WBBSE Madhyamik Vocational Automobile flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Class B fire

A fire involving flammable liquids such as petrol, diesel, paint or solvent. Water spreads it, so foam, CO2 or dry powder is used instead.

Axle stand rule

A jack only lifts a vehicle. Before any work underneath, the raised body must rest on rated axle stands, with wheels chocked against rolling.

Torque formula

Torque equals force multiplied by perpendicular distance from the pivot. A 200 newton force on a 0.3 metre wrench produces 60 newton metres.

Compression ratio

Swept volume plus clearance volume, divided by clearance volume. A 400 cc swept volume with 50 cc clearance gives a ratio of 9 to 1.

Four-stroke cycle order

Induction, compression, power and exhaust, completed in two crankshaft revolutions. The power stroke is the only one that produces useful work.

Firing order 1-3-4-2

The sequence in which cylinders of a common four-cylinder inline engine fire. It spreads power strokes evenly to smooth crankshaft torque and reduce vibration.

Valve overlap

The brief period near top dead centre when inlet and exhaust valves are both slightly open. It helps scavenge burnt gas and is larger in high-speed engines.

Stoichiometric air-fuel ratio

The chemically ideal petrol-air mass ratio, about 14.7 to 1. Richer mixtures near 12 to 1 give maximum power, leaner ones near 16 to 1 give economy.

SAE 10W-40 meaning

The 10W grade describes cold-flow behaviour, so the oil behaves like a 10-grade when cold. The 40 grade describes viscosity at 100 degrees Celsius.

Hygroscopic brake fluid

Glycol brake fluid absorbs moisture from the air over time. The absorbed water lowers the boiling point and can cause vapour lock under hard braking.

Gear ratio

Driven gear teeth divided by driving gear teeth. Overall ratio is gearbox ratio multiplied by final drive ratio, giving total speed reduction.

Positive caster effect

Tilting the steering axis so its top leans rearward improves straight-line stability and makes the steering self-centre after a turn.

Clutch slip symptom

Engine speed rises without matching road speed, especially on hills. Causes include a worn friction plate, weak diaphragm spring, oil contamination or no pedal free play.

Ohm's law

Voltage equals current multiplied by resistance. A 12 volt circuit carrying 5 amperes therefore has 2.4 ohms resistance and dissipates 60 watts.

Alternator charging voltage

With the engine running, the regulator holds roughly 13.8 to 14.4 volts across the battery. A reading below about 13 volts indicates a charging fault.

Parallel circuit rule

Voltage is the same across every branch while currents add. Vehicle lamps and accessories are wired in parallel so each receives full battery voltage.

Thermistor coolant sensor

A temperature-sensitive resistor whose resistance falls as coolant temperature rises. The engine control unit uses its signal to adjust fuelling and ignition.

Tread wear indicator

Raised bars moulded across the tyre grooves at 1.6 millimetres depth, the legal minimum in India. Tyres worn to these bars must be replaced.

Cold tyre pressure rule

Tyre pressure is set when the tyre is cold. Pressure rises about 0.1 bar for every 10 degrees Celsius increase once running.

Vernier least count

One main-scale division divided by the number of vernier divisions. A 0.5 millimetre main division with 50 vernier divisions gives 0.01 millimetre.

Micrometer least count

Screw pitch divided by the number of thimble divisions. A 0.5 millimetre pitch with 50 thimble divisions gives a least count of 0.01 millimetre.

Plastigauge use

A soft plastic strip placed across a bearing journal. After tightening, the crushed strip width is matched to a scale to read oil clearance.

Three-way catalytic converter

A converter that oxidises carbon monoxide and hydrocarbons while reducing oxides of nitrogen. It works efficiently only when the air-fuel mixture stays near stoichiometric.

BS-VI emission norm

India's Bharat Stage VI emission standard, effective from 1 April 2020. It sharply tightened nitrogen oxide and particulate limits, requiring systems such as SCR and DPF.

Hazardous workshop waste

Used oil, coolant, brake fluid and batteries that cannot go to drains or open ground. They must be handed to authorised recyclers for safe treatment.

Fire extinguisher colour code

Water extinguishers are red, foam cream, carbon dioxide black and dry powder blue. CO2 and dry powder suit fuel and electrical fires.

Diesel compression ignition

Diesel fuel ignites from the heat of highly compressed air rather than a spark. Compression ratios are typically 16 to 1 up to 22 to 1.

Common-rail injection pressure

Modern diesel systems store fuel at very high pressure, often above 1,500 bar, in a shared rail before the injectors atomise it into the cylinder.

Brake pedal free play

The small gap before the master cylinder piston moves. Too little holds the compensating port closed and causes drag; too much gives a low pedal.

Synchroniser baulk ring

A friction ring in a manual gearbox that matches shaft speeds before the dog teeth engage. A worn ring causes gear crunch, especially on downshift.

Wheel bearing play check

Rock the raised wheel at twelve and six o'clock. Detectable play, plus a rumble that changes when cornering, indicates a worn bearing.

Cast iron properties

An iron-carbon alloy with roughly 2 to 4 percent carbon. It is brittle but offers good damping and wear resistance, suiting cylinder blocks and liners.