A process of making physical objects directly from digital models, usually layer-by-layer; also known as additive manufacturing. 3D printing has been hailed as a technology that will ultimately revolutionize many sectors of industry, medicine, art, and other fields. As information technology significantly enhances freedom in the digital world, 3D printing brings this freedom to the physical world by blurring the boundary between information and physical products (Fig. 1). See also: Information technology

A mathematical technique, often done graphically, by which accelerations of parts of a mechanism are determined. In high-speed machines, particularly those that include cam mechanisms, inertial forces may be the controlling factor in the design of members. An acceleration analysis, based upon velocity analysis, must therefore precede a force analysis. Maximum accelerations are of particular interest to the designer. Although an analytical solution might be preferable if exact maxima were required, graphical solutions formerly tended to be simpler and to facilitate visualization of relationships. Today, for advanced problems certainly, a computer solution, possibly one based on graphical techniques, is often more effective and can also produce very accurate graphical output. See also: Force; Kinematics; Mechanism

A friction type of energy-conversion mechanism used to retard, stop, or hold a vehicle or other moving element. The activating force is applied by a difference in air pressure. With an air brake, a slight effort by the operator can quickly apply full braking force. See also: Friction

The control of certain environmental conditions including air temperature, air motion, moisture level, radiant heat energy level, dust, various pollutants, and microorganisms.

Lowering of air temperature for comfort, process control, or food preservation. Air and water vapor occur together in the atmosphere. The mixture is commonly cooled by direct convective heat transfer of its internal energy (sensible heat) to a surface or medium at lower temperature. In the most compact arrangement, transfer is through a finned (extended surface) coil, metallic and thin, inside of which is circulating either chilled water, antifreeze solution, brine, or boiling refrigerant. The fluid acts as the heat receiver. Heat transfer can also be directly to a wetted surface, such as waterdroplets in an air washer or a wet pad in an evaporative cooler. See also: Comfort temperatures; Heat transfer

A transportation vehicle that rides slightly above the Earth's surface on a cushion of air, also called a hovercraft. The air is continuously forced under the vehicle by a fan, generating the cushion that greatly reduces friction between the moving vehicle and the surface. The air is usually delivered through ducts and injected at the periphery of the vehicle in a downward and inward direction via a flexible skirt that creates a chamber where the high-pressure air cushion is formed (Fig. 1). Because the vehicle is not in contact with the surface, it has six dimensions of motion. See also: Degree of freedom (mechanics); Ducted fan

A component of most systems in which air is used for industrial processes, for ventilation, or for comfort air conditioning. The function of an air filter is to reduce the concentration of solid particles in the airstream to a level that can be tolerated by the process or space occupancy purpose. Degrees of cleanliness required and economics of the situation (life cycle costs) influence the selection of equipment. See also: Air; Air conditioning; Ventilation

Fuels derived from resources other than petroleum that are used to power automotive engines, or any vehicular technology that does not involve solely petroleum. Gasoline and diesel fuels are gradually being replaced by alternative fuels and technologies, including natural gas, propane, ethanol, biodiesel, hydrogen, and electricity. Technological advances in the automotive industry, such as electric and hybrid-powered vehicles, are helping to increase public demand for alternative fuels (Fig. 1). See also: Automobile; Diesel fuel; Gasoline

A self-propelled land vehicle, usually having four wheels, used primarily for personal transportation; also known as a car. Other types of motor vehicles include buses, which carry large numbers of commercial passengers, and medium- and heavy-duty trucks, which carry heavy or bulky loads of freight or other goods and materials. Most automobiles, and those discussed here, are powered by internal combustion engines (Fig. 1). However, electric vehicles, hybrid electric vehicles, and fuel-cell vehicles, which make up only a fraction of the automobiles on the road, are becoming more common. See also: Electric vehicle; Fuel cell; Hybrid automotive power systems; Internal combustion engine

An energy conversion device used to slow a vehicle, stop it, or hold it in position. The two systems are the service brake and the parking brake (Fig. 1), both of friction type. The service brake includes a hydraulically operated brake mechanism at each wheel. These wheel brakes are controlled by movement of the brake pedal, providing braking proportional to the applied pedal force. The parking brake is a mechanical brake operated through a separate hand lever or pedal; it applies parking-brake mechanisms usually at the two rear wheels. Most automotive vehicles have power-assisted braking, where a hydraulic or vacuum booster increases the force applied by the driver to the service-brake pedal. See also: Brake