VW Heads for Aircraft - Billet

[TiPi]

Funny you should say that, my next post (now this one) was going to address that very issue.

I'm going to digress here and give a little background that lays the basis for what I am going to describe.

What is our immediate goal? To get rid of waste heat. More specifically, to get rid of waste heat from areas where it can cause malfunction. For example, we really don't care, within reason, how hot our exhaust pipes get. They're usually steel or stainless and can stand the heat without losing their functionality. We are concerned about how hot the cylinder head, and more specifically some areas of the cylinder head, get. The areas of greatest concern are the EX valve, cracking between adjacent parts, for example valve seats that have different thermal profiles, EX valve seat and the EX valve guide. Taking these in order.

EX valves are not much of an issue in themselves anymore. A normally aspirated, gasoline burning engine is hard put to do any damage to a stainless steel or Inconel valve. You can hurt one over time but you almost have to try. Not to say that valve selection isn't important, it's just that if everything else is good, valves are the least areas of concern.

Cracking between valve seats or valve seats and spark plug holes is best taken care of by careful design and ensuring that the heat gradient between hotter and relatively cooler parts (EX seats and IN seats) is kept as low as possible.

EX valve seat and the EX valve guide; keeping these cool is the prerequisite for having a successful engine.

There are two ways of keeping the EX cool. The first, and the way most everyone focuses on, is to extract heat from these areas and discharge it to the air, either aluminum --> air with fins or aluminum --> water --> air with a radiator. The second way is to keep the heat away from those areas to start with. This is a design philosophy that the Japanese have used with great success.

After all that build up it's actually simple. Make the EX port as short as humanly possible. And then make it shorter. Get the hot gases into the exhaust system pipes as soon as possible. The longer the hot gases are in contact with the aluminum cylinder head the more they heat it up. Less contact, less heat build up , less heat to remove with our fins. This post is getting long and I still have more to cover but I can give examples of successful use of this design philosophy if anyone is interested.

Next, and this addresses wsimpso1 concerns about fins near EX ports; do away with the exhaust manifold flange. Thread the inside of the port and screw in an exhaust spigot. Triumph has been doing this with their 650 and 750 twins for decades. The EX manifold flange is a large mass of AL. It must be thick enough to provide threaded holes for the EX pipe and big enough around to provide an adequate sealing surface. By doing away with it you can cut fins right up to the outside of the EX port, exactly where they are needed. This also gets your fins much closer to the EX valve guide boss. Not many people that I have spoken with realize that this is the way that R4360 EX ports are made. The R4360 head has a threaded insert cast into the head and short EX spigots with square flanges screwed into the head. My belief is that they did it for the same reason, to put more fins closer to the EX port.

Attached is a kind of low res (didn't know what the rules were about pics) section view of an actual cylinder head designed and built for a customer. This engine is successfully meeting all of its' specs and is used in a research environment. The EX port is in red. The reason it looks bigger than the IN port is that this is really a 3 valve engine and only one IN valve is shown. To put the size in perspective, the cam bearing bore at the top is approx 1.0 inch (25mm). The recessed area at the right hand end of the EX port is the area where the exhaust spigot is threaded.

View attachment 121218

BMW used the reverse of this idea on their air heads: Externally threaded spigot with an alu nut with fins. The exhaust pipe can still reach inside the head as far as the port is straight and the finned nut adds a bit of cooling. If you use a liberal amount of a high-temp antiseize, removal was no problem.

 

[Vigilant1]

Whichever method that is less likely to leave an unwanted bit of galled, rusted, broken-off steel exhaust pipe/collar inside the AL head would have a distinct advantage, it seems to me.

 

[TiPi]

Whichever method that is less likely to leave an unwanted bit of galled, rusted, broken-off steel exhaust pipe/collar inside the AL head would have a distinct advantage, it seems to me.

The BMW design has the advantage of being able to cut the nut if required. The steel pipe should have 0.5 to 1mm of clearence to reduce the heat transfer into the head anyway, so should be easy enough to pull out.
Rotax recommends the removal of the exhaust pipe from the head and clean/re-lube the contact area at the 100h/annual so that could also be applied to keep it clean and removable.

 

[Vigilant1]

The BMW design has the advantage of being able to cut the nut if required. The steel pipe should have 0.5 to 1mm of clearence to reduce the heat transfer into the head anyway, so should be easy enough to pull out.

Is there a seal of some type at the point where the steel pipe terminates inside the head? Does the steel pipe rest inside a steel seat or shoulder pressed into the AL head, or something similar?
That part of the AL head surrounding the pipe would be a good candidate for a ceramic reflective coating, especially if it's not exposed to sooty exhaust gases.
Thanks.

