D-Pillar or D-Post
The vertical or sometimes diagonal roof supporting member located at the extreme rear of the roof or greenhouse structure on station wagons and some sedan models.

Die
solid or hollow form used for shaping materials by stamping, pressing, extruding, drawing or threading.

Diesel Engine
A diesel engine uses heavier weight components than gas engines to handle higher compression ratios. Typically, diesel engines run with greater efficiency and higher torque than similar size gas engines. These attributes lead to better fuel economy and towing performance. Diesel engines do not have spark plugs or carburetors. Instead glow plugs are used to preheat air in the cylinders to ensure easy starts. Once the engine is started, compression heats the fuel in the cylinders for combustion.

Dieseling
A condition in which gasoline continues to fire after the ignition has been shut off. In late-model engines, dieseling , or run-on, is caused by heat and the unusually high manifold pressure that result from retarding the spark at idle. In fuel-injected cars when the engine is turned off, fuel is automatically shut off, eliminating dieseling.

Differential
The gear assembly connected to the drive shaft that permits the wheels to turn at different speeds when going around a corner, while transmitting power from the drive shaft to the wheel axles.

Differential, Locking
The same attributes of a standard differential, except that when one wheel is slipping, the most torque is supplied to the wheel with best traction. A locking differential reduces the possibility of a vehicle becoming immobile when one driving wheel loses traction.

Directional Stability
A vehicle's ability to maintain a true course of travel despite bumps, crosswinds, uneven road surfaces.

Disc Brakes
Properly called caliper disc brakes, a type of brake that consists of a rotor that rotates at wheel speed, straddled by a caliper that can squeeze the surfaces of the rotor with brake pads near its edge. Disc brakes provide a more linear response and operate more efficiently at high temperatures and during wet weather than drum brakes.

Displacement
In an engine, the total volume of air or air-fuel mixture an engine is theoretically capable of drawing into all cylinders during one operating cycle. Generally expressed in liters or cubic inches. Engine displacement is equal to (bore) x (bore) x (stroke) x (number of pistons) x (.785).

Distributor A component of the ignition system, usually driven by the camshaft that directs high-voltage surges to the spark plugs in the proper sequence.

Double Wishbone Suspension ("A" Arm Suspension)
A system of independent suspension in which each wheel is located on a "knuckle" that is connected by ball joints to an upper A arm and a lower A arm. Usually, the lower A arms are longer. This system provides minimal changes in track and camber when the suspension is under load, as when going over bumps or in hard cornering.

Drag Coefficient
A measure of the aerodynamic sleekness of an object. Drag coefficient is signified by "dc.: The lower the number, the greater the aerodynamic efficiency. The higher the drag coefficient, the more a car's engine must work to keep a given road speed. Also known as "CD" for coefficient of drag."

Drive Shaft
The shaft that transmits power from the transmission to the differential in a rear-drive power train.

Drivetrain
The power-transmitting components in a car, including clutch, gearbox (or automatic transmission), driveshaft, universal joints, differential and axle shafts.

Dual Overhead Camshafts (DOHC)
A DOHC engine has two camshafts in each cylinder head; one camshaft actuates intake valves and the other actuates exhaust valves. The camshafts act directly on the valves, eliminating pushrods and rocker arms. This reduced reciprocating mass of the valve train enables the engine to build RPM more quickly. DOHC designs are typically high-performance, four valve per cylinder engines. (A four valve per cylinder two intake and two exhaust design helps the engine "breathe" more freely for increased performance.)

Dynamometer
A device which absorbs and measures the power derived by an internal combustion engine.

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