Exploring the Effects of Second Injection Timing and Split Injection Ratio on the Combustion and Emission Characteristics in a Dual-Fuel Spark Ignition (DFSI) Engine

Owing to the increasingly stringent limits on vehicular gaseous and particulate emissions, the Dual-Fuel Spark Ignition (DFSI) engine with Gasoline Port Injection (GPI) plus Ethanol Direct Injection (EDI) strategy has drawn growing attention for its potential to reconcile thermal efficiency with clean combustion. In this research, the effects of the EDI second injection timing (tDI2) and the mass percentage of the second stage (δDI2) on the combustion, fuel economy and emission characteristics were investigated experimentally on a turbocharged DFSI engine with tDI2 swept from −300 °CA to −140 °CA and δDI2 varied from 30% to 70%. The results show that retarding tDI2 prolongs the ignition delay (θF) monotonically by 1.01 degrees, while the maximum cylinder pressure (PM) and maximum in-cylinder temperature (TM) decrease by 0.026 MPa and 37.46 K, respectively. Notably, BSFCE and brake-specific emissions of CO (ECO) respond non-monotonically to tDI2, attaining the optima under tDI2 = −220 °CA and tDI2 = −180 °CA, respectively. The brake-specific emissions of NOx (ENOx) remain within a narrow interval of 19.28 g/kWh to 21.10 g/kWh, whereas brake-specific emissions of HC (EHC) rise monotonically with retarded tDI2. However, the peak concentration of particle number (NP) rises up to 1.16 × 106 n/cm3 when tDI2 = −140 °CA, nearly three times the level under −300 °CA. By raising δDI2 from 30% to 70%, comparatively moderate variations are introduced in the in-cylinder thermodynamic state. In addition, ECO and ENOx fall to 31.33 g/kWh and 19.38 g/kWh, and the peak NP concentration decreases from 5.01 × 105 n/cm3 to 3.23 × 105 n/cm3 with the nucleation-mode-dominated distribution morphology preserved, at the expense of a modest EHC increment from 2.67 g/kWh to 3.62 g/kWh. These findings could help clarify the relationships between EDI boundary conditions, pollutant generation, and combustion performance, offering practical optimisation strategies and calibration guidance for DFSI engines with a GPI plus EDI strategy.