Co-Optima’s premise is that current fuels constrain engine design—and thus engine efficiency. The researchers suggest that there are engine architectures that can provide higher thermodynamic efficiencies than available from modern internal combustion engines; however, new fuels are required to maximize efficiency and operability across a wide speed/load range.

The Co-Optima researchers are working to discover:

1. What fuel properties maximize engine performance?

2. How do engine parameters affect efficiency?

3. What fuel and engine combinations are sustainable, affordable, and scalable?

In an informational webinar in September, John Farrell, Laboratory Program Manager—Vehicle Technologies, National Renewable Energy Laboratory, and part of the Co-Optima leadership team observed that:

Put most simply, the Co-Optima goal is to introduce better fuels and better vehicles sooner. Essential to this is the recognition that current fuels available in the marketplace constrain engine efficiency and limit the potential fuel economy and greenhouse gas improvements available across the light, medium, and heavy duty transportation fleet.

The introduction of new fuels and engines is a formidable task and requires close coordination with stakeholders to ensure that the most promising options are identified. The path to realizing the goal is to build on the strong legacy of fuels and engines work sponsored by the vehicle and bioenergy technologies offices over the past several decades.

The project has two parallel thrusts: a near-term thrust on spark ignition (SI), with a 2025 commercial target, and a long-term thrust on advanced compression ignition (ACI), with a 2030 commercial target.

Thrust 1 builds on the significant amount of work carried out over the past several decades, focused on building advanced boosted, downsized, and downspeeded spark ignition engines. The Co-Optima team is working to extend the knowledge base resulting from simulations with petroleum-derived fuels and ethanol to the broader range of fuels and fuel properties available from biomass resources.

Thrust 2’s focus on advanced compression ignition engines includes not only advanced conventional compression ignition engines or diesel engine technologies, but also on the broad suite of technologies focused on kinetically controlled and low-temperature combustion approaches. These efforts basically take the high efficiency of the diesel engine and the low emission potential of the gasoline engine and try and find ways to capitalize on both efficiency and emissions potential.

While such technologies have demonstrated the potential for high efficiency and low emissions at the research labs, they are much less developed than the advanced spark ignition engine technologies and thus represent the higher risk and longer term approach being pursued in the Co-Optima effort, Farrell said. Further, noted Wagner, the two thrust may require very different fuel properties.

Co-Optima has very aggressive targets for market introduction. With a target 2025 introduction of Thrust 1 fuel and engine technologies—and recognizing that there is a lead time required to get the new fuels and engines into the marketplace—DOE is targeting having Thrust 1 R&D complete by 2019. With the 2016 start date, that gives the team three years to complete the R&D efforts in this area. Thrust 2, at a much earlier stage of technology development, will take longer to reach market readiness. DOE is targeting R&D completion in that area sometime in the middle of the next decade.

To address the complexity of this broad technology space, Co-Optima is relying on the collaboration of six multi-disciplinary teams:

• Low Greenhouse Gas Fuels. Low-GHG blendstock properties and pathway attributes

• Modeling and Simulation Toolkit. Simulation of fuel and engine parameter impacts on efficiency and emissions.

• Advanced Engine Development. Experimental assessment of impacts of engine parameters on efficiency and emissions.

• Analysis of Sustainability, Scale, Economics, Risk, and Trade. Environmental impacts, cost, scalability, feed logistics.

• Fuel Properties. Impact of fuel properties on engine performance.

• Market Transformation. Infrastructure and legacy fleet compatibility.

THRUST 1

For Thrust 1, the researchers are focused on fuel properties—not a specific molecule, Wagner said.

The one thing we didn’t want to do was to try to pick a molecule as a winner. We’ve been accused of that; people still say we are, but we’re not. We’re not going to say “Ethanol. Ethanol is the answer.” Maybe ethanol will meet the properties we come up with, maybe it won’t. It’s going to be more open than that. Let’s say we do come up with properties, and there is some fuel we can make in mass quantity that meets those properties. Five years from now, ten years from now, we want there to be an opportunity for other fuels to come on line that also meet those properties.

—Robert Wagner

Put another way, the Thrust 1 approach seeks to understand not just “what could we make” but “what should we make,” said Dan Gaspar from Pacific Northwest National Laboratory (PNNL) in the webinar. Thrust 1 began by casting a wide net, looking at biomass from all sources and the range of conversion processes, from fermentation and other biological approaches to thermochemical or catalytic approaches. Co-Optima is definitely not seeking to develop new conversion pathways, but is relying on what has already been done.

The team then focused on the types molecules that might be best fuels and came up with nine different molecular classes that are suitable for spark ignition blendstocks or fuels. These include traditional hydrocarbons such as paraffins or olefins, cycloalkanes, and aromatics. They also focused on a set of oxygenates, alcohols, furans, ketones, ethers, and esters suitable as fuels and have the potential to improve engine performance. The result was one the order of 400 potential blendstock candidates.

The researchers then used a tiered approach to prioritize candidates. This process includes consideration of a range of metrics spanning not only engine performance, but also environmental impacts, cost, scalability, infrastructure compatibility and so on.

From this fuel funnel, the researchers have currently identified 20 molecules or mixtures that will be evaluated as blends in the 10–30% range using BOB (blendstock for oxygenate blending) as the basis.

To rank promising candidates, the researchers developed an engine performance fuel merit function, which continues to evolve, Wagner said. The merit function includes a large number of properties that are important for engine performance (such as octane number, heat of vaporization, flame speed, lower volume fraction, Heaviside function and particle mass index) coupled to a series of expected values.

