Cars · Lesson 4 of 15Beginner9 min read

Cooling and Lubrication

Most of the energy in fuel turns into heat, and an engine full of hot, fast-moving metal needs two things to survive: a way to shed that heat, and a film of oil between every moving part.

Why an engine needs cooling

In Lesson 1 you saw that engine losses, mostly heat, eat 68 to 72 percent of a gasoline car’s fuel energy. Some leaves with the exhaust; the cylinder walls, head and pistons soak up the rest. Without a cooling system they would get hot enough to break down the oil film and damage the metal.

An engine also shouldn’t run too cold. It’s designed to warm up quickly and work best at its normal operating temperature, which is why the cooling system has a thermostat.

The coolant loop

The water pump pushes coolant through passages in the block and head called the water jacket, where it picks up heat. The hot coolant flows out through the upper hose to the radiator. There it spreads into many small tubes with thin fins, and air flowing through carries the heat away. At speed, the car’s motion pushes air through; when it’s moving slowly or stopped, the fan pulls air through instead. The cooled liquid returns through the lower hose to the pump.

The thermostat is a valve between the engine and the radiator. When the engine is cold it stays closed, keeping heat in the engine so it warms up fast. As the engine reaches operating temperature it opens and adjusts the flow to the radiator to hold a steady temperature.

Your heater runs on the same loop. Hot coolant passes through the heater core, a small radiator behind the dashboard, and a blower pushes cabin air across it.

A simplified model with a temperature gauge instead of degrees, since normal operating temperatures vary by engine. Try each scenario to see how a failed part shows up on the gauge.

Pressure and the radiator cap

The radiator cap has two spring-loaded valves. The pressure valve lets the system build a little pressure, and pressure raises the coolant’s boiling point, the same physics as a pressure cooker. The vacuum valve opens as the engine cools so coolant can flow back from the overflow reservoir instead of the hoses collapsing.

That pressure is also why you never open a hot cooling system. Steam and boiling coolant can spray out and burn you. Check the coolant level at the see-through reservoir, with the engine cold.

Coolant, or antifreeze

Coolant is a mix of water and antifreeze, usually ethylene glycol, plus corrosion inhibitors. Water alone would freeze in winter, and since water expands about 9 percent when it freezes, it could crack the engine. A half-and-half mix of ethylene glycol and water protects to about −34°F. Use the type and mix your owner’s manual specifies.

Ethylene glycol tastes sweet and is poisonous to swallow. Keep antifreeze bottles closed, off the ground, and out of the reach of children.

Lubrication: oil’s six jobs

Engine oil does more than make things slippery. It reduces friction between moving parts, cushions them with a film so metal doesn’t touch metal, carries away heat, helps seal the piston rings against the cylinder walls, carries dirt to the filter, and coats parts so they don’t corrode. Oil’s resistance to flow is its viscosity. It thins as it heats, so it has to be thin enough to flow when cold and thick enough to hold a film when hot.

In a car engine, oil is pumped under pressure. It sits in the oil pan. An engine-driven pump draws it up through a screen and pushes it through the oil filter into drilled passages, the oil galleries, that lead to the crankshaft and camshaft bearings and the valve train. A relief valve keeps the pressure from climbing too high, and a bypass valve lets oil around a clogged filter so the engine still gets some. The oil then drains back to the pan and the cycle repeats.

A pressure-fed oiling system. The pump pushes oil through the filter to the bearings; a relief valve keeps pressure from climbing too high. Source: FAA Aviation Maintenance Technician Handbook (piston engines oil the same way).

Oil pressure

Oil pressure has to be high enough to keep the film between bearing and shaft intact. A pressure sensor on the oil gallery lights the dashboard oil light when pressure drops. Losing oil pressure can mean the oil has run low or the pump has failed, and running an engine without oil pressure can ruin it. Worn bearings with too much clearance also let pressure fall.

Parts and what they do
Part	What it does	Signs of trouble
Radiator	Passes coolant through thin tubes and fins so air can carry the heat away.	Green, blue or orange drips; overheating.
Water pump	Circulates coolant through the engine and radiator.	Coolant leaks near the front of the engine, overheating.
Thermostat	Stays closed until the engine warms, then regulates flow to the radiator.	Stuck closed: overheating. Stuck open: slow warm-up, weak heat.
Hoses	Flexible links that carry coolant and let the engine move on its mounts.	Cracks, bulges, soft spots, leaks.
Fan	Pulls air through the radiator at low speed and in traffic.	Overheating in traffic but fine on the highway.
Radiator cap	Holds pressure to raise the boiling point; lets coolant return as it cools.	Coolant loss or boiling over.
Reservoir	Holds extra coolant and shows the level.	Level below the MIN mark when cold.
Heater core	A small radiator behind the dash that heats the cabin.	No heat, a sweet smell inside, fogged windows.
Oil pump	Pushes oil through the filter and passages.	The oil pressure light.
Oil filter	Removes particles from the oil.	Changed with the oil at the interval in your manual.
Oil pan	Holds the oil supply.	Drips of brown or black oil.

You canWhat you can safely do

Check the oil level (Check Your Engine Oil) and the coolant level in the reservoir with the engine cold (Check Coolant, Brake Fluid, and Washer Fluid).
Look over the hoses for cracks, bulges and leaks.
Change your own oil and filter once you’ve done it with help (Change Your Own Oil and Filter). Take the used oil to a collection site: the used oil from one oil change can contaminate a million gallons of fresh water.

StopWhen to stop

If the temperature gauge climbs above normal or you see steam, stop driving as soon as it’s safe and find out why.
Never remove the radiator cap or open any part of the pressurized system until it has cooled.
If the red oil pressure light comes on while driving, pull over when it’s safe and shut the engine off. Check the oil level once it has settled, and have the car checked before you drive it far. Your owner’s manual describes this light for your car.

Check yourself

Tap an answer to check it, or open “Show the answer.”

A cold engine’s thermostat is closed. Why?
Show the answer
So the engine warms up quickly. Keeping coolant out of the radiator holds heat in the engine so it reaches operating temperature fast.
Why does the cooling system run under pressure?
Show the answer
Pressure raises the coolant’s boiling point. Higher pressure means a higher boiling point, so the coolant can run hotter without boiling.
A car overheats in stop-and-go traffic but is fine on the highway. Which part is a likely suspect?
Show the answer
The fan. At highway speed, air rushes through the radiator by itself. In traffic, the fan has to do that job.
Which of these is NOT one of oil’s jobs?
Show the answer
Raising the coolant’s boiling point. Oil lubricates, cushions, cools, seals, cleans and protects against corrosion. The radiator cap’s pressure raises the coolant’s boiling point.
What does the oil pressure relief valve do?
Show the answer
Keeps oil pressure from climbing too high. It opens when pressure gets too high and sends extra oil back.

Go deeper

Caltech Question of the Week: Why Does an Engine Cooling System Have a Thermostat? A Caltech engineer on why engines have thermostats.
FAA Aviation Maintenance Technician Handbook, Powerplant, Chapter 6: Lubrication and Cooling Systems The FAA’s chapter on lubrication and cooling for piston engines (detailed).
Being an Adult 101: Check Coolant, Brake Fluid, and Washer Fluid Step-by-step coolant, brake fluid and washer fluid checks.
CDC/ATSDR ToxFAQs: Ethylene Glycol Ethylene glycol safety from the CDC.

Sources

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A free course from Teddy Rook Book. It explains how cars work in general and follows U.S. sources; your owner’s manual and your vehicle’s service information are the final word for your car, along with state and local rules.