WBBSE Madhyamik Vocational Automobile cheat sheet

Fire classes: A = solid combustibles (wood, paper), B = flammable liquids (petrol, diesel, solvent), C = flammable gases, D = combustible metals; water is forbidden on class B and on live electrical fires.

Fire extinguisher colour codes: water = red, foam = cream, CO2 = black, dry powder = blue; CO2 and dry powder are the standard choices for fuel and electrical fires in a workshop.

A wheel chock or scotch block placed fore and aft of a wheel prevents rolling; a jack alone is a lifting device, never a support - axle stands must carry the load before any work under a vehicle.

Personal protective equipment by task: safety goggles for grinding and battery work, face shield for compressed air and angle grinding, leather gloves for welding and hot components, ear defenders above 85 dB(A), steel-toe boots at all times.

Hand tool selection rule: spanner jaw size must match fastener across flats exactly; a loose fit rounds hex corners, and an open-end spanner is used where a ring spanner cannot seat fully.

Torque formula: torque (N·m) = force (N) × perpendicular distance (m); a 200 N force on a 0.3 m wrench gives 60 N·m.

Socket and drive sizing: 1/4 in drive for small fasteners up to about 12 mm, 3/8 in for general work, 1/2 in and above for suspension and cylinder-head fasteners.

Compressed air safety: never direct the jet at skin or eyes (air embolism risk), keep the pressure at the tool rating, and drain the receiver daily to remove condensate.

Battery charging area rules: hydrogen is released during charging, so the area must be ventilated and free of sparks or naked flames; acid spills are neutralised with sodium bicarbonate solution.

Four-stroke cycle order: induction (inlet open, piston down), compression (both valves closed, piston up), power (both closed, ignition, piston down), exhaust (exhaust open, piston up); two crankshaft revolutions per cycle.

Compression ratio = (swept volume + clearance volume) ÷ clearance volume; a 400 cc swept volume with 50 cc clearance gives 450 ÷ 50 = 9:1.

Swept volume of one cylinder = (π ÷ 4) × bore² × stroke; total engine capacity = swept volume × number of cylinders.

Diesel ignition is by compression heat, petrol by spark; a diesel compression ratio is typically 16:1 to 22:1 against about 8:1 to 12:1 for petrol.

Firing order spreads power strokes to smooth the crankshaft; common orders are 1-3-4-2 (four-cylinder) and 1-5-3-6-2-4 (six-cylinder inline).

Valve clearance is measured with a feeler gauge when the engine is cold (or at the maker's stated temperature) and is set at the rocker or tappet; too little clearance holds a valve open and loses compression.

Valve overlap is the period when inlet and exhaust valves are both open around top dead centre; it aids scavenging and is larger in high-speed engines.

Turbocharging uses exhaust gas energy to drive a turbine that spins a compressor, raising intake air density; a wastegate limits boost pressure.

Engine knocking (detonation) is uncontrolled end-gas ignition; causes include over-advanced timing, low-octane fuel, hot spots and excessive compression.

Petrol engine air-fuel ratio by mass: stoichiometric is about 14.7:1; rich mixtures (about 12:1) give maximum power, lean mixtures (about 16:1) give best economy.

Diesel injection timing: injection begins slightly before top dead centre and the injector atomises fuel at high pressure; common-rail pressure can exceed 1,500 bar.

Fuel pump types: mechanical diaphragm or eccentric-driven pump on older petrol engines, electric in-tank pump on modern petrol, and plunger/distributor or common-rail high-pressure pump on diesel.

A blocked fuel filter shows as loss of power at high load, hard starting and lean-mixture symptoms; the filter is replaced at the maker's interval, not cleaned.

SAE viscosity grade rule: the first number with W is cold-flow behaviour, the second is viscosity at 100 °C; 10W-40 flows like a 10-grade oil when cold and a 40-grade when hot.

API and ACEA service categories grade oil performance; a higher second letter in API (for example SN versus SL) denotes a later, more demanding specification.

Oil pump types are gear, rotor and vane; low oil pressure at idle with a warning lamp points to worn bearings, a faulty relief valve, a blocked strainer or low oil level.

