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US Army awards Cummins $87M contract to deliver the opposed-piston Advanced Combat Engine

The US Army has awarded Cummins an $87-million contract to complete the development of the Advanced Combat Engine (ACE), a modular and scalable diesel engine solution, capable of hybridization, that uses innovative opposed-piston (OP) technology to provide leap-ahead capabilities in power density and heat rejection not available in the current marketplace. (Earlier post.)


Coupling our opposed-piston technology with Cummins’s robust engine design, manufacturing, and new product introduction capabilities allows us to create a high performing and reliable engine for military operations. Achates Power is pleased to continue to support Cummins as a key technology partner in creating the next generation of engines for combat vehicles.

—Dave Crompton, President and CEO of Achates Power

The recently awarded Other Transaction Authority (OTA) contract builds upon a competitive multi-year effort from the US Army’s Ground Vehicle Systems Center (GVSC) to develop transformational powertrain technology that is power dense, thermally efficient, modular, scalable, and affordable enough to enable the toughest mobility, survivability, and lethality vehicle requirements.

Furthermore, the OTA allows for transitioning the new technology into the next generation of vehicle programs ahead of the programs’ launch dates.


ACE’s innovative OP technology provides a 50% increase in power density, a 20% reduction in heat rejection, and 13% improved fuel efficiency when compared to today’s best in class combat engine. ACE’s flexible layout options allow the engine to be configured in 3 cylinder, 4 cylinder, and 6 cylinder arrangements to deliver power ranging from 750 to 1,500 horsepower.

ACE can be integrated into hybrid architectures enabling commonality, thereby eliminating the expensive logistical burden of having multiple combat powertrains and facilitating the incorporation of new electrified technologies.

We are excited to continue our valuable partnership with Cummins on the development of the Advanced Combat Engine (ACE); what’s under the hood of the Army Ground Vehicles is also what drives our team’s efforts at enhancing capability development. This project is a key part of our research and development work, and we see ACE as a potential enabler for both traditional and hybrid electric powertrains applicable to current and future combat vehicles.

—Alfred Grein, Executive Director for Research & Technology Integration, US Army DEVCOM Ground Vehicle Systems Center



Nice to see this concept getting closer to production! The Germans got pretty far in the development of the OP concept already during WWII but they lost the war, as we know, and somehow, the industry lost interest in OP engines. Now Achates/Cummins are aiming to advance the technology one step further and it will be exciting to see the outcome.


The Junkers Jumo 205 was an outstanding engine. However, opposed piston engines were developed by many companies, e.g. Fairbanks Morse, Commer, Napier, Rolls Royce, Leyland, and KMDB. Today, KMDB in the Ukraine still markets a two-stroke multifuel engine with six cylinders 1500 HP version of the 6TD series of engine.


Gemini has one for small planes


So why isn't Cummings pushing a commercial version of this engine?

"ACE’s innovative OP technology **provides a 50% increase in power density,** a 20% reduction in heat rejection, and 13% improved fuel efficiency when compared to today’s best in class combat engine. "

Achates has unwisely positioned this technology as an efficiency advantage (small in reality) when it actually has been a power density advantage (large).



I just returned from the EAA (Experimental Aircraft Association) Oshkosh Airshow and while Superior had a display they apparently are no longer working on the Gemini Diesel and are just offering parts and remanufacturing for Lycoming and Continental engine. About 5 years ago, they were displaying the Gemini but I have not seen it for the past 3 or 4 years.

An interesting engine that was new this year was the Higgs Diesel. It not a true diesel in that it has spark ignition but it runs on diesel fuel. It is a different type of 2-stroke that has a stepped compression or pumping piston and a power piston in a single stepped bore. They have a 1600 hp V-12 that weighs only 740 lbs that and is designed to replace a Pratt & Whitney turbo-prop. They also had several different V-4s on display.

I worked on an opposed piston diesel engine first almost 30 years ago first on a DARPA contract to demonstrate our ability to manufacture CNC machined parts directly from a solid model. Then about 20 years ago, I consulted on designing and building parts for this engine but there was never sufficient funding to finish the engine.


They have the design but no government contract


Just a short note about the Junkers engines: Jumo 207 was far more advanced than Jumo 205. Among other things 207C had a turb-supercharger. In particular, an experimental version of the Jumo 207C (as basis for Jumo 224 development) could be mentioned. Below is an excerpt from one of the links below. We can leave Jumo 224 for later discussions... A power level of 2,210 hp was pretty good from an engine of this size, i.e. roughly similar as the most powerful versions of RR Merlin (e.g. Spitfire) and DB 605 (Messerschmidt Bf 109), in spite of that these engines had roughly double cylinder capacity.

“While the Jumo 224 was being designed, a Jumo 207C was tested to its limits to better understand exactly what output could be expected from the Jumo 224. Tests conducted in late 1944 found that with a 200 rpm overspeed (3,200 rpm), intercooling, modified fuel injectors, and 80% methanol-water injection, the Jumo 207C was capable of a 10-minute output at 2,210 hp (1,645 kW)—twice its standard rating of 1,100 hp (820 kW).”

Junkers Jumo 207 D-V2 In-line 6 Diesel Engine | National Air and Space Museum (
Junkers Jumo 224 Aircraft Engine | Old Machine Press

Kudos to SJC for actually participating in OP engine development, which I suppose is rare on this forum. My own experience is mainly limited to much more conventional diesel engines.

Tim Duncan

@SD, I also visited Superior both at Oshkosh. Their folks said the Gemini was way back on the back burner and was not being worked.

@charlesH 13% improvement to the best diesels would actually be huge. The Diesel engine company I work for would kill for this in their BTE sensitive markets. 1.13x0.45%=50.85%. ~50% is consistent with Fairbanks Morse OP using Achates tech, they say theirs is best in class. New EPA fuel efficiency requirements will make a few percent the difference between market access and obsolescence.

Little longer term 51% also approaches the PEAK efficiency of exotic & expensive PEM fuel cells. Achates has demonstrated BTE near peak over most of their load/speed map & has a hydrogen engine program. When considering the typical drive cycle it has very competitive BTE, lower build cost and installed worldwide Mfg base.

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