Air Cooled VW and Modern Emissions 1

[blakeclark]

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?

 

[patprimmer]

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.

Regards
Pat
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[MiketheEngineer]

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

 

[crerus75]

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.

 

[dgallup]

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.

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[patprimmer]

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.

Regards
Pat
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[blakeclark]

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

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.....

 

[patprimmer]

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.

Regards
Pat
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[blakeclark]

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).

 

[Lou Scannon]

Pat,
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

 

[patprimmer]

Hemi

Yes I did. Thanks for fixing it.

Regards
Pat
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[140Airpower]

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.

 

[GregLocock]

Um, we cool our 250 hp petrol engine with a sinlge elctric fan. I'm sure a bazillion VW Golf owners will be scratching their heads as well.

Cheers

Greg Locock


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[patprimmer]

Greg

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.

Regards
Pat
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[140Airpower]

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).

 

[blakeclark]

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.

 

[blakeclark]

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

 

[Fmangas]

140airpower
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.

 

[blakeclark]

Update:

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.

 

[140Airpower]

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.