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High Performance ICE Engines Roadmap

POSSIBLE FOUR-STROKE COMPRESSION IGNITION ENGINE IMPROVEMENTS

There have been a number of advances made in four-stroke internal combustion engines (ICEs) over the last 10 years, many of them resulting from Department of Energy (DOE)-sponsored studies. Among these, some of the most impressive gains have come from the SuperTruck I and SuperTruck 2 programs. As just one example, in this year’s DOE Annual Merit Review, Cummins and Daimler reported engine only status of 53.5 percent and 52.9 percent brake thermal efficiency, respectively. . . both with plans to exceed the 55 percent program target. This compares with an actual best-point brake thermal efficiency status of roughly 42 percent for their comparably sized engine available in 2007.¹

It would be worthwhile to consider which of the following SuperTruck improvements might be applicable to the ICEs used today in the Army’s ground combat vehicles, tactical vehicles, and mobile/stationary power plants:

• Improved high heat release rate combustion
• Variable valve timing/displacement-on-demand

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¹ Misc. Authors, 2020, U.S. Department of Energy’s (DOE) Vehicle Technologies Office (VTO) 2020 Annual Merit Review (AMR), Online, U.S. Department of Energy, https://www.energy.gov/eere/vehicles/annual-merit-review-presentations, accessed November 2020.

• Increased compression ratio/higher peak cylinder pressure
• High efficiency turbochargers
• Interstage cooling
• Electrified accessories
• Power cylinder friction reduction actions, such as thermal spray bores
• Variable displacement oil pump
• Split cooling
• Active piston oil nozzle jets
• Thermal barrier coatings

WASTE HEAT RECOVERY

Waste heat recovery takes advantage of energy that would otherwise be lost to the exhaust or cooling system to improve the system efficiency. This energy can either be supplied to the electrical system or to the crankshaft. Waste recovery systems could be deployed on ground vehicles and/or stationary power plants.

All major truck engine suppliers (Cummins, Volvo, Navistar, and Daimler) have included waste heat recovery in their SuperTruck programs. These systems typically are based on an organic Rankine cycle (using cyclopentane), including a superheater/expander, turbine, recuperator, and cooler. The associated brake thermal efficiency improvements are projected to range from 2 to 4 percent, depending on heat source content. For example, the Cummins waste heat recovery system is one of the most extensive, collecting heat from charge air, the EGR cooler, engine coolant, and the exhaust system.

Given that military engines do not run exhaust gas recirculation (EGR), the available waste heat will not be as great as that in these SuperTruck programs. Using only exhaust heat in lieu of exhaust heat plus EGR, it is estimated that the fuel efficiency benefit will be roughly half that of the SuperTruck programs. However, it will still be substantial enough to be worth considering.

Interestingly, Southwest Research Institute is working on a waste heat recovery system that uses supercritical carbon dioxide as its media in lieu of cyclopentane. It deploys a Brayton cycle with a compressor in lieu of the pump on the organic Rankin cycle. Southwest Research claims that this system has roughly three times the superior efficiency of the organic Rankine cycle. As such, work on this system should continue to be monitored for potential inclusion in a future Army program.

Another interesting waste heat recovery system is turbo compounding, where energy collected from a turbine is converted directly into mechanical energy and supplied back to the crankshaft. Volvo recently

introduced into production their next generation of turbocompounding on their D13 engine, claiming a 20 percent improvement in fuel efficiency.²

A further waste heat recovery advancement now under development is the SuperTurbo, which enables power transfer to and from the turbo shaft through a high-speed planetary traction drive and continuously variable transmission. At this year’s DOE annual merit review meeting, Caterpillar reported that they are using this SuperTurbo technology on their 13-liter concept engine for off-road applications. This particular Caterpillar concept engine also includes a motor/generator unit and high-speed flywheel to improve transient performance.

Capturing this energy would also help to reduce the heat signature of the Army’s combat vehicles.

HORIZONTALLY OPPOSED PISTON COMPRESSION IGNITION ENGINES

Development work on conventional four-stroke engines has been steady over close to a hundred years by many different original equipment manufacturers, universities, and national laboratories. In sharp contrast, development of opposed piston two-stroke compression ignition engines within the United States using computer-aided engineering tools has been much more recent starting with OPOC (opposed piston opposed cylinders) in the 1990s. The OPOC engine under development at that time had some inherent architectural flaws, but it led to the subsequent development of the Advanced Combat Engine.

