Air motor

A pneumatic motor (air motor), or compressed air engine, is a type of motor which does mechanical work by expanding compressed air. Pneumatic motors generally convert the compressed air energy to mechanical work through either linear or rotary motion. Linear motion can come from either a diaphragm or piston actuator, while rotary motion is supplied by either a vane type air motor, piston air motor, air turbine or gear type motor.

Pneumatic motors have existed in many forms over the past two centuries, ranging in size from hand-held motors to engines of up to several hundred horsepower. Some types rely on pistons and cylinders; others on slotted rotors with vanes (vane motors) and others use turbines. Many compressed air engines improve their performance by heating the incoming air or the engine itself. Pneumatic motors have found widespread success in the hand-held tool industry, but are also used stationary in a wide range of industrial applications. Continual attempts are being made to expand their use to the transportation industry. However, pneumatic motors must overcome inefficiencies before being seen as a viable option in the transportation industry.

Classification

Linear

In order to achieve linear motion from compressed air, a system of pistons is most commonly used. The compressed air is fed into an air-tight chamber that houses the shaft of the piston. Also inside this chamber a spring is coiled around the shaft of the piston in order to hold the chamber completely open when air is not being pumped into the chamber. As air is fed into the chamber the force on the piston shaft begins to overcome the force being exerted on the spring.Engineers Edge. Pneumatic actuator design and operation. Retrieved from http://www.engineersedge.com/hydraulic/pneumatic_actuator.htm As more air is fed into the chamber, the pressure increases and the piston begins to move down the chamber. When it reaches its maximum length the air pressure is released from the chamber and the spring completes the cycle by closing off the chamber to return to its original position.

Piston motors are the most commonly used in hydraulic systems. Essentially, piston motors are the same as hydraulic motors except they are used to convert hydraulic energy into mechanical energy.Vane-type motors. Retrieved from

Piston motors are often used in series of two, three, four, five, or six cylinders that are enclosed in a housing. This allows for more power to be delivered by the pistons because several motors are in sync with each other at certain times of their cycle.

The practical mechanical efficiencies attained by a piston air motor are between 40%-50%.

Rotary vane motors

A type of pneumatic motor, known as a rotary vane motor, uses air to produce rotational motion to a shaft. The rotating element is a slotted rotor which is mounted on a drive shaft. Each slot of the rotor is fitted with a freely sliding rectangular vane. The vanes are extended to the housing walls using springs, cam action, or air pressure, depending on the motor design. Air is pumped through the motor input which pushes on the vanes creating the rotational motion of the central shaft. Rotation speeds can vary between 100 and 25,000 rpm depending on several factors which include the amount of air pressure at the motor inlet and the diameter of the housing.

One application for vane-type air motors is to start large industrial diesel or natural gas engines. Stored energy in the form of compressed air, nitrogen or natural gas enters the sealed motor chamber and exerts pressure against the vanes of a rotor. This causes the rotor to turn at high speed. Because the engine flywheel requires a great deal of torque to start the engine, reduction gears are used. Reduction gears create high torque levels with the lower amounts of energy input. These reduction gears allow for sufficient torque to be generated by the engine flywheel while it is engaged by the pinion gear of the air motor or air starter.

Turbine motors

Air turbines spin the burr in high-speed dental handpieces, at speeds over 180,000 rpm, but with limited torque. A turbine is small enough to fit in the tip of a handpiece without adding to the weight.

Application

A widespread application of pneumatic motors is in hand-held tools, impact wrenches, pulse tools, screwdrivers, nut runners, drills, grinders, sanders and so on. Pneumatic motors are also used stationary in a wide range of industrial applications. Though overall energy efficiency of pneumatics tools is low and they require access to a compressed-air source, there are several advantages over electric tools. They offer greater power density (a smaller pneumatic motor can provide the same amount of power as a larger electric motor), do not require an auxiliary speed controller (adding to its compactness), generate less heat, and can be used in more volatile atmospheres as they do not require electric power and do not create sparks. They can be loaded to stop with full torque without damages. The efficiency of a rotary piston engine is highly dependent on mechanical energy losses. The value of mechanical losses, according to various estimates, can be 20 % of the energy supplied to the engine.

Historically, many individuals have tried to apply pneumatic motors to the transportation industry. Guy Negre, CEO and founder of Zero Pollution Motors, has pioneered this field since the late 1980s. Recently Engineair has also developed a rotary motor for use in automobiles. Engineair places the motor immediately beside the wheel of the vehicle and uses no intermediate parts to transmit motion which means almost all of the motor's energy is used to rotate the wheel.

History in transportation

The pneumatic motor was first applied to the field of transportation in the mid-19th century. Though little is known about the first recorded compressed-air vehicle, it is said that the Frenchmen Andraud and Tessie of Motay ran a car powered by a pneumatic motor on a test track in Chaillot, France, on July 9, 1840. Although the car test was reported to have been successful, the pair didn't explore further expansion of the design.The History of compressed air vehicles. (n.d.). Retrieved from

The first successful application of the pneumatic motor in transportation was the Mekarski system air engine used in locomotives. Mekarski's innovative engine overcame cooling that accompanies air expansion by heating air in a small boiler prior to use. The Tramway de Nantes, located in Nantes, France, was noted for being the first to use Mekarski engines to power their fleet of locomotives. The tramway began operation on December 13, 1879, and continues to operate today, although the pneumatic trams were replaced in 1917 by more efficient and modern electrical trams.

American Charles Hodges also found success with pneumatic motors in the locomotive industry. In 1911 he designed a pneumatic locomotive and sold the patent to the H.K. Porter Company in Pittsburgh for use in coal mines. Because pneumatic motors do not use combustion they were a much safer option in the coal industry.

Many companies claim to be developing compressed air cars, but none are actually available for purchase or even independent testing.

Tools

Impact wrenches, pulse tools, torque wrenches, screwdrivers, drills, grinders, die grinders, sanders, dental drills, tire changers and other pneumatic tools use a variety of air motors. These include vane type motors, turbines and piston motors.

Torpedoes

Most successful early forms of self-propelled torpedo