Odisha CHSE +2 Vocational Automobile Technology key terms

The ratio of the total cylinder volume with the piston at bottom dead centre to the volume remaining above the piston at top dead centre. Higher ratios usually improve thermal efficiency in petrol engines but risk detonation.

The exact position where the piston reaches its highest point in the cylinder bore during rotation of the crankshaft. Timing marks on the pulley or flywheel indicate this position for ignition and valve setting.

The small gap left between the valve stem tip and the rocker arm or cam follower when the valve is closed. It allows for thermal expansion and is set cold or hot as specified by the manufacturer.

The sequence in which the engine cylinders deliver their power strokes, chosen to balance loads on the crankshaft. Common sequences are 1-3-4-2 for four-cylinder and 1-5-3-6-2-4 for six-cylinder engines.

Abnormal combustion in which the end gas in the cylinder ignites spontaneously before the flame front arrives, producing a sharp metallic knock. Caused by low octane fuel, excessive advance or overheating.

Bore is the internal diameter of the cylinder, while stroke is the distance the piston travels between top and bottom dead centre. Together they determine engine displacement and its long-stroke or short-stroke character.

The small clearance between the release bearing and the clutch pressure plate fingers, felt as pedal travel before resistance. Too little free play causes clutch slip; too much causes incomplete disengagement and gear crunching.

Loss of torque transfer because the friction disc fails to grip the flywheel and pressure plate fully, causing engine speed to rise without matching road speed. Often caused by oil contamination, worn lining or incorrect free play.

A thrust bearing that presses against the rotating clutch diaphragm or pressure plate fingers when the pedal is depressed, converting pedal force into clutch disengagement while allowing rotation.

A device in a manual gearbox that uses cone friction surfaces to equalise the rotational speeds of the gear and the output shaft before the dog teeth engage, preventing gear clash during gear changes.

The relationship between the number of teeth on the driven gear and the driving gear, expressing how many turns of the input are needed for one turn of the output. It determines torque multiplication and speed.

A gear train that allows the two driven wheels to rotate at different speeds while transmitting torque from the propeller shaft, essential when a vehicle turns a corner.

The overall reduction between the gearbox output and the road wheels, usually set by the crown wheel and pinion in the differential. It trades top speed against acceleration and fuel consumption.

The tilt of the steering axis from the vertical when viewed from the side, with the top leaning towards the rear. It gives self-centring steering feel and straight-line stability.

The inward or outward tilt of the road wheel from the vertical when viewed from the front of the vehicle. Excessive camber causes uneven tyre wear and affects cornering grip.

The difference in distance between the front and rear of the two front wheels, measured at hub height. Slight toe-in or toe-out counteracts suspension flex and controls straight-line tyre scrub.

Adding or removing small weights on the rim so that the mass of the wheel and tyre assembly is distributed evenly about its axis, eliminating vibration at speed.

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

A temporary loss of braking effectiveness caused by overheating of the friction material or boiling of the brake fluid, common after repeated heavy braking on long descents.

A hygroscopic hydraulic fluid, usually glycol-based with a DOT rating, that transmits pedal force to the wheel cylinders. It absorbs moisture over time, lowering its boiling point and requiring periodic replacement.

The procedure of expelling trapped air from the hydraulic brake circuit by opening the bleed screws at each wheel while an assistant holds and releases the pedal, until clear bubble-free fluid flows.

The amount of charge a battery can deliver, expressed in ampere-hours, at a specified discharge rate. It indicates how long the battery can supply a given current before reaching its cut-off voltage.

The ratio of the density of the battery electrolyte to that of water, measured with a hydrometer to estimate state of charge. A fully charged lead-acid cell reads about 1.260 to 1.280.

A belt-driven generator that produces alternating current, rectified by built-in diodes, to charge the battery and supply electrical loads while the engine runs. Its regulator holds output voltage near 14 volts.

The distance between the centre and ground electrodes of a spark plug, typically 0.7 to 1.1 mm. An incorrect gap weakens the spark and can cause misfiring or hard starting.

A rating of a petrol's resistance to detonation, based on comparison with iso-octane and normal heptane. Higher numbers suit higher compression ratios and prevent knocking.

A measure of the ignition quality of diesel fuel, indicating how readily it self-ignites under compression. Higher cetane numbers give easier cold starting and smoother combustion.

The moment, measured in degrees of crankshaft rotation, at which the injector begins delivering diesel into the combustion chamber. Incorrect timing causes smoke, poor power and high fuel consumption.

A classification such as 15W-40 that describes how thick an oil is at low and high temperatures. The W number relates to cold cranking performance and the second number to operating temperature.

A gear, rotor or vane pump that draws lubricating oil from the sump and delivers it under pressure to the engine bearings and galleries. A pressure relief valve limits maximum pressure.

A spring-loaded pressure cap that raises the boiling point of the coolant by maintaining system pressure, and contains a vacuum valve that admits air as the engine cools to prevent hose collapse.

A temperature-sensitive valve in the cooling circuit that stays closed while the engine warms up and opens at a set temperature to route coolant through the radiator, regulating operating temperature.

A measuring instrument with a fixed main scale and a sliding vernier scale, used to measure outside, inside and depth dimensions to a least count of 0.02 mm or 0.05 mm.

A precision measuring instrument using a calibrated screw and thimble to measure small external dimensions, typically with a least count of 0.01 mm. The ratchet stop prevents over-tightening.

A set of thin steel blades of known thickness used to check small gaps such as valve clearance, spark plug gap or contact breaker points by trial insertion.

A wrench fitted with a scale or clicking mechanism that indicates when a specified tightening torque has been reached, ensuring bolts are tightened to the manufacturer's specification without stripping threads.

A diagnostic check in which a pressure gauge is screwed into each spark plug or injector hole and the engine is cranked, comparing readings to reveal worn rings, valves or head gasket leaks.

The Indian legislation that governs licensing, registration, insurance, traffic offences and penalties for motor vehicles, applicable in Odisha through state rules and enforcement by transport authorities.

Legally set limits on pollutants such as carbon monoxide, hydrocarbons, nitrogen oxides and particulate matter in vehicle exhaust, progressively tightened through Bharat Stage standards across India.

A mandatory motor policy that covers liability for injury, death or property damage caused to other people by the insured vehicle, but does not pay for the insured vehicle's own damage.

An exhaust device containing platinum, palladium and rhodium coatings that converts harmful carbon monoxide, hydrocarbons and nitrogen oxides into less harmful gases before release into the atmosphere.