High-Efficiency Mixing Controlled Compression Ignition Combustion of Propane DME Blends

The program is a multi-institutional effort led by the University of Wisconsin-Madison Engine Research Center, partnered with WM International and the University of Central Florida, aiming to revolutionize combustion technology for medium-duty commercial vehicles. The project’s primary objective is to develop a high-efficiency mixing controlled compression ignition (MCCI) combustion strategy utilizing propane and propane/DME blends. This development is crucial because current propane-fueled spark-ignited (SI) engines operate at substantially lower efficiencies (BTE of 31.5%) than comparable modern diesel engines (37.1%). The program goal is to achieve an efficiency greater than 37.1% and enable a 15% reduction in CO2 emissions compared to current diesel engines, along with a greater than 5% reduction in total cost of ownership. The core technical challenge involves overcoming propane’s low cetane number, which resists auto-ignition, especially at low loads, while simultaneously developing a robust fuel system capable of handling these liquefied gaseous fuels at extremely high injection pressures. The project addressed this by coupling engine modeling, fundamental chemical kinetics studies, and hardware development. A high-pressure propane/DME fuel system, featuring an improved dual pump architecture, was successfully developed, fabricated, and installed in the test cell, demonstrating operation at injection pressures exceeding 1100 bar, which is necessary for effective mixing controlled combustion. Furthermore, high-flow nozzles were specifically designed and calibrated for the direct-injection system. To address the challenge of achieving stable ignition on a low cetane fuel like propane under low-load conditions, Computational Fluid Dynamics (CFD) optimization identified an innovative solution: exhaust rebreathing. This strategy uses a custom-designed camshaft to re-open the exhaust valve during the intake stroke, trapping hot residual gases to preheat the charge. Simulations demonstrated that this mechanism, combined with an optimized injection system, enables stable MCCI combustion. Unassisted propane operation required highly elevated intake conditions (180 kPa intake pressure and 90 °C intake manifold temperature) for stability; however, with the rebreathe cam installed, stable compression ignition was achieved at typical engine intake conditions (125 kPa intake pressure and 45 °C IMT). Validated CFD models project that this optimized system achieves an indicated specific fuel consumption (ISFC) near 180 g/kW-hr, corresponding to a BTE greater than 38% and achieving a 16% reduction in Brake Specific CO2 (BSCO2) compared to the baseline LPG SI engine. Engine testing using the final hardware confirmed stable compression ignition operation on neat propane with combustion phasing similar to conventional diesel fuel. Complementing the engine development, fundamental research was conducted using a high-pressure shock tube facility to collect Ignition Delay Times (IDTs) for various propane/DME blends across high pressures (up to 110 bar) and relevant temperature ranges. These data were used to modify and validate chemical kinetic mechanisms, resulting in a computationally efficient reduced mechanism of 63 species for use in engineering CFD codes. Additionally, alternative ignition methods, such as ceramic glow plugs and spark plugs, were tested under energy-assisted compression ignition (EACI) strategies, both proving effective in enabling split-injection combustion at normal intake temperatures. Economic analysis suggests that while the MCCI powertrain hardware costs approximately $3,300 more than an SI engine, the projected fuel savings due to efficiency gains result in a quick payback period of roughly 1.5 years (about 1500 hours).