Maharashtra HSC Automobile Technology - Vocational (MSBSHSE) cheat sheet

Ladder frame chassis: two side members joined by cross members; resists bending and torsion, used on trucks and older SUVs.

Monocoque (unitised) body: body panels and floor pan carry loads; lighter and stiffer but harder to repair after collision.

Semi-monocoque: unitised body with partial subframes; common on modern passenger cars.

Wheelbase = distance between front and rear axle centres; track = distance between centre lines of left and right wheels.

Overhang = distance from axle centre to nearest end of vehicle; front overhang affects approach angle, rear affects departure angle.

Chassis types by function: conventional ladder frame, integral (monocoque), semi-integral, and tubular/space frame.

Body construction terms: pillars A/B/C, sill, bulkhead, firewall, wheel arch, crumple zone, passenger safety cell.

Cross member functions: support engine, radiator, gearbox and suspension loads, and stiffen the frame against torsion.

Four-stroke cycle order: suction, compression, power (expansion), exhaust - one power stroke per two crankshaft revolutions.

Compression ratio CR = (swept volume + clearance volume) / clearance volume.

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

Valve timing: inlet opens before TDC and closes after BDC; exhaust opens before BDC and closes after TDC.

Diesel cycle uses high compression (typically 16:1 to 22:1) and fuel injected near end of compression for ignition by heat of air.

Petrol SI engine uses spark plug; mixture ignited by spark before TDC - ignition advance measured in degrees of crank rotation.

Firing order examples: 4-cylinder inline 1-3-4-2; 6-cylinder inline 1-5-3-6-2-4; determines crankshaft and camshaft design.

Main parts: cylinder block, head, piston, rings, gudgeon pin, connecting rod, crankshaft, camshaft, flywheel, valves, timing drive.

Clutch purpose: engage and disengage engine from transmission smoothly; transmits torque through friction disc pressed by diaphragm spring.

Clutch free play is measured at the pedal; excessive free play causes incomplete disengagement, zero free play causes slip and wear.

Manual gearbox ratios: overall ratio = gear ratio × final drive ratio; lower gear = higher ratio = more torque, less speed.

Synchromesh uses baulk rings and cone friction to equalise shaft speeds before dog engagement.

Torque converter: impeller, turbine, stator; multiplication occurs when turbine speed is much lower than impeller speed.

Automatic transmission planetary gear set: sun gear, planet carrier, ring gear; ratio changes by holding or driving members via clutches and bands.

Final drive types: spiral bevel, hypoid, and worm; differential allows inner and outer wheels to rotate at different speeds in a turn.

Propeller shaft needs universal joints to handle angle changes; slip joint absorbs suspension movement.

Steering ratio = steering wheel rotation / road wheel rotation; higher ratio reduces effort, lower ratio gives faster response.

Ackermann principle: inner wheel turns through a larger angle than the outer wheel to avoid tyre scrub.

Caster angle gives self-centring; positive caster improves straight-line stability but increases steering effort.

Camber: inward (negative) or outward (positive) tilt of wheel top; excessive camber causes uneven tyre wear.

Toe-in/toe-out measured in mm or degrees; incorrect toe causes feathered edge wear.

Suspension types: leaf spring, coil spring, torsion bar, MacPherson strut, double wishbone, trailing arm; shock absorber damps spring oscillation.

Hydraulic brake: Pascal's law - pressure transmitted equally; braking force = pressure × piston area.

Brake fade from heat; bleeding removes air from hydraulic lines; DOT 3/DOT 4 glycol fluid is hygroscopic and must be replaced periodically.

Ohm's law V = I × R; power P = V × I = I²R = V²/R.

Lead-acid battery: 6 cells × 2.1 V ≈ 12.6 V at rest; specific gravity of electrolyte ~1.26 when fully charged.

Starter motor draws high current; solenoid engages pinion with flywheel ring gear and closes main contact.

Alternator rectifies AC to DC via diode bridge; voltage regulator holds charging around 13.8–14.5 V.

Ignition coil steps up battery voltage to several thousand volts by mutual induction between primary and secondary windings.

Lighting circuits: headlamp relay, dip/main switch, fuse protection; LED lamps draw lower current than halogen.

Sensors: MAP, MAF, TPS, O2 (lambda), coolant temperature, crankshaft position; ECU uses signals for fuel and ignition control.

Multimeter use: voltmeter in parallel, ammeter in series, ohmmeter on de-energised circuit only.

Cast iron: high carbon, good wear resistance and damping, used for cylinder blocks and brake drums.

Aluminium alloys: light, good thermal conductivity, used for pistons, cylinder heads, radiators.

Steel classifications by carbon: low (mild) up to ~0.3% C, medium 0.3–0.6%, high above 0.6%; higher carbon = harder, less ductile.

Alloy steels with Cr, Ni, Mo used for gears, crankshafts, springs; heat treatment (hardening, tempering, case carburising) alters properties.

Non-ferrous: copper for wiring, brass/bronze for bearings and bushes, zinc for galvanising, magnesium for lightweight castings.

