Luftkuhlung: How Do Air Cooled Engines Work?

If you’ve been a fan of European sports cars, or sports cars in general over the last 30 or so years, you know that air-cooled Porsches are the best. Or at least that’s what you’ve been told. Everybody says it, so it must be true. Right? Well don’t worry, we’re not here to debate if older Porsches are better than new ones. What we’re here to do is answer some questions that you might be too afraid to ask by now. Like what is an air-cooled engine? Why did they use it? Why don’t they use it now? Why does engine technology have to move so quickly? Am I getting old? What’s that damned noise the kids are listening to?!

Uh… Well, this is how air-cooled engines work.

To put it bluntly, they aren’t air-cooled. Not really, at least. The engines are oil cooled. But that’s probably being a little bit pedantic.

Every time a combustion cycle happens in an engine, it makes a whole bunch of heat. And most of that heat is wasted energy. Even the most efficient cars still turn somewhere around 60 percent of the energy from the tiny controlled explosion in the engine into heat. With temperatures in the cylinders reaching into the thousands of degrees, dissipating that heat is a big deal.

Fortunately for engineers and metallurgists, most of that heat stays in the exhaust and leaves through the tailpipe. It’s that heat that allows the catalytic converter to work, but that’s a bedtime story for a different day. What’s important today is that somewhere around 20% of the heat goes into the engine block and cylinder head.

With a water cooled engine, there are passages running around the inside of the block and heads. Those are full of liquid coolant, usually water and anti-freeze. Water has a really good heat capacity (meaning it takes a lot of energy to make water hot). So the water is heated by that extra engine heat and then travels to the radiator where it heats the air passing through, cools off, and starts the cycle again. That keeps the engine happy and the water temps under the boiling point.

In an air-cooled engine, there are no hidden passages for water. And sure there’s some oil in there, but oil can’t move as much heat away from the cylinders as water. So how do engineers prevent it from overheating and fusing into a metal cube? Surface area.

Air has a lower heat capacity than water, but it has another advantage. There’s more of it. Unless you’re in the ocean, that is. Air moving over an object will remove heat. Like a cool breeze from a fan on a warm day. It does have limitations, though.

Stick your arm into the breeze from that fan and you won’t get much cooler. Put your entire body in front of the fan and you’ll get much cooler. That’s because you’re exposing more surface area to the moving air.

Air-cooled engines do the same thing. Look at the engine on an air-cooled motorcycle (because you’re a lot more likely to see one of those than a Porsche engine case) and you’ll see what look like ribs evenly spaced up the cylinder. Those are called fins. They work like the fins inside your baseboard heaters at home, only instead of warming up your room they warm up the planet.

Those fins add more surface area to the engine’s case, which means more air is touching the engine. That means it can take away more heat. Hopefully keeping the engine cool. It’s why so many air-cooled engines are horizontally opposed, or “flat” engines. there’s more room around the cylinders for airflow. In a Vee configuration, there are four sides of the engine exposed to air. With a VW flat-four, there are eight sides of the engine exposed to the air.

Why did automakers move to air cooling over water? It’s simple. No, really. That’s it. It’s simple. No coolant passages inside the block to cast. No water pump. No coolant lines. No radiator. It’s why many piston-engined aircraft used air cooling. No worrying about a water pump failure at 5,000 feet.

So why did automakers drop it? Well, it isn’t all that efficient. You need a big fan on the engine and plenty of airflow. And if you make more hp than an economy car you need to start adding massive oil coolers and more oil capacity. Since it’s really the oil that’s doing a lot of the cooling.

Nevertheless, Porsche persisted until 1998, when the 996 arrived and brought water cooling to the 911. Why did they finally switch? There were a handful of reasons thrown around at the time. Emissions, noise–air cooled engines are louder than water-cooled–and before the flood of Cayenne cash came in the company couldn’t afford to keep developing it.

So that’s how air-cooled engines work. The fins add airflow and the oil does the bulk of the work. They’re noisy and they’re dirty, and that’s why they’re pretty much gone.

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