Tamil Nadu HSE Second Year Basic Automobile Engineering key terms

A sequence in which a piston makes four strokes (suction, compression, power, exhaust) and the crankshaft turns twice to complete one power event. Most automobile engines work on this cycle.

The distance a piston travels between top dead centre and bottom dead centre inside the cylinder. Multiplying stroke length by cylinder area gives the swept volume used to calculate engine capacity.

The ratio of total cylinder volume with the piston at bottom dead centre to the volume remaining at top dead centre. Higher ratios raise thermal efficiency but demand higher-octane petrol to avoid knocking.

Abnormal, explosive burning of the tail-end fuel-air charge after spark ignition, producing a metallic rattle and shock loads on bearings. Caused by low octane fuel, over-advanced timing, or excessive compression and heat.

The exact crank angles at which intake and exhaust valves open and close, usually quoted with valve overlap. Correct timing ensures full charging and complete scavenging of the cylinder at all speeds.

An exhaust-driven turbine coupled to a compressor that forces extra air into the engine, increasing power from a small displacement. A wastegate limits boost pressure to protect the engine.

A device that mixes petrol with air in the correct proportion using a venturi and jets before the mixture enters the intake manifold. Replaced by fuel injection on modern cars for better economy and emissions.

The mass of air supplied per unit mass of fuel. Petrol engines need roughly 14.7:1 for complete combustion; richer mixtures give more power, leaner mixtures give better economy but risk overheating.

An electrically operated nozzle that sprays atomised fuel into the intake port or directly into the cylinder under pressure. Pulse width, set by the ECU, controls the quantity delivered per cycle.

A diesel injection layout in which one high-pressure pump feeds a shared rail supplying all injectors. The ECU controls injection pressure and timing independently, allowing multiple pilot and main injections.

The crank angle, measured before top dead centre, at which the spark plug fires. It must advance as engine speed rises and retard under load to avoid detonation.

A threaded component carrying a centre electrode insulated by ceramic; a high-voltage arc jumps the gap to ignite the compressed mixture. Electrode gap and heat range must match the engine.

A step-up transformer that converts 12 V battery supply into the 15,000-40,000 V needed to jump the spark plug gap. Modern systems use one coil per cylinder fired by the ECU.

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, holding the engine at its design temperature.

A heat exchanger of tubes and fins that transfers heat from the circulating coolant to air drawn through it by the fan or ram effect. A pressure cap raises the boiling point of the coolant.

A mixture of water and ethylene glycol antifreeze with corrosion inhibitors that carries heat from the engine to the radiator. It also raises the boiling point and prevents freezing in cold weather.

A measure of an oil's resistance to flow, graded by SAE numbers such as 10W-30. The first number indicates cold-flow behaviour and the second the thickness at operating temperature.

A gear or rotor type pump driven from the crankshaft or camshaft that forces pressurised oil through galleries to bearings, camshaft and rocker gear. A relief valve limits maximum pressure.

A friction coupling between engine and gearbox that lets the driver engage drive smoothly and disconnect power for gear changes. Pressing the pedal releases the pressure plate from the driven plate.

The spring-loaded cover assembly bolted to the flywheel that clamps the friction disc against the flywheel face. Diaphragm spring types give lighter pedal effort than coil spring designs.

The number of teeth on the driven gear divided by the number on the driving gear. It sets the trade-off between torque multiplication and road speed in each gear.

A cone-and-sleeve device inside a manual gearbox that brings the selected gear and shaft to matching speed before engagement, preventing tooth clash and allowing silent gear changes while moving.

A fluid coupling with a stator that multiplies engine torque and slips smoothly at low speed, replacing the clutch in automatic transmissions. A lock-up clutch eliminates slip at cruising speed.

A gear set that divides drive between the two half-shafts while allowing them to rotate at different speeds during turns. Without it, tyres would scrub and the vehicle would resist cornering.

The shaft that carries drive from the differential or final drive out to a wheel hub. Independent suspension cars use constant-velocity joints at each end of the shaft.

A steering layout in which a pinion on the column turns a toothed rack, moving the track rods sideways to swivel the front wheels. It is compact, direct and gives good road feel.

The tilt of the steering axis from vertical, viewed from the side. Positive caster makes the wheels self-centre and improves straight-line stability but increases steering effort.

The inward or outward lean of a wheel from vertical when viewed from the front. Excessive camber causes uneven tyre wear across the tread and affects cornering grip.

The setting where the front edges of the two front wheels are closer together than the rear edges. Slight toe-in compensates for play and keeps the vehicle tracking straight.

A hydraulic damper that converts spring oscillation into heat, controlling bounce after the vehicle meets a bump. Worn dampers cause continued floating and longer stopping distances.

A suspension unit combining a coil spring, telescopic damper and the upper steering pivot in one assembly, commonly used at the front. It saves space and reduces unsprung weight.

An electronic system that senses wheel lock-up and rapidly modulates brake pressure, keeping the tyres rolling so the driver can still steer during hard braking.

The process of removing trapped air from hydraulic brake lines by opening bleed nipples and pumping fresh fluid through. Air is compressible and gives a spongy, ineffective pedal.

A rechargeable battery in which lead plates and dilute sulphuric acid produce about 2.1 V per cell. A 12 V car battery has six cells and supplies cranking current and standby power.

A belt-driven AC generator with a built-in rectifier that charges the battery and runs electrical loads while the engine runs. The regulator holds output near 14 V.

A heavy-duty series-wound DC motor that turns the flywheel ring gear through a pinion to crank the engine. A solenoid engages the pinion and closes the high-current contact.

A microprocessor module that reads sensor inputs and controls actuators such as injectors, ignition coils and idle valves. It stores fault codes that technicians read with a diagnostic scanner.

Adjustment of camber, caster and toe so the wheels sit correctly relative to the road and each other. Correct alignment prevents rapid or uneven tyre wear and steering pull.

A raised bar moulded across the tyre grooves at a legal minimum depth. When tread wears flush with it, the tyre must be replaced for grip and water clearance.

Adding small weights to the rim so the wheel-and-tyre assembly's mass is evenly distributed about its axis. Imbalance causes vibration that worsens with road speed.

Periodically swapping wheel positions on the vehicle, following the maker's pattern, so all tyres wear at a similar rate. It extends tyre life and keeps handling consistent.

The vehicle's self-check system and standardised diagnostic socket that report emission and engine faults as codes. Technicians use it to locate faults without dismantling components.

An oxygen sensor in the exhaust that compares exhaust oxygen with ambient air and signals the ECU to correct the air-fuel ratio. A failed sensor raises fuel consumption and emissions.

A trap in the diesel exhaust that collects soot and periodically burns it off at high temperature, a process called regeneration. Short-town running can clog it and trigger a warning lamp.

A vehicle driven by an electric motor powered from a rechargeable traction battery instead of an internal combustion engine. It has no clutch, gearbox or exhaust in the usual sense.

Recovering kinetic energy during deceleration by running the traction motor as a generator, feeding energy back to the battery. It extends range and reduces wear on friction brakes.

A high energy-density rechargeable battery using lithium compounds in the electrodes, the usual traction pack in modern electric cars. Cells are grouped into modules managed by a battery management system.

The remaining energy in a traction battery expressed as a percentage of full capacity, shown on the dashboard like a fuel gauge. It is estimated from voltage, current and temperature data.

A fuel made from vegetable oils or animal fats by transesterification, usable in diesel engines usually blended with mineral diesel. It gives lower particulate emissions but needs compatible seals and filters.

A rating of a petrol's resistance to detonation, with iso-octane as 100. Engines with high compression ratios require higher-octane fuel to avoid knocking.