Charge Cooling Effects on Knock Limits in SI DI Engines Using Gasoline/Ethanol Blends: Part 1-Quantifying Charge Cooling 2012-01-1275

Gasoline/ethanol fuel blends have significant synergies with Spark Ignited Direct Injected (SI DI) engines. The higher latent heat of vaporization of ethanol increases charge cooling due to fuel evaporation and thus improves knock onset limits and efficiency. Realizing these benefits, however, can be challenging due to the finite time available for fuel evaporation and mixing. A methodology was developed to quantify how much in-cylinder charge cooling takes place in an engine for different gasoline/ethanol blends.

Using a turbocharged SI engine with both Port Fuel Injection (PFI) and Direct Injection (DI), knock onset limits were measured for different intake air temperatures for both types of injection and five gasoline/ethanol blends. The superior charge cooling in DI compared to PFI for the same fuel resulted in pushing knock onset limits to higher in-cylinder maximum pressures.

Knock onset is used as a diagnostic of charge cooling. The experiment measures how much the intake air needed to be heated in DI mode to cancel out charge cooling and make the engine knock at the same conditions as PFI. The same maximum pressure at borderline knock was used to make the comparison between DI and PFI. It is additionally shown that when maximum pressure at borderline knock is the same between DI and PFI, all conditions relevant to knock are very similar. The temperature difference measured using this methodology corresponds to how much more charge cooling due to fuel evaporation took place in DI. The measured charge cooling increases with fuel ethanol content from 14 degrees C for gasoline to 49 degrees C for E85 (85% ethanol, 15% gasoline by volume). The amount of charge cooling as a fraction of the thermodynamic maximum was around 70% for all fuel blends. The starting intake air temperature for E50 and E85 was significantly higher compared to lower ethanol blends due to hardware limitations. It is anticipated that the high intake air temperatures contributed significantly to the high charge cooling numbers realized for high ethanol blends by making fuel evaporation faster. Computational Fluid Dynamics (CFD) modeling confirmed that intake air temperature indeed has a significant effect of on how much charge cooling is realized.

This paper is the first part of a two paper series looking at the chemical and charge cooling effects of gasoline ethanol blends on engine knock.