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Ecomotors Says Its OPOC Engine Could Deliver About 45% Greater Fuel Efficiency in a Class 8 Truck And With Tier 2 Bin 5 Emissions

The EM100 base module showcased at the ARPA-E summit. Click to enlarge.

EcoMotors International, a Khosla-funded startup working to commercialize an opoc (opposed-piston, opposed-cylinder) engine family (earlier post), showcased its EM100 (100mm cylinder bore) base module implementation at the ARPA-E Energy Innovation Summit in Washington DC last week.

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 suggests, while delivering emissions at the US Tier 2 Bin 5 level (the 50-state level in the US for diesel light duty vehicles).

The opoc engine operates on the 2-cycle principle, generating one power stroke per crank revolution per cylinder. Each module consists of 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 valvetrain components of conventional engines, offering a more efficient, compact and simple core engine structure, the company says. The power density is more than 1 hp per pound of engine weight. The fully balanced opoc engine can be run on any liquid fuel.

Cutaway diagram of the opoc engine. Click to enlarge.

The EM100 comes in different power configurations, said Jonathan Hurden, Chief Engineer - Engine programs, and with different emissions outcomes. The Ecomotors website describes a military spec version of the EM100 (no emissions requirement) with 325 hp (242 kW) of power and 664 lb-ft (900 N·m) of torque. The basic commercial power version of the engine offers 300 hp (223 kW) of power and 550 lb-ft (746 N·m) of torque, with a fuel economy improvement of 15% compared to a conventional engine, Hurden said. (These figures are all for diesel.)

Opoc modules can be combined through the use of an electrically controlled clutch, with select modules deactivated at different points in the operating cycle to optimize fuel consumption (cylinder deactivation, but on a module basis). The clutch assembly is housed between two engine modules, and is engaged when both modules are running to deliver power from both modules through the drivetrain.

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, it also eliminates parasitic power losses in the primary module. A dual module opoc offers a 45% improvement in fuel efficiency, according to EcoMotors. A dual module Class 8 truck would use two 240 hp (179 kW) modules (“because we don’t need more than 480 hp total”, Hurden said) and meet Tier 2 Bin 5 emissions requirements on diesel.

With no valvetrain, the opoc engine has 40% less friction than conventional valve-controlled engines. The engine design features 90% cylinder scavenging, a high-pressure fuel injection system, and an electrically controlled turbocharger, allowing it to run higher levels of EGR. Four features allow the opoc engine to achieve that high 90% scavenging:

  • Asymmetric port timing
  • Circumferential ports
  • Uni-flow air charging
  • Electronically-controlled turbocharging
The turbocharger unit with electric motor. Click to enlarge.

The electrically controlled turbocharger (ECT) incorporates an electric motor into the turbo assembly. In essence, it provides a supercharger, driven by the electric motor, as an adjunct to the exhaust-driven turbocharger. Boost pressure can be created by the electric motor, the turbocharger, or both.

The ECT effectively eliminates turbo lag because the electric motor provides much faster turbine response, and also provides boost when there is low energy from the exhaust flow. The motor is actuated by an electronic controller, which can be integrated with the engine control unit. When it is being spun by the turbocharger, the electric motor acts as generator, producing electricity.

While some two-stroke engines suffer from high oil consumption, the opoc engine’s oil consumption is 0.2 grams per kilowatt-hour, as compared to 0.4 grams per kilowatt-hour of a standard four-stroke engine, according to Ecomotors. Because the opoc is a direct gas exchange engine, the only components exposed to combustion gases are the piston top, rings and cylinder wall—less than in a conventional four-stroke engine, where lubricated components such as valve stems are exposed.

Ecomotors is also developing a smaller version of the opoc, the EM65 (65mm bore diameter), with 75 hp per module, and targeted for light duty vehicle gasoline and flex-fuel applications.



