Haryana BSEH Senior Secondary NSQF Automotive flashcards

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

Stoichiometric air-fuel ratio

The mass ratio of air to fuel that gives complete combustion. For petrol it is about 14.7:1; for diesel about 14.5:1. It is the reference point for lambda and mixture control.

Lambda value

The ratio of actual air-fuel ratio to the stoichiometric ratio. Lambda 1.00 means chemically correct mixture; above 1.00 is lean, below 1.00 is rich.

Compression ratio

The ratio of cylinder volume above the piston at bottom dead centre to the volume at top dead centre. It equals (swept volume + clearance volume) divided by clearance volume.

Torque formula

Torque equals force multiplied by perpendicular distance from the pivot. A 50 N force applied 0.4 m from the fastener axis produces 20 newton-metres.

Micrometer least count

For a metric micrometer with 0.5 mm main scale divisions and a 50-division thimble, each thimble division equals 0.01 mm. The reading is main scale plus thimble value.

Vernier caliper least count

Main scale division divided by number of vernier divisions. A 1 mm main scale with 20 vernier divisions gives 0.05 mm; with 50 divisions it gives 0.02 mm.

Four-stroke cycle order

Intake, compression, power and exhaust strokes. The crankshaft turns twice per complete cycle while the camshaft turns once, giving a 2:1 drive ratio.

Valve clearance setting

The gap between valve stem and rocker or cam, set cold unless specified. Typical values are 0.15 to 0.30 mm intake and 0.25 to 0.35 mm exhaust.

Torque-to-yield bolt

A head or main bolt tightened to a torque then rotated a further angle, stretching it into the plastic range. It must not be reused once stretched.

Cylinder taper limit

The maximum acceptable difference in bore diameter between top and bottom of the wear area, usually 0.05 to 0.10 mm. Beyond this, reboring or replacement is needed.

Clutch free play

The pedal travel before the release bearing contacts the pressure plate, typically 10 to 25 mm. Too little causes slip; too much causes gear clash.

Differential gear ratio

Ring gear tooth count divided by pinion tooth count. It multiplies gearbox output torque and determines final drive speed; typical backlash is 0.10 to 0.20 mm.

Synchroniser ring clearance

The gap between synchroniser ring and gear cone, normally 0.5 to 1.5 mm. Less than 0.5 mm means worn rings and causes gear crunch during shifts.

CV joint failure symptoms

A clicking noise on full steering lock indicates a worn outer constant-velocity joint. Vibration under acceleration points to the inner joint or a worn intermediate shaft.

Brake fluid hygroscopy

Glycol-based DOT 3 and DOT 4 fluids absorb moisture from air, lowering boiling point. DOT 4 boils near 230 C dry and 155 C wet, so replace every two years.

Steering free play limit

The maximum steering wheel movement at the rim before the road wheels respond, about 25 mm. Excess indicates worn tie rods, steering box or column joints.

Wheel alignment angles

Camber is vertical wheel tilt, caster is steering axis tilt fore and aft, and toe is the front-rear distance difference between wheels. Total toe is usually within 0.2 degrees.

Ohm's law and power

Voltage equals current times resistance; power equals voltage times current. A 12 V circuit drawing 5 A has 2.4 ohms resistance and dissipates 60 watts.

Series and parallel circuits

In series, resistance adds and current is common. In parallel, voltage is common, current divides, and total resistance is less than the smallest branch.

Battery state of charge

A fully charged 12 V lead-acid battery reads about 12.6 V open circuit. About 12.0 V indicates roughly 25 percent charge, so charging is required.

Voltage drop test

Measuring voltage lost across a connection or cable while current flows. Less than 0.5 V on starter positive and ground paths under load indicates acceptable resistance.

Three-way catalytic converter

A device that oxidises CO and HC while reducing NOx, working only within a narrow lambda window near 1.00. Lead and sulphur poison its active coating.

EGR valve function

Recirculates exhaust gas into the intake to lower combustion temperature and reduce NOx. Excessive EGR causes rough idle and misfire; too little raises NOx.

Diesel smoke colours

Black smoke means over-fuelling or restricted air. Blue smoke indicates lube oil burning. White smoke indicates unburnt fuel or coolant entering the combustion chamber.

Paint film thickness

Total dry film thickness of primer, base coat and clear coat is typically 80 to 120 micrometres. Excess thickness causes cracking and poor adhesion.

Spray gun settings

Typical gun distance is 15 to 20 cm with 50 percent overlap. A 1.3 to 1.4 mm fluid tip suits base coat; 1.4 to 1.8 mm suits primer.

OBD-II DTC format

The first letter gives the system: P powertrain, B body, C chassis, U network. The second character 0 means generic and 1 means manufacturer-specific.

Fuel trim interpretation

Long-term fuel trim above plus 10 percent indicates a lean condition such as a vacuum leak or weak pump. Negative trim indicates a rich condition.

Cylinder leakage test

Air at about 100 psi is fed into a cylinder at top dead centre. Leakage past the intake means intake valve; past the exhaust means exhaust valve; into the crankcase means rings.

Break-even quantity

Fixed cost divided by contribution per unit, where contribution is selling price minus variable cost. It gives the number of jobs needed to cover all fixed costs.

Job card contents

A record of customer details, vehicle registration, odometer, reported complaint, authorised work, parts used, technician signature and delivery date. It protects both parties.

Fastener grade marking

Markings such as 8.8 mean tensile strength 800 MPa and yield strength 80 percent of tensile. Grade 10.9 means 1000 MPa and 90 percent yield.