Quote:
Originally Posted by pete c
how do they fall in the efficiency area? is there a reason they are no longer made other than prohibitive production costs?
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Radial, or rotary? The Gnome is a rotary, where the crankshaft is solidly bolted to the fuselage and the rest of the motor spins around it. (Different from the Wankel rotaries to be sure!) The main reason it was successful was that the whole engine acted as a flywheel, which meant no extra (heavy!!) flywheel was needed. Engines at the start of WWI in particular needed a pretty massive flywheel to keep them running reasonably, and the rotary provided that with basically zero weight.
The efficiency was horrible, absolutely rotten. The mixture went in through the crankshaft (which was hollow for at least some part of it) and bounced around inside the crankcase. I seem to recall that it went into the cylinder, then back out again, then back in to be burned. Compression ratios were very low, and valve timing was just short of a joke. Not to mention the total-loss oiling system.
But the displacement was pretty large, and it was very light for an engine at the time.
Later aircraft often used other engine configurations. The Fokker D-VII, arguably the best fighter in the war, used an inline-6 water-cooled motor.
Radial engines were used throughout WWII because you could easily make them
really big (3350 cubic inches, about 55 liters, in the twin-row Wright Cyclone!!) and relatively compact. They were also very resistant to damage and would fly for a while with large chunks missing, and had no radiator that needed both airflow and protection.
I don't know what the efficiency was on those, but I would be surprised if we found it was very good.
Packaging up a radial in an automobile is a definite challenge. They are very wide, and unless you have a tubular cross-section you wind up with lots of wasted space by the time you fit one in your vehicle.
BTW, the fastest aircraft of WWII generally ran liquid-cooled V12 motors. Except for the very few jets, but that's another story.
-soD