Automotive Glossary of Terms - D -
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Definitions for automotive terms and illustrations from dead center to dynamometer .
DEAD CENTER
The extreme top or bottom of the piston stroke.
DETERGENT
An additive in engine oil to improve its operating characteristics.
DETONATION
An unwanted explosion of the air/fuel mixture in the combustion chamber caused by excess heat and compression, advanced timing, or an overly lean mixture. Also referred to as "ping".
DEXRON
A brand of automatic transmission fluid.
DIAPHRAGM
A thin flexible wall separating two cavities, such as in a vacuum advance unit.
DIESELING
The engine continues to run after the car is shut off; caused by fuel continuing to be burned in the combustion chamber.
DIFFERENTIAL
A geared assembly which allows the transmission of motion between drive axles, giving one axle the ability to rotate faster than the other, as in cornering.
DIGITAL VOLT OHMMETER
An electronic diagnostic tool used to measure voltage, ohms and amps as well as several other functions, with the readings displayed on a digital screen in tenths, hundredths and thousandths.
DIODE
An electrical device that will allow current to flow in one direction only.
DIRECT CURRENT (DC)
Electrical current that flows in one direction only.
DISC BRAKE
A hydraulic braking assembly consisting of a brake disc, or rotor, mounted on an axleshaft, and a caliper assembly containing usually two brake pads which are activated by hydraulic pressure. The pads are forced against the sides of the disc, creating friction which slows the car.
DISPLACEMENT
The total volume of air that is displaced by all pistons as the engine turns through one complete revolution.
DISTRIBUTOR
A mechanically driven device on an engine which is responsible for electrically firing the spark plug at a pre-determined point of the piston stroke.
DOHC
Double overhead camshaft.
DOUBLE OVERHEAD CAMSHAFT
The engine utilizes two camshafts mounted in one cylinder head. One camshaft operates the exhaust valves, while the other operates the intake valves.
DOWEL PIN
A pin, inserted in mating holes in two different parts, allowing those parts to maintain a fixed relationship.
DRIVE CYCLE TEST
A function of the vehicle on board computer that includes the testing of the monitored systems that requires a start up -- from a cold engine to a warm engine -- and the vehicle being driven.
DRIVE TRAIN
The components that transmit the flow of power from the engine to the wheels. The components include the clutch, transmission, driveshafts (or axle shafts in front wheel drive), U-joints and differential.
DRUM BRAKE
A braking system which consists of two brake shoes and one or two wheel cylinders, mounted on a fixed backing plate, and a brake drum, mounted on an axle, which revolves around the assembly.
DRY CHARGED BATTERY
Battery to which electrolyte is added when the battery is placed in service.
DVOM
Digital volt ohmmeter
DWELL
The rate, measured in degrees of shaft rotation, at which an electrical circuit cycles on and off.
DYNAMOMETER
Various devices used in testing a motor or engine for such characteristics as efficiency and torque, especially an instrument that measures current or the power of a motor by calculating the force between a fixed coil and a moving coil.