RE: Everything Radial Engines
[b]Rick:
A model sized radial could easily be made with the cam ring in the rear, but in practical manufacture it's much easier to put it in front. this allows a crank shaft of conventional single cylinder appearance, it does not require any support behind the plane of the cylinders. This also allows the use of a one piece master rod. Stronger.
Almost all full size radials have a device called a "Diffuser" behind the rear crank bearing. It looks like it would be a supercharger, but its purpose is to mix the fuel and air, prevent condensation of the fuel, with the result of one cylinder running lean and another running rich. A big eggbeater in effect, whipping the fuel/air into a nice meringue for the cylinders to feed on. The diffuser is normally run at a multiple of the crank speed so it will be effective at low engine speeds. With a single row radial it's just easier to have the cam rings in front and the diffuser in the back. There is still a drive through the diffuser for the accessories, the magnetos, generators, starter motor, fuel and hydraulic pumps, so forth.
On a two or four row radial we can't have the rear bank's valves operated from a front cam ring, so a two row engine could have the cams between the banks, but that would make the crank shaft longer, leading to harmonic vibration problems.
Attached pictures are a Pratt and Whitney R-1535.
First, as it arrived for overhaul. In this picture you don't see the push rod tubes, they have already been pulled. But the front bank does have a front cam ring. The rear bank cam ring is just behind the rear cylinders.
The second picture is the crankcase after the jugs were pulled. The small round holes near the bottom are the outlets from the diffuser fan.
Last picture is the completed engine installed in the airframe.
Bill.
EDIT>> The R-1535 was a 14 cylinder two row engine, used mainly on the Douglas SBD dive bomber. wr.