Nearly all larger gas combustion engines have a reverse gear, they don't run in reverse. They have a transmission that switches the direction of the driveshaft through mechanical gears. There are a number of motors, though, that are capable of running in forward and reverse. Some are set up to switch quickly from forward to reverse---or clockwise to counter-clockwise. Some can be switched with a little more effort.
Electric AC Motors
There are several kinds of electric motors, and a couple of those are types of AC motors: single-phase and three-phase. Any single-phase or three-phase AC motor can be modified to run in reverse. With a single-phase AC motor, if the starter winding leads are reversed, the motor will run in the opposite direction. Switching one of the hot legs on a three-phase AC motor will reverse its rotation. Many small AC motors are prewired to to switch back and forth between running clockwise and counter-clockwise. A common example is a sewing machine motor.
Electric DC Motors
As with AC motors, DC motors can run either clockwise or counterclockwise. With a DC motor, though, the direction of the motor changes by changing the polarity of the hot lead. As with AC motors, any DC motor can be modified to run in the opposite direction, and many are equipped with a switch to easily and simple change the direction of its operation. Cordless drills have DC motors that run in both directions.
Air motors are less common than electric or gas motors. In a sense, they are the opposite of an air compressor. They transfer pressurised air or gas into torque, and are useful motors for any application where there is an existing source of pressurised air. The motors consist of a simple cylinder, piston and crankshaft configuration. They have valves to let pressurised air in, actuate the piston and crankshaft, then let the air out. Small air motors are often equipped with a variable input valve. Switching the input valve will switch the rotation of the motor. Pneumatic tools used by fabricators and auto-mechanics are often air motors run on compressed air. Examples include air drills, wrenches and impact drivers.
Four-cycle engines, the kind in most cars, are not suited to running backwards. They can be designed to run in either direction, but once they're built, they're not suited to switch operating directions. A transmission with reverse is required to change the direction of their force output. Two-stroke gas engines and four-stroke diesel engines will run forward and backwards for short periods, but are not designed to; they'll stop running and it will likely damage them. A two-stroke diesel engine, though, can run clockwise and counterclockwise. Two-stroke diesels are rare. Small two-stroke diesels are even more rare, but if you want a combustion engine that can power a crank in either direction with no transmission, this is the engine---if you can find one.
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