Current Thrust 1 research areas include:

• Clarifying the effects of HoV on knock mitigation
• New insights into the meaning of RON and MON
• Improving HoV measurements for mixtures
• Investigating the impact of HoV on engine performance
• Lean and EGR dilution tolerance
• Fuel impacts on particulate emissions
• Fuel impacts on catalyst light-off temperatures
• Fuel impacts on low speed pre-ignition

Findings from these investigations will feedback to help calibrate the merit function.

THRUST 2

Thrust 2, which includes both gasoline-like and diesel like fuels, examines the compatibility of Thrust I fuels with Thrust 2 engine technologies and seeks to accelerate Thrust 2 technology development. Broadly, the effort seeks to discover the important fuel properties or performance metrics and how to measure or compute those metrics.

Researchers are working to identify new, or combined, metrics for autoignition that are not capture in the conventional RON, MON, CN metrics. The researchers are also seeking to clarify which fuel properties dominate specific performance measures, such as particulate formation. From an engineering perspective, this means determining how fuel properties and engine design/operating parameters interact, and how those interactions can be leveraged to optimize efficiency.

Thrust 2, said Wagner, is facing a decision point in March 2017.

So far, we have been very focused on Thrust 1. When we hit the decision point, we want to look at how we are doing on Thrust 1, what do we need to do going forward, and that will help dictate if we shift our focus more strongly to Thrust 2. Part of that will be coming up with a more defined Thrust 2 strategy.

— Robert Wagner

The field of advanced compression ignition combustion represents a very broad spectrum of technologies. As a general rule, ACI offers high efficiency and reduced particulate and NO_x emissions, but is often unstable and is hampered by a limited speed/load range. High combustion noise and high HC and CO emissions are also a problem, with lower exhaust temperatures representing a challenge for current emissions controls.

• Controllability is dominated by in-cylinder reactivity stratification. More reactivity stratification is better for stability. Reactivity (or ignition delay) is a function of the local equivalence ratio, local temperature, and fuel properties. High dilution may also lead to controllability challenges.

• Criteria pollutants are dominated by equivalence ratio stratification and fuel chemistry. A lower equivalence ratio and/or high dilution reduces NO_x and PM emissions but may result in an increase in unburned HC emissions. The fuel chemistry also impacts on emissions. For example, NO_x emissions from aromatics and olefins are greater than those from paraffins due to higher adiabatic flame temperatures. PM emissions from aromatics are greater than those from naphthenes which are greater than those from paraffins. The fuel reactivity resulting in HC emissions depends on the chemical composition of the fuel.

There are four Thrust 2 combustion mode strategies currently under investigation:

• Leaner lifted flame combustion (LLFC). LLFC is mixing-controlled combustion that does not form soot because it occurs at equivalence ratios < 2. LLFC delivers high efficiency, good controllability and low noise, is fuel-flexible, and EGR-tolerant for NO_x control. However, LLFC has yet to be sustained in an engine at moderate loads using a practical injector-tip configuration and fuel. The current approach for high-load operation included diced fuel injection (DFI): injecting fuel down a tube co-axial with the fuel jet. The Co-Optima researchers are looking into fuel properties that might help enable LLFC.

• Reactivity Controlled Compression Ignition (RCCI). The RCCI process uses in-cylinder fuel blending with at least two fuels of different reactivity and multiple injections to control in-cylinder fuel reactivity to optimize combustion phasing, duration and magnitude. (Earlier post.) The process involves introduction of a low reactivity fuel into the cylinder to create a well-mixed charge of low reactivity fuel, air and recirculated exhaust gases. Co-Optima is investigating fuels that would extend the load-range band supported by RCCI and also investigating the impact of fuels on phasing control.

• Gasoline Compression Ignition (GCI). Gasoline compression ignition ignites gasoline purely by compression, as with a diesel, rather by using a spark. It is a high-efficiency, low-temperature combustion mode that offers low engine-out NO_x and soot. (Earlier post.) GCI, however, is challenged by stable idle- to low-load operation (i.e., 0–2 bar BMEP) because it is challenging to ignite the low-reactivity gasoline purely through compression. Co-Optima is focusing on challenges impact load-load operation, as well as fundamentals.

• Low Temperature Gasoline Combustion (LTGC). LTGC is based on the compression ignition of a premixed or partially premixed dilute charge; it can provide thermal efficiencies and maximum loads comparable to those of turbo-charged diesel engines, and ultra-low NO_x and particulate emissions. Intake boosting is key to achieving high loads with dilute combustion, and it also enhances the fuel’s autoignition reactivity, reducing the required intake heating or hot residuals. However, at high boost the autoignition reactivity enhancement can become excessive, and substantial amounts of EGR are required to prevent overly advanced combustion.

There are multiple possible Thrust 2 paths forward: Thrust 2 fuel the same as Thrust 1; a Thrust 2-specific fuel; or an advanced diesel-specific fuel. What the program has not done to this point is add blended power or extended-range architectures (i.e., various forms of hybrid architectures, including engines optimized for range-extension, earlier post), Wagner said.

I think we need to add that into the mix, although we have not had enough conversations with industry around it. We have certainly thrown it out there as something to discuss.

—Robert Wagner

A second Co-Optima Listening Day (the first was earlier this year) is targeted for 19–20 January at Sandia Labs in California; the goal is to solicit stakeholder input on program goals and progress.