UPMSP Vocational Automobile flashcards

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

Compression ratio

Ratio of the cylinder's total volume with the piston at bottom dead centre to the volume at top dead centre. Petrol engines run roughly 8:1 to 12:1; diesels 16:1 to 23:1.

Four-stroke cycle

Intake, compression, power and exhaust strokes. The crankshaft turns twice (720 degrees) per complete cycle while the camshaft turns once, giving a 2:1 reduction.

Stoichiometric air-fuel ratio

The ideal mass ratio of air to fuel for complete combustion: about 14.7:1 for petrol and about 14.5:1 for diesel. Richer mixtures have excess fuel; leaner mixtures have excess air.

Engine capacity formula

Swept volume per cylinder equals (pi/4) times bore squared times stroke. Multiply by the number of cylinders for total capacity in cubic centimetres when bore and stroke are in centimetres.

Torque converter

Fluid coupling in automatic transmissions that transmits drive through oil. Its stator multiplies torque at low speed and it allows the engine to idle while the vehicle is stationary.

Differential function

Allows the inner and outer driven wheels to rotate at different speeds during a turn while still delivering torque to both, preventing tyre scrub and axle wind-up.

Synchromesh

A cone-clutch device that equalizes the speed of the gear and shaft before the dog teeth engage, preventing gear clash during manual gear selection.

Gear ratio

Driven gear teeth divided by driving gear teeth. Overall ratio equals gearbox ratio multiplied by final drive ratio; a numerically lower ratio gives higher road speed for the same engine speed.

Pascal's law in brakes

Pressure applied to a confined fluid is transmitted equally in all directions. Braking force equals system pressure multiplied by caliper or wheel-cylinder piston area.

Ackermann steering principle

The inner wheel turns through a larger angle than the outer wheel so all wheels roll around a common centre point, minimizing tyre scrub during a turn.

Caster angle

The tilt of the steering axis from vertical as seen from the side. Positive caster gives self-centring steering and straight-line stability; too much increases steering effort.

ABS function

Anti-lock braking system modulates brake pressure to prevent wheel lock-up. It preserves steering control during hard braking but does not always shorten stopping distance.

Ohm's law

Voltage equals current multiplied by resistance (V = I x R). Power equals voltage times current, or current squared times resistance, used to size fuses and diagnose voltage drops.

Alternator output voltage

A healthy charging system holds roughly 13.8 to 14.5 volts across the battery with the engine running. Lower output suggests a faulty regulator, diode or belt.

Cold cranking amps

The current a battery can deliver for 30 seconds at minus 18 degrees Celsius while keeping at least 7.2 volts. It indicates starting capability in cold conditions.

CAN bus

A twisted-pair serial network letting electronic control modules exchange data. A shorted line or missing termination can disable many vehicle functions at once.

Voltage drop test

Measuring voltage loss across a cable or connection while current flows. A drop above about 0.5 volts on a power or earth cable indicates excessive resistance.

Engine oil viscosity grade

A grade such as 10W-40: the first number is the winter (cold) grade and the second the hot grade. A higher second number means thicker oil at operating temperature.

Thermostat function

A temperature-sensitive valve that regulates coolant flow to the radiator, keeping the engine near its designed operating temperature of roughly 80 to 95 degrees Celsius.

Overheating causes

Low coolant, faulty thermostat, failed water pump, blocked radiator, slipping fan belt, or a blown head gasket. A head gasket fault often shows white exhaust smoke and coolant loss.

Wet compression test

Adding a little oil to the cylinder before re-testing compression. If the reading rises significantly, worn piston rings are indicated; little change suggests valve or gasket leakage.

Diagnostic trouble code

A code stored by the OBD-II system that identifies a faulty circuit or system, not necessarily the failed part. Pinpoint tests must follow before replacing components.

Tyre tread wear limit

The legal minimum tread depth is 1.6 millimetres, marked by wear indicators in the grooves. Pressure must be checked cold and tyres rotated to equalize wear.

Battery open-circuit voltage

A fully charged 12 V lead-acid battery reads about 12.6 volts at rest; about 12.0 volts indicates roughly 50 percent charge. Load or conductance testing confirms cranking ability.

Monocoque body

A unitary construction in which the body shell carries structural loads, used by most passenger cars. It differs from body-on-frame construction used on many commercial and off-road vehicles.

Crumple zone

A front or rear body section designed to deform in a controlled way during impact, absorbing energy while the reinforced passenger safety cell resists deformation.

Paint layer sequence

Bare metal, then primer for adhesion and corrosion protection, then colour base coat, then clear coat for gloss and UV protection. Each layer must cure before the next.

Orange peel paint defect

A textured, uneven finish caused by incorrect spray pressure, viscosity or distance between gun and panel. It is corrected by sanding and polishing or repainting.

Torque wrench use

Set the specified torque, tighten in the manufacturer's sequence, and never use it to loosen fasteners. Cylinder heads are usually tightened spirally from the centre outward.

Vernier calliper resolution

A common workshop measuring tool with typical resolution of 0.02 millimetres, used for external, internal and depth measurements. A micrometer resolves to 0.01 millimetres.

Tap drill size rule

For cutting an internal thread, the drill diameter approximately equals the nominal thread diameter minus the thread pitch. Lubricate and reverse the tap to clear chips.

Class B fire extinguisher

Used on flammable liquid fires such as petrol and oil. Foam, dry powder or carbon dioxide are suitable; water must never be used on burning fuel.