K.O.H.L.E.R COMMAND VTWIN CH750 based engine

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philr

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Oil coolers are stock on (all?) these V-twins. They do a good job of keeping the oil from breaking down and help with head cooling. To have a more substantial impact on CHTs (and, thus, continuous HP) would require a much larger flow of oil through the heads, to piston squirters, etc. Probably a high volume, low pressure oil pump would be needed in addition to the stock high pressure, low volume oil pump (plus a bigger heat exchanger, and air to feed it).

With 750cc, I think it will require "heroic measures" to achieve more than about 28 continuous HP (i.e no more than about 415F CHT, measured under the plug). Like most of the air cooled 4 strokes, heat rejection will be the factor that constrains the available continuous HP.
This is post #3 of this thread where I explain a potential modification for horse power gain " I forgot to state the displacement for the 2 options I am pursuing. 3.1" stroke = 852 cc and 3" stroke =824 cc. " I am not expecting to get much more out of 750cc but rather gain horsepower through added displacement with a 3" stroker crank and boring, resleeving the engine to larger bore. Other things that may add horsepower is new carbs and better airflow as the stock air cleaner set up is restrictive. Though the test for stated hp was done with no air cleaner installed so this is an investigation of what can be done with this engine to improve it from its stock configuration for aviation use.
 

philr

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Right. A smaller combustion chamber (including head and piston top) has more area relative to its contained volume than a larger combustion chamber would have. To the degree that HP correlates to displacement, the argument goes that a smaller combustion chamber has (relatively) more external surface area to get rid of heat. The counter-argument is that (relative to a larger displacement cylinder) the >inside< surface of the combustion chamber also has more area to take on heat (per unit displacement), and this >increases< cooling issues.

There's more on the "continuous HP is limited by heat rejection" discussion here, complete with a groovy table.

The thermal mass won't do anything for us in improving continuous HP. It's not even much use in short runs--as a practical matter the AL has very little heat capacity relative to the entire heat balance of the engine, it gets "used up" in a very little time and then we're staring at a CHT gauge as it goes into the red ...
The engine with stock piston crank setup has been run in a flying aircraft at over 5000 rpm ,using a speed reduction for the propeller, for take of power only and can reportedly hold that rpm for 3 minutes without over heating CHT does it stand to reason that adding displacement and running at lower rpm (3200-3600rpm) that it would generate more or less heat? I think piston speed is still slower with 3" stroke at 3600 rpm right?
 

Vigilant1

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The engine with stock piston crank setup has been run in a flying aircraft at over 5000 rpm ,using a speed reduction for the propeller, for take of power only and can reportedly hold that rpm for 3 minutes without over heating CHT does it stand to reason that adding displacement and running at lower rpm (3200-3600rpm) that it would generate more or less heat? I think piston speed is still slower with 3" stroke at 3600 rpm right?
Regarding the already flying Kohler 750 and PSRU: Do we know the power that engine is producing at 5000 rpm? Has it been fitted with a test club (either on the engine or the PSRU)? This would tell us a lot.
Piston speed/friction produces a negligible amount of heat. Almost all of the heat we need to get rid of comes from direct heat gain due to the burning of fuel. If we produce more mechanical HP (with more displacement, higher CR, better breathing, anything else) we also produce more heat because we are burning more fuel. If the existing CH750 heads are used with a bigger bore or longer stroke, they still have the same number/size of cooling fins as they had originally. Critically, the area of the head near the exhaust valve will see a higher heat intensity as we burn more fuel. The ability of aluminum to transfer heat is finite, and even increasing fin sizing and area has diminishing returns.
The situation with air cooled industrial engines in aircraft will probably be a lot like we've found with VW Type 1 engines. We increased the displacement from the stock engines a lot (about 50%). For continuous use, regardless of displacement, RPM, CR, etc, the continuous HP output is limited to about 70-75 HP due to the heat shedding ability of the cylinder heads (and this is with careful attention to air flow and baffling). These same engines produce over 200 HP on the street and track (without turbos or blowers, etc), where peak power demands are of short duration and the reliability isn't as critical.
The air cooled VW-based engines have proven to be very good, very economical, and very reliable aircraft engines IF their limitations (especially with regard to heat) are understood and accommodated. I suspect we'll find the same thing with industrial engines.
This is post #3 of this thread where I explain a potential modification for horse power gain " I forgot to state the displacement for the 2 options I am pursuing. 3.1" stroke = 852 cc and 3" stroke =824 cc. " I am not expecting to get much more out of 750cc but rather gain horsepower through added displacement with a 3" stroker crank and boring, resleeving the engine to larger bore. Other things that may add horsepower is new carbs and better airflow as the stock air cleaner set up is restrictive.
All these things can improve short term peak HP, but the engine"s continuous HP in acft use will still be limited by CHTs (actually, by the temp of the exhaust valve seat and the aluminum around it).
 
