Are Opposed Piston Diesels the Future of Military Powertain Technology?
AVL has announced that they’ve been awarded a $17 million dollar project by the National Advanced Mobility Consortium (NAMC) to do some R&D work with the US Army Tank Automotive Research, Development and Engineering Center (TARDEC).
Together, they’re going to work on an opposed-piston two-stroke single cylinder advanced combat engine technology demonstrator.
That’s a lot of words.
Essentially, they’re going to do develop an advanced engine which could power a number of military vehicles.
This is especially interesting for the engineering community because it’s an opposed-piston engine, not an advanced gas turbine.
We know that M1 Abram Tanks, for example, used gas turbine technology. Conventional wisdom has been that gas turbines are the way to get the most thermal efficiency out of any kind of automotive power system. That’s why they use it as turbo shafts in helicopters.
So what should we expect from an opposed-piston engine?
We know that the military favors a commonality of fuels, usually kerosene-based, JP type fuels. So it’s safe to assume this is going to be a diesel.
Opposed-piston diesels have been done before and been done successfully, notably in the early 1930s.
The Jumo Engine Company in Germany, Junkers Engine Division, developed an aviation opposed-piston diesel engine in the early ‘30s, the Jumo 204.
It’s a really interesting way of getting thermal efficiency from a mechanically simple device. If you use blower scavenging and a two-stroke diesel configuration you can do this without a valve gear. You also reduce the number of machine surfaces in the crank case, eliminating the cylinder head.
Essentially, you have single cylinders that hold two pistons, you’ve halved the number of effective cylinder bores that have to be in the machine and have only bearing saddles to hold the two crank shafts. The crank shafts are a disadvantage which you have to gear together to combine that power.
In the Jumo engine, they used only one of the crank shafts to power all the engine accessory drives, absorbing about 25 percent of the power in doing so. You still have to draw power off of one of the crank shafts though.
One alternative to that technology appeared in the ‘50s.
Rootes Group designed a diesel engine for Commer’s commercial truck division, called the TS3.
The TS3 is a really interesting technology where very large rocker arms are used to convert that reciprocating motion of the pistons going in and out and convey that to a single crank shaft in the center. It was a really useful kind of technology at the time.
It was however nicknamed “The Knocker,” which gives you a sense of how noisy the thing was.
Large sorts of high mass rockers, oscillating back and forth, looked like a neat solution to the problem, but I doubt they’re going to work in this application.
So is the future of US military engine technology going to move away from gas turbines and toward good old-fashioned diesel engines? I think it’s possible.
With the thermal efficiency gains that are now possible through improvements in lubricants, thermal efficiency and decreases in internal friction, I think the hundred-plus-year-old technology of reciprocating pistons and cylinders won’t be going away any time soon.