When will we have opposed cylinder engines?

Jon Lawson
Different stages of combustion in an OP engine (source: Achates)

The concept of opposed piston (OP) engines has been around over a century and a quick search on the global patents database reveals over 700 entries. The appeal is clear to see: no head means no wasting energy heating it up then wasting even more energy cooling it down. And no head gasket means better durability. Also this architecture lends itself well to more compact designs meaning more space for other things on board the vehicle.

During the last 5 years Volvo Truck, Cummins and Toyota have all taken out patents. One of the most active Tier 1 researchers in the area is Achates Power, based in San Diego, California. Starting out in 2004, the firm concentrates its efforts in car, truck, military vehicle and stationary engines for power generation.
EVP of Business Development Laurence Fromm says, “Despite Covid-19 and the work-at-home mandates at various US locations, we continue to make good progress. One project is a 1,000 hp Advanced Combat Engine for the US Army, being developed by the team of Cummins and Achates Power. That program is proceeding nicely with engine tests taking place in Columbus, Indiana (by Cummins); Warren, Michigan (at the US Army Ground Vehicle Systems Centre); and San Diego, by us. Vehicle tests are planned for next year. The strategic motivation of the program is a doubling of power pack density – twice as much power in the same space to substantially increase the ability of the combat vehicles to engage and evade the enemy.”
With the ability to run a variety of different fuels, it’s no surprise this type of engine has been interesting  for the military for decades. Throughout the cold war the Russian military experimented with OP engines in tanks. Potential disruptions to supply lines while behind enemy lines mean the greater fuel flexibility the better.
This also offers a further benefit, where environmentally-friendly fuels are concerned. Fromm continues, “We think renewal fuels have a lot of potential since they can combine carbon neutrality with the range, refuelling speed, infrastructure support advantages of a fossil-fuel liquid hydrocarbon. It would depend on the nature of the fuel, but we would not expect any different challenges than a conventional engine. We’ve primarily worked with diesel, JP-8 (the military grade heavy fuel) and gasoline. The gasoline work, interestingly, is without a spark plug - gasoline compression ignition - for diesel-like efficiency. We have also done some work with natural gas. While it would require some more development, it looks promising.”
Also with an eye on the environment, the company has teamed up with the California Air Resources Board (CARB) for a 450 hp heavy duty diesel demonstration program. The plan is to establish that the technology can cut both NOx and CO2 at the same time. Engine testing is happening in San Diego and Novi, Michigan (at development partner Aramco’s facility). Peterbilt has integrated an engine into one of its trucks, and is test driving it at its Denton, Texas facility. It will driven in service by Walmart next year. 
Fromm explains, “Engine testing confirms that we can meet future year (2024) EPA CO2 requirements while also meeting CARB’s ultralow NOx objective (90% reduction in NOx from current levels). We recently commissioned FEV to undertake a cost study – its conclusion is that the OP engine costs 11% less than a conventional engine with comparable power and torque. The main costs savings are due to 260 fewer parts in the OP engine (no cylinder head; head gasket, or valve train) and a 30% smaller aftertreatment system due to low-emissions characteristic of the OP engine.”
Meanwhile the company is also working with Nissan and the University of Michigan on a hybrid project, and a full-size pickup truck program is also on the cards, both DOE-funded programs. Argonne National Labs near Chicago is also testing one of Achates’ engines for another industrial firm.
So it is quite possible that in time the OP design may well become mainstream, especially for heavy duty applications.

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