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Air Cooled VW and Modern Emissions

Air Cooled VW and Modern Emissions

Air Cooled VW and Modern Emissions

Conventional wisdom has it that VW and Porsche gave up on the air cooled engine because they couldn't meet stricter emission standards. Regardless of whether that's accurate, what would it take to adapt more recent strategies and technologies to clean up an 80 year old design? I'm not just talking about fuel injection, electronic ignition and catalytic converters. Early 80's VW air-cooled had these technologies, but supposedly still ran into an emission wall at some point. I like Vee Dubs and I'm curious to figure out just how close these engines could get to new car specs by applying out-of-the-box thinking. I'd like feedback on my logic and approach.

Air cooled VW engines run hotter than water cooled engines. This seems to be the first major hurdle. Among other things, preignition is more difficult to control. To compensate, VW kept compression ratios low and in the old days ran slightly rich. Neither is good from an emissions standpoint. The other problem with running hot is abysmal tolerances compared to modern designs. Things expand when they heat up, and in a AC engine moving parts are very "far" apart when cold. Lots of nooks and crannies for hydrocarbons to hide and opportunities for fuel to go where it shouldn't.

Possible solutions:
1. Aluminum cylinders. Available aftermarket for a small ransom, builders are able to design in much more precise tolerances for rings and pistons due to much better heat dissipation than cast iron.
2. Liquid propane port injection. To me, this seems like a good way to "beat the heat". LPG injected as a liquid would significantly lower the charge temperature. Combined with propane's higher octane rating should allow for higher compression. Propane supposedly doesn't deposit carbon on the combustion chamber, so no "hot spots". Provided you can control preignition, a higher compression ratio should theoretically reduce operating temperatures (specifically exhaust temperatures). Finally, as a "clean fuel" there's supposedly less to clean up to start with.
3. LPG would also all but eliminate evaporative emissions.
4. Increase the stroke from 69mm to 82mm. Faster moving piston should increase fuel mixing. The longer stroke could also potentially allow for more valve overlap to increase internal EGR.
5. It goes without saying: A closed loop fuel injection system, high output electronic ignition and 3 way cat. (I also understand there's room in the head for two spark plugs per cylinder.)
6. Other ideas: An electric motor (borrowed from a Neighborhood Electric Vehicle) to operate the cooling fan independent of engine speed. Use a start/stop system to eliminate idle emissions, or even a micro-hybrid like GM's eAssist to help with low-end torque allowing cam to be optimized for a narrower power band. (No variable valve timing coming any time soon to aftermarket...)

I know to get to modern tailpipe standards, the devil is in the details. Without the endless funds to sort all of them out that alone would probably keep a solitary engine builder from achieving modern standards. But with the strategies I've proposed (or with whatever else can be dreamed up) how close to new car emissions could Herbie get?

RE: Air Cooled VW and Modern Emissions

I have no supporting data, but my belief is that air cooled engines have much more of a problem with NOxs than hydrocarbons UNLESS the compression ratio is to low for modern levels of performance and fuel economy.

I'm not convinced that aluminium cylinders help as even though they would expand more than a cast iron cylinder thereby allowing tighter clearances, they also flex more under load, especially at higher temperatures, and even cast iron cylinders are prone to blowing out to barrel shape bores once you up the compression and lean on them. I do have data on that in the form of engines with lots of blow by and measurements with dial bore gauges.

Of course the ecological advantage of air cooled is the substantially lighter engine and cooling system and consequential potential flow through to chassis. It's always a complex mix of compromises.

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RE: Air Cooled VW and Modern Emissions

Most light airplanes use air-cooled engines. Not sure of their emissions - but there are a lot of them...

RE: Air Cooled VW and Modern Emissions

The VW engine is oil-cooled about as much as it is air-cooled. On a stock engine, this means several things:
-The #3 cylinder runs hot because the oil cooler blocks airflow to it. It is the cylinder that is usually detonation prone, and I believe that the timing to this cylinder is retarded from the factory.
-Some parts are just not cooled enough. Cylinder heads in particular run hot, reducing valve, guide, and seat life.
-Tolerances have to account for the fact that cooling for some engine parts is sub-optimal. I don't know for sure but I suspect that things like ring end-gaps are set loose to account for this.
-Insufficient and/or uneven cooling leads to other problems. For example, if the wrong cylinder head studs are used (ones that have insufficient thermal expansion), the threads pull out of the crankcase and the cylinder/head interfaces leak. This was a problem with factory head studs for a while.

