NSSCO Motor Mechanics 6189 flashcards

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

Fire triangle

Three elements needed for combustion: fuel, oxygen and ignition source. Removing any one extinguishes the fire. Extinguisher choice depends on the fuel class.

COSHH

Control of Substances Hazardous to Health. Requires assessment of hazardous substances, exposure control, health surveillance and record keeping, with elimination and engineering controls before PPE.

Micrometer least count

Smallest reading increment. Metric micrometer: 0.01 mm, with 50 thimble divisions over 0.5 mm. Imperial: 0.001 in, with 25 divisions over 0.025 in.

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 gives 0.02 mm.

Compression ratio

Ratio of cylinder volume above the piston at bottom dead centre to volume at top dead centre: (swept volume + clearance volume) / clearance volume.

Four-stroke cycle

Induction, compression, power and exhaust strokes, completed in two crankshaft revolutions or 720 degrees. Each stroke is 180 degrees of crank rotation.

Two-stroke scavenging

Transfer of fresh charge from crankcase to cylinder while burnt gases leave through the exhaust port, as the piston uncovers the ports near bottom dead centre.

Torque-to-yield bolt

Head or main bolt tightened past its elastic limit by a specified torque plus angle, then replaced rather than reused because it has permanently stretched.

Plastigauge

Calibrated plastic strip placed across a bearing journal and crushed by the tightened cap. Its flattened width, compared with a scale, gives the oil clearance.

Ring end gap

Gap between the ends of a piston ring when fitted squarely in the bore. Too small causes ring breakage when hot; too large causes blow-by and oil consumption.

Multigrade oil 10W-40

Oil whose viscosity meets cold grade 10W at low temperature and hot grade 40 at 100 degrees Celsius. Viscosity index improvers give the wide temperature range.

Stoichiometric ratio

Chemically ideal air-to-fuel mass ratio for complete combustion: about 14.7:1 for petrol and about 14.5:1 for diesel.

Thermostat function

Valve that stays closed when the engine is cold to speed warm-up, then opens at its rated temperature, typically 82 to 90 degrees Celsius, to allow radiator flow.

Ohm's law

Current through a conductor equals voltage divided by resistance (I = V / R). Combined with P = V x I, it allows calculation of current, voltage drop and power in circuits.

Battery state of charge

Estimated from open-circuit voltage after rest: 12.6 V is 100 percent, 12.4 V about 75 percent, 12.2 V about 50 percent, 12.0 V about 25 percent for a lead-acid battery.

Alternator rectifier

Six diodes converting three-phase AC from the alternator into DC for the vehicle. A failed diode causes AC ripple and reduced charging output.

Clutch free play

Small clearance in the release mechanism, typically 10 to 25 mm at the pedal, ensuring the release bearing is not loaded when the clutch is engaged.

Differential action

Planet gears allow the two driven wheels to rotate at different speeds in a turn while the average of their speeds equals the crown wheel speed.

Final drive ratio

Number of crown wheel teeth divided by pinion teeth. Multiplied by the gearbox ratio it gives the overall ratio used to calculate road speed.

Ackermann steering

Geometry in which the inner wheel steers at a larger angle than the outer wheel, allowing all wheels to roll without scrub on a turn.

Tyre size 195/65 R15

Section width 195 mm, aspect ratio 65 percent, radial construction, rim diameter 15 inches. Sidewall height equals 65 percent of 195 mm, about 127 mm.

Static wheel imbalance

Heavier point of the wheel causes vertical bounce at speed. Corrected by fitting balance weights opposite the heavy point on a balancing machine.

Pascal's law in brakes

Pressure applied to a confined fluid is transmitted equally in all directions, so master-cylinder pressure acts equally at every wheel cylinder or caliper.

Brake fade

Loss of braking force caused by overheating of pads, shoes or discs, or by vapour lock in the hydraulic fluid. Friction recovers after cooling; vapour lock needs bleeding.

Monocoque chassis

Body and chassis combined into one stressed structure, lighter and stiffer than a separate ladder frame but requiring careful panel alignment and corrosion protection after repair.

Tread depth limit

Legal minimum commonly 1.6 mm across the central three quarters of the tread around the whole tyre. Below this, wet grip and drainage fall sharply.

Third-angle projection

Drawing convention in which the top view is placed above the front view and the right-side view to the right, as if the object were viewed through transparent planes.

Mechanical advantage

Load divided by effort. Efficiency equals mechanical advantage divided by velocity ratio, expressed as a percentage, and is always less than 100 percent in practice.

Gear ratio formula

Driven gear teeth divided by driving gear teeth. Speed of driven gear equals speed of driving gear multiplied by driving teeth divided by driven teeth.

Voltage drop test

Voltmeter connected across a cable or joint while the circuit carries current. More than about 0.2 V per connection indicates excessive resistance, often a corroded or loose terminal.

Parasitic drain

Current drawn from the battery when all circuits are off and modules have slept. Typically below 50 mA; higher values indicate a faulty module, lamp or alternator diode.

Brake judder cause

Usually disc thickness variation or warped disc, sometimes worn wheel bearings or a sticking caliper. Measured with a micrometer for thickness and a DTI for runout.