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philr

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Regarding the already flying Kohler 750 and PSRU: Do we know the power that engine is producing at 5000 rpm? Has it been fitted with a test club (either on the engine or the PSRU)? This would tell us a lot.
Piston speed/friction produces a negligible amount of heat. Almost all of the heat we need to get rid of comes from direct heat gain due to the burning of fuel. If we produce more mechanical HP (with more displacement, higher CR, better breathing, anything else) we also produce more heat because we are burning more fuel. If the existing CH750 heads are used with a bigger bore or longer stroke, they still have the same number/size of cooling fins as they had originally. Critically, the area of the head near the exhaust valve will see a higher heat intensity as we burn more fuel. The ability of aluminum to transfer heat is finite, and even increasing fin sizing and area has diminishing returns.
The situation with air cooled industrial engines in aircraft will probably be a lot like we've found with VW Type 1 engines. We increased the displacement from the stock engines a lot (about 50%). For continuous use, regardless of displacement, RPM, CR, etc, the continuous HP output is limited to about 70-75 HP due to the heat shedding ability of the cylinder heads (and this is with careful attention to air flow and baffling). These same engines produce over 200 HP on the street and track (without turbos or blowers, etc), where peak power demands are of short duration and the reliability isn't as critical.
The air cooled VW-based engines have proven to be very good, very economical, and very reliable aircraft engines IF their limitations (especially with regard to heat) are understood and accommodated. I suspect we'll find the same thing with industrial engines.

All these things can improve short term peak HP, but the engine"s continuous HP in acft use will still be limited by CHTs (actually, by the temp of the exhaust valve seat and the aluminum around it).
I have not been able to contact Ron Besse but here is a link to youtube videos of him showing the CH750 with his chain reduction drive and he says he has over 300 hours flying on his trike and he claims 5000 rpm and shows CHT for take off and demonstrates it with a static runup. There are two videos. Ron Besse 1 and Ron Besse 2
 

Vigilant1

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Rpm and cylinder head temp isn't enough to determine power made (and heat). Need to determine Brake Mean Effective Pressure (BMEP) with a dyno or something. A manifold pressure gauge might help.
+1. Calibrated clubs (aka "Eiffel Bar', "wood dyno", etc) are the gold standard, IMO, but even an accurate fuel consumption figure (actually measured) combined with the approx AFR from the O2 sensor Mr Besse mentioned would allow a fairly reasonable estimate of the HP.

Running that V-twin from a single carb: It would be interesting to see how his measured AFRs compared between the cylinders.
 
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philr

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+1. Calibrated clubs (aka "Eiffel Bar', "wood dyno", etc) are the gold standard, IMO, but even an accurate fuel consumption figure (actually measured) combined with the approx AFR from the O2 sensor Mr Besse mentioned would allow a fairly reasonable estimate of the HP.

Running that V-twin from a single carb: It would be interesting to see how his measured AFRs compared between the cylinders.
How can a accurate test be completed with a club since this is an air cooled engine which will depend on propwash for cooling?
 

Vigilant1

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How can a accurate test be completed with a club since this is an air cooled engine which will depend on propwash for cooling?
A gutsy electric fan can supply the needed air without much trouble. These engines are about 18" wide. The stock centrifugal fans on these engines use about 1 HP to move the required air into the baffling.
The distance between the heads and the crankshaft is about 10". At that radius, the prop isn't moving a lot of air, most of the cooling in flight will be due to airspeed, not prop blast. A beefy electric fan can generate the required breeze.

Or (cheaper), just leave the stock centrifugal fan in place and use standard fan equations to determine the power absorbed by the fan at the given RPM.

FWIW, we had a useful discussion on fans and on the cooling airflow requirements of airborne industrial engines starting here, and for a few pages after that.
 
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karmarepair

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+1. Calibrated clubs (aka "Eiffel Bar', "wood dyno", etc) are the gold standard
A couple of clubs in progress. The idea is to disable the governor and run these things full throttle. The RPM, the size, and a formula and you have the HP @ that RPM. Switch Clubs, rinse, repeat, soon you have a curve.
 

