RBSE Sec Voc Automotive flashcards

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

Fire class B

Fire involving flammable liquids such as petrol, diesel or solvents. Extinguish with foam, CO2 or dry powder; never use water, which spreads the burning liquid.

Micrometer least count

Pitch of the spindle screw divided by the number of thimble divisions. A 0.5 mm pitch with 50 divisions gives 0.01 mm resolution, added to the barrel reading.

Torque formula

Torque equals force multiplied by lever length, T = F x L. A 200 N force at 0.5 m from the pivot produces 100 N·m of turning effort.

Jack stand rule

A hydraulic jack lifts but never supports. Place rated stands under the manufacturer's jacking points on firm ground, chock the opposite wheel and lower the load onto the stands.

Compression ratio

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

Four-stroke cycle order

Intake, compression, power then exhaust, completed in two crankshaft revolutions. The camshaft turns once per cycle, at half crankshaft speed.

Valve clearance too small

Insufficient clearance holds valves slightly open when hot, causing compression loss, burnt valve seats and misfire. It also prevents the valve from seating and transferring heat.

Torque-to-yield bolts

Head bolts tightened to a low torque plus a specified angle stretch beyond their elastic limit. They must be replaced at every strip-down, never reused.

Blue exhaust smoke

Persistent blue-grey smoke indicates engine oil entering combustion, typically from worn piston rings, worn valve guides or failed valve stem seals.

Overall gear ratio

Gearbox ratio multiplied by final drive ratio. A 3.5:1 gear with a 4.1:1 final drive gives 14.35:1 overall, reducing engine speed for a given road speed.

Clutch free play

Gap between release bearing and clutch cover before the pedal starts disengaging. Too little causes slip; too much causes incomplete release and gear clash.

Torque converter stall

Maximum engine speed reached with the turbine held stationary and full throttle applied. It indicates converter multiplication and engine torque output.

CV joint clicking

Repeated clicking from a front wheel during full-lock acceleration indicates a worn outer constant-velocity joint, usually with a split boot and lost grease.

Differential ratio

Ring gear tooth count divided by pinion tooth count. A 41-tooth ring gear with an 11-tooth pinion gives a 3.73:1 ratio.

Hydraulic pressure formula

Pressure equals force divided by piston area, P = F / A. A larger master cylinder bore lowers line pressure for the same pedal force.

Brake fluid hygroscopy

Brake fluid absorbs moisture from the air, which lowers its boiling point and promotes corrosion. It is replaced on the manufacturer's time interval, not only by distance.

Camber angle

Inward or outward tilt of the wheel top viewed from the front. Excessive negative camber wears the inner tread; positive camber wears the outer tread.

Tyre centre wear

Wear concentrated on the middle of the tread indicates sustained over-inflation, which bows the tread outward and reduces the contact patch.

Steering free play

Movement at the steering wheel rim before the road wheels respond. Measured with wheels straight ahead, it reveals wear in tie rods, rack or steering box.

Ohm's law

Voltage equals current multiplied by resistance, V = I x R. It allows any one quantity to be calculated when the other two are known in a DC circuit.

Parallel resistance

Reciprocal of total resistance equals the sum of reciprocals of each branch: 1/Rt = 1/R1 + 1/R2. Total resistance is always less than the smallest branch.

Voltage drop test

Measuring voltage lost across a cable or connection while current flows. More than about 0.5 V on a main cable indicates excessive resistance.

CAN bus termination

Two 120-ohm resistors at the ends of the bus appear as about 60 ohms across the data lines when measured with the system powered down.

Lambda value

Actual air-fuel ratio divided by stoichiometric ratio. Lambda 1.0 means chemically correct mixture; below 1.0 is rich, above 1.0 is lean.

Radiator cap function

Pressurises the cooling system to raise the boiling point, and opens to release pressure or draw coolant back from the overflow bottle as the engine cools.

Stuck-open EGR valve

Exhaust gas flows into the intake at idle, diluting the mixture and causing rough idle, stalling and lean fuel-trim readings. It may also set a low-flow code.

Three-way catalyst

Converts carbon monoxide, hydrocarbons and nitrogen oxides simultaneously, but only when the mixture oscillates close to stoichiometric under closed-loop oxygen-sensor control.

OBD-II code prefixes

P denotes powertrain, B body, C chassis and U network faults. The code locates the circuit or system, not necessarily the component that failed.

Freeze-frame data

A snapshot of engine speed, load, temperature and fuel trim stored when a fault is set. Comparing it with live data helps locate intermittent faults.

Job card contents

Customer details, vehicle identification number, odometer, reported complaint, authorised work, parts fitted, labour time and signatures recording customer approval.

Break-even point

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

Gross profit margin

Revenue minus cost of goods sold, divided by revenue, expressed as a percentage. It measures workshop profitability before overheads such as rent and wages.