 

[rmeyers]

Is there a seal of some type at the point where the steel pipe terminates inside the head? Does the steel pipe rest inside a steel seat or shoulder pressed into the AL head, or something similar?
That part of the AL head surrounding the pipe would be a good candidate for a ceramic reflective coating, especially if it's not exposed to sooty exhaust gases.
Thanks.

The steel pipe (I actually always use stainless) is just screwed in until the threads run out. Since the threads are milled in the head and on the pipe you have full depth threads all the way to the bottom, since it's not a tap, there are no lead in threads. Just like TiPi said, we use high temp anti-seize. This system has been used on turboed engines with no leaks.

 

[rmeyers]

Is there a seal of some type at the point where the steel pipe terminates inside the head? Does the steel pipe rest inside a steel seat or shoulder pressed into the AL head, or something similar?
That part of the AL head surrounding the pipe would be a good candidate for a ceramic reflective coating, especially if it's not exposed to sooty exhaust gases.
Thanks.

We use an air gap of at least .125 in between the pipe and the head material. Ceramic reflective coating sounds like a great idea.

 

[Vigilant1]

If starting with a blank sheet and assuming these are single cyl heads for aircraft use on a flat twin, is it better to assume the cooling airflow will start at the end of the head where the stock exhaust port is, and that it will flow aft? Or, should we assume the flow starts "on top" and exits the fins underneath (where the pushrods are)?
For heads to be used on a 4 cyl Type 1, a "flow through" from top to bottom probably makes the most sense.

The biggest difference between the two approaches would be in the orientation of the fins covering the crown of the combustion chamber (under the rocker gallery). If the airflow goes straight aft, we don't need any cooling through-holes between the exhaust port and the intake port, just keep the air (and heat) moving through straight fins/channels from the hot side to the cooler side.

If we want one common design to be used on both 4cyl and 2 cyl engines, airflow channels between the ports (as on OEM heads) would be a must.

 

[TiPi]

To look at an example of the cooling complexities (and variations of installation), look at the evolution of the Jabiru heads. They have tried all sorts of different variations of fin thickness/spacing, size and shape. Many lessons can be learnt from them. The Gen 4 head/liner assy seems to have fixed some of the issues but they are still struggling with piston failures and other problems.

 

[rmeyers]

If starting with a blank sheet and assuming these are single cyl heads for aircraft use on a flat twin, is it better to assume the cooling airflow will start at the end of the head where the stock exhaust port is, and that it will flow aft? Or, should we assume the flow starts "on top" and exits the fins underneath (where the pushrods are)?
...
If we want one common design to be used on both 4cyl and 2 cyl engines, airflow channels between the ports (as on OEM heads) would be a must.

I can't answer that, I have no firm knowledge of what Hot Wings thinks is the most useful. My opinion, and it's just that, is that I agree with you, make the heads so that they can be used on either a 2 or 4 cylinder engine.

 

[Hot Wings]

what Hot Wings thinks is the most useful

What Hot Wings thinks seems to change day by day. His thinking is influenced by what he personally would like and limited by his experience and knowledge base.
A complete new kit based engine developed following the same "use existing parts" philosophy as the O-100, but based on more available VW and other mass produced automotive parts, is way beyond his allotted finances and time.

Don't let his limitations constrain others.

 

[JP Straley]

There are lots of lessons from the Suzuki bikes of the 1980s and early 90s. They raced four-valve bikes using air-oil cooling. The engines had two oil pumps, the secondary had high flow and low pressure, primarily cooled the heads through a large oil cooler. Finning was what I considered modest.

"R" mentions short exhaust ports. Good call, this is what Harley does on their newer big twins. Ceramic coatings on exh ports also helpful.

Third, consider the exh valve seats. Powder-metal seats by far the best choice. See "Dura-Bond" company products. Some of their seats have temp expansion coefficients higher than the aluminum cylinder head metal. Constant contact with the head, so constant heat transfer. (A seat loose in the pocket overheats immediately with failure soon too follow. Yikes, I've seen this!)

JPS

 

[Chucker]

& to state the obvious re: definition of thermodynamic efficiency:
they waste more heat = run hotter for the same HP.

Contrary to the design initiatve on the face of it.

smt

Although flatheads lose volumetric efficiency at higher RPM's I don't see any reason that they would create more waste heat. Efficient fuel burn in a flathead is challenging due to the shape of the combustion chamber. However, the fuel burned does as much work as it would in any other chamber. The valves are closed when the fuel burns. Waste heat is a function of friction and the length of the stroke. If you had zero friction and a long enough stroke, there would be no waste heat.

At the speed we want to turn our engines/propellers, the D-Motor is putting out some pretty impressive numbers with excellent fuel economy.

