Heat Transfer Advantage of Opposed-Piston Engines

To maximize the efficiency of an engine, one must minimize losses. Heat transfer to the cylinder and piston walls is a large source of heat loss in engines. Cooling systems keep these engine components from overheating, but all that heat being carried away by the cooling system is lost energy.
Many factors contribute to the heat transfer—and energy loss—of an engine, but a major one is the ratio of the surface area of the combustion chamber to the volume of the combustion chamber during combustion. The higher the ratio, the greater the heat loss as there is more surface area to absorb the heat from combustion. Indeed, one of the main reasons that larger engines are more efficient than smaller engines is that larger engines have a lower ratio of surface area to volume (since the denominator, or volume, increases as a cubic function and the numerator, or surface area, increases as a square function).
One of the inherent thermal efficiency advantages of opposed-piston engines, as described in this technical paper, is a favorable (i.e. low) surface area-to-volume ratio. To understand the source of this advantage, consider the following thought experiment. Compare the surface area and volume of an inline 6 (I-6) engine with a comparable three-cylinder, six-piston, opposed-piston engine (OP6). In the example below, the conventional engine has six cylinders, a displacement of 6.6 liters, and a stroke/bore of 1.16, typical figures for a medium-duty engine. From the geometry, the surface area at top dead center—primarily the piston crown and cylinder—and combustion volume are calculated.

In this example, a comparable opposed-piston, two-stroke engine with three cylinders, a displacement of 4.5 liters, and a stroke/bore ratio of 2.48 generates the same power and torque as a four-stroke, conventional engine with six cylinders, a displacement of 6.6 liters, and a stroke/bore of 1.16.

The comparable opposed-piston engine—that generates the same power and torque—has three cylinders, a displacement of 4.5 liters, and a stroke-to-bore ratio of 2.48. Since the opposed-piston engine is a two-stroke engine, it has a power stroke in each cylinder during each engine revolution. This increased power density enables a reduced displacement without exceeding peak cylinder pressure limits. Also, because of the combined motion of the opposed pistons, a high stroke-to-bore ratio is enabled without creating excessive piston speed.
Calculating the ratio of surface area to volume is straightforward: 3.2 cm-1 for the conventional engine and 1.9 cm-1 for the comparable opposed-piston engine. Eliminating the cylinder heads in the opposed-piston engine conveys a significant advantage in the ratio of surface area to volume. A significantly lower ratio of surface area to volume leads to significantly lower heat rejection to coolant, which leads to significantly improved engine efficiency.
The graph below compares the ratio of surface area to volume across a range of engine displacements. The upper curve is a conventional I-6 four-stroke ratio of surface to volume. The lower curve is the same ratio for a three-cylinder, opposed-piston engine of the same power and torque—the OP6 has a ratio of surface area to volume about 30% lower. Another way to look at it is that an opposed-piston engine has the same ratio of surface area to volume as a much larger conventional engine. For example, an opposed-piston engine sized to match the power and torque of a 4-liter conventional engine (sized to power a large auto or pickup truck) has the same ratio of surface area to volume as a much more efficient 12-liter conventional engine (sized to power a heavy-duty truck).

An opposed-piston engine has the same ratio of surface area to volume as a much larger conventional engine. For example, an opposed-piston engine sized to match the power and torque of a 4-liter conventional engine has the same ratio of surface area to volume as a much more efficient 12-liter conventional engine.

The favorable surface area-to-volume ratio of opposed-piston engines is just one source of inherent thermal efficiency advantage, but it is an important one that serves to improve opposed-piston engine efficiency across all engine sizes, applications and fuel types.

6 thoughts on “Heat Transfer Advantage of Opposed-Piston Engines

  1. Hello,
    I would see another ratio as still better to characterize energy efficiency: combustion chamber surface to power (or inverse, as you want).
    With that, one catches the full two-stroke power density advantage as well as the OP geometry advantage. (Also, one would catch the advantage of cylinder shutdown, if there were any.)
    For the non professional reader, it would be interesting to know the percentage of total non thermodynamic losses covered by heat transfer losses, for the two considered engine examples.

    • Hello,

      Actually I had the exact principle behind these heat transfer in opposed piston cylinder because even I to have got the same thought about opposed piston cylinder before I don’t know it was there in real ….
      actual process is in cylinder after fuel is compressed and the spark is given the compressed fuel is com-busted by layer by layer and this pressure of the com-busted gases apply pressure on the piston and the same amount of the pressure is going to be act on the upper wall of the cylinder which dispeats out in the form of heat which is a loss ..What if we replace the upper wall with another piston it is going to do some work by this we can increase efficiency and speed of the engine ..Another advantage is we can meet the higher compression ratios which we want..

