Curtiss Wright R-3350 Engine
Technical Data:
18 Cylinder (2x9) Radial Engine
3350 cubic inches
2800 HP
3250 HP with Turbo-Compound
Some people say that the Super Constellation had the best cowlings of all airplanes, free access from all sides...
This is the R-3350 with Turbo-Compound in a museum
A close-up of the Power Recovery Turbine (PRT) and the attached gears. Three of these are mounted around the motor, each one driven by six cylinders.
The Turbo Compound system is not a "turbo" as known from car engines. In a car, the turbo is used to compress the intake air - or in other words, to push more air-fuel mixture into the cylinders. On the R-3350, this is done by the so called Supercharger, a rotary compressor that can be seen on the picture above.

The Turbo Compound delivers it's energy directly mechanically to the crankshaft with the help of gears and fluid couplings. The PRT's (Power Recovery Turbines) deliver 20% more power to the engine from the "waste" energy of the exhaust gases.
You can download this exploded view of a R-3350 in big format (2000x1395)

Just click the pic...
An animated picture of a radial engine

Here you can see how pistons, rods and crankschaft work together. This is a radial engine which is electrically driven. Instead of cylinders there are electromagnets.

Because of the very short rods in this model, you can see that the movement of all the pistons is a bit different. This is also the case in the real engine. Rotating counterweights are installed to compensate this.

(Muellers Workshop)