Maharashtra HSC Automobile Technology - Vocational (MSBSHSE) key terms

The load-bearing framework of a vehicle that carries the engine, transmission, suspension and body, and absorbs the forces produced while driving. It excludes the body panels on separate-frame designs.

A body construction in which the panels and floor form a single stressed shell that carries all loads, removing the need for a separate heavy frame. Common in modern cars and scooters.

A chassis made of two long side members joined by cross members, resembling a ladder. It is strong and simple, and is widely used on trucks, buses and older utility vehicles.

A deliberately weak front or rear section of a body designed to fold and absorb impact energy in a collision, keeping the passenger cell intact. It works together with a rigid safety cage.

The strong, rigid structure around the passenger compartment, built from reinforced pillars, sills and roof rails, which resists deformation during a crash and protects occupants.

A transverse structural beam connecting the two side members of a chassis, giving it rigidity and supporting components such as the gearbox mounting or body floor.

The sequence of intake, compression, power and exhaust strokes that a piston completes in two crankshaft revolutions. Each stroke performs one distinct function in the engine's working cycle.

The ratio of the cylinder volume with the piston at bottom dead centre to the volume at top dead centre. Higher values generally improve thermal efficiency but demand higher-octane fuel.

Abnormal, explosive burning of the fuel-air charge after spark ignition, producing a metallic knocking sound and possible piston damage. It is caused by excessive pressure, heat or low-octane fuel.

The exact instants, measured in crankshaft degrees, at which the intake and exhaust valves open and close relative to piston position. It determines breathing quality and engine output.

A rotating shaft with lobes that push the valves open at the correct moment and control their lift. It is driven from the crankshaft by belt, chain or gears at half engine speed.

The small gap between the valve stem and the rocker or cam follower, set to allow for thermal expansion. Too little causes valves to leak; too much causes noise and reduced lift.

A friction device that connects and disconnects the engine from the gearbox, allowing smooth starts and gear changes. It transmits torque through a pressure plate and friction disc.

The number of teeth on a driven gear divided by the number of teeth on the driving gear. It determines the trade-off between output speed and torque in a transmission.

A fluid coupling with a stator that multiplies engine torque and allows the vehicle to stop without disengaging gears. It replaces the clutch in most automatic transmissions.

A gear assembly that splits drive between the two wheels of an axle while allowing them to rotate at different speeds when the vehicle turns a corner.

The last reduction gear set between the gearbox output and the road wheels, usually in the axle or transaxle. It multiplies torque and sets overall top speed.

The steering geometry in which the inner wheel turns through a larger angle than the outer wheel so both wheels follow circles about a common centre, reducing tyre scrub in turns.

The tilt of the steering axis from vertical, viewed from the side. Positive caster gives self-centring and straight-line stability but increases steering effort.

The adjustment of camber, caster, toe and steering axis inclination so that wheels roll correctly. Poor alignment causes uneven tyre wear and pulling to one side.

A setting in which the front edges of the wheels are closer together than the rear edges, viewed from above. It counteracts the outward splay produced by camber thrust.

The inward or outward lean of a wheel from vertical, viewed from the front. Excessive camber wears one side of the tyre and affects cornering grip.

A braking system that transmits pedal force through incompressible brake fluid to wheel cylinders or calipers, multiplying force by piston area ratios. Air in the fluid makes the pedal spongy.

The process of expelling trapped air from the hydraulic brake circuit by opening bleed nipples while pumping the pedal, until a solid stream of fluid appears.

The pump, driven by the brake pedal, that pressurises brake fluid and supplies it to the wheel brakes. It also contains a reservoir and often a tandem safety piston.

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

A belt-driven generator that produces alternating current, rectified by diodes to charge the battery and run electrical loads. Its output is regulated to about 14 volts.

A control device that limits alternator field current so charging voltage stays within safe limits, protecting the battery from overcharging and electrical components from excess voltage.

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

A sensor, usually magnetic or Hall-effect, that reports each wheel's rotational speed to the ABS or engine control unit for slip detection and speedometer input.

Electronic Control Unit: a microprocessor that reads sensor inputs and controls actuators such as injectors, ignition coils and idle valves according to stored maps and feedback.

A self-test system that monitors sensors, actuators and emissions components and stores fault codes readable through a diagnostic connector, aiding systematic troubleshooting.

A lead-acid storage device that supplies current for cranking and accessories and is recharged by the alternator. Its capacity is rated in ampere-hours and cold-cranking amps.

Controlled heating and cooling of a metal to change its microstructure and properties, such as hardening, tempering, annealing or normalising, without altering its chemical composition.

A heat treatment that heats metal and cools it slowly, usually in a furnace, to soften it, relieve internal stresses and improve machinability before further shaping operations.

A treatment that hardens only the outer skin of low-carbon steel, often by carburising, while leaving a tough core. Used for gears and shafts needing wear resistance with shock strength.

A measuring instrument with a main scale and a sliding vernier scale, used to read outside, inside and depth dimensions to a least count of 0.02 mm on typical workshop models.

A precision instrument using a calibrated screw to measure small thicknesses and diameters, commonly reading to 0.01 mm. Its reading combines the barrel and thimble scales.

The smallest measurement an instrument can read reliably, calculated from its scale divisions. It determines the precision of a reading and the acceptable rounding of results.

A test instrument that measures voltage, current and resistance, and often continuity. It is the basic tool for tracing electrical faults in charging, starting and lighting circuits.

Scheduled servicing such as oil changes, filter replacement, lubrication and inspections carried out at set intervals to prevent breakdowns rather than to repair failures already present.

A wrench that indicates or limits the tightening torque applied to a fastener, ensuring bolts such as cylinder-head bolts are tightened evenly to the manufacturer's specified value.

A set of thin metal blades of known thickness used to check small gaps such as tappet clearance, spark plug gap and contact breaker point gap by trial fit.

India's emission standards, aligned in stages with Euro norms, that set limits on carbon monoxide, hydrocarbons, nitrogen oxides and particulate matter from new vehicles.

An exhaust device with a platinum, palladium or rhodium coating that converts carbon monoxide, hydrocarbons and nitrogen oxides into less harmful gases through catalysed chemical reactions.

Pollution Under Control certificate issued after an exhaust gas test at an authorised centre, confirming a vehicle meets the prescribed emission limits for on-road use.

Exhaust Gas Recirculation: a system that routes a small portion of exhaust gas back into the intake to lower combustion temperature and reduce nitrogen oxide formation.