What you'll probably find is that you'll have to make sure that your starting point is in mechanically good condition to begin with (a no-brainer, but things like head studs pulling out can sneak up on any of us). There are/were lots of aftermarket parts for these cars, and I don't know which ones are best for addressing any shortcomings your particular year engine may have. Assuming a mechanically sound engine with no undue wear, EFI and a three way catalyst will do the most good, getting the engine to at least mid-1980s emissions levels or possibly better. After that the law of diminishing returns will make additional improvements increasingly difficult to achieve.

EGR might help if you identify NOx emissions as a problem. Some later model buses and maybe Beetles, especially California models, had catalysts and EGR, but I don't know what years.

Propane/LPG will probably be an expensive nightmare, especially for port liquid injection. The few aftermarket systems I've seen (Impco, mostly) use a fuel mixer "carburetor" in the intake tract. Because you have to use a dedicated, expensive tank, and because propane is really best suited to high-compression engines, this is probably a no-go.

RE: Air Cooled VW and Modern Emissions

There are still a fair number of air cooled motorcycles that meet (admittedly modest) emissions requirements.  Most have fuel injection and cats but some are still carburetted.  Their performance is uniformly dismal compared to their water cooled brethren.  

While I don't know of any direct AC/WC comparisons, you can find examples of motorcycle companies that make engines of both types with similar displacement and basic layout.  But they always change bore stroke ratio or the number of valves, etc. so it's never a perfect comparison.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Air Cooled VW and Modern Emissions

VW did a water cooled boxer in the Kombi for a few years. It was loosely based on the air cooled engine and had considerably more power.

The #3 cylinder being hat was fixed by removing the stock oil cooler and fitting a large external cooler or by the Superbeetle dog box style cooler that was outside the fan shroud that supplied the cylinders.

I was able to get all four cylinders pretty even by experimenting with thermal sensitive crayons and slight tweaks to baffles in the shrouds.

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RE: Air Cooled VW and Modern Emissions

Thanks everyone. Some very astute cautions to heed. "Expensive nightmare" caught my eye smile

One engine builder with extensive experience with the aftermarket aluminum cylinders regularly runs .016 ring gaps or less. These are essentially the same piston/cylinder that Porsche used on much, much more powerful (and cleaner) engines. If Porsche got them sorted for 400+ hp, that's good enough for me.... Yes, aluminum has twice the thermal expansion of cast iron - it also has has at least 3-5 times the thermal conductivity. Also, someone please check me on this, if using aluminum pistons wouldn't the rate of expansion be the same, keeping tolerances in check?

I've had numerous people suggest good quench for fuel mixing and efficiency - but I too wondered if it could contribute to higher HC production. VW in the '70s increased deck to something like .120, presumably as one strategy to lower HC emissions.

NOx, if it's a problem, seems like it could be dealt with both with proper tuning, adding egr, and 3-way cat.

Blow by - yeah, not good. Better ring tolerances is one solution, propane is the other. I've heard that there's not much carbon blowing by with propane, engine oil comes out clean at 10,000 miles. Also, since I don't actually have to pass anything, I might kinda look the other way on that and focus on the tail pipe.....

RE: Air Cooled VW and Modern Emissions

The pistons will still probably expand more but the difference will be less. This is because the pistons normally get less cooling. It also depends on the alloy.

Hyperutectic alloys expand less but are not so strong. Modern OEM engines normally have hyperutectic pistons so they can run tight clearances presumably for both emissions and quiet running.

I did VWs over 30 years ago, so I don't remember the clearances.

I do remember the GOOD studs. It was not that they expanded more from heat. They went from10mm down to 8mm studs but put I think 12mm threaded steel inserts in the case. This gave a stronger thread, but a weaker bolt that stretched easier so it did not pull the threads.

