The earliest ATVs were fairly primitive; they used two-stroke engines and didn't offer much in the way of convenience or comfort. They didn't offer much in terms of performance, either: The very first ATV, produced by Honda in 1970, had a mere seven horsepower. The popularity of ATVs grew immensely in the 1980s and performance improved over time [source: Casper]. Today's ATVs can have more than 50 horsepower, and racing ATVs have even more than that.
There are still ATVs on the market that use two-stroke engines, although they are much more advanced than two-stroke engines from 1970. The two-stroke engine offers more power and a much higher power-to-weight ratio than modern four-stroke engines. However, many ATVs today use the four-stroke variety, which require less maintenance and run much more cleanly.
In many modern ATVs, the engine is linked to a continuously variable automatic transmission (CVT). This means that the rider doesn't have to worry about shifting gears and the vehicle will automatically choose the best gear ratio for the speed, grade and amount of effort required (pulling a trailer full of logs up a hill is a lot different from a high-speed run down the straightaway at your local motocross track). If an ATV doesn't have a CVT (meaning it has a standard gearbox), there may be a foot-pedal shifter or a shifter on the handle bars.
In addition to two and four-stroke varieties, ATV engines come with either one or two cylinders, and with displacements typically measuring between 100 and 800 cc. It's important to note that safely riding an ATV requires a certain amount of body weight and physical strength, so the ATV's size and power should be matched to the rider. Disaster can strike, for example, when a young rider uses an overpowered or overly large ATV -- he or she simply can't control it.
In the next section, we'll cover ATV batteries.