D-Pillar or D-Post – The vertical or sometimes diagonal roof supporting member located at the extreme rear. The vertical or
sometime diagonal roof supporting member located at the extreme rear of the roof or greenhouse structure on station wagon
and some sedan models.
Diesel Engine – A diesel engine uses heavier weight components than gas engines to handle high 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.
Direct Injection – Many diesel engines feature direct injection (DI). The injection nozzle is placed inside and incorporates a
depression where initial combustion takes place. Direct injection diesel engines are generally more efficient and cleaner
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 surface 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).
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.
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
engines 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 increase performance.
Electronic Fuel Injection System – A system that injects fuel into the engine and includes anelectronic control unit to time and
Electronic Ignition System – An ignition system that uses transistors and other semiconductor devices as an electronic switch to turn
the primary current on and off.
Emissions (Exhaust) – Toxic gases created when an engine burns gasoline, emitted in the form of unburned hydrocarbons, carbon
monoxide and oxides of nitrogen.
Four Wheel Drive (4WD) – In a Four Wheel Drive System, a secondary transmission assembly, called a transfer case, is driven from
the main transmission. The transfer case distributes power to both axles to drive all four wheels. It is the heart of the Four-Wheel
Drive system. Four-Wheel Drive can be full-time, in which power is delivered to both axles at all times or part-time, where the driver
selects two or four-wheel drive. Four wheel drive is often combined with independent suspension systems and off-road type tires to
enhance drivability on rough, off-road terrain, or on-road drivability in unfavorable driving conditions.
Front-Wheel Drive (FWD) – A drive system where the engine and transaxle components apply the driving force to the front wheels
rather than the rear wheels. Benefits of Front-Wheel drive include: Maximized passenger space, enhanced cargo area, excellent drive
traction; particularly on wet or slippery surfaces, since the drive is through the front wheels, which carry a heavier load.
Fuel Injection – A method of delivering fuel under pressure into an engine’s combustion chamber. Fuel injection systems can be
single-point, multi-point, etc. Replaces carbureted system.
Fuel Injection, Electronic – A computer-controlled method of delivering fuel under pressure. The computer monitors signals from
coolant temperatures, manifold vacuum, exhaust oxygen sensor, and engine cranking sensor. It “tells” the injectors to release and
adjust the fuel to yield an air/fuel mixture assuring engine operation well matched with emission requirements, optimum fuel
economy and overall vehicle performance.
Fuel Pump – A mechanical or electrical device that draws fuel from the fuel tank and delivers it to the carburetor or injectors.