I never had leaky head problems, but I never ran the factory gaskets either over or under the cylinders. I always made sure the deck of the crankcase was dead flay and parallel to the crank axis, that both bores in the head where to exactly the same depth so they where flat in line with each other and I hand lapped each cylinder into its position in the head and crankcase then used a thin film of Loctite 515 under the cylinders to avoid oil leaks. I used nothing between the head and cylinder other than a very good fit.

The idea with quench as I understand it is you leave it real wide so it has zero quench to reduce hydrocarbon emmissions, but once you get to an effective quench distance of less than about 0.100" piston to head, the tighter it is the less the hydrocarbons are as there is less volume containing the quenched charge. I ran VWs with 0.032" piston to head. You might get away with less with aluminium barrels. I don't know how much the studs limit their expansion.

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RE: Air Cooled VW and Modern Emissions

Any experiences with ceramic coatings? I've heard coating the top of the piston can make a significant difference in piston temperature. And supposedly doesn't cost a fortune (so I'm a little skeptical).

RE: Air Cooled VW and Modern Emissions

Re 8mm studs, that makes perfect sense. They aren't weaker as in more failure prone necessarily, but "stretchier". That is exactly what you want in almost any bolted joint, as long as the ultimate strength is adequate. The fastener acts as a spring. The lower the spring constant, the less variation in preload with thermal expansion/contraction of the parent material (head). Sorry if I'm preaching to the choir, but you left that a little open.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

RE: Air Cooled VW and Modern Emissions

blakeclark, One caution, an electric fan probably will not work as the sole cooling fan for a VW engine. Look up the VW fan's power requirement. Electric fans are very seldom rated at more than a fraction of a horsepower (1 hp > 50 amps at 14v) where the hp requirement for cooling any engine is on the order of a few percent of the output power. Furthermore, the fan has to be a pressure blower, not just an air mover. It is highly likely you will get overheating on a level road at highway speeds and will certainly overheat on a grade. Don't even think about sustained high power.
I think your list of measures will clean up the VW, but probably not to today's standards. Air cooled engines are not the only casualties of the current standards for emissions, economy and power. The Mazda rotary bit the dust too.

RE: Air Cooled VW and Modern Emissions


You probably get a bit of assistance from the draft created by vehicle motion. A rear engine air cooled VW gets no such assistance, well at least not unless you have an external oil cooler mounted in the draft.

Water cooled engines also get some relief from "thermal inertia"? due to the mass of water in the cooling system giving an ability to absorb short term peaks and cool them over a longer time. I know this does not really cover towing a large load up a mountain, but it still helps a little.

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RE: Air Cooled VW and Modern Emissions

GregLocock, As far as I know, NOBODY cools ANY auto engine electrically under steady-state conditions.
Radiator fans are augmented greatly or made superfluous by road airflow when the engine is making even moderate power and require shop fans to help them when the engine is undergoing dyno pulls for any length of time. And that's JUST the radiator fan. A much bigger job is done by the water pump. Only drag cars can get away with electric pumps during the trip back after running for a few seconds.
I reiterate, it requires several percent of the output power to cool an engine. So, a 250hp motor will require about 7-8hp for cooling under prolonged max power (if the total cooling system is highly efficient). It can get by with less if the duty cycle is less.
As mentioned, an air cooled engine requires a pressure blower that can push a decent volume of air through the very tight restriction that the engine imposes because it does not have the low restriction, open area of a radiator. A VW may need as much as 10 inches H2O pressure at high air flows. Radiator fans are usually rated at 0 inches and produce much lower flow even at 1 inch (if rated at 1 inch).

RE: Air Cooled VW and Modern Emissions

My thoughts on electric cooling is more along the lines of a micro-hybrid setup. A dedicated battery bank, 6hp continuous electric fan motor (borrowed from a neighborhood ev vehicle), and a high output alternator all running at 48V. Quite an elaborate system to run a fan to be sure, but no reason it couldn't work.

RE: Air Cooled VW and Modern Emissions

If I didn't make it clear, the electric motor would drive the stock cooling fan in its original location.

