Mazda's five-decade-long, largely futile efforts on behalf of the rotary engine demonstrate how difficult it is to unseat the conventional piston engine. Yet new concepts continue to emerge, especially given the industry's relentless quest for efficiency. Most of these never make it past the design—or even crayon—stage. But here's one design that is well into development and shows considerable promise.



It's the Achates opposed-piston (OP) engine, a prototype of which was shown in a Ford F-150 pickup at the 2018 Detroit auto show. That engine had three long cylinders in a row, with one crankshaft below and another above the row of cylinders. A pair of opposed pistons reciprocated in each cylinder, each connected to its own crankshaft.

How It Works

The engine is a piston-port two-stroke, meaning that it has no valves. Instead, ports in the walls of the cylinders are uncovered when the pistons are at the bottom of their strokes. The lower ports are the intakes and the uppers are the exhausts, and the exhaust leads the intake by about 10 degrees to ensure that the exhausts open first. When both are open, a supercharger blows fresh air into the cylinder at one end to force the exhaust gases out the other end. Then the pistons start to move toward one another, covering the both ports and compressing the air. Fuel is injected directly into the cylinder when the pistons near the end of their travel. Thus, there are no intake and exhaust strokes as in a conventional four-stroke engine—only compression and power strokes. It is probably easier to understand by watching this animation, provided by the manufacturer.

The basic layout of the Achates engine has been around for some time. The German Luftwaffe started using similar Junkers Jumo opposed-piston diesels in large aircraft in 1932. A larger Fairbanks-Morse opposed-piston diesel powered most U.S. Navy submarines in World War II and still provides backup power on modern nuclear subs.

Making an Old Idea New

Achates has taken this basic design and improved the combustion characteristics, while also providing the ability to tailor the engine's operation to match different conditions and fuels. Much of the development has focused on diesel versions, but the company is also working on gasoline versions without spark plugs—the HCCI (Homogeneous Charge Compression Ignition) concept that many are developing but no one has perfected.

Theory suggests that an opposed-piston engine has an efficiency advantage because, since there are no cylinder heads, less heat is lost to the cooling system. Think about a conventional combustion chamber in which the surfaces consist of a piston crown, a short cylindrical circumference, and a cylinder head. In the OP design, each pair of cylinders comes together, so the heads are eliminated.

Furthermore, because each piston's combustion chamber connects with another's, the resulting chamber is thicker and less like a thin disc. Achates increases this advantage by using a stroke as much as 1.3 times as long as the bore. Achates carefully shapes the piston crowns to provide swirl and tumble to the intake charge to promote rapid and stable combustion.

Two opposing direct injectors—each delivering multiple carefully timed squirts—achieve a thorough distribution of fuel to burn most of the air and to limit particulate generation. A compression ratio of around 18.0:1 for the gasoline compression-ignition (GCI) version and a bit less for the diesel assures a good expansion ratio to extract maximum mechanical energy from each combustion event.

Superior Scavenging Is Key

The secret sauce to making this work is the ability to control the scavenging in the engine. The supercharger, which provides low-pressure air to scavenge the cylinders, has a two-speed drive and can also be bypassed. This flexibility allows the cylinder to be less than fully scavenged under certain conditions. For example, during a cold start, partial scavenging leaves plenty of combustion products from the previous cycle (internal exhaust-gas recirculation ) in the cylinder. That raises the combustion temperature and delivers a hotter exhaust stream to get the catalysts up to operating temperature.

Such a hotter combustion environment also helps combustion at low rpm and under light loads, which has been a problem area for conventional HCCI engines. At idle, for example, the retained exhaust gas is about 50 percent. At higher power outputs, the cylinder is more completely scavenged to keep peak combustion temperatures down, thereby limiting nitrogen-oxide (NOx) emissions. Also helpful is that, while the engine also has a turbocharger to take over from the supercharger at higher loads and to improve efficiency, the engine does not operate with much boost pressure. Instead the two-stroke design, with twice as many power strokes per revolution, lets the engine match conventional engine power without much boost, even when it is downsized by 20 to 30 percent in displacement.

Although there is no conventional valvetrain in this engine, overall friction is similar to a conventional engine because of the second crankshaft, the robust geartrain connecting the two cranks, and the need to drive the supercharger. Weight is also roughly comparable. The engine is taller than a conventional engine, but by tilting it, it generally fits under existing hoods.

More Efficient Than a Prius?

Achates claims that the GCI version of this engine will achieve a peak efficiency of 44 percent. That's only 10 percent better than the 40 percent efficiency claimed by the Atkinson-cycle engine in the current Toyota Prius, but Larry Fromm, Achates's executive vice president for business development, says that the Achates engine delivers that efficiency over much broader range of rpm and load than the Prius engine. For example, the Achates GCI engine in the Ford F-150 is said to achieve 30 percent better fuel economy on the EPA cycle than Ford's most efficient gasoline engine, the 2.7-liter EcoBoost V-6. That would mean a combined rating of about 29 mpg—or 4 mpg higher than Ford's 3.0-liter diesel V-6 in the rear-drive version of that truck.

Unfortunately, we're not likely to see an Achates engine under the hood of a car or truck for some years. Achates has no plans to build engines itself but intends to license its technology to established manufacturers. It already has a relationship with Fairbanks-Morse, which is using the Achates technology on a 12-cylinder, 24-piston, 5000-hp, 95,000-pound stationary diesel used for power generation. Fairbanks-Morse claims a thermal efficiency of 50 percent.

Achates is also working with Cummins to develop a 1000-hp opposed-piston engine to power combat vehicles for the U.S. Army. Fromm says that the company is in talks with several major automakers that are interested in the GCI version. But with nothing concrete yet, and with the lengthy development and testing times that new engines undergo, we won't see an Achates engine on the road for at least five years.

Automotive Applications Could Include Plug-In Hybrids

If you think that by then electric cars will have eliminated the need for internal-combustion engines of any kind, think again. If the engine's efficiency is realized in production, achieving 2025 federal fuel-economy standards might be easier and cheaper with the Achates design than by employing electrification. And, Fromm points out, opposed-piston engines are inherently smooth, so a single-cylinder, two-piston version could make an ideal range extender for a PHEV, especially since it can be made into a flat package tucked under a trunk or seat. Only time will tell if Achates has cracked the code for opposed-piston engines in cars and trucks.

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