Crank Phasing and the Impact on Engine Efficiency

In an Opposed-Piston (“OP”) Engine, two pistons come together within a cylinder and the piston crowns form the combustion chamber.  The most common and widely deployed OP Engine, often referred to as the ‘Junker’s style’ has two crankshafts, one on each side of the cylinder.  Typically the exhaust crank leads the intake piston for effective gas exchange: the exhaust piston opens the exhaust ports when it is near the end of the combustion stroke, allowing blow down.  Next, the intake piston opens the intake ports when it is near the end of its combustion stroke, enabling scavenging.  The two pistons come together for compression.  The crank lead can generally range up to 12° depending on engine design and overall optimization.

Achates Power has conducted a series of test to assess the effect of crank lead on engine efficiency.  With fueling and calibration kept constant, the exhaust crank lead was varied from 3° to 12°, and brake mean effective pressure, indicated mean effective pressure, pumping mean effective pressure, and friction mean effective pressure were analyzed.  The results are depicted in the graph below: there is virtually no difference in BMEP, IMEP, PMEP, and FMEP across the entire exhaust lead sweep.

The historical record:

Martin and Pirault, in their well-researched book, “Opposed Piston Engines: Evolution, Use, and Future Application” describe many OP Engine variations in great detail.  The Jumo 205E, for example had a 9° exhaust crank lead and “set many long distance records…[it] remains the most efficient piston aero engine in aviation.”

Later in the book, Martin and Pirault describe the Fairbanks Morse 38D engine.  At the time of publication, it had 12° exhaust crank lead.  Fairbanks Morse recently announced an upgraded version of the engine which it describes has “best in class fuel efficiency”.

These two examples – perhaps the most widely deployed and best known OP Engines – demonstrate that OP Engines are the most efficient engines in their class, and support the experimental results that exhaust crank lead has little impact on efficiency.

At Achates Power we have recently reviewed two presentations that mistakenly claimed that overall mechanical efficiency of an Opposed-Piston Engine dropped off with increased exhaust crank lead.  The error in their analysis was simple.  Yes, these presentations correctly claim that the torque from the intake crank decreases as exhaust crank lead increases.  However, the torque provided by the exhaust crank increases proportionally and thereby effectively compensates for the intake torque reduction. This factor was ignored in each of those presentations.  Bottom line, total torque at the power takeoff, which is equivalent to the sum of the two crankshaft torques less any gear connection losses remains roughly constant.

Such a corrected analysis can be done kinematically looking at the in-cylinder pressure traces against the position of each piston to its top dead center, confirming our measured data and the experience of others.

2 thoughts on “Crank Phasing and the Impact on Engine Efficiency

  1. The Junkers was diesel and thereby requires a much higher compression than is possible with a petrol engine which mitigates the lower calorific value (think that is the right term) of diesel by comparison to petrol which is why diesel engines use less fuel than petrol engines for a given power output. This also means less fuel is needed for a given distance which reduces weight and allows for less wing drag (less lift). Therefore I suspect, at a guess, the reason for the Junkers engine being the most efficient piston aircraft, if indeed the claim is true, is probably more to do with it being diesel than being OP. The Junkers OP was also exceptionally well balanced (no engine vibration) and perhaps this also allowed for a lightweight air-frame which again allows for less wing drag etc. So, Junker efficiency is likely down to being diesel and having low drag wings rather than being an inherent feature of being OP. Also it was quoted about OP that it was the most efficient in its class but said class was not mentioned so it is not possible evaluate the statement (because potentially many classes could relate to OP engines).

    • Nick,
      There are inherent efficiency advantages to a compression ignition (aka, diesel) engine over a spark ignited engine, and the OP Engine has additional efficiency advantages over a four-stroke compression ignition engine. Fabien Redon’s post on our OP GCI project lays out some of that advantage, you can check his post here. As or carrying less fuel, or designing a lighter aircraft, these are advantages that would be on top of the inherent efficiency advantages of the Junkers engine in an aircraft (just like a vehicle using our OP Engine would realize additional efficiency advantages from lightweighting, optimized tires, and other fuel savings technologies.)

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