D Motor - GTLAC

LF26 4 cylinder 2700 cc liquid cooled engine COMPACT By using the flathead engine instead...

www.g-tlac.com

 

[wsimpso1]

Although flatheads lose volumetric efficiency at higher RPM's I don't see any reason that they would create more waste heat. Efficient fuel burn in a flathead is challenging due to the shape of the combustion chamber. However, the fuel burned does as much work as it would in any other chamber. The valves are closed when the fuel burns. Waste heat is a function of friction and the length of the stroke. If you had zero friction and a long enough stroke, there would be no waste heat.

Flathead engines, sigh. They breath less well, combustion chamber shape is lousy for ignition and propagation, they have more combustion chamber area per unit volume, etc. How does all of this relate to thermodynamic efficiency?

They breath poorly, so you mix more exhaust gases in with the incoming charge and we do not get as much incoming charge. This dilution of charge and lower amount of O2 and fuel means lower pressures for less power and it means lower peak and cyclic temperatures. Tsource/Tsink being critical to thermodynamic efficiency, lower temps absolutely means lower efficiency;

Then the bigger area exposed to the hot gases means more heat is lost to the combustion chamber wall, which is also lost thermodynamic efficiency. More area in a flat head? Yes. At firing in an OHV engine, there is the piston crown, a tiny sliver of cylinder wall, the valve crowns, and the remaining area of the head after the valves are subtracted. On a flat head, you still have the piston crown, cylinder wall, valve crowns, but then you have the entire area of the piston and the valves again. Heat goes out the head and that wasted heat then subtracted from energy available to drive the piston down.

Yes, there is wasted heat in the friction, both from the pistons sliding in the bores and shafts running in their bearings, but if you got rid of all that friction, you would only make a modest improvement in efficiency. If you do not believe it, run the numbers on energy in pistons scraping and journals running, and then compare it to power. One of the interesting events in talking an engine lab class is running the integral of the P vs t curves and comparing it to the power produced. When the engine is running unrestricted, P vs t power and measured output power are pretty close to one another without even worrying over removing bore and bearing losses.

I suspect we are missing the huge amount of waste heat in a piston engine. Exhaust gases carry away a bunch of energy - even after expansion allows us to extract a bunch of energy, these gases are more than 1000 F hotter than ambient temps. Capturing that power is doable - turbo chargers make it into boosted manifold pressures, and turbo compound engines were a thing for a while, but they inherently have pretty high losses there too.

At the speed we want to turn our engines/propellers, the D-Motor is putting out some pretty impressive numbers with excellent fuel economy.

Fix that to say "for the weight and fuel burn, the D-Motor is ... " and we agree. They traded away some efficiency in exchange for a lighter engine and it may make sense for some customers.

 

[rmeyers]

Hate to hijack my own thread, but Oh heck yes to everything Billski said above. Miserable efficiency.

I'm actually fascinated by flatheads. They have several advantages that may make them desirable for aircraft use.

1. Potential for significantly reduced complexity. I'm looking at you valvetrain!
2. Potential for reduced weight through careful design.
3. Potential for a physically smaller engine.
4. Potential for a reduced cost engine. That is, can reduced complexity translate into reduced cost with small production numbers?

These things are all advantageous for aircraft use. Notice that I prefaced all of the 'advantages' with the word 'potential'.

Now to the disadvantages:
1. Miserable thermal efficiency
2. Miserable thermal efficiency
3. Miserable thermal efficiency (and did I mention miserable thermal efficiency?)

I hear stories all of the time about how somebody's Dad's flathead Ford in the 50's used hardly any gas. Well that's because it made hardly any power.

One thing that I have wanted to try is using a full ceramic head on a side valve engine. Seems like it could be a good use of ceramic. No moving parts, just a slab. Would take really careful design and a lot of small fasteners to keep unit loading/pressure down to acceptable levels, but I think that it could be done. Don't get me wrong, it would help thermal efficiency slightly but still nowhere near overhead valve levels. Just maybe fuel efficiency would't be quite so bad.

All that being said, there may be a small market for an engine like that.

 

[Pops]

liked the design of Revmaster's 3000cc engine.

 

[Martti Mattila]

{re: Phlatties):

Besides intrinsically reducing the thermodynamic efficiencies/(HP) at a given displacement, I'm pretty sure a re-design to flat head/sidevalve design would be far more complex than a pushrod overhead valve replacement head for an existing motor. You've simply moved the thermal issues down to the cylinder, to an area which is much more sensitive to distortion due to asymmetry & often a generally more restricted/limited area for heat rejection

Especially if there is a preference to use aluminum for the cylinder. (has it even been tried successfully?) With CI cylinders, there is still the matter of a cast design, in which the bores stay suitably round & straight, at both ends of the scale from cold to running temperature.

