A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
D
Dampener: A device to reduce oscillations or vibrations. A
kind of damper.
Damper: Component that reduces the oscillations of a spring in a suspension system. A device that smoothes out fluctuations.
Damping: The reduction of oscillations or smoothing out of fluctuations.
Damping Chamber: An air space in which a damper flap moves that acts to reduce oscillations.
Darlington Circuit: Two transistors connected so that the first switches the second. Used to switch high currents.
Dashpot: A damper used on some variable venturi carburettors.
Data Link: General term for a connection to a scanner or similar diagnostic tool.
Databank link: A connection to an electronic source of data.
DC Direct Current: An electrical current that flows in one direction only. Conventionally from the positive pole to the negative pole.
De-energised: Switched off - usually refers to a coil or relay.
De-ionised: Used to describe pure, neutral water used for topping up batteries.
Dead Axle: A non-driving axle. Supports the vehicle and carries freely rotating wheels.
Dead Center: Top or bottom piston positions in the cylinder when the crankshaft is at 0 degrees or 180 degrees of rotation.
Deceleration: Opposite of acceleration. Reducing velocity or speed. Slowing down.
Decelerometer: Brake Meter for recording rate of deceleration during braking for test purposes.
Deck (Block): The top of the engine block forming the mating face for the cylinder head.
Deep Socket: Same as a normal socket but longer to allow it to be used on a component such as a spark plug.
Deflection: Applied to springs to indicate the amount of movement away from the original position.
Degrees of Advance: Refers to crankshaft rotation degrees as linked to ignition timing.
Delta Wound Stator: An alternator stator winding with the three-phase coils connected at the ends in order to give a large current output.
Density: A measure of the compactness of a substance in relation to a given volume at a given temperature. Heat and pressure affect the density of a given mass of material.
Desiccant Bag: Chemical used to absorb moisture. Often used in AC systems to remove moisture from the refrigerant.
Detent: A position holding device consisting of a spring and locking piece that locates in a groove. A locating device for holding sliding gears in mesh in a manual gearbox.
Detonation: Ignition of fuel in an engine that is too early or too rapid. Produces a knocking or pinking sound.
DI: Direct injection of fuel, usually diesel, into the combustion chamber.
Dial Gauge: A gauge used to indicate lateral movement of a component, on a clock (dial) type display.
Dial Test Indicator: A gauge used to indicate lateral movement of a component, on a clock (dial) type display.
Diaphragm: Flexible disc or membrane which deflects under pneumatic or hydraulic pressure and, in so doing, imparts linear motion to a centrally-located rod or other form of mechanical actuator, as in an air brake or carburettor.
Diaphragm Spring: Dished clutch spring applying clamping force and disengagement linkage.
Die: Tool used to cut an external thread.
Dielectric: Used in a capacitor as an insulator between the two conductive plates.
Diesel: Heavy fuel oil for diesel engines. Compression ignition engine.
Diesel 'Knock': Engine noise produced by very rapid rise in cylinder pressure following injection and ignition of the fuel in a diesel engine.
Dieseling: A condition in a petrol engine when it continues to run after the ignition has been turned off. Also known as running on or run on and usually occurs in a very hot carburettor engine where the fuel cannot be fully shut off.
Differential: Relating to or using a difference. Operating on differences in pressure. Bevel gear set that allows two wheels to rotate at different speeds.
Differential Pressure Valve: A valve that sets its position depending on the difference between two pressures, for example, two fuel pressures or fuel pressure and manifold pressure.
Differential Pressure Valves: Valves that operate under the control of TWO external pressures or the difference (differential) between them.
Digital: A signal current with either an 'on' or 'off' voltage. Digital instruments which show a numerical value.
Digital Converters: Electronic devices that convert analogue signals to digital or digital signals to analogue.
Diode: An electronic one way valve with polarity bias manufactured from a semiconductor material.
Diode Pack: A collection of diodes that act as a rectifier. A minium of six diodes in the pack are needed for a three phase alternator.
Dip Switch: A switch used to select main beam or dipped beam headlights. Sometimes called a dimmer switch.
Dipped Beam: The lower of the two headlamp beams, dipped to avoid blinding the drivers of oncoming vehicles.
Dipstick: A stick used to dip in a fluid and check its level!
Direct Injection: Diesel engine design where fuel is injected directly into the cylinder.
DIS: Distributorless ignition system that uses an ignition coil or coils directly connected to the spark plugs. The ignition is triggered by the ignition module on signals from the ECM (Engine control module).
Disc Brake: Brake in which external friction pads are brought to bear on the faces of a rotor or disc, usually by the clamping action of a caliper.
Discard Dimension: The lowest safe value for a component before it needs to be replaced. Used for brake components such as drums and rotors.
Discharge Time: Refers usually to how long a capacitor will take to release all its stored energy. It can be calculated by multiplying the capacitance by resistance and by five (t = 5CR).
Disengagement: The act of removing a connection between two gears of through a clutch.
Displacement: The swept volume of a cylinder between the top and bottom positions of the top of a piston.
Displacer Unit: A hydraulic suspension component that contains gas under pressure an a fluid.
Distilled: Purified by evaporation and condensation.
Distributor: A component in the ignition system that houses the primary circuit switching device and the secondary circuit distribution components.
