Mr_Grubby

I think I am right in saying that Radial Engines always had an odd number of cylinders.

If so, can somebody please explain why ?

Thanks,
Mr G.

RatherBeFlying

At the risk that fading brain cells have dropped some vital details, I recall that the firing order would be to the next cylinder after 180 degrees.

With an even number of cylinders, the engine might get confused as to which way to turn.

A firing order to adjacent cylinders would generate some interesting stresses on the mount

PaperTiger

The total number of cylinders in a radial could be even, by arranging them in separate rows. The Pratt & Whitney series (and probably others) indicated the number of cylinders by the designation - an R2800 had 28 (4 rows of 7 cyls), R3600 (4 x 9) and so on.

Lu Zuckerman

R XXXX does not indicate the number of cylinders. It designates the cubic displacement or close to it. The R-2800 has two rows of nine cylinders. The R-1830 has two rows of nine cylinders where the R-2600 has two rows of nine cylinders. The individual banks have an uneven number of cylinders as postulated in the first post.

All of this through the fog of time.

sycamore

Paper Tiger
Sorry to disillusion you but US engines are designated by capacity and type
P& W R-985- is a Pratt&Whitney Radial of 985 cu.ins capacity. It is also a single - row 9 cylinder.
A Wright R3350 is an 18 cylinder Radial of 3350 cu.ins capacity.double -row(2 x 9).
A Packard V-1650 is a licence built Merlin.
A P&W R-4360 IS A 28 CYL, 4- row radial( 4x7).
The USA system for designating engines is based around capacity,whereas our system is more HP based,and capacity in litres.(conversion factor for cu.ins - ltrs = x16.38)
As a guess you can probably guesstimate the number of cylinders by dividing capacity by 110 for small engines and 140-155 for larger one s.
Lu Z will probably to put me straight if i`m talking bO*****S.

Synthetic

I think it is to do with having two pistons changing direction at the same time. With an even number of cylinders, they do, which would produce a more severe vibration. With pistons as large as the ones they used, this would have been a significant factor. This is the principle behind five cylinder car engines (Volvo et. al.).

The direction of rotation is governed by the inlet and outlet valve and ignition timing, and the starter usually gives the engine a clue

PaperTiger

I should have figured it couldn't be that simple, sycamore.

Tim Zukas

Suppose you built a single-row eight-cylinder radial (a four-stroke-cycle radial, which I assume the well-known radials were). All the cylinders connect to the same crank-- right? So the #1 piston starts its downstroke, then the #2, then the #3... but as you go around the circle you'd like power strokes to alternate with intake strokes. So if #1 fires, then #2 does its intake, #3 fires, and so on around to #7 fires, #8 intakes... and then #1 is scheduled for an intake. So now what?

But with an odd number of cylinders that problem doesn't arise.

A typo in the previous post: the R-1830 has two rows of seven cylinders, of course.

Lu Zuckerman

I’m straining my brain on this one. I attended mechanic school in 1949. What is quoted above is quite possibly the reason but the internal geometry of the engine may also come into play. On a radial engine the connecting rods are not quite the same as on a conventional internal combustion engine. On an auto engine the crank throw passes through a 360-degree path and the connecting rods pass through the same path. On a radial engine there is what is called a master rod and this is attached to the crankshaft which is similar to that of an auto engine. The center of the master rod passes through a 360-degree path. The other rods are called articulating rods and they are attached to the master rod at the periphery of the master rod and as such when the master rod passes through a circular path the articulating rods pass through an elliptical path. I believe this type of set-up can not accommodate an odd number of articulating rods. (The master and the articulating rods total up to an uneven number).

Because the articulating rods pass through an elliptical path the firing of the spark plugs must be altered so that when the engine passes through two revolutions all of the plugs fire at the same position of the piston. This is accomplished by the use of a compensated cam on the magnetos, which is profiled to cause the points on the mags to open at the correct time. The cam rings on the engine are profiled so that the valves open and close at the same time to compensate for the uneven movement of the pistons.

At least I think this is correct.

Check this out: http://www.howstuffworks.com/radial-engine2.htm

411A

And then we have the most advanced radial of 'em all, the Curtis-Wright R3350- DA/EA series, eighteen fuel injected cylinders and three power recovery turbines pumping out (largest version) 3,400 HP. SFC 'round about 0.36 pounds fuel/HP/Hr.

However, in order for them to churn out all that HP, 115/145 fuel was required.

Worked good....provided you didn't run short of oil...1.5 gal/hr as I recall.

pigboat

Check out http://www.enginehistory.org/
It's the Aviation Engine Historical Society site. Great stuff. In the gallery section, one member has done some CAD images of the R3350TC, including the PRT's.
There's also an article on the development of the R2800, and the problems they encountered with crank shaft torsional flexing.

Lu Zuckerman

The R 3350 Turbo compound was the first A/C recip engine to develop more horsepower than its' cubic displacemnent. I remember long ago reading a US Navy publication that they had the first TC R-3350 to exceed 1000 hours installed on the wing. In that article it stated 3500 horsepower.

Chuck Ellsworth

And of course they sound good.

Cat Driver

GotTheTshirt

I notice all the radials referred to are of US origin
Its like Mr P & W had a patent on them
Having worked many hours ( and nights ) on Mr Pratts fine products I'm afraid they could not hold a candle to the Bristol Beauties.

When we were getting between 800 and 1,000 hours on the wing from Pratt we were getting 2000 hours + on Centaurus.

We never changed a cylinder in anger on a Centaurus. Bit different from Pratts ! On the 1830 we got plenty of practice and you just hoped it wasn't a bottom cylinder on your shift.

Incidentally for the uninitiated the Bristols were sleeved valved

18-Wheeler

(tongue firmly in cheek here)

The reason the English engines lasted longer is because they'd NEVER rust at all, not with the amount of oil that comes out of them!

The only radial I've had anything to do with is the Alvis Leonides, in the Twin Pioneer.

GotTheTshirt

18 wheeler,

Alvis are Ok but you cant beat a pair of Bristols

Jhieminga

Hey, I've got that on a T-shirt!!

Lu Zuckerman

Are you referring to the engine or something else?

sycamore

Lu
Seems like we on this side of the pond need to keep edificatin` you N.Americans.
Bristols-- large , round and firm,--- usually found on the front of girls. Depends upon age( of subject), but usually ,Jupiter,Mercury,Hercules, or Centaurus, in ascending order of capacity---- or size of hand for measuring..

Lu Zuckerman

Thats what I was alluding to when I said engines or something else.

I believe it comes from Cockney rhyming slang.