What are the simple differences between engines?

What are the differences between a radial and rotary engine (excluding Wankel engines)?

• I know that in rotatory engines the cylinders spin round and in radial they do not. I was wondering what other differences exist and what advantages and problems this creates.

• Answer:

  Radial were used in aircraft because they were easier to air cool, as each cylinder was exposed directly to incoming airflow, at least until the mutli-row engines of the 50's. Rotaries took that a step further by spinning the entire engine, so the individual cylinders were exposed to more air as the cylinder spun around the crankshaft. The rotary arrangement created much better cooling, particularly at lower speeds. Having a consistent amount of cooling allowed the engines to be built to tighter tolerances because there was less thermal expansion. Given the metallurgy of the time, consistent cooling also made the engines more reliable. Spinning the entire engine created a number of problems: The spinning mass of the engine acted as a huge gyroscope that had tremendous impact on the plane's ability to maneuver. Any plane using a rotary had a limited ability to turn in one direction, depending on the rotation of the engine. Most of these planes could not make abrupt left turns. Carburetion and lubrication were challenges on an engine that was spinning. Because of this, these engines did not have a throttle - they ran full throttle or not at all. Rather than a throttle the pilot had an button that interrupted the ignition. To reduce power for landing, the engine would be constantly "blipped" - turned on and off. To lubricate these engines, a constant supply of oil was delivered to the cylinders, which slung it into the air in a fine mist. The plane was constantly enveloped in a mist of this oil. To make matters worse, the lubricate used was Castor oil, a natural laxative. Flying behind a rotary had predictable results on the pilots digestive tract.