Split Injection of High-Ethanol Content Fuels to Reduce Knock in Spark Ignition



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Authors Abstract
Spark ignition engines have low tailpipe criteria pollutants due to their stoichiometric operation and three-way catalysis and are highly controllable. However, one of their main drawbacks is that the compression ratio is low due to knock, which incurs an efficiency penalty. With a global push towards low-lifecycle-carbon renewable fuels, high-octane alternatives to gasoline such as ethanol are attractive options as fuels for spark ignition engines. Under premixed spark ignition operating conditions, ethanol can enable higher compression ratios than regular-grade gasoline due to its high octane number. The high cooling potential of high-ethanol content gasolines, like E85, or of ethanol-water blends, like hydrous ethanol, can be leveraged to further reduce knock and enable higher compression ratios as well as further downsizing and boosting to reduce frictional and throttling losses.
In this work, a split injection strategy is used, where 5-15% of the total injected fuel is injected during the compression stroke with E75 gasoline (75% ethanol, by volume) and hydrous ethanol (190 proof ethanol, 92% ethanol, 8% water, by mass). These experimental results indicate that injecting a small amount of E75 or WE92 during the middle of the compression stroke such that the fuel cannot completely homogenize with the air before the spark fires results in knock mitigation that exceeds the amount of knock mitigation expected by evaporating that amount of fuel in air in a premixed setting. This split injection strategy achieved a 1.3% relative fuel conversion efficiency benefit with E75 at 19 bar IMEPn and a 1.4% relative fuel conversion efficiency benefit at 15 bar IMEPn with WE92. Overall, these results indicate that a split injection strategy with a high cooling potential alcohol fuel can further increase a fuel’s effective octane number, enabling highly boosted, high compression ratio stoichiometric spark ignition operation.
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Gainey, B., Gandolfo, J., Gao, M., and Lawler, B., "Split Injection of High-Ethanol Content Fuels to Reduce Knock in Spark Ignition," SAE Technical Paper 2023-01-0326, 2023, https://doi.org/10.4271/2023-01-0326.
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Apr 11, 2023
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Technical Paper