Thermally stratified compression ignition is a new advanced, low-temperature combustion mode that aims to control the heat release process in a lean, premixed, compression ignition combustion mode by controlling the level of thermal stratification in the cylinder. Specifically, this work uses a mixture of 80% ethanol and 20% water by mass, referred to as “wet ethanol” herein, to increase thermal stratification via evaporative cooling of areas targeted by an injection event during the compression stroke. The experiments conducted aim to both fundamentally understand the effect that a late cycle injection of wet ethanol has on the heat release process, and to use that effect to explore the high-load limit of thermally stratified compression ignition with wet ethanol. At an equivalence ratio of 0.5, injecting just 8% of the fuel during the compression stroke was shown to reduce the peak heat release rate by a factor of 2, subsequently avoiding excessive pressure rise rates. Under pure homogeneous charge compression ignition using wet ethanol as the fuel, the load range was found to be 2.5–3.9 bar gross indicated mean effective pressure. Using a split injection of wet ethanol, the high-load limit was extended to 7.0 bar gross indicated mean effective pressure under naturally aspirated conditions. Finally, intake boost was used to achieve high-load operation with low NO_x (oxides of nitrogen (NO or NO₂)) emissions and was shown to further increase the high-load limit to 7.6 bar gross indicated mean effective pressure at an intake pressure of 1.43 bar. These results show the ability of a split injection of wet ethanol to successfully control the heat release process and expand the operable load range in low-temperature combustion.

Wet ethanol is a biofuel that can be rapidly integrated into the existing transportation sector infrastructure and have an immediate impact on decarbonization. Compared to conventional hydrocarbon fuels, wet ethanol has unique fuel properties (e.g., short carbon chain, oxygenated, high heat of vaporization, no cool-flame reactivity), which can actually improve the efficiency and engine-out emissions of internal combustion engines while decarbonizing. In this work, wet ethanol 80 (80% ethanol, 20% water by mass) was experimentally studied at high loads under boosted conditions in compression ignition to study the tradeoffs in efficiency and emissions based on boosting and injection strategies. Specifically, this work explores the potential of adding a third, mixing-controlled injection at high loads. The results indicate that adding a third, mixing-controlled injection results in combustion stabilization at high loads, where the peak pressure limit of the engine is a constraint that requires combustion phasing to retard. However, since the heat of vaporization of wet ethanol 80 is ~6% of its lower heating value, evaporation of fuel injected near top dead center imposes a thermodynamic efficiency penalty by absorbing heat from the working fluid at a time in the cycle when adding heat produces net work out. Additionally, the mixing-controlled injection increases NOx emissions. Therefore, the amount of fuel injected in the mixing-controlled injection should be limited to only what is necessary to stabilize combustion. Ultimately, by using wet ethanol 80 in a triple injection strategy, a load of 22 bar IMEPn is achieved with a net fuel conversion efficiency of 42.2%, an engine-out indicated specific emissions of NOx of 1.3 g/kWh, and no measurable particulate matter, while maintaining a peak cylinder pressure below 150 bar.

Ethanol, n-propanol, isopropanol, n-butanol, isobutanol, and sec-butanol are six potential carbon-neutral fuels of the future. One application of these carbon-neutral fuels is in high-dilution spark ignition. To understand the potential of these fuels in high-dilution spark ignition, this work experimentally determines the spark ignition lean flammability limit of each fuel with no external, cooled exhaust gas recirculation (EGR), and with 20% external, EGR. The spark ignition lean flammability limit in this work is defined as the excess-air ratio that results in a coefficient of variance of gross indicated mean effective pressure greater than 5%. This is done on an engine with a compression ratio of 12.5, using an intake pressure of 1 bar and an intake temperature of 320 K. It was found that ethanol had the leanest lean limit, due to its high flame speed, followed by n-propanol, isopropanol, and sec-butanol, which all had similar lean limits. n-Butanol and isobutanol had the richest lean limits due their high knock propensity and low flame speed, respectively. The lean limit of each fuel decreased with external, cooled EGR addition, with ethanol as the least sensitive and isopropanol as the most sensitive to EGR addition. Overall, using a high dilution strategy increased the cycle efficiency for each fuel. Ethanol, n-propanol, isopropanol, and sec-butanol all showed promising performance and are great candidates to be combined with an advanced high-dilution SI strategy to enable high-dilution SI with a carbon-neutral fuel.