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Direction of Piston Engine Rotation - Which Direction is More Efficient?

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Direction of Piston Engine Rotation - Which Direction is More Efficient?

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Old 01-02-2012, 03:05 AM
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Bundubasher
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Default Direction of Piston Engine Rotation - Which Direction is More Efficient?

Are piston engines turning clock wise (as seen standing behind the plane) more fuel/power efficient than engines turning counter clockwise? - and will rotation direction make a difference if you are in Southern or Northern hemisphere?

What determines the design consideration for rotation direction of an engine?

And what about jet engines?

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Old 01-03-2012, 07:32 PM
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Default RE: Direction of Piston Engine Rotation - Which Direction is More Efficient?

Rotational direction efficiency is dependent on many other factors. Induction port design/orientation, front/rear/sleeve induction, etc.

With a front induction engine, the connecting rod can kind of mask the hole in the crankshaft with some timings. On some racing engines you'll see the counterweight scalloped out to mitigate this masking. I believe Nelson, Jett, and many of the eastern european engines do this to some of their engines.

With a sleeve induction engine (like some of the older ignition and diesel engines, ex. the GHQ), rotation wouldn't matter at all since the flailing crank and rod aren't in the incoming fuel charge's path. These type engines are can be happy running in either direction.

On some engines, like the Cox reed valve .049's where the cylinder port alignment is variable and dependent on minor machining variances, rotation direction might help in one example, and hurt in another.

I don't think the rotational direction impacts are much more than a few hundred RPM one way or another even in the extreme case of racing engines turning 30K. For sport type engines, its going to be almost negligible. Proper breakin and blueprinting would be worth more.
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Old 11-19-2018, 06:57 PM
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most engines have small offsets in the piston pin location that makes one direction more efficient than the other. between that and the port timing or cam timing, the direction of rotation is thus dedicated. it makes no difference which direction the engine actually turns as far as spinning a prop,..... the above details will produce the direction of crank rotation that is intended for the design of the prop,....in the case of our plane engines, the props are designed to be spun counter-clockwise, looking at the front of the plane, so the offsets are built into the parts to produce the best performance in that direction.. in some engines, these offsets are so small that the engine will run backwards, but it won't give nearly the performance as if it were running the direction it is intended to run. case in point,...the old Cox. 049. you could run that engine either way because the offsets were "o" and it didn't matter which way it turned. 4-cycle engines with valves and cams have a very dedicated direction of rotation and most of them won't even start in the opposite direction because valve timing is then reversed and not coordinated to the stroke phase of the piston. there are engines designed to run in reverse rotation,....this done by using a special cam and piston offset designed for reverse rotation. these are used on twin engine planes so that the torque of the engines are counter balanced. for these engines, prop pitch is also reversed. the twin engines used in PT boats were built "CW" and "CCW" rotation to facilitate a straight running boat, when run wide open.
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Old 12-07-2018, 08:01 AM
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Engine design is optimized for the direction of rotation intended for that specific engine. Induction timing, exhaust timing and ignition timing all play a part as do a multitude of other variables. Some engines, Cox .049 comes to mind, will run backwards as good or almost as good as they do forward. The more complicated the engine, i.e. 2 stroke vs 4 stroke, the more difficult it is to make it run backwards. However, most any engine design can be reversed ON PAPER. And when built from that, will run as well as the original.
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Old 12-07-2018, 08:22 AM
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Originally Posted by r ward View Post
........................ the twin engines used in PT boats were built "CW" and "CCW" rotation to facilitate a straight running boat, when run wide open.
Beg to differ. The Elco 80 foot PT boat was the most produced PT during WWII. From day one it had THREE Packard built Rolls Royce Marine engines in it. Not the Merlin engine found in the Mustang, Spitfire and other planes of WWII. The Higgins built boats also used 3 Packard engines. Elco (326 boats) and Higgins (199 boats) built pretty much ALL of the US built PT boats. Other manufacturer designs, especially foreign, may have differed. But the primary British manufacturer, Vosper 70' and 73' boats also used 3 engines. Surprisingly they used Parckard engines as Rolls Royce was too busy producing their Merlin aircraft engine.

All three engines turned the same direction. One screw (prop) was reversed via the transmission on that engine. I believe it was the starboard screw but I'm not sure and don't care to look it up.

Last edited by Appowner; 12-07-2018 at 08:36 AM.
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