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The heartbeat of the global economy is commercial vehicles, and as the economy grows, so does the demand for on-road trucks, off-road construction equipment, and agricultural vehicles. These vehicles are almost entirely powered by compression ignition engines using fossil diesel fuel. In the face of the global climate crisis, this presents a real challenge: how do we provide efficient and productive commercial vehicle powertrains while reducing criteria and greenhouse gas (GHG) emissions? Full electrification of these vehicles faces many hurdles, such as cost, weight, operating hours, lack of infrastructure, and time to implementation. Thus, the most pragmatic and impactful way to reduce emissions in the near term is by using lower carbon intensity fuels, such as ethanol, methanol, natural gas, propane, hydrogen, or ammonia.
Using these fuels in heavy-duty engines as a substitute for diesel fuel is very challenging, because these fuels are poor direct replacements for diesel fuel. These fuels all have very low cetane numbers, which means they are very hard to autoignite and more suitable to spark ignition (SI) engines. But SI engines are NOT suited to heavy-duty applications because of the knock-limited peak torque, potential for catastrophic pre-ignition when highly boosted, poor torque density, poor torque response, low thermal efficiency, high exhaust temperatures, and high heat rejection. The combustion process used in conventional diesel engines is lean, mixing-controlled combustion (MCC). It is highly desirable for heavy-duty vehicles to use an engine that employs this combustion strategy—regardless of the fuel—because the engine will maintain the performance and operational characteristics of a diesel engine, such as high efficiency, no fear of knock or pre-ignition, snap torque, high torque at low speed, low cyclic variability, and robust combustion. An engine that has these characteristics, we like to say, “runs like a diesel”, which all stems from the mixing-controlled combustion process. Thus, an innovative combustion system is needed that will allow low-cetane fuels to ignite readily and be used in a non-premixed MCC strategy, just like the diesel engine today.
Using the CONVERGE computational fluid dynamics (CFD) modeling software, our engine combustion research group at Marquette University has been working to develop such an innovation known as prechamber enabled mixing-controlled combustion (PC-MCC). Illustrated in Figure 1, the concept uses a conventional compression ignition engine with high-pressure direct injection and adds an actively fueled prechamber igniter. The igniter contains a fuel injector, a spark plug, a small prechamber volume, and orifice passageways between the prechamber and main chamber. The high-pressure direct injector and prechamber injector use the same low-cetane fuel source. Figure 2 shows the operational strategy of PC-MCC with ethanol fuel compared to conventional diesel combustion (CDC). During the compression stroke, the prechamber is fueled with ethanol, while air from the main chamber is forced into the prechamber by piston motion. Closely coupled to the direct injection timing near top dead center, the prechamber is sparked, and the prepared charge is burned by rapid flame propagation. This combustion process elevates the pressure of the prechamber and promotes hot jet flames that are ejected into the main chamber. The penetrating jets impinge and subsequently ignite the direct-injected ethanol fuel, which would otherwise not autoignite.
As shown in Figure 3, the direct-injected ethanol, once ignited by the prechamber jet flames, burns in a mixing-controlled manner with a rate of combustion like that of diesel fuel. This is the ultimate goal: to allow the engine to “run like a diesel” by reproducing the diesel engine combustion process, but running on low-cetane fuels like ethanol, methanol, or even hydrogen and ammonia.
An animation of the CFD-predicted PC-MCC combustion process with ethanol fuel is illustrated in Figure 4. The prechamber jet flames are ejected toward the direct-injected ethanol fuel, igniting the ethanol fuel sprays very quickly and establishing a mixing-controlled, diffusion-style combustion process that is typical of a modern diesel engine.
This concept is currently under development with the assistance of two federal grants. The first is from a United States Department of Energy Vehicle Technologies Office award (DE-EE0009872), where the concept is being developed to convert diesel engines to be flex-fuel and run on gasoline/ethanol, while maintaining performance and dramatically reducing GHG emissions. The second is from the Advanced Research Projects Agency Energy’s (ARPA-e) REMEDY program (DE-AR0001528), which aims to reduce methane emissions, a powerful GHG, from natural gas engines by radically changing the combustion process to PC-MCC. The institutions working on these projects together with Marquette University are John Deere, Mahle Powertrain, the University of Wisconsin-Madison, Czero, ClearFlame Engines, and the Missouri Corn Merchandising Council. The CFD modeling has led to several publications by the team, showing how the CONVERGE simulations were used to determine the prechamber’s characteristics—such as volume, number of holes, hole size, and jet targeting—and the general PC-MCC operating strategy for prechamber fueling, injection timing, spark timing, and direct injection timing [1-5].
The modeling tools provided by CONVERGE were quintessential for performing the detailed CFD modeling needed to develop this advanced combustion concept. CONVERGE’s automatic mesh generation dramatically reduces the simulation setup time and complexity, allowing for rapid simulation development and analysis with confidence in the meshing strategy. Further refinement to the mesh is achieved through fixed grid embedding in user-defined regions of interest within the domain and CONVERGE’s Adaptive Mesh Refinement (AMR), which is able to resolve cell-to-cell gradients in temperature and velocity. AMR is especially useful when modeling the combustion process within the prechamber, the resultant high-intensity jets, and subsequent jet-spray induced combustion process. The predicted combustion process is captured using CONVERGE’s detailed chemical kinetics solver SAGE, which is fully coupled to the flow solution for accurate results and efficient solution times.
Based on the CFD modeling, a prototype PC-MCC engine was constructed and tested separately on pure ethanol fuel and natural gas, demonstrating a robust mixing-controlled combustion process with both fuels and highlighting the fuel-agnostic nature of the technology. Photos of the prototype hardware and recorded test data are shown in Figure 5. The tests with the prototype PC-MCC hardware corroborates the findings from the CONVERGE CFD simulations: that PC-MCC can be a fuel-agnostic, low-carbon engine technology for the future of heavy-duty engines, both on-road and off-road and for stationary power generation.
[1] Dempsey, A., Chowdhury, M., Kokjohn, S., and Zeman, J., “Prechamber Enabled Mixing Controlled Combustion – A Fuel Agnostic Technology for Future Low Carbon Heavy-Duty Engines,” SAE Paper 2022-01-0449, 2022. DOI: 10.4271/2022-01-0449
[2] Zeman, J., Yan, Z., Bunce, M., and Dempsey, A., “Assessment of Design and Location of an Active Prechamber Igniter to Enable Mixing-Controlled Combustion of Ethanol in Heavy-Duty Engines,” International Journal of Engine Research, 24(9), 4226-4250, 2023. DOI: 10.1177/14680874231185421
[3] Zeman, J., and Dempsey, A., “Characterization of Flex-Fuel Prechamber Enabled Mixing-Controlled Combustion With Gasoline/Ethanol Blends at High Load,” Journal of Engineering for Gas Turbines and Power, 146(8), 2024. DOI: 10.1115/1.4064453
[4] Nsaif, O., Kokjohn, S., Hessel, R., and Dempsey, A., “Reducing Methane Emissions From Lean Burn Natural Gas Engines With Prechamber Ignited Mixing-Controlled Combustion,” Journal of Engineering for Gas Turbines and Power, 146(6), 2024. DOI: 10.1115/1.4064454[5] Zeman, J., and Dempsey, A., “Numerical Investigation of Equivalence Ratio Effects on Flex-Fuel Mixing Controlled Combustion Enabled by Prechamber Ignition,” Applied Thermal Engineering, 249, 2024. DOI: 10.1016/j.applthermaleng.2024.123445.
