Co-Optimization of Fuels & Engines: A Transportation Future with Science in the Driver’s Seat Mapping a Viable Route Forward for Affordable, Efficient, and Clean Fuels and Engines

by John Farrell, Robert Wagner, Chris Moen and Dan Gaspar (U.S. Department of Energy) ... For the past four years, the U.S. Department of Energy (DOE) has been funding the CoOptimization of Fuels & Engines (Co-Optima) initiative, focused on identifying fuel properties and engine parameters that impact engine efficiency with the goal of accelerating the commercial introduction of new fuel options that are scalable, sustainable, affordable, and compatible with existing vehicle and retail/distribution infrastructure. The Co-Optima initiative aligns the diverse expertise, world-leading science, and one-of-a-kind science facilities of the national laboratories and universities with significant engagement with industry and government stakeholders. Results are now available that will inform the debate surrounding which fuel/engine options should be considered for the United States in the future.

Scope of Effort
The Co-Optima initiative brings together nine national laboratories and more than 20 university and industry partners to identify the benefits that can be achieved through simultaneous improvements to fuel properties and engine operation. Key goals of the initiative for passenger vehicles include identifying pathways to a minimum 10% increase in fuel economy relative to an expected 2030 baseline (which represents a 35% increase over a 2015 baseline), saving U.S.
consumers $20 to $30 billion each year through improved vehicle fuel efficiency, diversifying the supply of domestically sourced fuel, and significantly decreasing greenhouse gas emissions from transportation. Although not limited to bio-based fuels, pathways that utilize domestic biomass resources including perennial energy crops, forestry and agricultural residues, or other waste resources can offer even greater reductions and provide long-term sustainable options.

Co-Optima research builds upon and leverages current industry megatrends. U.S. automakers are increasingly replacing conventional (“naturally aspirated”) spark ignition (SI) engines with smaller engines paired with a turbocharger, which are capable of delivering the same power output with improved emissions and fuel economy. A key challenge to realizing the maximum efficiency potential of these turbocharged (or “boosted”) SI engines is mitigating engine knock—spontaneous ignition of the unburned fuel/air mixture that can be very damaging to engines.

Co-Optima research to date has been focused on identifying the fuel properties and engine parameters that mitigate knock and thus maximize boosted SI efficiency, emissions, and performance. While CoOptima is also addressing co-optimization of advanced compression-ignition combustion approaches, this discussion is limited to boosted
SI engines.

For background purposes, gasoline sold at the retail pump is marketed as regular, mid-grade, or premium based on its “pump octane” or anti-knock index (AKI). AKI is the average of two numbers: Research Octane Number (RON) and Motor Octane Number (MON). Together, they make up the numerator of the familiar equation on the pump: (R+M)/2. Both RON and MON are fuel performance metrics established in the 1930s to measure the ability of a fuel to mitigate
engine knock under different operating conditions. It is well established that RON is a more relevant metric for assessing anti-knock performance in modern boosted SI engines,5 though MON is still useful for some engine operating conditions (e.g., aggressive, high-load driving).

Co-Optima research is framed around three interrelated questions:
• What fuels do engines want?
• What fuels should we make?
• What is practical in the real world?

• Central Engine Hypothesis: There are engine architectures and combustion strategies that provide higher thermodynamic efficiencies than are available from modern internal combustion engines; new fuels are required to maximize efficiency and operability across a wide speed / load range.

• Central Fuels Hypothesis: If we identify target values for the critical fuel properties that maximize efficiency and emissions performance for a given engine architecture, then fuels that have properties with those values (regardless of chemical composition) will provide comparable performance.

Co-Optima researchers have shown that all the high-performing boosted SI blendstocks can be produced from various resources, including conventional petroleum, natural gas, and renewable domestic feedstocks that offer significantly lower lifecycle greenhouse gas emissions and other benefits, such as increased domestic fuel supply and diversity. Because the state of technology readiness of biomass-sourced blendstocks is much less developed than conventional fossil-based blendstocks, we have analyzed 24 biomass-derived boosted SI blendstocks against 17 metrics assessing economic benefits, technology readiness, and environmental viability.

The findings of these analyses indicate that all of the top-performing blendstocks have the potential to reduce
life-cycle greenhouse gas emissions by at least 60% compared to petroleum-based fuels. At a 30% blend level, this equates to an 18% reduction in emissions for the finished fuel. This information provides the biofuel community a comprehensive and consistent comparison of the feasibility and technical research and development barriers to commercializing promising boosted SI blendstocks. READ MORE