Compression stroke injections of ethanol-gasoline blends for knock reduction in spark ignition

Abstract

Ethanol is a low carbon biofuel that has a lower carbon intensity, higher knock resistance, and higher evaporative cooling potential than gasoline. Due to its low lifecycle emissions and compatibility with spark ignition engines, blends of gasoline and ethanol provide a promising avenue toward reducing CO₂ emissions. Ethanol is also an effective octane improver of gasoline, which can enable elevated compression ratios and advancing spark timings thereby improving engine efficiency. Knock constraints limit the effectiveness of these two parameters, thus knock mitigation strategies must be deployed. An injection strategy was previously proposed and developed by the authors specific to the high heat of vaporization of high ethanol content gasoline blends in which a small fraction of the total fuel mass is injected during the compression stroke to cool the end-gas to reduce knock propensity. This strategy has been previously tested and studied with high cooling potential fuels. However, there is a need to evaluate the strategy with low cooling potential fuels. It is shown that when 10% of the total fuel mass is injected during the compression stroke with E10, a maximum spark timing advance of ∼1.0° with E10 (10% ethanol, 90% gasoline) was achieved versus ∼3.0° with E50 (50% ethanol, 50% gasoline). A thermodynamic model and pressure-temperature trajectory analysis was used to demonstrate that the heat of vaporization of E10 was not high enough to overcome the temperature associated with compressing a slightly leaner mixture prior to the compression stroke injection.