UPMSP High School Automobile flashcards

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

Compression ratio

The ratio of total cylinder volume to clearance volume, calculated as (swept volume + clearance volume) divided by clearance volume. Petrol engines typically run lower ratios than diesel engines.

Four-stroke cycle order

Induction, compression, power and exhaust strokes, completed in two crankshaft revolutions. In a spark-ignition engine the plug fires near the end of compression; in a diesel engine fuel is injected there.

Valve overlap

The period near top dead centre when both inlet and exhaust valves are slightly open. It helps scavenging at speed but excessive overlap causes rough idling and exhaust backflow at low engine speed.

Piston ring functions

Compression rings seal combustion gas against blow-by; oil control rings scrape excess oil from the bore. Rings also conduct heat from the piston to the cylinder wall.

Turbocharger wastegate

A valve that bypasses exhaust gas around the turbine once the desired boost pressure is reached. It protects the engine and turbocharger from excessive boost and detonation.

Clutch slip symptom

Engine speed rises without a matching increase in road speed, especially under load or uphill. Common causes are a worn friction disc, weak pressure plate springs or oil contamination of the lining.

Gear ratio formula

Driven gear teeth divided by driving gear teeth, equal to input speed divided by output speed. Overall ratio is the gearbox ratio multiplied by the final drive ratio.

Synchromesh purpose

Cone-shaped rings frictionally equalise the speed of the gear and shaft before the dog teeth engage. This allows smooth gear selection without double-declutching in a constant-mesh gearbox.

Differential function

Allows the inner and outer driven wheels to rotate at different speeds during a turn while still transmitting torque to both. It contains crown wheel, pinion and planet and sun gears.

Torque converter parts

Impeller, turbine and stator inside a fluid-filled housing. It replaces the clutch in an automatic transmission and multiplies engine torque at low vehicle speeds.

Ackermann steering principle

In a turn the inner road wheel steers a larger angle than the outer wheel, so both wheels turn about a common centre. This reduces tyre scrub and steering resistance.

Caster angle effect

The tilt of the steering axis from vertical. Positive caster gives self-centring and straight-line stability; too little causes wander, too much increases steering effort.

Camber and tyre wear

Camber is the inward or outward lean of the wheel from vertical. Excessive negative camber wears the inner tread edge; excessive positive camber wears the outer edge.

Spring versus damper

The spring supports vehicle load and absorbs road shocks; the damper controls spring oscillation. A worn damper lets the body bounce and reduces tyre contact with the road.

Tyre size 165/70 R14

165 is section width in millimetres, 70 is aspect ratio as a percentage of width, R means radial construction and 14 is rim diameter in inches.

Pascal's law in brakes

Pressure applied to a confined fluid is transmitted equally in all directions. Force at each wheel cylinder therefore equals hydraulic pressure multiplied by its piston area.

Spongy brake pedal

A pedal that feels soft and travels further than normal, usually caused by air in the hydraulic circuit. The cure is bleeding the brakes, starting at the wheel farthest from the master cylinder.

Brake fade

Loss of braking effect when linings or fluid overheat, common during repeated hard braking. Disc brakes resist fade better than drums because they cool more quickly.

ABS function

Anti-lock braking modulates hydraulic pressure using wheel speed sensors and solenoid valves to prevent wheel lock. It preserves steering control during hard braking but may lengthen stopping distance on gravel.

Ohm's law and power

Voltage equals current multiplied by resistance. Electrical power equals voltage times current, also current squared times resistance; a 12 volt, 55 watt lamp draws about 4.6 amperes.

Lead-acid battery cell voltage

Each cell produces about 2.1 volts, so a 12 volt battery contains six cells in series. Electrolyte specific gravity falls as the battery discharges and is checked with a hydrometer.

Series and parallel circuits

In series the same current flows and voltages add; in parallel the voltage is common and currents add. Vehicle accessories are connected in parallel, with the chassis acting as earth return.

Alternator precaution

Never disconnect the battery while the engine is running. Without the battery as a load, voltage spikes can destroy the rectifier diodes and the voltage regulator.

Stoichiometric air-fuel ratio

The chemically ideal mass ratio of air to fuel for complete combustion: about 14.7 to 1 for petrol and about 14.5 to 1 for diesel. Rich mixtures have excess fuel, lean mixtures excess air.

Exhaust smoke diagnosis

Black smoke indicates an over-rich mixture or restricted air intake; blue smoke indicates burning oil; white smoke indicates coolant entering the combustion chamber or unburnt diesel.

SAE 15W-40 meaning

The 15W figure describes cold-flow behaviour, where lower numbers flow better in winter. The 40 figure is the viscosity grade at operating temperature; higher numbers are thicker when hot.

Thermostat function

A temperature-controlled valve that stays closed when the engine is cold for rapid warm-up and opens at its rated temperature to send coolant through the radiator.

Cooling system overheating causes

Low coolant level, slipping or broken fan belt, stuck-closed thermostat, blocked radiator core, failed water pump or an air lock after refilling. Never open a hot pressurised radiator cap.

Vernier calliper least count

The smallest measurement the instrument can read, commonly 0.02 millimetres. Reading equals main scale value plus the coinciding vernier division multiplied by the least count.

Feeler gauge use

A set of thin metal blades of known thickness used to measure small gaps such as valve clearance, spark plug gap and contact breaker points. The blade should be a light sliding fit.

Cylinder head bolt sequence

Head bolts are tightened in stages to the specified torque, usually spiralling from the centre outward. This distributes clamping force evenly and prevents head warping or gasket failure.

Workshop fire extinguisher choice

Petrol and solvent fires need foam, dry powder or carbon dioxide, never water. Electrical fires need carbon dioxide or dry powder because these do not conduct electricity.