RE: Air Cooled VW and Modern Emissions

Beg to differ on cooling auto style engine in stationary applications. It becomes a matter of how much heat rejection the radiator has at a given forced airflow. This forced airflow can be zero if there is enough heat rejection in the radiator.

RE: Air Cooled VW and Modern Emissions


A little more research turned up a very interesting fact. The very late model mexican beetles (2000+) were imported in small numbers to Europe where they met Euro 3 emissions. Herbie would be proud.

RE: Air Cooled VW and Modern Emissions

blake, that setup will work fine, IMHO. You won't need the full 6 hp most of the time and the ability to closely control fan speed and temperature is a great asset. The fact that you are using the stock fan avoids the pitfall of trying to use a low pressure fan.

Fmangas, I agree that the amount of heat rejection both of the engine and of the radiator determines how much radiator fan you need. My eyes were opened when I noticed how small my Civic radiator was compared to my older Civic, and then, the fan rarely even came on. The engine was more thermally efficient and the whole system was obviously better.

Don't neglect to include the water pump in the calculation of cooling power. With an air cooled engine, it's all up to the fan, but with a water cooled engine, most of the power draw is in the pump.

RE: Air Cooled VW and Modern Emissions

Hi Blake

Before you could even contemplate putting a vehicle through an emissions test a certain amount of upfront work is required on an engine dyno, fitted with an emissions analyser, to ensure that the engine out emissions are full optimised. This in itself is a significant and costly task.

When the engine is in the car and emissions development has begun one of the biggest challenges, and arguably the most important part of the emission test, is the cold start and catalyst heating phase; with the most critical points being lighting the catalyst off to its operating temperature as quickly as possible. This would necessitate you fitting the catalysts as close as possible to the exhaust ports as possible and/or utilising Secondary Air Injection to ensure as much heat as possible is transferred to the front face of the cat as quickly as possible.

During the afterstart/warm up (open loop fuelling) phases, minimising engine out pollutant emissions becomes crucial since, with cold catalysts, all of these pass straight out of the exhaust and into the sample bag. To enable the closed loop control to happen as quickly as possible the lambda sensor is required to be lit off as quickly as possible. However, after the cold start there is a risk of water condensate thermally shocking the heated cell in the sensor, so full heating power is held off for a period called the Dewpoint Time. If you accepted that longevity will be compromised you could calibrate the heating control so that the sensor is heated as quickly as possible, allowing closed loop control sooner.

Once the catalysts are lit off, and the fuelling is closed loop, the next challenge is the fuelling control whilst the cycle is driven. The catalyst deals with the three pollutants, NOx, CO & HC by way of two reactions: Reduction (NOx - N2 + O2) and Oxidization (HC - H20 + CO2 +CO), it does this with either a deficit or excess of O2 in the feed gas. This is achieve by alternately 'dithering' the fuelling slightly rich (for reduction reactions) or slightly lean (for oxidization); the popular notion that the fuelling is constantly held at stoich. is slightly misleading. The calibration of this dither is very application specific.

One issue that you would find is that to know whether you were making improvements each of the above would need to be calibrated and checked over an emissions cycle, with bagged emissions and also second by second. Which would be pretty bloody expensive!

Also I don't believe that the fact that the engine is air cooled is an issue for emissions per se but more for OBD, which utilises coolant temp as an enable condition for so many diagnostics and coolant temp itself is fairly easy to diagnose.


RE: Air Cooled VW and Modern Emissions

Actually, I don't believe either Porsche or VW were forced to abandon air cooling because of emissions -although they might eventually have had to do that. The Beetle succumbed to more modern designs like the Rabbit and Sirocco. Porsche had insurmountable cooling challenges going to an air cooled 4-valve head.

