Internal Combustion Engine Cooling: Air, Liquid and Why Cars Chose Water

Internal combustion engine cooling removes the waste heat that an engine cannot turn into motion. Engines use either air or liquid for the job, and most cars and large engines rely on a water-based coolant that gives up its heat to the air in a radiator. Without that removal, metal parts and lubricants would overheat and fail.

Why does an engine need cooling?

An engine converts only part of its fuel energy into motion, so the rest leaves as heat that must be carried away. Cooling keeps temperatures low enough for metal and lubricant to survive, and it evens out heat so no single area becomes a dangerous hot spot.

Some waste heat is actually useful, because it carries energy through the engine in much the same way that outflowing water keeps a water wheel turning. Engines with higher efficiency send more energy out as motion and less as heat, but every heat engine still needs a cooling path to operate.

Combustion temperatures are hotter than the materials that form the engine can withstand, and they are hot enough to set lubricants alight. Hot climates make the problem harder, because the air used for cooling may range from well below freezing to 50 degrees C.

Liquid carries heat far better than air, but both methods survive because engines must work in every climate and every job.

How do air and liquid cooling compare?

Liquid cooling moves heat more smoothly and lets designers shape coolant passages around the hottest areas, but it needs a pump and a radiator. Air cooling is lighter and simpler, yet it needs far more surface area and airflow to shift the same amount of heat.

Water carries heat far better than air. It has a higher heat capacity per gram and per volume, and its thermal conductivity is roughly four times that of oil. Air has much lower heat capacity, less than a tenth of water's conductivity, and a viscosity around 200 times lower, so it moves easily but carries little heat on each pass.

The arithmetic is unforgiving. Air cooling needs ten times the surface area of a liquid system, and the airflow must be about 2000 times faster. A recirculating air fan therefore needs ten times the power of a water pump, which is one reason liquid cooling dominates in cars.

Water also boils at about the temperature an engine should run at. Boiling absorbs a great deal of heat with very little rise in temperature, which helps a single stream of coolant hold several hot parts at nearly the same temperature. Air behaves differently. Passed over several hot objects in turn, it warms at each step, so the first part may be over-cooled and the last under-cooled.

Boiling has a downside. Once water boils, steam bubbles form that insulate the metal, and cooling drops suddenly. Because steam can recondense as it mixes with the rest of the coolant, a temperature gauge can still read an acceptable figure while local areas are hot enough to cause damage.

What goes into engine coolant?

Most liquid-cooled engines run a mixture of water and glycol-based antifreeze, along with rust inhibitors and other additives. Some antifreezes contain no water at all and rely on propylene glycol, or a blend of propylene and ethylene glycol, to carry heat and resist freezing.

Common coolant families are named IAT, OAT and HOAT, which refer to the corrosion inhibitor technology in each formula. Raw water can carry sediment or salt that clogs passages and damages metal, which is why marine engines often pass their coolant through a closed loop cooled by the surrounding water.

Propylene glycol coolant shrinks slightly when it crystallises rather than cracking the engine. It stays liquid to -55 degrees C and has a service life above 10,000 hours, which is about the working life of many engines.

How are the moving parts cooled?

The block and cylinder head are cooled directly by the main coolant, but moving parts such as the pistons depend largely on the lubricating oil. The crankshaft and connecting rods shed a smaller share of their heat through the oil and through conduction into the block.

High performance engines often spray extra oil up under the piston crowns just to draw heat away.

Liquid-cooled engines can vary the size of passages through the block, so the flow suits each zone. Areas near the combustion chamber and around the exhaust ports usually need the most cooling. Air-cooled engines can match that only by using more closely spaced fins, which are difficult and expensive to manufacture, and air-cooled motorcycles lean heavily on oil cooling of the cylinder barrels as well.

How does a car control its engine temperature?

Cooling systems adjust their output because the load on a road vehicle changes quickly. Adjustable shutters in the airflow, a fan that runs either independently or through a clutch, and a thermostatic valve that blocks coolant flow while the engine is cold work together to hold the engine near its target temperature.

