Why were all radial engines about the same width?

[Builder 2010]

With the problems of oil pooling in the bottom cylinders, why did the Germans use so many inverted V engines? I would think that all of those cylinders would be susceptible to pooling. Plus with the crank on top, lubricating that would be more difficult since there would be no crankcase oil reservoir.

As I read the whole thread, it seems to me that stoke's big impact was torque, not HP. The larger the lever, the greater the torque. Everything has a tradeoff.

Lots of modern engine stuff we take for granted was developed in WW2 radial and water-cooled engines. Roller cams, super and turbo-charging, sodium filled valves, high compression, direct fuel injection and water injection. The one development that never made it to terrestrial vehicles; turbo-compounding. The R3350 turbo-compound was the final evolution of the reciprocating aircraft engine. Not only did they supercharge the incoming airflow, but then they took the exhaust, spun a turbine with it and used that turbine through a fluid coupling to actually impart energy to the crankshaft. They gained another 15% HP increase. Used in Lockheed Super Gs, and DC-7s it was the epitome of reciprocating design. I believe it came on a little too late for inclusion in the B-29. There are cutaways in several museums including the Franklin Institute in Philadelphia. I saw this engine years ago and just couldn't figure out what that cans were and what all the complexity was all about.

 

[Elvis]

I'm motivated by my interest in the use of aero engines in tank development. A 1m wide engine would be really nice for a low-profile tank though the bump on the rear deck of the Hellcat wasn't a major issue for that design. But most radial of the WWII and pre-war era were about 1.35m-1.40m wide, regardless of the number of cylinders, displacement of each cylinder, etc. Does anyone know why this is? And does anyone know what would be the consequences of using shorter connecting rods, especially with regard to power and durability?

My default assumption would be that the diameter was chosen not based on the needs of the engine since that would change with the number and size of cylinders, but on the typical diameter of a typical fuselage of a plane with a single engine and a single pilot.

Higher "side loading" of the cylinder and a shorter dwell time, is the downfall of a shorter connecting rod.
Side loading is the pressure the piston exerts on the cylinder itself when the crank throw is on the upstroke.
Its higher with a short rod because the shorter length makes the rod "lay over" more. As the crank comes around and starts to push the piston upwards, the initial force pushes the piston into the side of the cylinder wall. A longer rod would be at a steeper angle, so the force from the crank would be more upwards and less sideways.
This can also effect rod bearing wear, as the side load pushes against that bearing, on the other end.
The dwell time is shortened because the shorter rod allows the piston less time to sit at top dead center (as measured in crankshaft degrees). The shorter dwell time allows less flame to develop, so it tends to "chase" the piston more, than with longer rod.
This is why little hot rod tricks, like using 6" rods in your 350 Chevy, are so popular.
The upside of a shorter connecting rod is it allows a stiffer and more compact cylinder block to be built.
Hope that helps you understand that aspect of your question.


Elvis

 

[kmccutcheon]

Side loading is the pressure the piston exerts on the cylinder itself when the crank throw is on the upstroke.

More important is the side loading during the power stroke. If taken to its extreme, the side load breaks down the oil film separating piston and cylinder and causes piston scuffing.

Both Curtiss and Wright Aeronautical unsuccessfully tried to produce small diameter air-cooled radials in the 1920s before the two companies merged. Wright developed the P-1 for the U.S. Navy in 1925; it was the first Wright engine to bear the Cyclone name. It was a 9-cylinder radial that displaced 1,654 cubic inches and was supposed to have produced 406 hp at 1,650 rpm. Its big claim to fame is that its diameter was no larger than the 200 hp Wright J-5. Heavy piston side loads caused by the short connecting rods required to achieve its small diameter, combined with troublesome valve gear and lack of a supercharger, resulted in only one engine being built. The Wright P-2, a more conventional design that appeared the next year, achieved 435 hp at 1,800 rpm. It had longer connecting rods, a supercharger, and improved cylinders based on the work of Sam Heron at McCook Field. All of these Wright engines were eclipsed by the Pratt & Whitney Wasp, which was introduced in 1926.

