Engineer Motor Vehicles Maintenance key terms

The sequence of intake, compression, power and exhaust strokes completed in two crankshaft revolutions, during which every piston movement performs one distinct function of the engine's working cycle.

The ratio of cylinder volume above the piston at bottom dead centre to the volume remaining at top dead centre, indicating how far the charge is squeezed before ignition.

The proportion of the theoretical cylinder capacity actually filled with fresh air-fuel charge on the intake stroke, used to judge breathing quality of an engine.

The angular positions of the crankshaft at which intake and exhaust valves open and close, including overlap, set to suit gas flow at the engine's intended operating speed.

Abnormal explosive burning of the end gas ahead of the flame front, producing a sharp metallic knock, caused by excessive pressure, heat or too low an octane rating.

An exhaust-driven turbine coupled to a compressor that forces extra air into the cylinders, raising power output; it requires lubrication, cooling and often a wastegate to limit boost.

A fuel system in which a high-pressure pump charges a shared rail feeding electronically controlled injectors, allowing independent control of injection pressure and timing.

The crankshaft angle, measured before or after top dead centre, at which the spark or injection event occurs, chosen to obtain peak cylinder pressure just after top dead centre.

Loss of positive drive between engine and gearbox because the friction disc cannot transmit the applied torque, shown by rising engine speed without matching road speed.

A fluid coupling with a stator that multiplies engine torque at low speed and lets the vehicle creep or stop in gear without a mechanical clutch.

A cone-and-sleeve device that frictionally matches the speed of the gear being selected to the output shaft before the dog teeth engage, preventing gear clash.

The reduction between the gearbox output and the driven road wheels, usually through a crown wheel and pinion, which trades rotational speed for torque.

A gear train that divides drive between two wheels while allowing them to turn at different speeds when cornering, essential to avoid tyre scrub and transmission wind-up.

A differential that deliberately restricts speed difference between the driven wheels, usually through friction plates or a viscous coupling, improving traction on low-grip surfaces.

Adjustment of camber, caster, toe and steering axis inclination so that wheels roll correctly, minimising tyre wear and giving straight-line stability and self-centring.

The inward or outward tilt of a wheel's vertical plane from true vertical when viewed from the front, affecting tyre wear and cornering grip.

The amount by which the front edges of a pair of wheels are closer together than the rear edges, set to offset the tendency of wheels to splay outward while rolling.

An electronic system that senses wheel lock during braking and modulates hydraulic pressure at each wheel, preserving steering control and shortening stopping distance on most surfaces.

Loss of braking effectiveness caused by overheating of friction linings or brake fluid, often after repeated heavy stops or sustained downhill braking.

A suspension arrangement combining a telescopic damper with a coil spring and locating the wheel through a lower wishbone, common on front wheels of passenger cars.

A hydraulic damper that converts spring oscillation into heat, controlling rebound and compression so that the vehicle settles quickly after disturbance.

The relationship stating that current through a conductor equals voltage across it divided by resistance, the basis for calculating loads, voltage drop and circuit protection.

A live measurement of potential difference across a circuit component or connection while current flows, revealing excessive resistance that a static ohmmeter reading would miss.

Current drawn from the battery when the ignition is off and systems are supposedly asleep, which flattens the battery overnight if it exceeds the manufacturer's limit.

A twisted-pair serial bus over which electronic control units exchange short digital messages, reducing wiring and allowing modules to share sensor data and fault information.

A stored alphanumeric identifier set by an electronic control unit when a monitored signal or circuit falls outside expected limits, guiding the technician's diagnostic path.

A test instrument displaying voltage against time, used to inspect sensor waveforms, injector pulses and bus signals that a multimeter's averaged reading cannot reveal.

A measurement of peak cylinder pressure obtained by cranking the engine with a gauge in each plug or injector hole, used to identify worn rings, valves or head gasket failure.

A test in which compressed air is admitted to a cylinder at top dead centre and the percentage escaping is measured, with audible leaks locating the faulty sealing path.

The temperature at which brake fluid vaporises and forms compressible bubbles; wet fluid absorbs moisture and boils at a much lower temperature, causing a spongy pedal.

An exhaust after-treatment device trapping soot from diesel combustion, periodically regenerated by raising exhaust temperature to burn off accumulated deposits.

A system returning a metered portion of exhaust gas to the intake, lowering combustion temperature and thereby reducing oxides of nitrogen formation in both petrol and diesel engines.

A regulated measurement of exhaust constituents such as carbon monoxide, hydrocarbons, smoke opacity and lambda, confirming that a vehicle meets statutory limits before registration or licensing.

A document opened for each vehicle visit recording customer details, reported symptoms, authorised work, parts used and technician time, forming the basis for invoicing and warranty claims.

A published time allowance for a defined repair operation, used to price labour independently of the actual time a particular technician takes to complete the job.

A calculated forecast of parts, materials, labour and overheads for a proposed repair, given to the customer for approval before work begins.

A documented process identifying workplace hazards, judging the likelihood and severity of harm, and recording controls to reduce risk to an acceptable level.

A supplier document listing a substance's hazards, safe handling, storage, first-aid and disposal requirements, consulted before using chemicals such as solvents, paints or battery acid.

A formal written authorisation required before high-risk tasks such as hot work, confined-space entry or work on energised equipment, confirming precautions have been taken.

A procedure isolating energy sources and fixing personal locks and warning tags so machinery or vehicles cannot be started while someone is working on them.

An electronic system that detects wheelspin during acceleration and reduces engine torque or applies individual brakes, helping the driver maintain grip and directional control.

A rotating shaft carrying lobes that open and close the engine's valves in the correct sequence and lift, driven from the crankshaft by belt, chain or gears.

A heat exchanger cooling compressed intake air after the turbocharger, increasing its density so more oxygen enters the cylinder and power rises.

A gearbox shifting ratios without driver clutch operation, typically using planetary gearsets and a torque converter controlled hydraulically or electronically.