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philr

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A couple of clubs in progress. The idea is to disable the governor and run these things full throttle. The RPM, the size, and a formula and you have the HP @ that RPM. Switch Clubs, rinse, repeat, soon you have a curve.
Are you planning on using mostly a stock CH750? If not what are your plans?
 

llemon

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Right. A smaller combustion chamber (including head and piston top) has more area relative to its contained volume than a larger combustion chamber would have. To the degree that HP correlates to displacement, the argument goes that a smaller combustion chamber has (relatively) more external surface area to get rid of heat. The counter-argument is that (relative to a larger displacement cylinder) the >inside< surface of the combustion chamber also has more area to take on heat (per unit displacement), and this >increases< cooling issues.

There's more on the "continuous HP is limited by heat rejection" discussion here, complete with a groovy table.

The thermal mass won't do anything for us in improving continuous HP. It's not even much use in short runs--as a practical matter the AL has very little heat capacity relative to the entire heat balance of the engine, it gets "used up" in a very little time and then we're staring at a CHT gauge as it goes into the red ...
I've been doing a bit of statistical analysis on aircraft piston engines.

Here is Area/HP vs displacement, for lyc/con air cooled, direct drive NA engines fitted to aircraft.
engines area hp dis.png

And here is the same for a much larger selection of engines. Again all of them are air cooled, direct drive and naturally aspirated and fitted to production aircraft.
engine area hp dis lots.png

Area is in sqin calculated for bdc. Perhaps unsurprisingly, industrial engines are pretty much where you would expect.
industrial engs.png

Assuming an Area/HP ratio of 2.75, which is about the average for lyc/con/Ul/Jabiru, one might get ~32hp for a ~45cuin industrial. If VW conversions are taken as a guide then the Area/Hp ratio degrades to ~3, giving 30hp as realistic for a 45 industrial.
 

Vigilant1

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Thanks for that. Do you happen to recall the background on some of the outlying cases/dots?
 

llemon

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Thanks for that. Do you happen to recall the background on some of the outlying cases/dots?
I assume you mean that one way up high, pushing near 10?

That is the Lawrance A-3, opposed 2 cylinder of 150 displacement and a bore/stroke of ~4x6. It was put on non-flying navy trainers during WW1, then found its way into the surplus market and early light aircraft.

For the lycoming/continental chart, the outliers are things like the A-40. I'd assume those are just cases of the engines dying before being fully developed.
 

TFF

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When the A-40 was built, did they even use a slide rule to design it? Very much a design where they knew how to do it, but don’t know the depth of what it could take. That and metallurgy was still hit or miss. Simple hand poured castings. Machining tools were not as precise and easy to use. Today you could dump 15% of the weight off the bat. Without it where would airplanes be today?
 

BBerson

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The A-40 is about the same weight as a same size (2liter) VW conversion. (144 pounds from wiki)
Both have cast iron cylinders. The A-40 crank is light with no center main.
 

Tiger Tim

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the outliers are things like the A-40. I'd assume those are just cases of the engines dying before being fully developed.
The A40 went as far as it was going to go. The later single ignition models had 40hp, then the dual ignition raised that to... also 40hp. Heat rejection was long an issue on those engines, which is why there are large fins on everything that isn’t the crankcase and special head gaskets are required that are thermally conductive enough to keep heat moving out to the heads. Seriously, the very earliest A40s ran hot enough to damage and fail magnetos before they were able to identify the issue was the cylinder heads. Further to that, the single-piece crankcase meant there could be no centre main bearing so the crank shaft is loaded from the rear and runs on just two mains. I’ve heard the front main tends to wallow out but haven’t experienced it on any of ours yet.

Don’t get me wrong, the little A40 is cute as a button, historically significant, and almost exclusively attached to airplanes I want to fly. The thing is that when you go through the design and start addressing all the things that can be improved you end up with an A65, and look where that engine architecture has taken us.
 

blane.c

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The A40 went as far as it was going to go. The later single ignition models had 40hp, then the dual ignition raised that to... also 40hp. Heat rejection was long an issue on those engines, which is why there are large fins on everything that isn’t the crankcase and special head gaskets are required that are thermally conductive enough to keep heat moving out to the heads. Seriously, the very earliest A40s ran hot enough to damage and fail magnetos before they were able to identify the issue was the cylinder heads. Further to that, the single-piece crankcase meant there could be no centre main bearing so the crank shaft is loaded from the rear and runs on just two mains. I’ve heard the front main tends to wallow out but haven’t experienced it on any of ours yet.

Don’t get me wrong, the little A40 is cute as a button, historically significant, and almost exclusively attached to airplanes I want to fly. The thing is that when you go through the design and start addressing all the things that can be improved you end up with an A65, and look where that engine architecture has taken us.
Could part of the heat issue have been fuel? It was designed to run on 72 octane?
 
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