Two-stroke engines mix oil with petrol because crankcase scavenging leaves no separate sump; four-stroke engines use a sump, pump and filter circuit.

Crankcase ventilation (PCV) returns blow-by gases to the intake; a blocked PCV raises crankcase pressure and causes oil seal leaks.

Gear ratio = driven gear teeth ÷ driving gear teeth; overall ratio = gearbox ratio × final drive ratio.

Manual gearbox synchronisers match shaft speeds by friction before the dog teeth engage; a worn baulk ring causes gear crunch on downshift.

Torque converter principle: engine drives the impeller, fluid drives the turbine, and a stator redirects flow to multiply torque at low speed; a lock-up clutch removes slip at cruise.

Clutch slip is diagnosed when engine speed rises without road speed; causes include worn friction plate, weak diaphragm spring, oil contamination and no free play.

Steering ratio = steering wheel rotation ÷ road wheel rotation; a lower ratio gives faster response with higher effort, a higher ratio gives easier effort with slower response.

Wheel alignment angles: camber is the inward or outward tilt in front view, caster is the steering-axis tilt in side view, toe is the difference in front and rear track of the wheels.

Positive caster improves straight-line stability and self-centring; excessive negative camber wears the inner edge of the tyre.

Suspension types: MacPherson strut (integrated spring and damper), double wishbone, torsion bar, leaf spring for load carrying, and anti-roll bar to limit body roll.

A shock absorber damps spring oscillation; a worn unit shows as continued bouncing after a push test, oil leakage on the body and uneven tyre cupping.

Hydraulic principle: pressure is equal throughout a confined fluid, so force = pressure × piston area; a larger wheel-cylinder area gives more braking force at the shoe.

Brake pedal free play that is too small holds the compensating port closed, causing drag and overheating; too much free play gives a low, spongy pedal.

Brake fluid is hygroscopic; absorbed moisture lowers the boiling point and causes vapour lock and a spongy pedal when hot.

DOT 3 and DOT 4 are glycol-based and must not be mixed with DOT 5 silicone fluid; DOT 5.1 is glycol-based despite the number.

Disc brakes are self-adjusting and self-cleaning; drum brakes need a star-wheel adjuster or automatic adjuster strut to keep the shoe-to-drum gap correct.

ABS prevents wheel lock by modulating pressure per wheel; it does not shorten stopping distance on loose gravel, where a locked wheel builds a wedge of material.

Stopping distance = thinking distance + braking distance; thinking distance = speed × reaction time, and braking distance rises with the square of speed.

Parking brake on a hill-holder or drum system acts mechanically by cable on the rear shoes or a separate drum; a stretched cable gives excessive lever travel.

Brake bleeding order follows the maker's sequence, typically furthest from the master cylinder first; air in the circuit is the main cause of a spongy pedal.

Ohm's law: V = I × R; power P = V × I = I²R = V² ÷ R. A 12 V circuit drawing 5 A has 2.4 Ω resistance and dissipates 60 W.

A 12 V lead-acid battery has six cells of about 2.1 V each in series; a fully charged open-circuit reading is about 12.6 V, and 12.2 V or lower indicates partial discharge.

Series circuit: current is common, voltages add. Parallel circuit: voltage is common, currents add. Vehicle loads are wired in parallel so each gets full battery voltage.

Alternator output is three-phase AC rectified by a diode pack; the regulator holds about 13.8 to 14.4 V, and a reading below 13 V with the engine running points to a charging fault.

Starter motor draws several hundred amperes; a slow crank with dim lamps indicates a discharged battery, high-resistance earth strap or worn brushes.

Relays and fuses protect circuits: a fuse is rated just above the normal load current, and a relay lets a small switch current control a large load current.

Sensor types: potentiometer (throttle position), inductive or Hall-effect (crank and cam position), thermistor (coolant temperature, resistance falls as temperature rises), and lambda/zirconia (oxygen in exhaust).

LED and halogen lamps are polarity sensitive or heat sensitive; CAN bus wiring uses twisted pairs to reject electrical noise, and a shorted bus line disables many modules.

Earth return fault diagnosis: voltage drop testing across a connection under load reveals resistance that a voltmeter on open circuit will not show.

