Scientists, engineers, and analysts with the US Department of Energy’s Co-Optimization of Fuels & Engines (Co-Optima) initiative are examining how simultaneous improvements to fuels and engines can improve efficiency and reduce emissions and costs for the entire on-road fleet, including light-duty (LD), medium-duty (MD), and heavy-duty (HD) internal combustion vehicles that are likely to make up the majority of the US automotive market for decades to come.

After completing a major body of research focused on boosted (also known as turbocharged) spark ignition (SI) engines in Fiscal Year (FY) 2018, Co-Optima’s FY 2019 LD research and development (R&D) shifted to multimode solutions that employ multiple engine operating modes to maximize engine efficiency and fuel economy. A report released this week highlights the most significant of many Co-Optima R&D accomplishments from FY 2019, with details on findings that straddle LD, MD, and HD technologies:

• Multimode Ignition Strategies to Boost Fuel Economy. SI engines operate most efficiently in high-load conditions—e.g., rapid acceleration, climbing a hill, pulling a trailer—and lose efficiency in low- and intermediate-load conditions that make up most real-world driving. Advanced compression ignition (ACI) combustion modes and engine technologies hold promise to boost fuel economy and cut emissions under these low- and intermediate-load conditions, while maintaining combustion control and performance.

  In FY 2019, Co-Optima LD research explored multimode approaches that alternate between boosted SI and ACI methods to maximize efficiency under given conditions. The team is starting to investigate how these strategies can be applied to MD and HD engines.

  
  

  Left: Simulation of spark-assisted compression ignition (SACI) engine operation with E30 (gasoline with 30% ethanol) during mixed deflagration (blue areas) and autoignition (red areas) combustion. The gradient across the center of each simulation represents the equivalence ratio. Right: Heat-release rate (in joules per crank angle degree [CAD]) versus mass of fuel burned for individual engine cycles in the deflagration-only mode (green lines) and the mixed deflagration and autoignition mode (red lines), showing the dominance of autoignition above 50% mass burned in mixed-mode cycles. Figure by Chao Xu, ANL

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• Fuel Metrics to More Accurately Predict Performance. Co-Optima research has helped reveal limitations in conventional fuel metrics, such as research octane number and motor octane number, in predicting performance under certain ACI combustion conditions. The team has developed new measurement protocols to more accurately assess these factors.

Trucks make up just 4% of all US automobiles, but MD and HD vehicles—from big rigs to delivery vans—account for more than 25% of transportation-related fuel consumption, and diesel averages 44 cents more per gallon than gasoline. After three years with a strong focus on LD vehicles, in FY 2019 the Co-Optima team made a planned shift to increase research related to reducing costs and improving efficiency and emissions of trucks.

The new Co-Optima report details MD and HD breakthroughs including:

• Ducted Fuel Injection for Virtually Sootless Diesel Combustion. The mixing-controlled compression ignition (MCCI) technology used in today’s diesel engines is highly efficient, but it requires expensive catalyst and filter technology to control engine-out particulate matter and nitrogen oxide emissions—both major causes of respiratory conditions. Co-Optima researchers showed that partially premixing fuel and air before ignition using ducted fuel injection (DFI) technology results in a dramatic reduction of soot emissions.

  Soot reduction is decreased by an additional order of magnitude when combined with oxygenated fuel components, making it nearly indetectable. Further development and testing is needed to adapt DFI to engines for real-world use, but this approach combined with oxygenated fuel could translate into cleaner, more cost-effective MCCI engines that run on diesel with renewable content.

• Renewable Feedstocks to Produce Commercially Viable Diesel Blends. To date, few biomass-based diesel fuel alternatives are viable for widespread commercial use and able to be produced at scale. The Co-Optima team’s molecular-level investigation has led to the identification of several new fuels with desirable properties.

  One promising option uses renewable corn stover as a feedstock for a biofuel called 4-butoxyheptane, which shows potential for competitive production costs, high performance, low soot and greenhouse gas emissions, and compatibility with existing fueling infrastructure. The Co-Optima team is also assessing the value of these fuel blends to refiners, to make sure that candidates under investigation have the potential for commercial-scale production.

The first-of-its-kind Co-Optima initiative is designed to provide American industry with the scientific underpinnings needed to maximize vehicle performance and efficiency, leverage domestic fuel resources, and reduce life cycle emissions. The Co-Optima initiative is sponsored by DOE’s Vehicle Technologies Office and Bioenergy Technologies Office, and partners include the National Renewable Energy Laboratory, Argonne, Idaho, Lawrence Berkeley, Lawrence Livermore, Los Alamos, Oak Ridge, Pacific Northwest, and Sandia National Laboratories, and more than 20 university and industry partners.