Air Cooled Water jacket engine ?

[Pilot-34]

I would like a inline six.
Would there be a advantage to a water jacket six with cooling fins built into the block ?
I’m thinking with a water jacket would help eliminate hotspots and even the heat distribution.
Fins of course would disperse heat like in any air cooled engine.
Would a radiator even be needed ?

 

[Dan Thomas]

The deHavilland Gipsy Queen inverted inline six-cylinder aircraft engine from the 1930s:



Look at the density of finning on those cylinders and heads. And then consider that your auto-engine conversion will be, most likely, a cast-iron affair that relies extensively for its structural strength on the water jackets and that cast iron is very difficult to weld (I've done lots of it). The Gipsy cylinders were machined steel, heads cast aluminum. Some really old heads were cast bronze.

Another drawback to large inline engines is the weight of the crankshaft and crankcase as these have to be fairly massive to maintain rigidity. It's one reason we don't see the old straight-eights in cars and haven't for nearly 70 years. Straight sixes are heavy enough. When I was young I considered using a Toyota Land Cruiser inline six, as it seemed to be the lightest for its power and a direct-drive setup might work. But it would weigh more than twice as much as the deHavilland Gispy Major inverted inline four-cylinder I was flying behind at the time, in an Auster MK6. The weight was unacceptable.

An Auster:

 

[WonderousMountain]

It might be possible to have a water-jacket-radiator, but my advice would be to circulate the coolant with some pump. Also, you may want to switch in high heat pistons, and aftermarket sleeve.
With all that, and an aluminum block (better heat rejection), You could potentially have a winner. Note: You'll also need an Oil-Jacket-radiator.

 

[rv6ejguy]

Some modern straight six BMW engines are just over 300 pounds dry for the long block. Some versions produce over 500hp from the factory. Technology has progressed somewhat since the '30s.

 

[Pilot-34]

Yes I was thinking newer tech, steel cylinder liners, aluminum block, rods etc. Lots of fins , think motorcycle style and of course a pump to circulate the coolant within the block or even to a radiator if needed .
Whatever , however would be most efficient or adaptable,

 

[TFF]

I can’t see it helping, and being a nightmare to cast and machine. AN average watercooled engine is running half the temperature an aircooled one does without all that.

 

[Niels]

Some of the nicest german mopeds called SOLO used this system for years .Standard power was 2.1 hp from 50ccm 0r 40 horsepower per littre

SOLO monkey

 

[Pilot-34]

I can’t see it helping, and being a nightmare to cast and machine. AN average watercooled engine is running half the temperature an aircooled one does without all that.

Really?
You don’t see any advantages to a liquid cooled engine ?
Why would it be difficult to cast and machine ?

 

[Dan Thomas]

Technology has progressed somewhat since the '30s.

I posted the Gipsy Queen to show that it had already been done and that it ain't done anymore for some good reasons. A modern, lightwight inline six would work, alright, but it would need a PSRU to get the HP out of it, and creating one that is strong and safe and light isn't easy.

 

[wsimpso1]

Would there be a advantage to a water jacket six with cooling fins built into the block ?
I’m thinking with a water jacket would help eliminate hotspots and even the heat distribution.
Fins of course would disperse heat like in any air cooled engine.
Would a radiator even be needed ?

I think you are not talking about the sort of engines we currently have. We have:

• Air cooled engines - The oil is cooled using a heat exchanger while the cylinders and heads run at 300-450 F and transfer a bunch of engine heat directly to the air. Because the difference in temperature between heads and ambient air is so large, the amount of finning we can fit around the heads is enough while keeping a fairly compact package and staying below creep temperature of the head material;
• Water cooled engines - The oil is cooled using a heat exchanger while the coolant is circulated through the cylinders and heads and then to a radiator where it is washed over with ambient air. The temperature difference between radiator and ambient air is much smaller than for the air cooled engine, so the radiator is commensurately bigger. Modern radiators pack a lot of fin area in and very high heat conduction, but the limitation of lower temp differences is still there.

I think what you are proposing is to put a water jacket between the combustion chamber surfaces and the air cooling fins. The fin area and air handled would have to be at least triple in order to move the same amount of heat rejected overboard. And this is assuming you can move as much heat from the combustion chamber structures at the same temperatures on the combustion chamber walls. You would also have to stir the water (forced convection) in the jacket to make it move heat well. If you rely on natural convection, you will not move enough heat from the combustion chamber walls until temperatures get much higher than in the current types. So, the fin areas would go way up, you would have more weight, more volume, have to move more air to achieve cooling - sounds like a poor scheme.

To achieve cooling with minimum package weight and size we have known for a long time that we need to minimize thermal resistance heat source and heat sink. In an air cooled engine, the path to the fins has to be short, the fins large, the material of high thermal conductivity, and the air well managed. In water cooled engines, the path to the water has to be short, the water moving quickly and turned over frequently, the radiators of low thermal resistance and high air handling, and the air through it well managed. To place water around the cylinders only to put fins around the outside is raising the heat path length and increasing thermal resistance, which either forces much larger cooling fin areas than for conventional air cooled engine or raises combustion chamber wall temperatures. Both bad. This is why you do not see this scheme in the market.

