂ emissions with higher combustion efficiency could be achieved.