Quoted: Radial engines. Seems like they only came in odd number of cylinders. What is the reason and are there even number engines?

twonami, the major reason for the odd number of cylinders (3, 5, 7, 9, 11, etc.) on a true radial is because the timing. The firing order always skips every other cylinder. It takes two it takes 720 degrees (two complete rotations of the prop/airscrew for every cylinder to fire. That makes radials run much smother, cooler, more reliable, and it's actually the simplest (less parts) than any other timing order.

IE: On a seven cylinder radial the firing order is generally 1 - 3 - 5 - 7 - 2 - 4 - 6 - 1 - 3 - 5 - 7 - 2 - 4 -6 - 1 ....


ETA: Side note - the very first radials (WWl era), sometimes worked by firing every third cylinder. But they could get away with it then because they had a monstrous amount of mass turning so they to carry over to every third cylinder (the crankshaft had a large flange that was the actual engine mount -- it was bolted to the firewall and the engine itself turned (the prop was bolted to the engine block). It all sounds crazy now, but it actually worked very well. The downfall of that system was the humungus amount of gyroscopic precession it generated. The craft wanted to keep going straight, and was a nightmare to change course very quickly. NOT a good thing in a dogfight!