Date of Award
12-2024
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Automotive Engineering
Committee Chair/Advisor
Benjamin Lawler
Committee Member
Brian Gainey
Committee Member
Robert Prucka
Committee Member
Zoran Filipi
Committee Member
Jiangfeng Zhang
Abstract
Even though most of the effort towards implementing low-carbon alternative fuels has been directed towards heavy-duty vehicles that are difficult to electrify, the high-autoignition resistance of these fuels make them challenging to combust in compression ignition engines. However, several fuel candidates, such as ethanol and methanol, are ideal fuels for spark ignition engines. With the demand for electric vehicles slowing down and increasing recognition of the merits of hybrid powertrains, there is a need to maximize the performance of these fuels for spark ignition engines, which are likely to remain the combustion strategy of choice for hybrids due to their cost and robustness. In contrast to gasoline, ethanol and methanol are classified as high cooling potential fuels that induce a large charge cooling effect upon injection in the cylinder. This means that the knock-free operating range, which traditionally limits gasoline spark ignition operation due to engine damaging auto-ignition events, can be significantly extended, allowing for higher compression ratios or optimal combustion phasing to maximize efficiency.
At high enough compression ratios, knock will re-emerge as an issue with these high cooling potential fuels, thus there is a need to develop knock-mitigation strategies. In particular, it is essential to utilize the cooling potential of these fuels since it is thermodynamically impossible to evaporate all of the fuel in the air during the intake stroke under typical ambient conditions for spark ignition engines. With a theoretical temperature drop of approximately 100 K, some fuel will instead evaporate off the metal surfaces rather than the air. This work demonstrates the difficulty of evaporating a high cooling potential fuel versus gasoline in the intake stroke. It is also shown that extending the injection timing window to include the compression stroke can enable more optimal combustion phasing and increase efficiency by improving the utilization of the fuel’s cooling potential and inducing stratification in the end-gas to suppress knock. Additionally, it was experimentally demonstrated that low cooling potential fuels are not suitable candidates for this compression stroke injection strategy due to their low heat of vaporization. A Pressure-Temperature trajectory analysis with a zero-dimensional thermodynamic model revealed the unburned gas temperature can be reduced when the heat of vaporization is high enough to counteract an increase in unburned gas temperature from lean compression prior to the compression stroke injection. It was found that compression stroke injections were more effective at suppressing knock with high injection pressures compared low injection pressures. Different mixture preparation strategies were also studied using the compression stroke injection strategy.
Next, the effectiveness of cool exhaust gas recirculation (EGR) as a knock suppression technique for stoichiometric spark ignition operation was evaluated for ethanol-gasoline blends. It was found that EGR resulted in higher knock propensity with high ethanol content fuels, which meant that the combustion phasing needed to be retarded to maintain a constant knock intensity. As the ethanol content decreased, EGR became more effective at suppressing knock, which resulted in advanced combustion phasing. Post-three-way catalyst EGR was found to be more effective at suppressing knock compared to pre-three-way catalyst EGR. Chemical kinetic modelling revealed that nitric oxide significantly increased the reactivity of the cylinder. Experimentally, increasing the nitric oxide concentration with both ethanol or gasoline increased knock propensity and retarded combustion phasing. In contrast, iso-octane demonstrated a combustion phasing advance due to decreased knock propensity at high nitric oxide concentrations.
Neat ethanol, hydrous ethanol (92% ethanol, 8% water by mass), wet ethanol 80 (80% ethanol, 20% water by mass), and methanol were studied under different operating conditions to evaluate the general performance of high cooling potential fuels in a spark ignition engine with a 14.8 compression ratio. Wet ethanol 80 was the most knock resistant of the ethanol-water blends, which resulted in high efficiency operation at high loads due to advanced combustion phasing. From a lifecycle emissions perspective, leaving water in ethanol during the distillation process can be beneficial. It was found that wet ethanol 80 and methanol are not interchangeable in a spark ignition engine, despite having similar combustion characteristics in compression ignition. This was because of methanol’s fast flame speed and higher knock resistance versus wet ethanol 80. Finally, a high compression ratio spark ignition engine fueled with methanol achieved similar or higher efficiency compared to a mixing-controlled compression ignition engine fueled with methanol.
Recommended Citation
Gandolfo, John, "Low Carbon, High Cooling Potential Alcohol Fuels in a High Compression Ratio Spark Ignition Engine" (2024). All Dissertations. 3847.
https://open.clemson.edu/all_dissertations/3847
Author ORCID Identifier
0000-0003-1459-5079
