Why engines need oil

An engine is full of metal parts sliding and spinning against each other, thousands of times a minute. The crankshaft turns in its bearings, the pistons slide up and down the cylinders, and the camshafts push the valves open. Metal rubbing on metal at that speed would get hot, wear fast and, within minutes, weld itself together. Mechanics call that seizing.

Engine oil stops that happening. It does several jobs at once:

• It keeps the parts apart. A thin film of oil between them means metal slides on oil instead of on metal.
• It cools. Oil carries heat away from places the coolant can’t reach, such as the bearings and the underside of the pistons.
• It cleans. It washes away soot and specks of metal and carries them to the filter.
• It seals. A film of oil on the cylinder walls helps the piston rings seal in the pressure from the burning fuel.
• It protects. It stops the inside of the engine rusting and corroding.

Step through with the numbered buttons, then change the engine speed and try cold oil and low oil.

1. Sump. The oil, about 4.5 litres (1 gallon) in a typical engine, sits in the sump bolted under the engine. A pipe called the pickup reaches down into it, with a strainer on the end to keep big bits out.
2. Pump. The oil pump, often a pair of gears turned by the engine, draws the oil up and pushes it on. The faster the engine turns, the harder it pushes, so the pressure rises with the revs. A relief valve stops it going too high, at about 5 bar (73 psi).
3. Filter. The oil goes through the oil filter, a canister of folded paper that catches the dirt the oil has picked up.
4. Bearings. Passages drilled through the engine, called galleries, take the oil to every bearing on the crankshaft and camshafts. The close-up shows the clever bit. The turning shaft drags oil round with it into a narrow wedge under itself, and the wedge pushes back hard enough to lift the shaft clear. It rides on a film of oil a few thousandths of a millimetre thick, about a twentieth of the thickness of a hair. As long as the oil keeps coming, the metal surfaces never touch.
5. Drain. Oil also sprays up onto the valves and camshafts and squirts onto the underside of the pistons to cool them. It all drains back down to the sump, carrying heat and dirt with it, and goes round again.

Try Cold oil. Cold oil is many times thicker than hot, so the pressure shoots up and the film is thicker. But thick oil is slow to reach everything, so the first few seconds after a cold start are when an engine wears most. That’s why it’s kinder to drive gently until it has warmed up.

Now try Low oil. The pickup sucks in air, the pressure collapses and the warning light comes on. In the close-up, the film is gone and the shaft is grinding on its bearing.

The most important thing about an oil is how thick it is, which is called its viscosity. Too thin, and the film breaks down under load. Too thick, and it’s slow to flow on a cold start and wastes power being pushed round.

The trouble is that oil thins as it heats up. Oil that’s just right at 100°C (212°F) would be as thick as syrup on a frosty morning. Modern multigrade oils have additives that keep their thickness steadier, and their grade shows how they behave at both ends:

• In 5W-30, the 5W is the cold, or winter, rating. The lower the number, the more easily it flows when cold.
• The 30 is the rating at running temperature. The higher the number, the thicker it stays when hot.

Use the grade in your car’s handbook. Modern engines are built with tight gaps for thin oil, and the wrong oil can wear them faster. Most new cars use synthetic oil, made rather than refined straight from crude oil. It lasts longer and copes better with heat.

• Low oil. Most engines burn a little oil, and older engines burn more. Check the level every month or so with the dipstick, with the engine off and the car on level ground: pull it out, wipe it, push it back in, then pull it out again and read where the oil comes to between the marks. Top up with the right oil if it’s near the bottom one.
• The oil warning light. A red oil can on the dashboard means the oil pressure has dropped. Pull over as soon as it’s safe and switch the engine off. Driving on, even for a few minutes, can ruin the bearings. Some cars also have an amber light that just means the level is getting low.
• Old oil. Oil slowly fills up with soot, dirt and unburnt fuel, and heat wears out its additives. Leave it too long between changes and it can turn into a thick black sludge that blocks the narrow galleries. Change the oil and filter as often as the maker says, usually once a year, or sooner if you drive a lot. Many modern cars work it out for themselves and tell you on the dashboard.
• Leaks. Oil can seep from gaskets and seals as they age. Look for drips under the engine, or a burning smell as oil lands on the hot exhaust.
• Blue smoke. Blue-grey smoke from the exhaust means oil is getting into the cylinders and burning, often past worn piston rings or valve seals.
• Oil and coolant mixing. A creamy, mayonnaise-like sludge under the oil filler cap can mean a failed head gasket, letting coolant into the oil. A little after lots of short trips in winter can just be condensation, but if coolant is going missing too, get it checked.