Daily or pre-start checks: engine oil level on level ground with the engine off, coolant level when cold, brake fluid level, tyre pressures and visible damage, lamp function.

Tyre pressure rule: check cold, and expect about 0.1 bar rise per 10 °C; under-inflation increases rolling resistance and shoulder wear, over-inflation wears the centre tread.

Tread wear indicators sit at 1.6 mm; the legal minimum in India is 1.6 mm, and wear bars appear across the grooves at that depth.

Service intervals are set by distance or time, whichever comes first; oil and filter, air filter, fuel filter, brake fluid and coolant each have separate intervals.

Tyre rotation pattern depends on drive layout and directional tread; cross-ply and radial tyres must not be mixed on the same axle.

Wheel bearing play is checked by rocking the raised wheel at 12 and 6 o'clock; play plus a rumble that changes on cornering indicates a worn bearing.

Battery maintenance: keep terminals clean and lightly greased, check electrolyte level in serviceable types, and never top up with acid - use distilled water only.

Coolant concentration is checked with a hydrometer or refractometer; a 50:50 glycol mix gives protection to about −37 °C and raises the boiling point.

Road test after service confirms brake balance, steering return, gear selection and absence of warning lamps before the vehicle is released.

Least count of a vernier calliper = 1 main-scale division ÷ number of vernier divisions; a 0.5 mm main division with 50 vernier divisions gives 0.01 mm.

Micrometer least count = pitch of screw ÷ number of thimble divisions; a 0.5 mm pitch with 50 divisions gives 0.01 mm.

Vernier reading = main scale reading before the vernier zero + (vernier division coinciding × least count).

Cylinder bore is measured with a bore gauge or inside micrometer at several heights and two perpendicular directions to detect taper and ovality.

Feeler gauge measures valve clearance and gap widths; plastigauge measures bearing oil clearance by the width of a crushed plastic strip.

Steel contains up to about 2 % carbon; cast iron has higher carbon (about 2 to 4 %) and good damping and wear resistance, which suits cylinder blocks and liners.

Aluminium alloy is used for pistons and cylinder heads because of low density and good heat conduction; it needs a cast-iron liner or coated bore for wear resistance.

Copper is used for wiring and brass for bushes and terminals; bronze bearings resist wear and are used in bushes and thrust washers.

Material properties: hardness resists indentation, toughness resists shock, ductility allows drawing into wire, and brittleness means fracture with little deformation - cast iron is brittle, mild steel is ductile.

Petrol engine exhaust gases: carbon monoxide (CO), hydrocarbons (HC), oxides of nitrogen (NOx), carbon dioxide (CO2) and particulates; a three-way catalytic converter cuts CO, HC and NOx together only near stoichiometric mixture.

Diesel exhaust adds smoke and particulate matter; a diesel particulate filter traps soot and regenerates by burning it at high exhaust temperature.

BS-VI (Bharat Stage VI) emission norms in India took effect from 1 April 2020 and tightened NOx and particulate limits, driving the use of SCR and DPF systems.

Used oil, coolant, brake fluid and batteries are hazardous waste; they must go to authorised recyclers, never to drains or open ground.

Waste hierarchy: prevent, reduce, reuse, recycle, recover, dispose - the last option is the least preferred.

Right to information and honest billing are core workshop ethics: the customer is told what work is needed, what was done, and what parts were replaced.

Personal data and vehicle records must be handled confidentially, and a technician must not test-drive a customer vehicle for private use.

Greenhouse gases from vehicles are reduced by correct tyre pressure, regular servicing, removing excess load and avoiding unnecessary idling.

Noise control in the workshop: compressors and grinders are enclosed or isolated, and work is scheduled to limit disturbance to nearby residents.

Fire classes: A = solid combustibles, B = flammable liquids, C = flammable gases, D = combustible metals; water is never used on class B or live electrical fires.

Extinguisher colour codes: water red, foam cream, CO2 black, dry powder blue; CO2 and dry powder suit fuel and electrical fires.

A jack lifts only; axle stands must support the vehicle before any work underneath, and chocks stop the wheels rolling.

PPE by task: goggles for grinding and batteries, face shield for air and grinding, leather gloves for welding and hot parts, ear defenders above 85 dB(A), steel-toe boots always.

