Lubricants Questions and Answers

Answer: A

Correct answer: reduce friction between moving surfaces

Reducing friction is the primary function, and everything else follows from it: less friction means less heat generated and less wear, and the film that separates the surfaces also carries away heat and excludes contaminants. Those are consequences of interposing the film, not separate purposes for which a lubricant is chosen.

Answer: B

Circulating oil carries heat from the pistons and bearings to the sump or the cooler and sweeps soot and wear metal to the filter. It also neutralises acids formed by combustion, which is why the detergent additives matter.

Answer: C

Correct answer: heat and wear

Friction converts mechanical work into heat, and the asperities that resist the motion are progressively torn away, which is wear. Both occur together and both grow with load, so a lubricant that reduces friction reduces both at once. No load is light enough to eliminate either entirely.

Answer: D

Correct answer: thick enough that no asperities touch

In hydrodynamic lubrication the relative motion drags oil into a converging gap and generates a pressure that lifts the surfaces apart, so the film is thicker than the combined roughness and no metal touches. Friction is then that of the oil alone and wear is essentially nil, which is why bearings are designed to run in this regime.

Answer: A

The oil is squeezed out until only an adsorbed layer a few molecules thick remains, and it is held there by its affinity for the metal rather than by pressure. Oiliness rather than viscosity is the property that matters.

Answer: B

Vegetable and animal oils have long polar molecules that anchor to the metal, which is why a little is blended into a mineral oil for such duty. A high viscosity is no substitute when the film has already been squeezed out.

Answer: C

Correct answer: react with the metal to form a low-friction film

Extreme pressure additives contain sulphur, chlorine or phosphorus and react with the metal at the high local temperature of an asperity contact, producing a soft sulphide or phosphide film that shears easily. The reaction is deliberate and localised, occurring only where and when the contact becomes severe.

Answer: D

It is the single most important property of an oil for hydrodynamic duty, since it decides the film thickness the bearing can build. Too high a viscosity wastes power in churning and too low a value lets the film collapse.

Answer: A

Thermal agitation loosens the molecules' hold on one another, so an oil thin enough to circulate when cold may be too thin to carry the load when hot. Reconciling the two is the whole problem that multigrade oils address.

Answer: B

Correct answer: how little its viscosity changes with temperature

The viscosity index is a measure of stability rather than of viscosity: a high index means the oil thins comparatively little as it warms. It is what matters in an engine that must be pumped when cold and must still carry load when hot, and it is raised by viscosity index improver additives.