Plastics/composites: PP and ABS for trim, glass-fibre reinforced plastic for panels; light but low heat resistance.

Gasket materials: copper, cork, rubber, MLS (multi-layer steel); chosen for temperature and pressure conditions.

Bearing materials (white metal, bronze) need low friction, embeddability, and conformability.

Vernier calliper least count = 1 main scale division − 1 vernier division; typical 0.02 mm.

Micrometer least count = pitch / number of divisions on thimble; typical 0.5 mm pitch, 50 divisions = 0.01 mm.

Vernier height gauge and depth gauge use the same least-count principle as the calliper.

Dial indicator (plunger type) reads run-out, end play and backlash; must be zeroed before use.

Feeler gauge measures valve clearance and spark plug gap; combination of blades gives total thickness.

Torque wrench: tighten to specified N·m; torque = force × perpendicular distance.

Tolerances: fit types - clearance, interference, transition; hole-basis and shaft-basis systems.

Surface finish Ra in micrometres; lower Ra = smoother surface.

Periodic service intervals: engine oil and filter, air filter, fuel filter, brake fluid, coolant, spark plugs per manufacturer schedule.

Engine oil level checked with dipstick on level ground with engine off; level between MIN and MAX marks.

Tyre rotation pattern depends on drive type; tread depth minimum 1.6 mm; pressure checked cold.

Common faults: engine overheating (coolant level, thermostat, water pump, radiator), no start (battery, starter, ignition/fuel), poor mileage (air filter, tyre pressure, O2 sensor).

Brake diagnosis: spongy pedal = air in system; pulling to one side = seized calliper or uneven pad wear; vibration = warped disc.

Battery testing: open-circuit voltage, load test, specific gravity; low voltage under load indicates replacement.

Coolant: 50:50 antifreeze-water mix; never open radiator cap when hot.

Diagnostic approach: verify complaint, gather symptoms, inspect, test, repair, verify.

Personal protective equipment: safety goggles, gloves, safety shoes, ear protection, respirator when painting or grinding.

Jack stands must support the vehicle after jacking; never work under a vehicle supported only by a jack.

Fire classes: A solid, B liquid, C gas, D metal; CO2 and dry powder suitable for fuel and electrical fires.

Battery safety: hydrogen gas is explosive; disconnect negative terminal first and reconnect last.

Exhaust gas contains carbon monoxide; run engines only with ventilation or extraction.

Hand tools: spanners (open-end, ring, combination), socket set, torque wrench, screwdrivers, pliers, hammers, chisels, files.

Measuring and diagnostic equipment: multimeter, compression tester, timing light, tyre pressure gauge, scan tool.

Lifting equipment: hydraulic jack, hoist, crane; check rated capacity and sling condition before use.

ABS prevents wheel lock by modulating brake pressure; wheel speed sensors feed the ECU.

Traction control and ESP use ABS sensors plus steering angle and yaw sensors to correct skids.

Airbag system: crash sensors trigger inflator; SRS components require disarming before service.

CRDI common rail diesel: high-pressure pump feeds rail; ECU controls injector timing and duration electronically.

MPFI petrol: fuel injected into intake port; ECU calculates injection duration from sensors.

Hybrid/EV basics: traction battery, inverter, motor-generator, regenerative braking; high-voltage cables are orange.

Turbocharger uses exhaust gas energy to drive compressor; wastegate limits boost pressure.

Emission-related systems: catalytic converter, EGR, PCV, EVAP, DPF; each reduces specific pollutants.

Petrol exhaust pollutants: CO, HC, NOx, CO2, particulates; diesel adds smoke and PM.

Three-way catalytic converter converts CO to CO2, HC to CO2 and H2O, and NOx to N2; needs stoichiometric (lambda = 1) mixture.

EGR recirculates exhaust gas into intake to lower combustion temperature and reduce NOx.

PCV valve routes crankcase blow-by gases back to intake to reduce HC emissions.

EVAP system traps fuel vapour in a charcoal canister; purge valve releases it to intake.

Diesel particulate filter traps soot; regeneration burns it at high temperature.

BS-VI norms in India limit CO, HC, NOx, PM; OBD-II monitors emission-related components.

Environmental measures: lead-free petrol, low-sulphur fuel, catalytic converters, alternative fuels (CNG, LPG, ethanol, biodiesel).

Practical record should document aim, tools used, procedure, observations, calculations, and conclusion.

Dismantling and assembly: follow manufacturer sequence, label parts, use correct spanners, torque to specification.

Common practical tasks: valve clearance adjustment, brake bleeding, tyre change, battery charging, engine oil change.

Viva questions often cover function of parts, reason for a procedure, and safety practice.

Measurement practical: use vernier calliper and micrometer, record readings with least count, state units.

Electrical practical: test battery, check charging voltage, trace a lighting circuit with a multimeter.

Project work: choose a fault-finding or servicing task, record data, analyse results, and present a report.

Report should include date, vehicle details, tools, safety precautions, findings, and recommendations.