I guess we're just going to have to agree to disagree on this one. Unless you work for Ecomotors, we're just two outsiders theorizing based on what we've heard, read, and beleive based on past experience. What I can say is that it is basically my job to keep tabs on engine technology and in particular air handling systems (super/turbochargers). None of my advanced engineering contacts in industry, whether at research/development companies or engine/vehicle manufacturers, in either light duty or heavy duty are seriously considering OPOC engines. Everyone acknoledges there are some advantages, however as people who see several concepts a year all claiming to save the world, they have keen eyes for the challenges that accompany those benefits. In most cases, the costs or deficiences of a concept are equal or greater than the upside, resulting in little or no gain for anything but niche applications and thus no reason to change course. I won't be surprised to see an OPOC in production somewhere in the next 10 years, but I'll be shocked if makes up 1% of vehicle sales, light, heavy or otherwise.
As for your 3 points.
1-I have the luxury of using GT Suite and have done the analysis over a range of engine operating conditions. A more thermally efficient engine will help with turbocompounding, but its not dramatic

2-the modularity is a form of cylinder deactivation which can be applied to most recipricating engines in one way or another. This is not exclusive to an OPOC and thus it can't claim benefit because of it over another engine architecture

3-A seperate downstream turbine doesn't change the results much vs one turbine powering both the compressor and extracting supplemental power. It can be a little easier to optimize each component for its specific task in terms of size, inertia, expansion ratio and a variety of other turbomachinery specific terms, but there is a fundamental issue of a turbine requiring a pressure differential across it to extract power, thus imposing backpressure on the engine, resulting in negative engine pumping work. This exists whether the turbine is shared with the compressor or is a supplemental and downstream.

See you on the next interesting topic. =)

Tim Duncan

Some rough estimates indicate this engine (EM100) would be over 5 feet wide if mounted in a vehicle for rear wheel drive. Why not cut off one side? What issues may arise from that. It would fit even more conventionally if it were in half and rotated 90 deg so the crank was on the bottom. Or keep both sets of cylinders but change the housing from an in line to a 60 deg vee.


Tim, you may be onto something.


Tim Duncan,

The pattakon PatOP engine at
is the correct "half" engine you are looking for:

Full balance of the inertia forces and moments.
Better lubrication and lower lubricant consumption.
Additional time for the injection and the combustion of the fuel.
Zero total force on the main bearings of the crankshaft.
Built-in volumetric scavenging pump (if desirable) for wider rev range than the four-stroke.

Manousos Pattakos


I imagine the reason for the electrical turbocharger is a way to provide intake pressure at start up. This is needed because the Ecomotor cannot use crankcase scavenging.

The description of the ecomotor mentions uniflow air flow as well as assymetric port timing. I assume that in the practice of Junkers et al, the airflow thru the engine is along the axis of the cylinder from intake port at one end controlled by one piston to the exhaust port at the other end controlled by the other piston. I assume that the intake piston lags the exhaust piston by about 11 degrees a la Jumo 205 (11 degrees).

Two things concern me: One is the distance between the main bearings. There are 3 throws with 4 connecting rods in between. The other is the imbalance in the reciprocating weights. The inner pistons are on a common pin and oscillate together. The outer pistons weigh more (I assume given their length) and lag the inner pistons, so it seems vibration is unavoidable.
Given the popularity of opoc in tank engines and it's stellar performance in the Deltic and Jumo engines I am excited to see how this pans out.


This engines is now in prototype form and has big players backing it, inc one large truck and engine maker and Bill Gates. They are VERY well funded and expect this engine to be as a hybrid range extender and with two modules in large vehicles.

The clutch will give great fuel benefits without doubt.

If they used the opposing cylinders on the same centre line using the Revetec cam lobes, no little end bearings are needed (simpler) and no piston side thrust. The inner pistons can be the same piston connected by rods, and the same with the outer pistons. The package will be much smaller. Efficiency will be improved.

The electric turbo will benefit as it claws back wasted energy and gives the necessary pressure at all times.

Overall I can see this engine about meeting the makers claims. Put the Revetec cam lobes on and it will shine.

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