These are the engine types present in Re-Volt, known as Class in the Car selection screen and frequently misunderstanded as Rating. Classes are classifications in Re-Volt which describes a motor type or fuel/power of certain R/C car. They do not affect the cars' driving abilities, but make them produce different engine sounds during the race. Also, Electric-powered cars tend to be mostly from the Rookie and Amateur ratings, while Glow-powered cars are typically the higher-ranked cars. The classes seen in the Car selection screen are Electric, Glow and Special, but there is also the engine class of the Clockwork which uses an unique engine sound (clockwrk.wav), even if in the car selection screen and parameters of the car were set/displayed as an electric one.
Like the real Electric Motors, this type of engine is powered by batteries and produce an hiss sound (wavs\electric.wav). It's used by most of the R/C cars in the game, most from Rookie and Amateur ratings, except by Aquasonic. Evil Weasel, Bertha Ballistics, Pole Poz and Rotor are the exceptions from the other ratings.
Glow is a type of fuel designed for internal combustion engines, made from methanol, nitromethane and oil. It's also known as Gasoline or Petrol. This type is more powerful than Electric, making cars reach higher speeds (refering to real life cars). It's usually set for most cars from Semi-Pro and Pro ratings.
This type describes a class that does not fit into any of the above. It was set for Probe UFO and also attributed to Trolley. Even though, Probe UFO is the only car able to produce the special engine sound (wavs\ufo.wav), while any other car set to this class produces the glow sound instead (wavs\petrol.wav). Evil Toyeca is another car that has the special engine attributed to it.
In theory, it works as a real Clockwork motor. That's a non-listed type in the game, but has its own sound (wavs\clockwrk.wav). For some reason, the only car able to produce this sound is the Clockwork of the Clockwork Carnage, while the rest of the clockworks produces the default electric sound.