By Christopher A. Sawyer
David Johnson has an enviable résumé. He launched an all-new diesel in Ford’s Super Duty pickup before heading to GM where he worked on clean diesel programs for Opel, Saab and Renault passenger vehicles. Navistar was next. There he was program manager for the MaxxPro line of Mine-Resistant Ambush-Protected (MRAP) vehicles used in Iraq and Afghanistan, before becoming vice president of product operations for military and export markets. In 2008, he went to Achates (a-kay-tees) Power as president and CEO, and was given the task of using that experience to accelerate the development of an opposed-piston two-stroke diesel engine. These engines were prevalent in the early part of the 20th century, but fell afoul of shortcomings like high hydrocarbon emissions and heavy oil consumption as that century progressed. However, until their deficits outweighed their plusses, opposed-piston two-strokes were more efficient than conventional piston motors.
“Why even bother?,” he repeats. “I asked myself the same question when I came to Achates, but the engine is very sound in terms of both physics and thermodynamics.” Not only are there no valves, there’s no valvetrain to introduce parasitic losses or add to the parts count. Also missing is a cylinder head that increases heat losses in the engine by removing temperature from the combustion chamber and releasing it to the atmosphere. Then there’s the long stroke-to-bore ratio. And the list goes on.
According to Johnson, “Many in the industry said: ‘You can’t have efficient combustion chambers in the void between the two pistons, and all two-strokes have trouble with oil consumption.’ They were historically accurate, but we believed that if we applied the engineering, materials and other improvements made to conventional engines to our motors, we could overcome these obstacles. Plus, in not knowing what couldn’t be done, we did it.” The Achates engine has demonstrated the ability to meet EPA10 and Euro6 emission standards.
The fuel efficiency gains versus conventional engine designs are not single-digit increases. Johnson claims gains on the order of 10%-20%. That’s significant. And that’s against the most efficient engines in the world. “Typically we show, on average, a 15% benefit,” he says. That number, it should be noted, is against four-stroke diesels of similar output. There has not been an apples-to-apples comparison to a gasoline engine, but Johnson thinks the rule-of-thumb difference would be on the order of 50%.
Achates does not have fully developed engine designs ready for production, though they are working on them with potential customers. “Based on the parts reduction and downsizing that comes with that, and a two-stroke’s greater power density per cc, our forecasts say Achates engines will be less expensive than conventional four-stroke designs,” he says. But Johnson also acknowledges that conventional two-stroke diesels fell out of favor in the 1970s when emissions regulations began to come into effect, and the four-stroke took the lead because each engine event (intake, compression, ignition, exhaust) is discrete, while a two-stroke accomplishes them in half the crankshaft time. However, he points out this was before the advent of modern microelectronics, which made controlling these events a problem, and this accelerated the shift toward four-stroke diesels.
Having opposed pistons creates an interesting problem: the pistons moved outward from the centerline of the engine, and toward the outer ends of the engine block. This suggests the Achates motor has two crankshafts. “Some of the opposed-piston engines have had two crankshafts,” says Johnson, “and some have just one. In the engines we run, we use two cranks, though we have looked at one. To us, the most important point is to have the two pistons in the one bore, and from there we do an application study to figure out which is the best engine architecture to get those piston motions converted into torque at the crankshaft.” Theoretically, it would be relatively simple to join the two cranks to a central power takeoff, but this might increase parasitic losses higher than the benefit this layout creates. “Believe me,” says Johnson, “we’ve looked at all of the possibilities, and have a number of solutions on the shelf.”
Inserting the fuel into the system is also a challenge. There are pressures, flow patterns, heating rates and other variables that must be studied to create the piston dome design, injection timing, spray pattern and more. “Since we are a diesel, we always will be direct injection,” he says. “We are not mixing the fuel and air before it enters the cylinder. We bring in the fresh air, often mixed with cooled EGR (exhaust gas recirculation), and residual exhaust gas.” That’s because, unlike a four-stroke engine, a two-stroke doesn’t fully scavenge exhaust gasses from the cylinder. The intake and exhaust ports are opened simultaneously, and exhaust gas goes out one end of the cylinder while fresh air flows in the other end. Depending on the pressure differential between the intake and exhaust ports, this determines how quickly the air flows through, how completely it is scavenged, and what remains in the cylinder as the pistons start to close off the ports. After that you have compression, the injection of the fuel and combustion.
