A new opposed-piston diesel engine design yields more power and better efficiency from a smaller displacement, and is coming soon to a tank near you.



There’s a new breed of engine coming for the USA’s armed forces, and it could pave the way for more powerful and efficient engines in broader applications.

It’s called the Advanced Combat Engine (ACE). A co-development of Cummins and Achates Power, it’s radically different to a typical diesel engine that we all know. It’s the result of a $US47.4 million contract, aimed at producing the next generation of engine for military use.

Opposing pistons

Instead of having six or eight pistons all in separate cylinders, all in a row or V turning a common crankshaft, this new engine design places two pistons in each cylinder. These pistons work opposite of each other, sharing the same air/fuel mixture and resulting combustion.

Horizontally-opposed, each side of pistons turns it’s own crankshaft. These crankshafts are connected via gearsets, which then turns a single output shaft.



Opposed-piston engines aren’t a new idea. Some might remember the ‘Commer Knocker’ from the 1950s and 1960s, and know of Junkers airplanes in the 1920s and 1930s. However this new design brings an old idea into the modern world, capable of improved power, efficiency and emissions.

Four cylinders in this diesel engines make up 14.3 litres of overall capacity , with peak power of 1000 horsepower (745kW) at 2600rpm. This number is no doubt helped along by two turbochargers and a gear-driven supercharger. Yes, twin-charged, like an old Volkswagen 1.4TSI.

The engine will be modular for larger and smaller applications, with a four-litre version making 223kW mentioned. Go bigger, with a 20-litre, 12-piston unit, making 1118kW.

This new engine will be coupled to a more efficient transmission in tank applications, which can be up to 90 per cent efficient in terms of power loss. John Tasdemir, from the US Army Ground Vehicle Systems Centre , told National Defence Magazine that some tank transmissions can be as bad as 55 per cent.



No valvetrain?

Nope. In fact, because of the opposed-piston layout, this engine has no cylinder head whatsoever. Instead, it uses a two-stroke combustion cycle, like your backyard whipper snipper. That means fuel and air is ingested, combusted and expelled in two strokes of the piston, in half the time of a more common four-stroke engine.

Instead of using things like camshafts, valves and lifters to control gas flow and compression, combustion gasses are moved into and out of the cylinder via ports placed strategically on cylinder walls. As the pistons move up and down during operation, these ports effectively open and close. Incoming air pushes out exhaust gasses, and fuel is then injected directing into the combustion chamber as the pistons close in on each other.

Once again, two-stroke diesel engines aren’t a new design. Companies like Detroit had great success with large and powerful two-stroke diesel engines in large trucks for many years. While powerful, old Detroit diesels were never considered leaders of efficiency and emissions.



Hybrid power

This new combat engine will include a hybrid powertrain of sorts, with an integrated starter generator that sits in between the engine and transmission. It’s quoted at having as much as 160kW available, and will be able to fuel a 600-volt auxiliary power system with lithium-ion batteries.

These new generation batteries are touted to offer twice the power density at a quarter of the weight, and can last as much as ten times longer than a lead acid battery.This development is interesting, as it follows in the same vein of mass electrification of passenger and commercial vehicles for road use.

Such a system has more potential in the future for military vehicles, like an EV mode for silent operation: Tasdemir explains:

“Our next set of programs are focused on electrification of combat vehicles, including hybrid systems that can offer silent mobility and these long periods of silent watch,” he said. “The better storage we have, the longer mobility we can get or the longer silent watch. So it’s certainly a stepping stone to that capability.”



More efficiency

This new-generation opposed-piston engine has roughly 25 per cent better fuel efficiency and heat rejection, compared to a typical diesel engine. This is because of better thermal efficiency from two pistons and no cylinder head, which also allows for a better combustion ‘chamber’ design.

This opposed-piston design effectively gives the engine more power density, meaning it develops more power for the given size, weight and capacity. In comparison, Cummins’ own B903 engine is a 14.8 litre 90-degree V8, which makes a maximum of 750 horsepower when twin-turbocharged.

The ACE engine is more compact and box-shaped, lending itself to a more compact overall packaging compared to a V or inline, especially in terms of overall height. This means new, clean-sheet platforms will be designed around this more powerful and compact engine, with scope for extra space, maneuverability, capability or armour.



Applications

This ACE engine is slated for use in the Bradley Fighting Vehicle and M88 armoured vehicle so far, although smaller and larger iterations could be used in a variety of different applications. Achates Power is also developing a 2.7-litre variant of the engine for non-military use, which has been fitted to a test Ford F-150. This size makes 201kW and 650Nm, and is touted to be more efficient than what is typically used.

There is also a 10.6 litre variant being developed for trucks, which has earned Achates Power a USD$9 million grant, making 335kW and 2,372Nm. Along with that huge amount of torque, Achates Power claims a 15 per cent reduction in CO2 and 90 per cent reduction in NOx.


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