The United States Armed Forces are constantly seeking to maintain their advantage over opposing armies by upgrading and improving their combat equipment, including the Bradley Family of Vehicles and the Next Generation Combat Vehicles.
As part of the enhancement program, the National Advanced Mobility Consortium has awarded Cummins Inc. a $47.4 million contract for the design and development of a revolutionary new type of diesel engine to be used by the U.S. Army. Designated the Advanced Combat Engine (ACE), the project will develop an engine that is lighter and more efficient than those currently in use.
Wayne Eckerle, vice president of corporate research and technology at Cummins, says “We are confident we can achieve significant improvements in mobility, power, range and fuel economy, creating combat vehicles that are safer, faster and have clear advantages in the field.”
Cummins will team up with California-based Achates Power to develop the new engine. Achates Power has already designed a two-stroke combustion cycle, opposed-piston engine that works without a valve train.
Cummins, Inc.
Headquartered in Columbus Indiana, Cummins Inc. is a global leader in power equipment and engines. The corporation is composed of multiple business units that specialize in the design, manufacturing, distribution, and service of diesel and natural gas engines. Their expertise includes related technologies, such as controls, fuel systems, air handling, emissions, filtration, and electrical power generation systems.
The company serves customers in approximately 190 countries and territories and employs 55,400 people worldwide. In 2016, earnings were $1.39 billion on sales of $17.5 billion.
Achates Power, Inc.
Founded in 2004, Achates Power, Inc. has designed and developed significantly improved internal combustion engines (ICE) that are more cost-effective and fuel-efficient than standard models, while reducing greenhouse gas emissions. The San Diego-based company employs an experienced staff of engineers and scientists, using leading-edge testing, simulation, and analysis tools.
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Opposed Piston, Opposed Cylinder (OPOC) Engine
Opposed piston engines using a two-stroke cycle have been around for many years. Oechelhäuser produced a 600 hp., two-stroke gas engine that was installed at the Hoerde ironworks in Germany as early as 1898.
The OPOC engine has two interlaced pistons in each cylinder. Each piston is divided into two parts, one moving inside the another in opposite directions, which creates the compression stroke. The intake stroke is created when the opposing ends of the other piston are moving apart to admit air in the gap.
The two back and forth motions are simultaneous, making it a two-stroke engine instead of the more conventional four-stroke engine. The two pistons in a single cylinder accomplish the work of the two pistons in two normal cylinders and apply the equivalent motion to the crankshaft. It produces a high-power density or high ratio of power to the mass of the engine.
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The Advanced Combat Engine (ACE)
Achates CEO David Johnson claimed the company’s new 270-hp, 479 ft-lb supercharged turbodiesel opposed-piston engine, available in 2018, will improve efficiency by 30 percent over current diesel engines with similar output, and will be more economical by 50 percent than the best gas engines.
The engine features three cylinders, each containing two opposed pistons that are connected to geared crankshafts, one at the top and the other at the bottom of the engine and a common output.
The air-fuel mixture is compressed between the pistons. Intake ports are exposed at the bottom of the piston strokes, and a direct fuel injection system adds diesel as the piston heads come together.
Achates states that eliminating the head reduces heat loss and augments the engine’s thermal efficiency. Cummins says, for the ACE, it is aiming for a 21 percent reduction in thermal rejection, a 50 percent power density increase, and a 13 percent hike in fuel efficiency compared to current engines.
Efficiency
Thermal efficiency is improved due to the low ratio of combustion-chamber surface area to cylinder displacement volume. Without a cylinder head, less combustion heat is rejected to the cooling system, and more goes to propelling the vehicle.
Without cylinder head bolts that tend to distort the bore, there is less ring friction. The lower peak operating speed also contributes to lower friction.
The location of the ports around the cylinder at the top and bottom of the pistons’ strokes means the pistons are not required to do any pumping work. The supercharger and turbo operate more efficiently with some of the ports always open.
The engine can be made lighter since operation at lower cylinder pressures does not require hardening to the same extent as a conventional four-stroke diesel.
Moving Parts
In an OPOC engine, the moving parts are much less complicated than an internal combustion engine where an intricate mechanism is required to time the intake valve and exhaust valve openings. In the OPOC engine, the valves are replaced with holes in the side of the cylinder. The piston sliding motion covers and uncovers the holes and, therefore, eliminates the need for a complicated valve mechanism.
Ecomotors, an OPOC engine manufacturer, estimates that the number of moving parts in its engine is 62—much less than the 385 moving parts in a typical internal combustion engine. Fewer moving parts translate to lower maintenance costs and a more reliable engine.
Reduced Emissions
With lower peak cylinder temperatures and pressures, less nitrous oxides and other pollutants are produced in the cylinder.
The two fuel injectors are designed to spray across the cylinder instead of onto the hot pistons, reducing the quenching effect that causes particulate formation. The design also includes a long piston stroke, which promotes the complete burning of hydrocarbons.
Lower Costs
The overall assembly is simpler without a cylinder head and valve train, even with the addition of a second crankshaft and connecting gears. The engine production will require only minimal retooling of existing engine plants.
The new engine can be implemented with a smaller cooling system since it produces less heat. Catalysts can also be downsized with fewer emissions to control. The engine’s broad range of torque and efficiency should permit it to be used with less expensive transmissions.
Implementation
During the Obama administration, new standards for the 2025 Corporate Average Fuel Economy (CAFE) were released, requiring automakers to meet an average fuel efficiency of 54.5 miles per gallon for new cars and trucks. Achates states that the engine will meet the new CAFE regulations and will cost about $1,000 less than modifying the current engine models.
For the military, the 600 hp Cummins VTA-903T currently used in the Bradley Fighting Vehicle is targeted for replacement by the ACE. Engine testing will be conducted by the U.S. Army Tank Automotive Research, Development and Engineering Center (TARDEC) in 2019.
Conclusion
New technologies like electric motors, hydrogen fuel cells, or compressed air may eventually replace the internal combustion engine, but their development takes time. While electric motors are probably our best bet for the future, ICEs are the near-term workhorses that power our vehicles. What we need is something practical: a better internal combustion engine. The OPOC technology offers just that—a lighter, more fuel-efficient and less polluting engine for use in automobiles and military applications.
