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Co-Optima team publishes details on its boosted SI engine research and figure of merit

For the past four years, the US Department of Energy’s Co-Optima team (earlier post) has focused research efforts on turbocharged (or “boosted”) spark-ignition (SI) engines, which is the dominant combustion approach for light-duty vehicles such as passenger cars and light trucks. This research involves identifying the fuel properties and engine parameters that mitigate knock and emissions, while maximizing efficiency and performance.

Co-Optima researchers recently detailed the findings of this research in an open-access paper in the journal Progress in Energy and Combustion Science. The paper provides previously unrevealed scientific details from the recently completed Co-Optima research into boosted SI engines.

The result of this research is a unified merit function, which is a tool that allows new fuels development from biomass or other feedstock to be evaluated for efficiency potential on the basis of critical fuel properties. Combined with a unique abundance of fuel property data on emerging biofuels, the merit function has changed how researchers think and talk about fuels.

The Co-Optimization of Fuels and Engines (Co-Optima) initiative from the US Department of Energy aims to co-develop fuels and engines in an effort to maximize energy efficiency and the utilization of renewable fuels. Many of these renewable fuel options have fuel chemistries that are different from those of petroleum-derived fuels. Because practical market fuels need to meet specific fuel-property requirements, a chemistry-agnostic approach to assessing the potential benefits of candidate fuels was developed using the Central Fuel Property Hypothesis (CFPH).

The CFPH states that fuel properties are predictive of the performance of the fuel, regardless of the fuel’s chemical composition. In order to use this hypothesis to assess the potential of fuel candidates to increase efficiency in spark-ignition (SI) engines, the individual contributions towards efficiency potential in an optimized engine must be quantified in a way that allows the individual fuel properties to be traded off for one another.

This review article begins by providing an overview of the historical linkages between fuel properties and engine efficiency, including the two dominant pathways currently being used by vehicle manufacturers to reduce fuel consumption. Then, a thermodynamic-based assessment to quantify how six individual fuel properties can affect efficiency in SI engines is performed: research octane number, octane sensitivity, latent heat of vaporization, laminar flame speed, particulate matter index, and catalyst light-off temperature. The relative effects of each of these fuel properties is combined into a unified merit function that is capable of assessing the fuel property-based efficiency potential of fuels with conventional and unconventional compositions.

… The merit function is applicable, to the first order, to both conventional petroleum-derived fuels, bio-based fuels, or blends of the two. Further, the merit function applies to all types of stoichiometric SI engine architectures that use a fixed rc , including both boosted and naturally aspirated engines. As a result, this is a tool that can be used to guide producers of alternative fuels towards formulations that can enable the highest efficiency engine operation.

—Szybist et al.

The article was authored by researchers from multiple National Laboratories, including Oak Ridge (ORNL), Argonne (ANL), Lawrence Livermore (LLNL), Sandia (SNL), and the National Renewable Energy Laboratory (NREL).

The Co-Optima initiative is sponsored by the Office of Energy Efficiency and Renewable Energy’s Vehicle Technologies and Bioenergy Technologies offices. Co-Optima partners include ANL, Idaho National Laboratory, Lawrence Berkeley National Laboratory, LLNL, Los Alamos National Laboratory, NREL, ORNL, Pacific Northwest National Laboratory, SNL, and more than 20 university and industry partners.


  • James P. Szybist, Stephen Busch, Robert L. McCormick, Josh A. Pihl, Derek A. Splitter, Matthew A. Ratcliff, Christopher P. Kolodziej, John M.E. Storey, Melanie Moses-DeBusk, David Vuilleumier, Magnus Sjöberg, C. Scott Sluder, Toby Rockstroh, Paul Miles (2020) “What fuel properties enable higher thermal efficiency in spark-ignited engines?” Progress in Energy and Combustion Science, doi: 10.1016/j.pecs.2020.100876


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