RE: Air Cooled VW and Modern Emissions

I'm not convinced that airccooled engines died due to exhaust emissions- even Nox.
For good debate we should consider the last of the aircooled Porsche 911s rather than an antiquated VW beetle with it's carburation etc.
The biggest hurdle in meeting modern petrol emissions regs is the cold catalytic converter light off period without a doubt. Yes, the air cooled engine will have higher metal temperatures and therefore slightly higher combustion temperatures which will lead to higher Nox emissions, but by this time the engine is warm and a modern 3 way catalytic converter does a good job in mopping up the Nox, HCs and CO.
if you look at the Porsche M64 engine in the 993 alot of things lend themselves WELL to modern emissions:
Very short ceramically lined exhuast ports. This has been done obviously so that the cooling fins can cope with the thmeral heat rejection but a fortunate side effect is it rejects more heat to the catalysts which aids rapid cat warm up.

Porsche 911 engines have had to be quite savagely oversquare (100 mm bore, 76.4mm stroke) which leads to large ring lands and therefore high crevice volume. However you can;t cite this for a reason for its death as the replacement watercooled 996 engine is watercooled by sports the same oversquare bore-stroke dimensions.
One thing that is correct was that it was very difficult to remain aircooled and migrate to a 4 valve/cylinder layout. This meant that in order to get the air flow into the chamber it was beneficial to have a oversquare chamber for bigger valves. Qversquare chambers aren't as efficient in terms of surface to volume ratio but again this isn't a reason for this aircooled engines death.
Porsche did the right thing with this oversquare chamber by adopting a twin plug layout for faster burn and to migrate knock limit and they pushed the compression ratio to as high as they could (something that Chrysler didn't do initially with their twin plug Hemi).

I still believe a 3 valve layout (2 inlet 1 exh) would have been possible with the aircooled layout-specially if oil spraying was utlised. This would get you to almost as high as a 4 valve specific output.

I like aircooled engines- as when vehicles get old one of the potential sources of problems is the radiator/cooling system and with aircooling you have a more robust cooling system over time.

Aircooled engines do have a certainly noise associated with them- the fins can cause ringing which isn't always perceived as agreeable.
And being constrained to a 2 valve layout does restrict ultimate BMEP and BHp/litre.

In anyway, the 993 aircooled Porsche will remain one of my all time favourites and I have zero intention on 'upgrading' to a 996 or 997 type car.


Sideways To Victory!

RE: Air Cooled VW and Modern Emissions

Pat, getting rid of water is indeed the surest way to get rid of water leaks. And, cracking is rare in air cooled engines since the worst overheating catastrophe that can befall a liquid cooled engine, loss of coolant, can't happen. But overheating of a correctly functioning system can happen, especially to automotive air cooled engines since the cooling capacity usually has a relatively low absolute limit.

Marquis, your thoughts about a 3-valve head are interesting. IMVHO, the worst problem I've seen for cooling an automotive air cooled engine is getting adequate airflow through the engine at the horizontal fin stratum of the exhaust port seat and combustion chamber top. Having side-by-side ports (like VW and Corvair) reduces this flow by over 50% compared to having ports in line (like Porsche flat 6). Comparing the Corvair flat 6 with side-by-side ports to the Porsche, it appears that the temperature-limited hp potential (not the breathing-limited potential) of the Porsche is at least 40% greater and I would attribute most of the difference to the greater and better placed air flow.

There would be two ways to do a 3-valve arrangement, having both intakes side-by-side with the exhaust in their lee or having the two intakes in line with the exhaust to the side. In the first arrangement, the intakes can be separated to allow airflow to the exhaust. In the second arrangement the stagger between the two intakes and the exhaust could allow a generous path around the exhaust to the inside of the combustion chamber top.

RE: Air Cooled VW and Modern Emissions

"cracking is rare in air cooled engines"

I think owners and shops specializing in Type 1 V-dub and 8 and 9 bolt heads Triumph twin MC engines of the 60s and 70s might not use the word "rare" when describing the incidence of cracks forming between valve seats, and even spark plugs, even with stock engines in "normal" service.

same forum, different engine



I guess even big HOnda singles are not immune

RE: Air Cooled VW and Modern Emissions

Yes Tmoose, I thought about those cases. Too much concentration of heat in a small area. There are air cooled engines that can sustain softening of the metal without cracking. I think it is a design problem that is avoidable with air cooling.

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