Charge Cooling Effects on Knock Limits in SI DI Engines Using Gasoline/Ethanol Blends: Part 2-Effective Octane Numbers 2012-01-1284
Spark Ignited Direct Injection (SI DI) of fuel extends engine knock limits compared to Port Fuel Injection (PFI) by utilizing the large in-cylinder charge cooling effect due to fuel evaporation. The use of gasoline/ethanol blends in direct injection (DI) is therefore especially advantageous due to the high heat of vaporization of ethanol. In addition to the thermal benefit due to charge cooling, ethanol blends also display superior chemical resistance to autoignition, therefore allowing the further extension of knock limits. Unlike the charge cooling benefit which is realized mostly in SI DI engines, the chemical benefit of ethanol blends exists in Port Fuel Injected (PFI) engines as well. The aim of this study is to separate and quantify the effect of fuel chemistry and charge cooling on knock.
Using a turbocharged SI engine with both PFI and DI, knock limits were measured for both injection types and five gasoline-ethanol blends. A methodology was developed and used to calculate unburned mixture temperatures including charge cooling for each fuel.
The experimental results were used in conjunction with a knock limit predictive model. The Livengood-Wu autoignition integral approach was employed in the model along with the Douad-Eyzat correlation for autoignition time. The octane number (ON) in the pre-exponential term of the correlation was adjusted for each ethanol blend to fit the experimental results. An “Effective Octane Number” (ONEFF) is thus obtained for each blend. The increase in ONEFF is a measure of the chemical benefit from increasing ethanol content. The ONEFF results show diminishing returns in ethanol chemical benefit above about E40 (40% ethanol by volume).
A methodology is also presented to quantify the thermal benefit from charge cooling in SI DI engines as an “Evaporative Octane Number”. The autoignition integral is used to predict what octane rating would be needed to prevent knock if there were no charge cooling. The results show that the antiknock thermal benefit increases monotonically with ethanol content from 5 octane numbers for DI gasoline to 18 octane numbers for E85.
This paper is the second part of a two paper series looking at the chemical and charge cooling effects of gasoline ethanol blends on engine knock.