Karnataka II PUC Automobile - Vocational (KSEAB) key terms

A wrench that indicates the amount of turning force applied so a fastener can be tightened to a specified value without over-stressing it. Used for cylinder heads, suspension and wheel nuts.

A set of thin steel blades of known thickness used to measure small gaps such as valve clearances, spark plug gaps and contact breaker points. The blade that just slides in gives the gap.

A measuring tool with a fixed main scale and a sliding vernier scale that reads outside, inside and depth dimensions to a precision of usually 0.02 mm.

A precision measuring instrument using a calibrated screw and thimble to measure small thicknesses or diameters, typically to 0.01 mm, used for pistons, journals and shims.

Bore is the internal diameter of a cylinder; stroke is the distance the piston travels from top dead centre to bottom dead centre. Together they determine engine swept volume.

The volume displaced by a piston moving through one full stroke, calculated from bore and stroke. Total swept volume of all cylinders is the engine's cubic capacity.

The ratio of total cylinder volume above the piston at bottom dead centre to the volume remaining at top dead centre. It strongly affects power, efficiency and fuel octane requirement.

The sequence of intake, compression, power and exhaust strokes, each completed in two piston strokes, giving one power stroke per two crankshaft revolutions in a spark-ignition engine.

The small gap between the valve stem tip and the rocker or cam follower, allowing for thermal expansion. Too little causes leakage and burning; too much causes noise and lost lift.

The precise crankshaft positions at which the intake and exhaust valves open and close, expressed relative to top and bottom dead centre, including overlap around the end of the exhaust stroke.

Abnormal metallic combustion noise caused by spontaneous ignition of the end gas ahead of the flame front, favoured by low octane fuel, high compression or excessive spark advance.

A scale rating a petrol's resistance to knocking, comparing it with isooctane and normal heptane mixtures. Higher octane number means greater resistance to detonation in spark-ignition engines.

A measure of how readily a diesel fuel self-ignites under compression. Higher cetane number gives shorter ignition delay, easier cold starting and quieter combustion.

The mass proportion of air to fuel supplied to the engine. Chemically correct or stoichiometric ratio for petrol is about 14.7 to 1; richer or leaner mixtures affect power, economy and emissions.

A device that mixes petrol with air using a venturi and jets, supplying a combustible mixture to the engine. Mixture strength is varied by throttle, choke and economiser circuits.

Delivery of fuel under pressure through an injector into the intake port or cylinder, metered electronically. It gives more precise air-fuel control than a carburettor and is standard on modern vehicles.

A diesel injection system with a single high-pressure fuel rail feeding all injectors. Electronic control of injection pressure and timing allows multiple injections per cycle.

The moment, in crankshaft degrees before or after top dead centre, at which the spark or fuel injection occurs. Correct timing balances power, economy and knock avoidance.

A step-up transformer that converts low battery voltage into the high voltage needed to jump the spark plug gap. Modern engines often use one coil per plug.

A device screwed into the combustion chamber that provides an electrode gap across which the ignition coil's high voltage produces the spark to ignite the mixture.

A heat exchanger that transfers heat from engine coolant to the air stream, using tubes and fins. It maintains coolant temperature within the design range together with the thermostat and fan.

A temperature-operated valve that stays closed while the engine is cold and opens at a set temperature to allow coolant to circulate through the radiator, speeding warm-up and regulating temperature.

A mixture of water and antifreeze, usually ethylene glycol with corrosion inhibitors, circulated to remove engine heat. It raises boiling point and lowers freezing point.

A measure of an oil's resistance to flow, graded by SAE numbers. Multigrade oils such as 10W-30 behave thin when cold and thicker when hot, suiting Indian conditions.

A gear, rotor or vane pump driven by the engine that pressurises oil and forces it through the filter and galleries to bearings and other moving parts.

A replaceable element or cartridge that traps metal particles, sludge and dirt from the lubricating oil to protect bearings and oil passages from wear.

A friction device that connects and disconnects the engine from the gearbox, allowing smooth take-up of drive and gear changes. It consists of flywheel, friction plate, pressure plate and release mechanism.

The ratio of teeth on a driven gear to those on the driving gear, determining the trade-off between output speed and torque. Lower ratios give more torque and less speed.

A gear train that divides drive between two half-shafts while allowing them to rotate at different speeds, essential when a vehicle turns a corner.

The tilt of the steering axis from vertical, viewed from the side. Positive caster improves straight-line stability and self-centring of the steering after a turn.

The inward or outward tilt of the wheel from vertical when viewed from the front. Correct camber ensures even tyre wear and proper contact patch loading.

The setting in which the front edges of the wheels are closer together than the rear edges, offsetting the tendency of driven wheels to splay outward and reducing tyre scrub.

Anti-lock braking system: wheel speed sensors and a hydraulic modulator rapidly release and reapply brake pressure to prevent wheel lock, preserving steering control during hard braking.

Loss of braking effectiveness after repeated or prolonged use, caused by heat build-up reducing the friction coefficient or vaporising brake fluid.

The depth of grooves on a tyre's running surface, which channels away water. Very low depth increases aquaplaning risk; Indian rules require a legal minimum for road use.

The amount of charge a battery can deliver, quoted in ampere-hours, with cold cranking current indicating its ability to start an engine in low temperatures.

A belt-driven generator using a rotating field and rectifier diodes to produce direct current that charges the battery and powers electrical loads while the engine runs.

A high-torque electric motor that engages a pinion with the flywheel ring gear to crank the engine until it fires, then disengages automatically.

On-board diagnostics: a standardised self-test system that monitors sensors and emissions components, stores fault codes and lets a scan tool read live data.

A test instrument combining voltmeter, ammeter and ohmmeter functions, used to trace open circuits, check battery voltage and verify sensor resistance during diagnosis.

An exhaust after-treatment device that traps soot from diesel exhaust and periodically burns it off during regeneration, reducing particulate emissions.

An exhaust device with a platinum-group catalyst that converts carbon monoxide, hydrocarbons and oxides of nitrogen into less harmful gases before release.