  2. Hi, in the above analogy a conventional 6.6L four stroke engine is compared with an opposed piston (OP) 4.5L two stroke engine, where both engines have the same power. But since a two stoke has twice the firing strokes as a four stroke for the same RPM, the two stroke engine should be closer to half the size of the four stroke engine if all other aspects are the same. However, two stroke valve timing (unlike four stroke valve timing) is limited to the crank sine wave which is not best for efficiency and is probably why the quoted OP has to be 2/3rds the size of the quoted four stroke to produce the same power instead of being closer to half the size. Also, the combustion surfaces have to be cooled down to the same workable parameters irrespective of the engine being conventional or OP. Therefore, whether the necessary heat dissipation is through the cylinder head or through the OP pistons it must amount to the same heat loss. OP engines may actually require extra piston cooling (by squirting oil, or more oil, under the piston crown) by comparison to conventional engines where a lot of heat can dissipate through the cylinder head. Since OP pistons brush past OP valves apertures there is inevitably a deposit of oil in the exhaust aperture which must cause smoking when the exhaust exits the cylinder. For these reasons it seems OP designs are best suited to (1) Single speed applications (to mitigate against the inherently poor valve timing), (2) Environments where pollution is not an issue, (3) Maximising power for a given size and weight where efficiency is not an issue. (1)(2)(3) seem to be the traditional uses of OP engines and I would be keen to know of any new developments that would significantly broaden OP application and I would also welcome feedback concerning my above conclusions about OP engines – it’s a very interesting subject.

    • Nick,
      Lots to answer here so I’ll try to quickly address some in a broad sense and that may address the larger comment you posted. You’re right in theory about the engine size, but there are additional considerations factored into that (BMEP, costs, components, pressure, etc). There are no valves in our OP Engine, all breathing is managed through ports which are “opened/closed” with our pistons and the airflow is managed through our turbocharger and supercharger (a four-stroke is a pump, our two-stroke is not a pump). Part of our efficiency advantage is because there is no cylinder head, subsequently we are better able to use the heat that is generated in combustion (a cylinder head is inefficient). Oil consumption (“exhaust…smoking…”) is not an issue for our engine – the short answer is that we’ve spent a lot of time and effort addressing oil consumption and are on par with “conventional” four-stroke SI engines available today. Linked here is an ASME tech paper on the topic, and we will have a new Oil Consumption specific paper presented at ASME’s Fall Internal Combustion Engine Conference in San Diego this November.
      The OP Engine is actually very efficient and clean across the entire engine cycle (all speeds and loads); our demonstration project that we’re developing for the California Air Resource Board is specifically running in communities where pollution is an issue as we will beat the voluntary Low NOx requirement in CA while also providing increased efficiency in a 10.6L HD Engine.
      As for continuing developments, we are hard at work every day working on development of the OP Engine, pure R&D and for customer development programs. We will continue to share that information when we can.

  3. I would think that because the engine is a diesel the valve timing is less critical. Since it is a forced air induction using a supercharger and turbocharger system it is even less critical on valve timing. With the older types of opposed piston engines injection timing was not variable because the pump was controlled by an engine driven cam. This new system can use high pressure rail and electronic injection to insure fuel timing is variable for load and rpm. I would think that would improve efficiency considerably.
    I like the idea of diesel power and with the opposed piston setup more of the heat is used pushing pistons instead of wasted in heating the head. Yes it still requires cooling for the cylinder and piston but no more than any other engine and due to the better use of heat it may require less cooling. Expansion of the burning gas in the cylinder will cool it and more expansion cools it more. That means less waste heat.
    Its a great idea but blind and dogmatic eco freaks don’t like diesel despite its advantages and lower overall emission of toxics compared to gasoline. They will strangle the engine with unneeded emission controls to keep it from being used as they are trying to do with all the rest of the diesels in use.

  4. In the Carnot cycle the work extracted is related to the amount of cooling of the combustion gases by expansion. If this engine is more efficient it should be reflected by greater difference between combustion and exhaust gas temperatures. Much of the heating of cylinder walls and head is from exposure to the hot gas after the work has already been extracted (exhaust stroke and ducting exhaust gas through the head to the exhaust manifold). Does this engine have lower exhaust gas temperature?

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