Recognizing the potential for further improvements, the Army has set some aggressive mid-term targets (i.e., through 2035) for this technology, including significant improvements in heat rejection, power density, and brake specific fuel consumption. To achieve those targets, it is recommended that the following actions be considered:

• Higher fidelity combustion computational fluid dynamics modeling—for improved indicated specific fuel consumption.
• Improvements in conjugate heat transfer models—to ensure even temperature distribution along the bore with minimum hot bore distortion; also for piston temperature predictions; needed to achieve increased power density.

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² Volvo Truck North America, 2019, “Volvo Trucks Introduces Enhanced Turbo Compound Engine in VNL Models,” https://www.volvotrucks.us/news-and-stories/pressreleases/2019/august/volvo-trucks-introduces-enhanced-turbo-compound-engine-in-vnlmodels/, accessed November 2020.

• Complete engine thermal surveys and hot bore distortion measurements to correlate against CAE (computer-aided engineering) thermal models (note that cold bore distortion measurements are easy to do with a PAT gauge³; physical hot bore distortion measurements are possible but time-consuming and expensive)
• Form honing—to provide even less hot bore distortion at rated power
• Some genetic algorithm studies to improve the in-cylinder combustion recipe (only practical after improvements in combustion CFD modeling)
• Potential use of metal matrix composites for pistons—higher strength and toughness at high temperature, improved thermal conductivity, reduced coefficient of thermal expansion (enabling reduced piston skirt to bore clearance), better skirt conformability, and lower reciprocating mass (Note: possible use of titanium metal matrix materials in lieu of aluminum metal matrix)
• Possible use of Tenneco’s EnviroKool™ technology, which decouples the cooling media in the gallery from engine oil, thereby avoiding oil degradation problems due to hot undercrown temperatures⁴
• Use of titanium or metal matrix composite (MMC) piston rods for reduced reciprocating mass
• Improvements in thermal barrier coatings (on MMC piston crowns) to minimize heat transfer; this requires a combination of low thermal conductivity and low specific heat
• Improvements in piston undercrown cooling to better manage temperatures within safe material limits
• Much higher power e-Turbos for improved air handling plus ability to recover energy from the exhaust
• Potential use of artificial intelligence/machine learning models to optimize the MMC properties used in the piston and conrods
• Additional work on friction . . . use of iron-based thermal spray bores, perhaps use of some advanced diamond like coatings on piston skirts, bearings, rings, etc.
• Perhaps some architectural changes, such as a longer conrod length to stroke ratio and added crankshaft offset to minimize piston side forces on the bore

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³ A PAT gauge is a type of inclinometer used to measure distortions in bore holes.

⁴ K. Westbrooke and D. Konson, 2020, “What is the Future for Diesel?” presentation at the Diesel Technology Forum, Tenneco, August 20, https://www.dieselforum.org/files/dmfile/future-of-diesel-presentation-final.pdf.

• Perhaps using free piston technology to effectively eliminate piston side forces on the bore almost entirely and reducing power cylinder friction
• Possible use of additive manufacturing for pistons to provide cooling gallery and localized skirt compliance opportunities not possible with traditional machining processes

Of particular importance, there have been several instances in past OP2S engine combustion studies where the combustion CFD studies have suggested design directions that were subsequently proven on dynamometer to be incorrect. Examples include studies of injector spray angle, number of holes, hole sizing, and piston crown shape.

As a first step, it is suggested that these faulty CFD studies be closely examined to determine the “root cause” for their failure. The fault may lie in one of the submodels, such as the injector spray break-up model. Perhaps only some revisions to the “tuning constants” used in these models may be needed. But perhaps a more extensive rewrite of the code will be required. Since most original equipment manufacturers use commercially sourced CFD code, the most likely candidates to resolve this issue will be the software suppliers (e.g., Convergent Science, AVL, FEV, etc.), Sandia or Argonne National Laboratory, or a major university (e.g., University of Wisconsin Engine Research Laboratory, MIT, etc.).

Once these models have been corrected, “analysis led design” can be much more effective, enabling combustion optimization approaches such as genetic algorithms. This will minimize the number of required hardware iterations to achieve the targets.