Diverter Valve: Used on air injection systems to control the air feed according to the engine running conditions. Also known as a 'dump valve'.
Divider: A digital circuit that, for example, reacts to only every second input hence dividing it in two.
DLC: Data link connector for connecting a diagnostic tool to a vehicle's electronic systems.
Dog Clutch: Pegs or similar used to transfer drive rotation from one component to another.
Dog Drives: Pegs or similar used to transfer drive rotation from one component to another.
DOHC: Dual overhead camshaft. Sometimes double overhead camshaft.
Donut Coupling: A rubber universal joint shaped like a donut.
Double-Cardan Universal
Double-declutch: A technique that was necessary before the use of synchromesh mechanisms. During a gear change the clutch is lifted as the gear mechanism moves to neutral, the engine is revved (if changing down), the clutch is now depressed and the new gear selected.
Dowel: A locating pin between two mating components. Can be solid or hollow.
Down Draft: A carburettor where the choke tube (venturi) is fitted vertically.
Downshift Valve: Automatic transmission valve to give a downshift under specific drive conditions.
Drag Link: Steering linkage between the steering box drop arm (Pitman arm) and the drag arm on the steering swivel.
Drain Plug: Removable plug used to let fluid out (engine oil for example).
Drain Tap: A simple valve used to allow a fluid to be removed.
Drain Tube: A tube used to, for example, remove moisture from an air conditioning evaporator.
Drive Axle: An axle that transmits a drive to the wheels. Also known as a live axle.
Drive Belt: Traditionally called the fan belt. A vee or multi-vee belt to take drive from the engine to auxiliary components such as an alternator or water pump.
Drive Flange: A flange that is used to transmit drive from one component to another. The flange on the end of a propshaft is a good example.
Drive Gear: A toothed wheel to pass drive to another toothed wheel.
Drive
Drive Line: Rotary shafts carrying drive to the wheels.
Drive Motor: A motor that supplies a driving force. Term usually refers to an electric motor.
Drive Pinion: A small gear that drives another gear.
Drive Plate: A plate that takes the place of the flywheel on an engine such as to transfer drive to a torque converter.
Drive Pulleys: Pulleys that transfer drive from one shaft to another.
Drive Ratios: Ratio between gears. For example 2:1 or 1.3:1.
Drive Shafts: Open shafts carrying the drive from the final drive to the driven wheels.
Driveline: Transmission components in the line between the engine and wheels.
Driveline Wind-up
: Description of how a vehicle in four-wheel drive (with no centre differential) can build up spring tension in its transmission components. This is because the front wheels may rotate at a slightly different speed to the rear wheels.
Driven Plane: The plane (like a flat sheet) through which the drive operates.
Driven Plate: The clutch disc that is driven by the pressure plate.
Driveshafts: Open shafts carrying the drive from the final drive to the driven wheels.
Driving Beam: The upper of the two headlamp beams. Also main beam and upper beam.
Driving Thrust: Forces generated by the engine and transmission.
Drop Arm: The output arm on a steering gearbox. Also known as a Pitman arm.
Dropping Resistors: Resistors used to reduce voltage to a component.
Drum Brake: Brake in which friction blocks or brake shoes lined with friction material are brought to bear on the periphery of a drum or cylinder. In most vehicle applications the shoes are brought into contact with the inner periphery of the drum.
Dry Clutch: A friction clutch that runs in an open housing. As distinct from a wet clutch that runs in an oil bath.
Dry Liner/Dry Sleeve: (US: dry sleeve) Hard metal engine cylinder liner or insert that is not exposed to contact with cooling water.
Dry Sump (Oil Pan): An engine lubrication system where the reservoir of lubricating oil is held in an oil tank. The sump (oil pan) below the engine is scavenged with a separate pump for return to the oil tank. As distinct from a wet sump (oil pan) where the reservoir of lubricating oil is kept in an oil pan below the engine.
DTI: Dial Test Indicator.
Dual Bed Converter: A three way catalytic converter with one stage coated with platinum and palladium and the other with platinum and rhodium. Converts HC and CO to H and CO2 and NOX to N and O.
Dual Circuit Brake System: A brake system that has two independent hydraulic circuits so that one continues to act if the other fails. The split of dual circuits can be front and rear, diagonal, split front and one rear, and two complete circuits.
Dual Master Cylinder: A brake master cylinder with two separate chambers for a dual circuit hydraulic system.
Duct: Pipe, tube or trunking.
Ductility: The property of a material that allows it to be drawn into wires.
Ducting: Large pipes or similar. Usually refers to the air intake system components.
Dump Valve: See diverter valve.
Duty Cycle: The percentage length of time that an actuator or solenoid is energised by an electronic control unit. Refers to the pulse width.
Dwell Angle: The angle of distributor cam rotation when the contact breaker points are closed.
Dwell Meter: Measures the dwell angle.
Dwell Period: The angle of distributor cam rotation when the contact breaker points are closed. This term is used when the dwell angle is not a fixed value. Used for electronic ignition systems.
Dynamic: A term that indicates components that are in motion. The opposite of static.
Dynamo: Machine converting mechanical energy into electrical energy. DC.
Dynamometer: A measuring device for engine power output. A brake dynamometer measures the output at the engine flywheel to give the term brake horse power (bhp). A rolling road dynamometer measures the output at the vehicle wheels and is used for test and diagnostic purposes.