“That’s where we have deployed computational fluid dynamics (CFD) extensively,” says Johnson, “in order to manage and understand how to make the air flow through the engine in a way that is optimized for current requirements and maximum possible efficiency.” Achates also created their own fuel lab so that they could characterize fuel spray and atomization on their own. This allowed them to tailor the atomization, droplet size, spray pattern, etc. for the engine’s combustion chamber design. “In that way,” he says, “we didn’t have to rely on a supplier industry that wasn’t very interested in opposed-piston engines.”
“We can, and have, run with multiple injections [of diesel], especially when we do studies for out light-duty [vehicle] customers. For them, noise is a critical parameter.” Nevertheless, the most efficient method is a single, optimized injection of fuel with the quickest possible burn. And when that can be done without creating undue noise or high NOx emissions from a high-temperature combustion, it is done. This makes us no different than any other engine maker in the industry,” he says. “They run the injection single-shot whenever they can.”
A lot of effort has gone into reducing the amount of work the engine has to do in order to move air into and out of the cylinders. “It really comes down to the fact that we don’t need to scavenge the cylinder after each combustion event,” claims Johnson. “Depending on where you are operating the engine, we sometimes have a significant advantage, and at other times we have a modest advantage.”
Conventional four-stroke gasoline engines are having to add technology in order to get closer and closer to diesels in terms of fuel efficiency, and Achates claims its engine is already ahead of today’s best diesels. “Because of our unique geometry, we can put multiple injectors into a single combustion chamber,” says Johnson. “There are no valves to get in the way, which makes for interesting opportunities.” Achates can add injectors around the bore as well as along the length of the bore. “We can tailor this to the fuels being used, and do things like an early injection to create a homogenous charge situation before the bulk of the fuel is introduced.”
An Achates four would have two cylinders, not four, however, friction would be a little higher due to the gear train connecting the two cranks. On the other hand, the combustion efficiency is so much higher that this offsets the higher drag. In addition, though wider than an inline or v-type engine, the Achates unit has a lower height, center of gravity, and a shorter overall length. Packaging is a plus, not a minus.
How the engine mounts in the chassis depends on the application. It can be laid down, stood on its end or anything in-between. However, it’s likely that light-duty trucks or automobiles would mount the engine flat. The fact that it is shorter in some measures and longer in others, and displaces less for the same output, means automakers might want to take a look at new ways to drop it into the vehicle. With vehicle size becoming critical to meeting future fuel economy regulations, taking a hit on vehicle width, packaging the engine partially in front of—or behind—the wheels, or even sitting it lengthwise takes care of that concern.
“The Achates engine rejects less heat, meaning radiators can be smaller. The nose can be lower, if you decide to put it up front in a conventional position. Vehicles today,” Johnson expounds, “have been optimized around the current crop of engine designs, which haven’t fundamentally changed for decades. However, that will change once a company decides it wants to embrace this technology.” Imagine the possibilities… like a four-cylinder/eight-piston mid-engined, two-stroke diesel sports car.
But what about fuels other than diesel? Is the Achates engine doomed to run on straight diesel or a mix of it and biodiesel? “I’m certain that, whatever fuels are available at the pumps of the local fuel station in 2050, they will run better and more efficiently in an Achates engine,” says Johnson. “If you can make diesel combustion work,” he continues, “you can make any fuel work.”
Every engine has a tradeoff between output and emissions, including the Achates. But as after treatment costs fall, these savings are passed along to all engines, even the opposed-piton two-stroke Achates motor. However, Johnson says that, though the fundamentals and after treatment strategies are basically the same, the greater efficiency of his company’s motor adds to the value proposition. All it takes is a vehicle maker ready to take the plunge. Just don’t expect it to happen quickly, even if the trigger was pulled today, it might take six years or more for the Achates engine to enter service; just in time for the second—tougher—half of the recently passed CAFE legislation to take effect. In the meantime, watch the skies for drones and the battlefield for vehicles powered by these engines. The military, which lives or dies on the battlefield cost of fuel, is very interested in the Achates engine.