Development of an Efficient Conjugate Heat Transfer Modeling Framework to Optimize Mixing-Limited Combustion of Ethanol in a Diesel Engine

Abstract

Abstract Long-term petroleum prices and increasingly-stringent emissions regulations are driving manufacturers and users alike to consider alternatives to diesel-fueled engines. Mixing controlled combustion of alcohol fuels, such as ethanol, has been identified as a promising technology based the low propensity for particulate and NOx production, but the higher heats of vaporization and auto-ignition temperatures of these fuels make their direct use in diesel engine architectures a challenge. However, because alcohol fuels do not form appreciable levels of soot even in mixing-controlled (MCCI) mode, and because stoichiometric air/fuel ratios (AFR) can be used to simplify NOx aftertreatment, engine design optimization efforts can be targeted to maximize thermal efficiency. Therefore, to realize the potential of alcohol-fueled combustion, engineering insight is required to understand how design parameters, such as increased engine insulation, piston bowl geometry, or spray targeting, should be optimally utilized. In this work, a computational fluid dynamics (CFD) modeling framework is developed and validated in order to identify pathways to improve the performance of an ethanol-fueled engine operating in an MCCI mode at a stoichiometric AFR. To evaluate the use of TBCs as an engine insulation method, a simplified 1-D conjugate heat transfer (CHT) modeling framework is employed. The CFD model is first validated against baseline engine data over selected inlet air heating temperatures for two piston bowl-injector configurations that define the extrema of the design space. The addition of the 1-D CHT model only increases the computational expense by 15% relative to traditional approaches, yet offers more accurate heat transfer predictions over constant temperature boundary conditions. The model is then used to explore the efficacy of injector orientations and piston bowl geometries in improving the indicated thermal efficiency of alcohol fueled compression ignition engines. Using a design of experiments approach, several candidate designs were identified that improved fuel-air mixing, shortened the combustion duration, and increased thermal efficiency. The most promising design was then fabricated and tested in a Caterpillar 1Y3700 Single Cylinder Oil Test Engine (SCOTE). The engine testing confirmed the findings from the CFD simulations, and found that the co-optimized injector and piston bowl design yielded over 2-percentage point increase in thermal efficiency at the same equivalence ratio (0.96) and over 6-percentage point increase at the same engine load (10.1 bar indicated mean effective pressure), while satisfying design constraints for peak pressure and maximum pressure rise rate.