RBSE Prav Voc Automotive flashcards

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

Compression ratio

Ratio of cylinder volume at bottom dead centre to volume at top dead centre. Calculated as (swept volume + clearance volume) divided by clearance volume. Higher ratios improve thermal efficiency but raise knock risk in petrol engines.

Stoichiometric air-fuel ratio

The mass ratio of air to fuel that gives complete combustion. Approximately 14.7:1 for petrol by mass. Lambda equals actual air-fuel ratio divided by stoichiometric ratio; lambda 1.00 is ideal for catalytic converter efficiency.

Four-stroke cycle

Intake, compression, power, and exhaust strokes. The crankshaft turns twice per complete cycle while the camshaft turns once, giving a 2:1 camshaft-to-crankshaft ratio. One power stroke occurs per cylinder per two revolutions.

Valve overlap

Period near top dead centre when intake and exhaust valves are both open. It helps scavenge exhaust and fill the cylinder at speed. Excessive overlap at idle causes rough running and low manifold vacuum.

Brake mean effective pressure

Average pressure acting on the piston during the power stroke, derived from torque and displacement. It allows comparison of engine load between engines of different sizes and is independent of engine speed.

Clutch slip point

The pedal position where the friction disc begins to transmit torque. Free play before this point is typically 10 to 25 mm. Insufficient free play causes slippage and premature release bearing wear.

Ackermann steering geometry

Steering linkage arrangement that turns the inner front wheel through a larger angle than the outer wheel. All wheels then share a common turning centre, reducing tyre scrub and improving low-speed manoeuvring.

Caster angle

Longitudinal tilt of the steering axis from vertical. Positive caster places the steering axis contact ahead of the tyre contact patch, improving straight-line stability and self-centring after a turn.

Torque converter lock-up

Clutch inside the converter that mechanically connects turbine to impeller at cruise speed. It eliminates hydraulic slip, reducing fuel consumption and heat generation while improving engine braking response.

Wheel toe setting

Difference in track width measured at the front and rear of the tyres at hub height. Toe-in or toe-out is adjusted last in an alignment sequence, after camber and caster are set.

Ohm's law

Current through a conductor equals voltage divided by resistance, written V = I × R. Combined with power P = V × I, it allows calculation of circuit current, voltage drop, and component wattage.

Voltage drop test

Measurement of voltage lost across a conductor or connection while current flows. More than 0.5 V on a main power or ground circuit indicates excessive resistance requiring repair.

CAN bus termination

Two 120 ohm resistors at opposite ends of the network. Measured in parallel with power off, the bus should read about 60 ohms. Incorrect termination causes communication faults and multiple module codes.

Alternator charging voltage

Regulated output typically between 13.8 and 14.5 volts at idle with electrical loads off. Higher or lower readings suggest regulator, wiring, or battery faults rather than normal operation.

NTC coolant sensor

Negative temperature coefficient thermistor whose resistance falls as coolant temperature rises. The engine control unit converts resistance to voltage and uses it for fuelling, ignition timing, and fan control.

Pascal's principle

Pressure applied to a confined fluid is transmitted equally in all directions. In hydraulic brakes, pedal force divided by master cylinder area gives system pressure, which acts on larger caliper pistons.

ABS wheel speed sensor

Sensor, usually inductive or Hall effect, that reports wheel rotational speed to the ABS control unit. A faulty signal disables anti-lock function and illuminates the ABS warning lamp.

Electronic stability control

System comparing steering angle, yaw rate, and lateral acceleration. When the actual path differs from the driver's intent, it brakes individual wheels and reduces engine torque to restore stability.

Brake fade

Loss of braking effectiveness caused by heat, typically from overheated pads or fluid. Friction coefficient falls and fluid may boil, producing a spongy pedal. Cooling and replacing contaminated components restores performance.

Parking brake adjustment

Setting cable free play at the equaliser or caliper lever so the brake holds the vehicle on a specified gradient. Excess free play reduces holding force; too little causes drag and overheating.

Tyre tread wear limit

Minimum legal tread depth of 1.6 mm across the central three-quarters of the tyre. Worn tread increases wet stopping distance and aquaplaning risk, and is a common inspection failure point.

Torque-to-yield bolt

Fastener tightened past its elastic limit and used once only. It is usually tightened to a torque plus a specified angle. Reusing it risks loss of clamping force and joint failure.

Compression test procedure

Remove all spark plugs, hold the throttle open, and crank a set number of strokes. Compare cylinders; variation above about 10 percent or readings below specification indicate wear or leakage.

Battery state of charge

Open-circuit voltage of a lead-acid battery indicates charge level. About 12.6 volts is fully charged; below 12.4 volts indicates partial discharge. Charge before load testing for accurate results.

Micrometer reading

Precision measuring instrument reading to 0.01 mm using a barrel scale and thimble. Used for measuring shaft diameters, brake disc thickness, and bearing journals where a caliper is not accurate enough.

Feeler gauge

Set of thin metal blades of known thickness used to measure gaps such as valve clearances and spark plug gaps. The correct clearance is the largest blade that fits with slight drag.

Cylinder leakage test

Compressed air applied to a cylinder at top dead centre. Air escaping into the intake, exhaust, crankcase, or coolant identifies leaking valves, rings, or head gasket without dismantling the engine.

Multimeter continuity mode

Function that checks for a complete low-resistance path, often with an audible tone. Used on de-energised circuits only; measuring resistance on a live circuit can damage the meter.

Hydrogen charging hazard

Lead-acid batteries release hydrogen during charging. Keep flames and sparks away, ventilate the area, and connect the negative terminal last to avoid arcing near the battery.

Hazardous waste segregation

Separating used oil, coolant, brake fluid, solvents, and batteries into distinct containers. Mixing streams prevents recycling and breaches disposal regulations; never pour to drain.

Diagnostic trouble code format

Five-character code starting with P, B, C, or U for powertrain, body, chassis, or network. The first digit indicates generic or manufacturer-specific, the second whether it is ISO or manufacturer defined.

Fuel trim interpretation

Short and long-term fuel trim show control unit correction to air-fuel ratio. Positive long-term trim indicates a lean condition such as a vacuum leak; negative trim indicates a rich condition.

Hybrid high-voltage safety

High-voltage circuits in hybrids and electric vehicles commonly operate at 100 to 650 volts DC. Orange cabling marks them. Isolate the system and use insulated tools and personal protective equipment before repair.

Regenerative braking

Recovery of kinetic energy during deceleration by using the motor as a generator to charge the battery. Friction brakes handle the remainder, so they may corrode from low use in electric vehicles.

ADAS automation levels

Six levels from L0 no automation to L5 full automation. L2 provides combined steering and speed assistance but requires constant driver supervision; the driver remains responsible for the vehicle.

Break-even point

Number of jobs or units needed for revenue to equal total costs. Calculated as fixed costs divided by selling price per job minus variable cost per job. Below this point the business operates at a loss.

Job card contents

Workshop record containing customer details, vehicle identification number, odometer, reported complaint, work performed, parts used, and technician signature. It supports invoicing, warranty claims, and later diagnosis.

Gross profit margin

Revenue minus cost of goods sold, divided by revenue, expressed as a percentage. Labour usually carries a higher margin than parts, so workshop profitability depends on labour utilisation and efficiency.