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Navistar reaches development agreement with EcoMotors International on Opposed-Piston, Opposed-Cylinder Engine

Cutaway diagram of the opoc engine. Click to enlarge.

Navistar International Corporation has reached a development agreement with EcoMotors International in support of the company’s opoc (Opposed-Piston, Opposed-Cylinder) engine architecture. EcoMotors International’s first product targeted for commercial application is a turbo-diesel version of the opposed-piston, opposed-cylinder engine.

The opoc engine comprises two opposing cylinders per module, with a crankshaft between them—each cylinder has two pistons moving in opposite directions. This design configuration eliminates the cylinder-head and valve-train components of conventional engines, offering an efficient, compact and simple core engine structure. With a two-module application configured at the appropriate power level (to deliver a combined 480 hp), the opoc unit could deliver about 45% better fuel efficiency compared to a conventional heavy-duty diesel engine in a Class 8 truck, the company has suggested, while delivering emissions at the US Tier 2 Bin 5 level (the 50-state level in the US for diesel light duty vehicles). (Earlier post.)

We continue to be on the cutting edge of technology and our development agreement with EcoMotors once again demonstrates our commitment to develop new, innovative approaches to the commercial vehicle industry. Our company has a long history of pushing the envelope to deliver state-of-the-art, customer-focused solutions and we see great promise in EcoMotors’ breakthrough engine design.

—Dan Ustian, Navistar chairman, president and chief executive officer

EcoMotors’ patented engine design creates a family architecture that will operate on a number of different fuels, including gasoline, diesel, natural gas and ethanol. The opoc’s new opposed piston-opposed cylinder direct gas exchange operation provides the emissions benefits of 4-cycle engines, the simplicity benefits of 2-cycle engines, the power density of the less well known opposed piston engine, and the developments in electronics and combustion technology all tied together in a new and proprietary engine architecture.

Greater power engine density delivers an important benefits will result, including: lower weight; smaller size; lower material costs; lower friction; greater fuel efficiency; lower emissions; and lower heat rejection. The opoc engine has very high power density of nearly one horsepower per pound.

The engine is perfectly balanced, enabling stackable power modules. This modular displacement capability can yield high efficiency, plus much less NVH than a conventional engine of comparable power. With 50% fewer parts than a conventional engine (including no cylinder heads or valvetrain), the design also allows for low-cost, efficient manufacturing and increased operating durability.

EcoMotors’ intellectual property also includes an electrically controlled turbocharger technology which incorporates an electric motor in the turbo assembly to regulate boost pressure resulting in:

  • Improved combustion efficiency to meet emissions;
  • Electrically controlled variable compression ratio;
  • Improved vehicle fuel economy;
  • Enhanced vehicle drivability due to improved low-end torque;
  • Eliminates Turbo lag; and
  • Waste heat recovery by generating electricity.

Electrically Controlled Clutch. The development in clutch technology enables customers to take advantage of the engine’s modular displacement capability. The clutch assembly is housed between two engine modules, and is engaged when vehicle power demands require both modules to deliver power. When the power of the second module is not needed, the clutch is disengaged, allowing the second engine to stop completely. This not only improves fuel economy by reducing parasitic losses, but also improves the efficiency of the primary module.



Holy anti-vaporware batman! Looks like a novel engine could become a reality! (I know, opposed piston engines were used in Junkers, but still...)

This is a huge step forward for EcoMotors. I wish them the best. Hopefully we'll see an OPOC on the road in a commercial vehicle sometime in the next couple of years!


It makes a lot of sense for Navistar to get involved since there is the potential for a diesel engine revolution for medium & large trucks as well as defense applications. Big $$$ here potentially.


The R & D looks headed in the right direction with the emphasis on commercial diesel applications. If they can pull this off for medium & heavy duty vehicles, there is the potential for a diesel engine revolution.


Wasn't that type of engine used in farm tractors some 50+ years ago?


"the opoc unit could deliver about 45% better fuel efficiency compared to a conventional heavy-duty diesel engine in a Class 8 truck, the company has suggested"

Big claim, I believe it when I see it. While a reduction in weight and the number of parts that create friction can reduce fuel consumption I don't see anything in this engine that rates 45% better fuel efficiency.

Nick Lyons

al_vin: What you said.

This claim seems to stem in part from the ability to run only a single module when at part-load, thus running closer to maximum efficiency more of the time, as well as further reducing frictional losses. However, a 45% improvement seems highly unlikely, especially for diesel, which is not throttled at part load.

Despite my doubts about the claimed boost in efficiency, I do like the OPOC design and wish them well. Isn't this potentially the perfect light aircraft engine (lightweight, simple, robust)?