The coolant and radiator also hold a little heat, which smooths out brief spikes during short bursts of hard driving. When an engine does overheat, some controls shut it down or limit it to half throttle. Modern electronic controls use throttle position to anticipate a temperature rise and reduce power before the engine runs too hot.

Why did cars move from air to liquid cooling?

Before World War II, water-cooled cars and trucks often boiled over while climbing mountain roads, and some routes had repair shops ready for overheating engines. The change came at the start of the war, when the US military needed dependable vehicles and engineers traced the boil-overs to the water pump.

The pump used a leaky graphite-packed rope seal, inherited from steam engines, that drained water until circulation stopped. Wartime vehicles used carbon-seal water pumps that did not leak, and the boil-overs stopped. Air cooling had already gained a following across Europe, and Volkswagen advertised its air-cooled cars in the United States as not boiling over, which helped them sell well.

Liquid cooling later became the norm for most automotive engines. Tougher exhaust emission laws in the 1960s and leaner fuel mixtures made air-cooled engines harder to keep both clean and quiet. Subaru chose liquid cooling for its EA flat engine when that unit appeared in 1966.

Where else is air cooling used?

Aircraft still favour air cooling because weight matters so much. Many propeller-driven aircraft with internal combustion engines remain largely air-cooled. Rotary engines, whose cylinders spin and move air across them even when the aircraft stands still, were popular until the end of World War I, but they had serious stability and efficiency problems.

Radial engines, which let air flow around each cylinder, served until the end of World War II, when gas turbines largely replaced them.

Motorcycles are lighter than cars, so both cooling methods are common on them. Some sport bikes combine air and oil cooling, with oil sprayed beneath the piston heads. For boats, seawater can cool the engine directly, but sediment and salt can damage passages, so many vessels pass coolant through a heat exchanger cooled by the water around them.

How big can air-cooled engines get?

Air-cooled engines struggle to scale up, because cooling air needs far more surface area than liquid does. Nearly all of them are under 500 kW (670 hp), while large liquid-cooled engines exceed 80 MW (107000 hp). The Wartsila-Sulzer RTA96-C, a 14-cylinder diesel, is one example of that larger class.

Air cooling is also harder to combine with low emissions and low noise, which is one of several reasons most road vehicles use liquid.

What are low heat rejection engines?

Low heat rejection engines, also called adiabatic engines, are experimental designs that aim to lose less heat in order to improve efficiency. They are usually diesels with combustion chamber parts lined with ceramic thermal barrier coatings, and some use titanium pistons because titanium conducts heat poorly and is light.

These engines remain a research area rather than a common production type.

A few designs remove the cooling system and its parasitic losses altogether. Developing lubricants that survive the higher temperatures involved has been a major barrier to commercial use, which explains why the idea has stayed largely experimental for several decades.

How do the cooling methods compare?

Each cooling method suits particular engines, and reliability can decide the choice more than efficiency does. The table below summarises the main trade-offs between the air and liquid systems covered in this article. Neither method wins on every measure, so the right choice depends on what the engine has to do.

Reliability sits at the heart of many decisions. Where a failure would be severe, as with aircraft, designers may accept lower efficiency, shorter time between rebuilds and more noise in exchange for slightly higher reliability. Cars typically put a premium on power and weight, which is why most of them have water-cooled engines.

What else shapes a cooling system's design?

Cost, weight, reliability and durability all press on the design, and the cooling system must satisfy each of them without starving the engine. Every part needs suitably low temperatures, because the whole engine fails if one part overheats. Designers therefore balance the cooling system against everything else the vehicle has to do.

Reliability can matter more than efficiency. Failure rates typically rise as the square of the number of failure points, so a simpler system with fewer parts has an advantage. Air cooling is forgiving in one respect, since small air leaks reduce cooling only slightly, while a liquid system depends on keeping its coolant inside the loop.