Mechanical engineering is a discipline of engineering that applies the principles of engineering, physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the design, production, and operation of machines and tools. It is one of the oldest and broadest engineering disciplines.
The engineering field requires an understanding of core concepts including mechanics, kinematics, thermodynamics, materials science, structural analysis, and electricity. Mechanical engineers use these core principles along with tools like computer-aided engineering, and product lifecycle management to design and analyze manufacturing plants, industrial equipment and machinery, heating and cooling systems, transport systems, aircraft, watercraft, robotics, medical devices, weapons, and others.
Four-wheel drive, All-wheel drive, AWD, 4WD, or 4×4 ("four by four") is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine.
In abbreviations such as 4×4, the first figure is normally the total wheels (more precisely, axle ends, which may have multiple wheels), and the second, the number that are powered. Syntactically, 4×2 means a four-wheel vehicle that transmits engine power to only two axle-ends: the front two in front-wheel drive or the rear two in rear-wheel drive.
A flat tire (British English: flat tyre) is a deflated pneumatic tire, which can cause the rim of the wheel to ride on the tire tread or the ground potentially resulting in loss of control of the vehicle or irreparable damage to the tire. The most common cause of a flat tire is puncturing of the tire by a sharp object, such as a nail, letting air escape. Depending on the size of the puncture, the tire may deflate slowly or rapidly.
The Michelin PAX is an automobile run-flat tire system that utilizes a special type of rim and tire to allow temporary use of a wheel if its tire is punctured. The core of Michelin's PAX system is the semi-rigid ring installed onto the rim using special equipment. It provides support to the tire and its sidewall to allow emergency operation at limited speed until such time the tire can be replaced. Cars that use the system are as exotic as the Bugatti Veyron EB 16.4 super car to the Rolls Royce Phantom to more pedestrian minivans such as the Honda Odyssey and Nissan Quests.
Prior to the late 1990s introduction of the Michelin PAX System run flat technology, both Michelin and Goodyear had introduced the a "zero pressure" run flat technology, meaning that a pneumatic (air pressure supported) tire could support itself with no air pressure. The new zero pressure tire was a modified standard tire, constructed with a sidewall that was much stiffer and heavier, so as to support the weight of a car running at speed without careening out of control. The heavier sidewalls and special bead construction allowed a driver to drive a car with little or no air at a limited speed (approximately 50 mph) over some specified distance to a service station, or at least off the roadway out of immediate danger. Consider that in a conventional pneumatic tire, loss of pressure at speed would result in the collapse of the soft tire sidewalls such that the metal wheel edges would slice through the collapsed sidewalls. This would likely result in an accident, possibly fatal, as well as the destruction of the tire, and possibly the wheel.