Date of Award
Doctor of Philosophy (PhD)
The production of alcohol fuels from bioderived feedstocks and the performance of next generation stratified low temperature combustion (LTC) modes for internal combustion engines are two research areas that have recently undergone rapid growth independently. Now, there is a need to bridge these two fields and identify the optimal combustion strategy for these low-carbon and carbon-neutral alcohol fuels as well as potential synergies. The large set of next generation stratified LTC modes are generalized into two groups based on how the heat release process proceeds in the compositionally stratified combustion chamber: lean-to-rich or rich-to-lean burn stratified combustion. It was found that the C1-C4 alcohol fuels are prime candidates to enable lean-to-rich burn stratified combustion based on their high cooling potentials and lack of cool flame reactivity (pre-ignition reactions). Previous experimental work by the author showed that a lean-to-rich burn stratified combustion mode, thermally stratified compression ignition (TSCI), can be enabled using a split injection of wet ethanol to gain control over the heat release process. The current work further investigates TSCI with wet ethanol experimentally on a diesel engine architecture, finding that the effectiveness of TSCI’s heat release control strategy is not affected by the use of external, cooled exhaust gas recirculation or intake boost. Further, it was shown that the effectiveness of TSCI’s heat release control strategy is highly coupled to the hardware used. Specifically, an injector whose spray targets high local heat transfer regions in the cylinder during the compression stroke is more effective at controlling the heat release process than an injector whose spray targets the adiabatic core. Additionally, a piston whose geometry allows regions with high compression stroke heat transfer to be distinct from the adiabatic core, such as a re-entrant bowl piston, will also increase the effectiveness of TSCI’s heat release control strategy. Using a split injection strategy to enable TSCI is not the only way to increase natural thermal stratification and control the heat release process. In this work, high-load LTC is experimentally enabled with wet ethanol on a light-duty gasoline engine architecture by employing a side-mounted, single hole injector with a relatively low injection pressure in a fairly quiescent combustion chamber. The low mixing propensity of this architecture results in a self-sustaining increase of thermal stratification that allows the high-load limit of LTC to be oxygen limited rather than noise limited. Following the experimental work with TSCI with wet ethanol, the LTC performance of seven bio-synthesizable C1-C4 alcohol fuels (methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, and sec-butanol) is experimentally characterized, showing that with the exception of n-butanol, the LTC performance of these fuels are similar, implying the remaining six fuels could form an equivalence class of fuels for LTC. To further explore this possibility, two previously proposed LTC fuel metrics are considered: critical compression ratio, a metric that describes the ignition propensity of a fuel in LTC, and normalized φ-sensitivity, a metric that describes how the local ignition delay time responds to a change in φ. The critical compression ratio, experimentally measured on a cooperative fuel research (CFR) engine, was shown to accurately predict the HCCI ignition propensity of the alcohol fuels near the critical compression ratio operating conditions. Similarly, the normalized φ-sensitivity showed the potential to predict the effectiveness of a fuel to control the heat release process of LTC using small amounts of in-cylinder stratification. The normalized φ-sensitivity could then serve as a blending benchmark for multi-alcohol water fuel blends.
Gainey, Brian, "The Role of Low Carbon Alcohol Fuels in Advanced Combustion" (2020). All Dissertations. 2728.