Why No Air Cooled Car Engines?

An air cooled car engine has a few advantages; you save the weight of a radiator, water pump, coolant, etc. Plus, you never have to worry about cold temperatures and diluted antifreeze causing a block crack. However, there are currently no air-cooled engine cars sold in the USA. Automakers are always trying to reduce the weight and thus save gas-why wouldn’t a modern air-cooled engine design work?
The Czech auto firm Tatra had an all-aluminum V-8 engine in 1935, so it can be done. Have any recent attempts been made?

It can be done in theory but you are greatly overstating the benefits of such a design. The original Volkswagen Beetle was air cooled but it was also very simple. People still praise it for being easy to work on but that isn’t really a compliment compared to today’s car. You shouldn’t need to work on your car at all the vast majority of the time if you design it right in the first place. That is what modern Hondas, Toyotas and other reputable brands achieve routinely.

Water cooling can remove heat from the engine much more efficiently than air cooling. Even modest sedans today have power that outstrips something like an old Volkswagen Beetle by a huge margin and they are much more reliable.

The potential weight savings isn’t very important overall either. It does take some additional fuel to get a heavier vehicle up to speed but the overall difference isn’t as much as you may expect. The heavier vehicle also carries more momentum and will not require significantly more fuel when you are looking at a difference of a few hundred pounds or less. To put it another way, do you see your gas mileage plummet when you have someone riding in your passenger seat? The difference between air cooling and water cooling is usually less than that. Tractor trailers can weigh well over 50,000 pounds (about the same as 13 cars or SUV’s) and yet they still get about 6 miles per gallon on average. Weight and fuel economy don’t scale linearly.

The reason VW stopped selling the original Beetle in the U.S. was that they had too much difficulty making their air-cooled engine meet the newer U.S. environmental emissions laws.

One of the reasons is that air-cooled engines tend to have a wider normal operating temperature range than liquid-cooled engines. Your typical water and ethylene glycol mix draws heat away from the engine much more quickly and efficiently than air does, so a liquid system with a thermostat tends to keep the engine fairly close to a constant operating temperature.

Since wider operating temperatures are normal for an air-cooled engine, the parts have to be designed to expand and contract more. This means that when the engine is running a bit cooler, the gaps inside the engine are larger, and more oil seeps into the cylinders, causing emissions problems.

Another problem you run into is that at the higher engine temperatures inside an air-cooled engine, you end up with more nasty stuff in the exhaust. As the combustion temperature increases, you actually end up with fewer unburned hydrocarbons, which you would think would be a good thing, but the problem is that you end up with significantly higher emissions of oxygen nitrates like nitrous oxide, nitrous dioxide, etc. and these are very bad for the environment.

There are things that you can do to combat these negative effects, like running the engine rich to lower the combustion temperature and choking the exhaust with a bigger catalytic converter to try to deal with the nitrogen oxides, but this ends up negating the fuel savings that you originally got with the air-cooled engine by not wasting mechanical energy on a cooling system.

VW was the last manufacturer making an air-cooled car engine, and they dropped out of the U.S. market in 1980 and stopped all Beetle production in 2003.

Motorcycles are also switching to liquid-cooled engines. Most of the current air-cooled engines are older designs. The newer ones are mostly going liquid-cooled.

More recent than the Beetle which was manufactured until 2003, you mean?

I don’t know, maybe you need a bigger heat exchange surface with airflow vs fluid cooling, so there isn’t really a weight saving?

Wouldn’t oil-cooled motorcycle engines do given your conditions?

A european one would weigh in at nearly 100,000lbs but still achieve 10mpg (albeit with bigger gallons).

There used to be some air cooled truck engines (Magirus Deutz - video ) but they cannot reach the current standards on noise levels. The water jacket helps a lot with suppressing noise.

Had a VW Beetle during my university years. That was a sweet little car, but the disadvantages of its air cooling were noticeable. When trapped in a traffic jam in the heat of summer, I had to crank the heating fully up in order to prevent the engine from overheating. Sounds easy from a desk but is not so nice when trapped in a sunblasted tin box with no sufficient fan power (and, of course, no A/C). Also, the third cylinder, being the least cooled because of its position, has always been the Achilles’ heel of Beetle engines.

On top of that, gas consumption was really high - the amount it took to get 34 BHP moving, around 18 to 15 mpg, would nowadays be more than sufficient to run a heavy sedan. Same with oil. The expansion/shrinking as described by engineer_comp_geek above sounds like a very plausible explanation.

Opposing benefits? Well, it was really easy to repair and maintain because of the low number of engine parts and a low complexity. Today, I have to bring my car to the shop to have the lightbulbs changed… Nevertheless, I think this bit of additional driving does not use as much additional gas as my little Beetle needed constantly.

