Combustion Robustness Characterization of Gasoline and E85 for Startability in a Direct Injection Spark-Ignition Engine 2012-01-1073

An experimental study and analysis was conducted to investigate cold start robustness of an ethanol flex-fuel spark ignition (SI) direct injection (DI) engine. Cold starting with ethanol fuel blends is a known challenge due to the fuel characteristics. The program was performed to investigate strategies to reduce the enrichment requirements for the first firing cycle during a cold start. In this study a single-cylinder SIDI research engine was used to investigate gasoline and E85 fuels which were tested with three piston configurations (CR11F, CR11B, CR15.5B - which includes changes in compression ratio and piston geometry), at three intake cam positions (95, 110, 125 °aTDC), and two fuel pressures (low: 0.4 MPa and high: 3.0 MPa) at 25°C±1°C engine and air temperature, for the first cycle of an engine start. The goal of this work was to simulate, test and investigate the engine's initial combustion cycle (1st fired cycle) including engine speed, manifold pressure, fuel pressure and ambient temperature (FTP testing temperature). The results revealed several trends with respect to hardware and fuel interactions: for gasoline, a bowl piston enabled lower injected equivalence ratio (φ) starts, while it had lower IMEP compared to flat top piston for the same equivalence ratio (φ). For E85, increasing the bowl piston compression ratio (CR) reduced the IMEP for the same φ. The flattop piston seemed to prefer a constant intake valve position across different fuels, thus may be considered a good flex-fuel piston. The higher CR bowl piston did not show a benefit at low fuel pressure starts, but showed significant improvements in high fuel pressure starts.