Torque (N·m) = force (N) × perpendicular distance (m); 200 N on a 0.3 m wrench gives 60 N·m.

Socket drive sizing: 1/4 in for fasteners up to about 12 mm, 3/8 in for general work, 1/2 in and above for suspension and cylinder-head bolts.

Compressed air is never directed at skin or eyes; drain the receiver daily and keep pressure at the tool rating.

Battery charging releases hydrogen, so ventilate the area and remove sparks and flames; neutralise acid spills with sodium bicarbonate.

Four-stroke order: induction, compression, power, exhaust, with two crankshaft revolutions per cycle.

Compression ratio = (swept + clearance volume) ÷ clearance volume; 400 cc swept with 50 cc clearance gives 9:1.

Swept volume per cylinder = (π ÷ 4) × bore² × stroke; total capacity = swept volume × cylinder count.

Diesel ignites by compression heat at about 16:1 to 22:1; petrol ignites by spark at about 8:1 to 12:1.

Firing orders: 1-3-4-2 for four-cylinder inline, 1-5-3-6-2-4 for six-cylinder inline.

Valve clearance is set with a feeler gauge at the stated temperature; too little clearance holds a valve open and loses compression.

Valve overlap is the period both valves are open near top dead centre; it aids scavenging and grows with engine speed.

A turbocharger uses exhaust energy to drive a compressor and raise intake density; the wastegate limits boost.

Detonation is uncontrolled end-gas ignition, caused by over-advanced timing, low-octane fuel, hot spots or excessive compression.

Stoichiometric petrol air-fuel ratio is about 14.7:1 by mass; about 12:1 gives maximum power and about 16:1 gives best economy.

Common-rail diesel injection pressure can exceed 1,500 bar, and injection starts slightly before top dead centre.

Pump types: diaphragm or eccentric petrol pump, in-tank electric petrol pump, plunger or common-rail high-pressure diesel pump.

A blocked fuel filter gives hard starting, lean symptoms and power loss at high load; it is replaced, not cleaned.

SAE 10W-40 flows like a 10-grade oil when cold and a 40-grade oil at 100 °C.

API and ACEA categories grade oil performance; a later API second letter (SN versus SL) means a more demanding specification.

Low oil pressure at idle points to worn bearings, a faulty relief valve, a blocked strainer or low oil level.

Two-stroke engines mix oil with petrol because crankcase scavenging leaves no separate sump.

A blocked PCV valve raises crankcase pressure and causes oil seal leaks.

Gear ratio = driven teeth ÷ driving teeth; overall ratio = gearbox ratio × final drive ratio.

Synchroniser baulk rings match shaft speed by friction; a worn ring causes gear crunch on downshift.

In a torque converter the impeller drives fluid to the turbine and the stator multiplies torque at low speed; a lock-up clutch removes slip at cruise.

Clutch slip shows as rising engine speed without road speed; causes are a worn plate, weak spring, oil contamination or no free play.

Steering ratio = steering wheel rotation ÷ road wheel rotation; a lower ratio is faster but heavier.

Camber is front-view tilt, caster is side-view steering-axis tilt, toe is the front-to-rear track difference of the wheels.

Positive caster improves straight-line stability and self-centring; excessive negative camber wears the inner tyre edge.

Suspension types include MacPherson strut, double wishbone, torsion bar, leaf spring and anti-roll bar.

A worn damper shows continued bouncing after a push test, oil leakage and cupped tyre wear.

Force = pressure × piston area; a larger wheel-cylinder area gives more braking force at the shoe.

Too little pedal free play holds the compensating port closed, causing drag and overheating; too much gives a low, spongy pedal.

Brake fluid is hygroscopic; moisture lowers the boiling point and causes vapour lock when hot.

DOT 3 and DOT 4 are glycol-based and must not be mixed with DOT 5 silicone; DOT 5.1 is glycol-based.

Disc brakes self-adjust and self-clean; drum brakes need a star-wheel or automatic adjuster for shoe-to-drum gap.

ABS modulates pressure per wheel to prevent lock but does not shorten stopping distance on loose gravel.

Stopping distance = thinking distance + braking distance; thinking distance = speed × reaction time, and braking distance rises with the square of speed.