The Curtiss small-diameter radial was the 12-cylinder two-row air-cooled H-1640 Chieftain that was intended to be a Wasp-beater and produced 600 hp at 2,200 rpm. This engine actually made it into production, but short connecting rods had again been used to lower its diameter, resulting in vibration. The engine also reportedly had cooling issues.

 

[Shortround6]

My default assumption would be that the diameter was chosen not based on the needs of the engine since that would change with the number and size of cylinders, but on the typical diameter of a typical fuselage of a plane with a single engine and a single pilot.

Getting back to this assumption, it is in error as the physical size of the engines of the late 20s had very little to do with the size of the fuselage and had just about everything to do with the needs of the engine itself as most, if not all, engines used in any large numbers were used on a variety of aircraft (mostly biplanes) from single seat "pursuits" to torpedo bombers to multi seat transports, all single engine, plus a number of 2 and 3 engined aircraft. Most of the radial engines of the 30s (at least the successful ones) were based very heavily off the older 1920s engines. Minor changes to cylinder dimensions and geometry were done. Only a few companies tried to change dimensions by much when going from a single row 9 cylinder to a two row 14. Two row ten cylinder engines (a double 5) were rare as the two throw crank and bigger crankcase rarely paid off compared to a single row 9.

 

[wuzak]

The physical size of the cylinders also limits the diameter the engine can be.

Where the cylinder mounts to the crankcase the cylinders are quite close together.

You could use fewer cylinders and get a smaller diameter, but then you are losing capacity and power.

Also, they weren't all the same diameter. The Armstrong-Siddeley Deerhound was around 44", The Wright R-1820, R-2600, R-3350 were all about 55-56" and the R-4090 (22 R-3350 cylinders) was about 58".

The Pratt & Whitney R-1830 was about 48" and the R-2800 about 52.5".

 

[Shortround6]

Actually if you drop from 9 cylinders to 7 cylinders you don't usually make the engine smaller in diameter. You just have more space between the cylinders. Most companies used the same stroke=crank-throw. they used the same cylinder barrels and pistons. Same heads and pushrods from the same diameter cam rings, etc. Same rod length (some may have used the same slave rods even if the master was different?)

Deerhound was a special case and showed some of the complications. 21 cylinders for 38.2 liters. The Hercules was 38.7 liters from 14 cylinders (as was the Gnome-Rhone 14K-N-R series) the Wright R-2600 was 42.7 liters.
Using 50% more cylinders and an extra crank throw/longer crankcase for less displacement relies too much on high rpm and low drag. (cooling issues aside).

P& W was able to make a 32.8 liter 14 cylinder engine that ran at 2700rpm that was 49in diameter.(R-2000) and would make about 1300hp on 87 octane fuel.

the engine used in the M-3 Stuart was the Continental R-670 which was 42 1/2 in in diameter using 7 cylinders of 5 1/8 in bore and 4 5/8 in stroke. So you had roughly a 9 in swing on the crankshaft and 9 in worth of stroke in the cylinders on each side. the other 22-22 1/2 inches were the rod lengths, the height of the piston crown above the piston pin center and the height of the cylinder heads.

note sparkplug placement. Cylinder heads include valves and valve gear and intake/exhaust ports. None of which can be shrunk much.

Please note that even a 90-150hp flat four or flat six used in light aircraft has about the same width as a WW II liquid cooled V-12 of over 1000hp. expecting to get a small diameter/width opposed engine or radial isn't going to happen.

 

[Elvis]

Shortround, I don't follow your logic.
How does an engine with a listed stroke length of 4 5/8" have 9" worth of swing?
A crank with a 4 5/8" stroke, has a throw that is 2 5/16" long (centerline of rod pin to centerline of main).
The "swing" would still be 4 5/8", because that's the length of the stroke.
Sorry, maybe its the wording you used, but I just don't comprehend.

Elvis

 

[Shortround6]

You comprehend just fine.

I am the one who suffered a brain fart

 

[Elvis]

It's ok. Happens to all of us. You're allowed......this time ( )

 

[MiTasol]

I am the one who suffered a brain fart

Been there, done that, got the tee shirt