Oil isn’t as good a coolant as water/glycol; the specific heat is lower, meaning that for X amount of heat imparted to Y amount of oil or water/glycol, the temperature of the oil will increase more than the temperature of the water/glycol. So for effective cooling, you need to move more oil around more quickly. It’s been done, but it’s becoming more and more rare. For example, the boxer engine in BMW motorcycles was oil-cooled from about 1993 until 2012 (it was actually air- and oil-cooled; air provided most of the cooling, with a small heat exchanger for oil cooling to provide the small degree of controllability required to maintain a relatively steady operating temperature). Even that’s going away though: in 2014 they switched to a water-cooled boxer engine. As emissions regulations for motorcycles tighten up, air cooling will go away, and likely so will oil cooling.

The fins are typically aluminum, and they’re thin with substantial air gaps, so compared to the extra weight of casting coolant channels into the block, plus a water pump, plus water, plus a radiator, there may be some weight savings. It’s worth noting that a lot of aircraft engines are air-cooled. Note however that aircraft aren’t subject to the same stringent emissions regulations that passenger cars are. Engineer_comp_geek has hit on what is probably the biggest reason you won’t see air-cooled engines in future passenger cars: it’s impossible to control the emissions as tightly as needed. But the inability to tightly control operating temperature also means that compression ratio has to be limited, lest the engine start knocking - and limiting the compression ratio means limiting efficiency, a big no-no these days.

Along with the air cooling is the wider operating temperature range, as mentioned earlier. The problem with this is that the engine tolerances have to be looser, since the relative expansion range would be greater. (IANAM, but it seems the air cooled (possibly cooler) cylinders vs. uncooled pistons means a wider gap between piston and cylinder. This is OK for small aircraft engines that run generally at 75% or more of power all through the flight - but for a VW where it probably idles half the time, in anything from 100F to -30F, the tolerances are a lot bigger; this limits he life cycle. Fortunately, with bigger tolerances, it runs in almost any circumstances. (I suspect this latter issue is what causes the pollution problem - it can run with less than optimal fuel burning, resulting in poorer pollution performance?)

Reminds me of the old joke about how to identify VW Bug drivers.

Their ankles were burnt and their hands frost bitten.

Maybe I missed it but no one mentioned the Porsche 911? These were all air/oil cooled until 1998. IIRC they had to switch to water cooling to meet emissions standards.

There have been a number of rear and mid-engine air cooled designs. Already mentioned is the impossible task of keeping an engine running at a set temperature with air cooling.

Not just emissions, but European noise regulations, too. Beyond that, they had basically hit the limit of what could be done in terms of specific power outputs with the air-cooled M64 because (for technical reasons I don’t fully understand) they couldn’t add more than two valves per cylinder.

I drove a Citroen Dyane from 1975 to 78:


All of the snags of air-cooling mentioned above were present- cooling was barely adequate in summer, and the only sort of control was putting a bit of cloth over the air intake in winter. It was noisy, and the 602cc engine gave little performance. You could wind it up until the speedo needle passed 80 mph and disappeared, but overtaking was a fraught matter of alarm-clock and calendar.

Having said that it was a good little car for an impoverished student; remarkable fuel economy. But it was replaced by a water-cooled Renault 5 TS.

We have now moved on a little: http://www.autocar.co.uk/car-review/skoda/octavia-vrs

I had a 66 Bug. Fuel economy was great (got about 30 highway). But its top speed was literally something like 65 (with no wind or hills).

According to Wikipedia, your Bug had a 1.3-liter engine that was good for 50 horsepower. This implies operating at a high percentage of full load when cruising, which is a recipe for decent fuel economy in spite of the sloppy sealing and low compression ratio (7.1) required to cope with air cooling.

Close. I’m thinking it was more like 1200cc and 40ish HP.

Maybe even 1100cc. I do remember when rebuilding the engine that I had to pay EXTRA for the parts because it wasn’t the common size at the time.

People like to make fun of the old bugs…I know I do. But I rebuilt the engine over the weekend for a few hundred dollars and a handful of shitty tools and a few pages of some shitty manual.

That car has a carb that was smaller than some I’ve seen on weed wackers.

I own a 74 Super Beetle. I haven’t checked the mileage precisely, but it’s somewhere up around 30-ish. It does top out at about 65 mph, maybe 70 if you really push it. It would probably go a bit faster if it had a 5 speed transmission.

Years ago my dad had a friend who bragged that he could take the engine out of his Bug in 90 minutes. My dad’s response? “Big deal - I don’t have to.”

I drove an air-cooled Bug (model long-forgotten) around South Africa in 1978. No cooling problems, and I enjoyed the very light steering. :dubious:

I went off the breed when their scary handling was a major factor in the death of a man I much respected: