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Engine Physics (?) Question: HO-Vertical


Bud Tierney
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Horizontal opposed engines are not my interest, but've noted their bore usually wider than stroke is long (31/2x3 etc), while verticals are usually the opposite (3x31/2 etc)  ..

Is this because of some engine physics property??

Many thxx for  satisfying my idle curiosity...

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As an amateur, I think the shorter stroke on the horizontally opposed engines was strictly due to external dimensions.  Most chassis were not that wide.

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I doubt the cylinder orientation has anything to do with whether the engine is oversquare or undersquare. More likely older, lower RPM engines use a longer stroke. Modern, higher RPM engines with better bearing materials tend to use shorter strokes and longer bores. Of course, that's an oversimplification of only one consideration. There are also things like rod angle, valve shrouding in small bore cylinders, cooling, and general packaging, for example.

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I think Tinindian and Joe hit it - Packaging.  The longer strokes require a wider engine that is hard to package.  This also applies to all the horizontally opposed aircraft engines.

 

The 6 cylinder Franklin helicopter engine that was modified for the Tucker 48 had the following specs:

Bore: 4.5 in (114.3 mm)
Stroke: 3.5 in (88.9 mm)
Displacement: 335 cu in (5.49 l)

 

Two examples of modern aircraft engines:

Lycoming 4 cylinder aircraft engine

Bore: 5.125 in (130 mm)
Stroke: 4.375 in (111 mm)
Displacement: 361 cu in (5,916 cc)

 

Continental 6 cylinder aircraft engine:

Bore: 5.25 in (133.35 mm)
Stroke: 4.00 (101.6 mm)
Displacement: 520 in³ (8.51 litres)

Edited by Stude Light (see edit history)
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Very early engines, like before WW1, usually had a slightly larger bore than stroke. Bore of 4 1/4 and stroke of 4 inches might be typical. Then around 1912 - 1915 the quality of fuel (octane) went way down and low compression, long stroke engines became the most popular with dimensions like 3" bore X 5" stroke. The long stroke engine was the norm until the late 1940s and early 50s when better high octane fuels allowed high compression, short stroke engines to be developed. In that era something like a 4"bore and 3 1/2" stroke might be typical.

 

Cylinder layout did not come into it, except an inline six or straight eight could take advantage of the small bore, long stroke configuration to reduce the length of the crankshaft and make it stiffer, and reduce overall engine length. This was not so important in opposed piston or V type engines. Even so, I think you will find the bore/stroke ratio is more related to when the engine was first made or designed and what kind of fuel it was meant to burn.

Edited by Rusty_OToole (see edit history)
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I suspect the state of metallurgy and relative ignorance of balancing limited engines to low piston speed  and rpm.  A longer stoke with these limitations was a way to increase output.  With the poor road conditions and low average speeds of the day, torque was more important than HP and long stroke engines are torquier (all else being equal)....

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On 7/16/2020 at 3:36 PM, FLYER15015 said:

That is why we see so many Subaru engines in home built aircraft rather than Buick straight 8's.

 

Ha! I'm sure weight has nothing to do with the decision!😄

 

45 minutes ago, MikeC5 said:

I suspect the state of metallurgy and relative ignorance of balancing limited engines to low piston speed  and rpm.  A longer stoke with these limitations was a way to increase output.  With the poor road conditions and low average speeds of the day, torque was more important than HP and long stroke engines are torquier (all else being equal)...

 

And Octane available. Once we had high octane, the 12 and 16 cylinder engines went away for large cars. Of course, BMW brought back the 12 from time to time.....

 

And don't forget the Taxable Horsepower rating of engines. Many countries based car tax on engine Taxable Horsepower, where a small bore long stroke of identical cubic  inches of a short stroke wide bore engine would be cheaper to pay the tax upon. Hmm, seems wordy. The small bore was cheaper to pay the taxing authority.

 

https://en.wikipedia.org/wiki/Tax_horsepower

 

Many early engines, I'm speaking of 00s and 10s,were long stroke horizontal engines. Not opposed, as they were only one or two cylinder.....😉  Just ask Brass Is Best.

 

Where does Henry's experimental X-8 fit?🙃

Edited by Frank DuVal (see edit history)
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