Billski

 

[rv6ejguy]

I posted the Gipsy Queen to show that it had already been done and that it ain't done anymore for some good reasons. A modern, lightwight inline six would work, alright, but it would need a PSRU to get the HP out of it, and creating one that is strong and safe and light isn't easy.

The BMW S58 produces 280hp at 2700 rpm so you could do that direct drive with a simple and light bearing housing to take the prop loads.

 

[rv6ejguy]

Interestingly in flight testing by Dan Horton with his IO-390 and my Subaru EJ22T, the results show that the radiator is much more efficient per unit volume than typical air cooled engine finning despite about half the Delta T. The maximum effectiveness on the air cooled installation was only about 68% and these outlet temps were skewed by some heating of the discharge air by the exhaust system so effectiveness is somewhat lower than that in reality. The Subaru installation showed a peak effectiveness of 92%.

Russell Sherwood's 230HP Subaru powered Glasair cools well with similar or less inlet area than most similar HP Lycoming powered aircraft.

The radiator has much higher thermal gradients at the tube and fin junction than an engine air cooling fin by virtue of the thinner sections involved. There is much more fin area per unit volume. Many air cooled engines also have steel barrels which have poor rates of heat transfer in this area.

The pressure drop across the rad is also less than the air cooled engine. Part of this is to do with the straight flow path in the rad vs. the 180 turn required on most opposed air cooled engines. The thick cast fins on most Lycoming and Continentals are not nearly as good as the sawed fins on the high powered radial engines of the '40s and '50s.

 

[wsimpso1]

Ross points out what I alluded to. Modern radiators are very good while aircooled cylinders and heads have serious limitations.

The big issue with making a finned head cool is that while the root end of each fin is at head temperature, the fin temperature gets lower and lower as you go further out the fin. Much of its area does not have the high temp differences implied by simply looking at CHT. Well, unless the fins are simply too small, and then it is game over anyway.

In a modern radiator, the tubes are small and virtually the same temperature everywhere. The cooling fins between tubes are short and heated by tubes at both ends, so they tend to be pretty hot everywhere too, so while the temp difference between fins and air looks more modest, they are pretty good. Add in modern design and manufacture plus good ducting and they can be substantially more efficient than aircooling. Or if you use old style radiators and oblique diffusers, they can be a lot worse.

None of this changes the fact that a water moderated system with cooling fins will require roughly three times the fin area of a conventional air cooled system because the delta T between base of the fin and cooling air is about a third of what it would be conventionally. This is assuming you can actually fit the larger fins in and also assuming that you do not get to the point of the system becoming an insulator instead of a cooler, which also can happen...

Billski

 

[Pilot-34]

I think you are focusing on the minor and missing the major .
What are the advantages of a liquid cooled engine?
What are the disadvantages?
Same with the air cooled ?
Could we get most of The advantages of both ?
Perhaps we need to use a different liquid ?

 

[TFF]

I see plenty of advantages to liquid cooled. So much advantage, that having anything air cooled except the radiator as a disadvantage. Cooling fins on a watercooled engine is a stop gap and a nightmare to cast.

 

[Pilot-34]

You keep saying it would be a nightmare to cast.
Why ?
Why would they be a stop gap

 

[bmcj]

The big issue with making a finned head cool is that while the root end of each fin is at head temperature, the fin temperature gets lower and lower as you go further out the fin.

But that begs the question, are they cooler further out because of distance or because they are dissipating heat better out there where the air is cooler (hasn’t been heated by a longer trip through the fins)? Of course, another takeaway from that line of thought is that shorter fins Are lighter and they work better because the cool air gets in closer to the source of the heat where the delta-T can be maintained at a higher level.

 

[rv6ejguy]

But that begs the question, are they cooler further out because of distance or because they are dissipating heat better out there where the air is cooler (hasn’t been heated by a longer trip through the fins)? Of course, another takeaway from that line of thought is that shorter fins Are lighter and they work better because the cool air gets in closer to the source of the heat where the delta-T can be maintained at a higher level.

Yes, the air is heated more as it moves down the fin towards the head which is hottest so that not ideal as far as Delta T and heat transfer goes but all you can do is add more and/or taller fins. The finning on the big Wright and Pratt radials is amazing and necessary given their specific output which is over one hp/ cubic inch in some cases. That would be 360 hp for a Lyc. 360. I don't think it would survive long in the climb at that output.

The power density and specific power capability of liquid cooled engines is far superior to air cooled. My lowly Subaru puts out 1.2hp/ cubic inch and has no problem sustaining that indefinitely, even in the climb. An air cooled engine would be seriously challenged there.

There are advantages and disadvantages to air and liquid cooling but both can work really well if optimized.

 

[BBerson]

Need liquid convection direct to the fins like heat pipes or sodium filled exhaust valves.

 

[pictsidhe]

But that begs the question, are they cooler further out because of distance or because they are dissipating heat better out there where the air is cooler (hasn’t been heated by a longer trip through the fins)? Of course, another takeaway from that line of thought is that shorter fins Are lighter and they work better because the cool air gets in closer to the source of the heat where the delta-T can be maintained at a higher level.

Thermal resistance causes the temp drop. It's a design challenge balancing air flow, pressure drop, fin area and thermal resistance. Water cooling is able to move heat from the engine to whatever size fins you want with a minimal temperature drop. Better cooling with less drag is possible with water cooling. The downside is added complexity.