Brian P

This is a piston-ported engine. How are they getting around the lubrication of the cylinder walls leading to oil either getting into the combustion chamber or going out the exhaust? Detroit Diesel never figured that one out (and they had 50 years of production to do so), and they only had this situation on the intake ports (where most of said oil would at least be going through a combustion cycle).


This engine has a huge potential to increase the HP/Pound ratio (2 strokes, no valvetrain, can run twice as fast because of the limited limited course of piston, self balanced architecture well suited to high speed) The potential as a diesel in light aircraft or helicopter is real. It is a uni-flow 2 stroke very effective. Still I wonder about the emissions problem with the oil going in the exhaust as well as the lubrication of the pistons when it comes to car use. For truck they can use a reformer on the exhaust to reduce the soot and unburned oil into CO that can be recycled in the engine.


Would love this as a range extender for the power density and it's lack of vibration- could a small hundred lbs gen-set get me the 40-50 KWH I'd like.

Henry Gibson

I have been calling for the use of smaller, even tiny, OPOC engine-generators as range extenders for several years now. The big versions for diesel trucks will be interesting as the original big versions were thought of for helicopters.

As someone mentioned they might be a good diesel aircraft engine. The formerly Thielert diesel aircraft engine is cutting the fuel costs and extending the range of single and double engined aircraft.

I have an old encyclopedia that shows a very large piston engine operating on the same principle.

The Commer Knocker is an opposed piston engine that used levers to a single crankshaft. The TS3 was widely used, but the larger TS4 in test was destroyed in the test stages by Chrysler which had bought Commer and the engines were superior to the Chrysler contracted ones from Cummins and other suppliers. Four of 14 TS4 prototypes were hidden from the Chrysler management after their destruction with all documentation was ordered. ..HG..


al_vin and Nick Lyons:

From other websites I've seen the claim that the claim is that "the elimination of the valvetrain reduces 40% of the friction". I think a journalist must have gotten it wrong or something. It may be that according to the accounting, the valvetrain can be included in the losses attributed to friction and indeed may amount to 40%.

The figure I usually see is that the valvetrain consumes 10% of the power of an engine, so a truck engine with 45% efficiency could see a 10% of 45% efficiency increase, which is 4.5 ~ 5% increase to around 50%. Maybe a journalist just added 40% and 5% somewhere.

It is impossible for this engine to be 45% more fuel efficient, I only expect an increase in fuel efficiency of a little more than 10% (elimination of valvetrain and better airflow through ports instead of valve openings), or an 5% increase in thermodynamic efficiency from 45% to 50%.

I don't think even part load efficiency can explain the bogus 45% figure.


The PatOP engine (at ) with a basic module comprising one only cylinder and two only opposed pistons is better balanced than the two-opposed cylinders four-opposed-pistons basic module of the OPOC engine of EcoMotors.

The PatOP engine provides some 20% additional time, as compared to the OPOC of EcoMotors, for more efficient injection, penetration, vaporization, and combustion of the fuel spray (at high revs the additional time enables higher peak power, at medium-low revs the additional time enables better efficiency and lower emissions).

The PatOP engine has constantly zero total force on the main bearings of the crankshaft (the connecting rods of the two opposed pistons remain constantly parallel, have normal size and are heavily loaded only in tension, i.e. they are pulling-rods).

The PatOP engine intergrades a “volumetric”, piston-type scavenging-pump that enables a flat torque curve in a wider rev range. The turbo-charging is an option.

The PatOP engine takes the thrust loads away from the hot cylinder walls and away from the ports, enabling far lower lubricant consumption and better lubrication, just like the giant cross-head Sulzer engines; because the PatOP is a cross-head engine, yet a short one (for 79.5mm bore and 64+64=128mm stroke, the PatOP is only 500mm long); the inner-exhaust piston skirt of the OPOC of EcoMotors cannot help thrusting heavily onto the hot cylinder wall, over the exhaust port openings, increasing lubricant consumption and decreasing piston / cylinder life.

Can somebody from EcoMotors or Navistar tell what are the advantages of the OPOC engine over the PatOP?
Or over the OPRE engine (at ) ?

Manousos Pattakos


"thrusting heavily onto the hot..." whoa there, buddy!


The claimed efficiency increase comes from the lack of heat loss to the non-existent cylinder head. To conceptualize this:

Take a picture of an OPOC engine and draw in a wall between the two pistons. Yes, so that both pistons compress against their own head. Note the tremendous increase in surface to volume area of the combustion chamber.... a thermodynamic loss that is huge. Now, remove the wall between the cylinders and recalculate....

Basic physics/thermodynamics predicts a considerable increase in efficiency....only time will tell how much....45% claims seem high but even 20 percent would be huge. Heck, the 40 mpg Corolla/Civic... etc would now be at 50 mpg...... that would make $5 gas easy on the pocket.

No valves, they do have to be two strokes, but my outboards are in daily use and run for 10 years easily so.... maybe no worries there??

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