I'm starting to wonder if going the opposite direction would not be simpler & more productive overall, if the merits of re-locating the intake and exhaust ports are strong enough. For an opposed twin, there seems little merit to other than *perhaps* better finning, considering a VW based design. Just add some better finning and leave it alone.

For the 4, all things considered and re-locating the ports, an OHC design starts to look like the easy solution.
Weight would increase by aprox one extra camshaft & a gear (or belt) train. A belt design might almost be offset by the reduced casting material for bosses and the parts including rubber & metal, of the pushrod system.

smt

Belgian D motor has tried it and I would take that motor in any day if I had the money, but you bring up many notable issues and a word of reason.

 

[Martti Mattila]

If starting with a blank sheet and assuming these are single cyl heads for aircraft use on a flat twin, is it better to assume the cooling airflow will start at the end of the head where the stock exhaust port is, and that it will flow aft? Or, should we assume the flow starts "on top" and exits the fins underneath (where the pushrods are)?
For heads to be used on a 4 cyl Type 1, a "flow through" from top to bottom probably makes the most sense.

The biggest difference between the two approaches would be in the orientation of the fins covering the crown of the combustion chamber (under the rocker gallery). If the airflow goes straight aft, we don't need any cooling through-holes between the exhaust port and the intake port, just keep the air (and heat) moving through straight fins/channels from the hot side to the cooler side.

If we want one common design to be used on both 4cyl and 2 cyl engines, airflow channels between the ports (as on OEM heads) would be a must.

I have worked with Air Conditioning for long time and there I have learned how the heat exchange works most efficently, air should med first cooler parts and continue to flow towards hotter parts and by that means got hotter all the way. Now it would be very bad to bring the air in the hottest spot first and after that air dont take any heat away because it is all ready hotter than surface it passes. Still most of the motorcycles do just that. I will re route the cooling air in my NSU 1200 project according to my AC. knowledge. It might cause some heat unbalance and distortion but as a heat exhanger it should work better, btw. NSU has steel inserts in their long ex. ports. Kids who raced in Finland took them off from causing restriction to flow.

 

[Vigilant1]

I have worked with Air Conditioning for long time and there I have learned how the heat exchange works most efficently, air should med first cooler parts and continue to flow towards hotter parts and by that means got hotter all the way. Now it would be very bad to bring the air in the hottest spot first and after that air dont take any heat away because it is all ready hotter than surface it passes. Still most of the motorcycles do just that. I will re route the cooling air in my NSU 1200 project according to my AC. knowledge. It might cause some heat unbalance and distortion but as a heat exhanger it should work better, btw. NSU has steel inserts in their long ex. ports. Kids who raced in Finland took them off from causing restriction to flow.

Some observations:
1) Our primary goal in this case is to prevent the hottest part of the head (the area around the exhaust valve seat) from getting so hot that the strength of the aluminum is compromised. A secondary goal is to help equalize temperatures between the hot side of the head (exhaust) and the cool side (intake). This reduces thermal stresses and cracking. Both of these goals are best achieved by introducing the cooling air to the hot side of the head first and flowing that air over the cooler part of the head later. In the case of an air cooled cylinder head under typical conditions, the air exiting from the baffling after passing over the head will still remain >much< cooler than the coolest part of the head, so heat exchange is still occuring.

2) What you've written is accurate, and would apply IF our goal was to remove as much heat from the head as possible. Your way would result in the air leaving the engine having the highest temperature. But, that's not the goal (see above).

 

[Martti Mattila]

Thanks to remarks, maybe I must silence AC. engineer in me. I weight some cyl. heads in my shop, BMW R 100 2,85 kilograms, Citroen Visa 652 2,8 kg. with rockers, VW type 4 6,5 kilos full head, type 1 5,2 kg. full head. VW type four exhaust goes downwards like real aircraft engines, witch remains me that airflow in Lyc. or Cont. is like the bad one, but we use fuel to cool them. VW type 4 is famous to drop exh. valve guides or seats.

 

[Martti Mattila]

Twenty five years ago there was side valve twins developed in U.S. so one in Finland got idea to make side valve cylinders to VW based twin. He made cylinders with ports from billed aluminium and for heads he made plywood casting plug and pros made casting by that. He hope to gain weight savings and a narrower engine with that design but those billed cylinders build up weight and he also got idea that he should make cylinders same way as this side valve radial what plans were offered that time. Also project would need some money to put valve stems etc. in place so he rest the project. Now he has tooling to make all the work by him self but no time. You can see ther`es not much space for valve springs and valve cap adjusting must be done by grinding. Cooling fins are made with circular saw. Cylinder liners and pistons were from Ford, liners from Transit and pistons from Mustang.