The parking brake acts mechanically by cable on rear shoes or a separate drum; a stretched cable gives excessive lever travel.

Air in the hydraulic circuit is the main cause of a spongy pedal; bleeding follows the maker's sequence, usually furthest from the master cylinder first.

Ohm's law V = I × R; power P = V × I = I²R = V² ÷ R; a 12 V circuit at 5 A has 2.4 Ω and dissipates 60 W.

A 12 V lead-acid battery has six cells of about 2.1 V; about 12.6 V open circuit is fully charged and 12.2 V or lower is partly discharged.

Series: current common, voltages add. Parallel: voltage common, currents add; vehicle loads are wired in parallel.

The alternator rectifies three-phase AC through a diode pack; the regulator holds about 13.8 to 14.4 V, and below 13 V running indicates a charging fault.

A starter draws several hundred amperes; slow cranking with dim lamps indicates a discharged battery, high-resistance earth strap or worn brushes.

A fuse is rated just above normal load current; a relay lets a small switch current control a large load current.

Sensor types: potentiometer for throttle position, inductive or Hall-effect for crank and cam position, thermistor for coolant temperature, zirconia for exhaust oxygen.

CAN bus uses twisted pairs to reject noise; a shorted bus line disables many modules.

Voltage drop testing under load reveals resistance that an open-circuit voltmeter reading will not show.

Check oil level on level ground with the engine off, coolant when cold, and tyre pressures when cold.

Tyre pressure rises about 0.1 bar per 10 °C; under-inflation wears the shoulders and over-inflation wears the centre tread.

Tread wear indicators appear at 1.6 mm, the legal minimum in India.

Service intervals are set by distance or time, whichever comes first; oil, air filter, fuel filter, brake fluid and coolant have separate intervals.

Do not mix cross-ply and radial tyres on the same axle; rotation pattern depends on drive layout and directional tread.

Wheel bearing play is checked by rocking the raised wheel at 12 and 6 o'clock; rumble that changes on cornering confirms wear.

Keep battery terminals clean and greased; serviceable batteries are topped up with distilled water only, never acid.

A 50:50 glycol coolant mix protects to about −37 °C and raises the boiling point.

A road test after service confirms brake balance, steering return, gear selection and no warning lamps.

Vernier least count = one main-scale division ÷ vernier divisions; 0.5 mm with 50 divisions gives 0.01 mm.

Micrometer least count = screw pitch ÷ thimble divisions; 0.5 mm pitch with 50 divisions gives 0.01 mm.

Vernier reading = main scale before vernier zero + (coinciding vernier division × least count).

Bore gauge or inside micrometer readings at several heights and two perpendicular directions reveal taper and ovality.

Feeler gauges measure valve clearance and gaps; plastigauge measures bearing oil clearance from the crushed strip width.

Steel holds up to about 2 % carbon; cast iron holds about 2 to 4 % and offers damping and wear resistance for blocks and liners.

Aluminium alloy suits pistons and heads for low density and heat conduction but needs a liner or coated bore for wear.

Copper is used for wiring, brass for terminals and bushes, bronze for wear-resistant bushes and thrust washers.

Hardness resists indentation, toughness resists shock, ductility allows drawing into wire, and brittleness means fracture with little deformation.

Petrol exhaust carries CO, HC, NOx, CO2 and particulates; a three-way catalytic converter cuts CO, HC and NOx only near stoichiometric mixture.

Diesel exhaust adds smoke and particulates; the DPF traps soot and regenerates by burning it at high exhaust temperature.

BS-VI norms took effect in India on 1 April 2020 and tightened NOx and particulate limits, driving SCR and DPF use.

Used oil, coolant, brake fluid and batteries are hazardous waste and must go to authorised recyclers, never to drains.

Waste hierarchy: prevent, reduce, reuse, recycle, recover, dispose.

Honest billing and clear explanation of work needed, work done and parts replaced are core workshop ethics.

Customer data and vehicle records are confidential, and a technician must not use a customer vehicle privately.

Correct tyre pressure, regular servicing, less excess load and less idling cut greenhouse gas emissions.

Enclose or isolate compressors and grinders, and schedule noisy work to limit disturbance to neighbours.