High compression at altitude

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7/17/2022

Ahoy, Homebuilt Aircraft & Kit Plane Forum, below you will find a dialogue between myself, and the smartest engineer who I have ever known. (Name redacted). He is the gifted designer of airplane engines, PSRU’s, and a lot of other projects. Please read my bottom discussion first. He ‘anticipated’ my idea.





Hello Bob.



RE the engine compression, I have done exactly what you described here, and well before electronic engine

controls, when a pilot could be relied upon to manage the MAP correctly.

And it did provide an increase in both power and efficiency at altitude.

One can even apply a bit more than the 21" limit depending on the OAT.

And I actually COULD achieve a 14:1 static CR in my O-200-R

Hope you stay well.

Best;



Ahoy, ****

A few thoughts on naturally aspirated airplane engines at altitude:



An airplane ‘likes’ to cruise at a greater altitude than sea level, say 10,000 feet altitude without supplemental pilot oxygen , which Is 20.6” mercury atmospheric pressure.



What if one increases the compression ratio such that (with modern control of mixture, timing), you have the same detonation margins at WOT/10,000” as the original engine has at sea level. A (defeatable) sea level throttle stop at 20.6 “ map ‘should’ see no detonation at sea level, but with the increased compression ratio, a not serious loss of power. (????)



WOT at 10,000 feet should see less loss of power with the high compression engine than the standard engine. I looked at some ‘standard’ hp loss with altitude formulas, but they don’t seem at all applicable in this case. The amount of O2 in the air this altitude is less, so there will be a power loss, but, how-much???

I suggest that such an engine mod, if successful and of benefit, would be simple, and quite easy to live with.

Enjoy /s/ Bob Belter -- [email protected]
 

wsimpso1

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I recommend that you go through the course information of an Engines class taught be an Engineering College. You will learn much, and may come up with some practical background on piston engines that you can put to work. Specific to your OP:

Compression ratio increase is one of the few things we can do to an existing engine that both increase power and increase fuel efficiency. Thermodynamic efficiency of any engine is increased by increasing compression ratio and thus makes more power from a given amount of air and fuel. This is inherent to all piston engine design, essential to efficiency, and known for a long time. CR of 4 were common a century ago because the gasolines available would only support that level, and power per pound of fuel and per pound of engine were much lower than with the higher octane fuels we have had since the 1920's.

Trouble is detonation margins get used up - manifold pressures must be reduced and/or spark must be retarded and/or higher octane fuels become necessary. If one is trying to make a given power, it may or may not be attainable.

Max airplane efficiency (not max power) is nominally achieved in our current normally aspirated engines at altitudes of 7500 to 12000 feet not because atmospheric pressure is at some sweet spot, but because the power available at full throttle at those altitudes is still large enough for good cruise speeds but is small enough that we can run lean of peak and not have pre-ignition and detonation. It is a compromise between benefit and loss. Once it was known, engines tend to be developed around that need. There is nothing magic about this altitude. Fly higher, the air is thinner, we can run without the throttling losses, and lean of peak, and get nice low BSFC numbers. Trouble with higher efficiency is that power available gets lower as the amount of oxygen in each intake stroke drops. For higher altitudes we extract power from either the crankshaft or the exhaust gases to run a compressor to get enough manifold pressure. Some of the most efficient engines ever put in production made really low BSFC at 35 In Hg at high altitude cruise and much higher pressures for take off and for climb. Nothing magic about 20 In Hg.

In addition, we have the altitude effect of the airframe becoming more efficient at altitude, to a point.

Combine the airframe liking higher and engine power available dropping, and efficiency goes up, power goes down, and at some point, you can not go higher while your indicated airspeed is at Vy. That is Absolute Ceiling...

Other factors you may not have considered.

I kind of doubt that you can go with 65% of sea level manifold pressure and then bring power levels back up to 100% (for takeoff and climb) with compression ratio. Yes, CR raises power, but the effect tends to be small compared with putting more fuel and oxygen. The exercise of seeing how much CR is needed is left to the student;

Just because you have limited manifold pressure to say 10000 foot levels does not mean the engine will make that 10000 foot power when run at lower elevations.

Air on the bottom of the piston and outside the exhaust pipe at 10000 feet is at 20 InHg too. Exhaust gases have to exit the engine against not 20 InHg, but against local atmospheric pressure - scavaging of the combustion chamber will be marginally decreased. Intake work to draw the piston down with 20 In Hg above the piston and 29.92 In Hg below will increase. These greater losses mean that 10000' power will require more manifold pressure than is present at 10000 feet.

Now I get to bad part of all this. There really is very little new under the sun. Literally millions of smart folks passionate about aviation and engines have been thinking about getting more range or more speed or both out of engine driven machines since for more than a century. It is possible that one might come up with something really new that nobody else has? Sure. New patents are filed all the time. New schemes work. But they will take expanding upon present knowledge. Something as simple as running more compression ratio to make better use of a given amount of air per cycle has been done and was well known about a century ago. Yes it can be done, but is it actually of benefit? Are other things making this change long obsolete?

To evaluate ideas in anything resembling a decent manner, we had better already be in touch with the knowledge base on how these things work, and the experiential base on what has been done and found successful or unsuccessful. Then, maybe, just maybe, something truly new and beneficial can be created. I know about creativity - I have Patents and have sat on Patent Committees while employed by a couple large famous companies. Almost every session, we would have to tell folks that their baby was not as beautiful as they thought. Many times these folks were proposing things already patented or that had already been into and then out of production as their usefulness was exceeded by more recent creations. Some were broken hearted, a couple of them were immediate colleagues who were inconsolable for weeks, others were downright furious with us, and few went back to work with a strengthened outlook for the "new" in their areas of knowledge and work. I hope you are in the last group...

Billski
 
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Pops

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They don't like hearing that their idea was tried in 1927 and it didn't work out. Time and effort wasted in re-inventing the wheel.
But, never stop.
 

trimtab

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Every ivtec out there already has variable valve timing that, in effect, provides variable CR (albeit small). They are already sort of an Atkinson cycle. Exaggerating the effect would make a fully Atkinson style capability to map the CR for every MP.

As long as the goal is efficiency, and it so much a power gain.

Adding O2 is immediately more effective at both power and efficiency. I've commented before on the use of an adsorber, with small net gains for a single cylinder four stroke test bed. A loss of 3-5 inHg in MP was more than compensated by the increase in O2, even with a crude adsorber setup and dyno. Every 5% of O2 enrichment (1% of total) is roughly equivalent to 1k' of altitude reduction in terms of the O2 molecules in each charge. However, the effective CR drops as well with lowered MP.

In my setup, at a mile high, it is very easy to overheat the engine by adding O2 and fuel while maintaining peak EGT, showing where this all ends up. Namely, if you had an engine that could maintain the heat rate (fuel flow at peak EGT), the ability to cool can still become the limiting factor at altitude.
 

PMD

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I know about creativity - I have Patents and have sat on Patent Committees while employed by a couple large famous companies. Almost every session, we would have to tell folks that their baby was not as beautiful as they thought. Many times these folks were proposing things already patented or that had already been into and then out of production as their usefulness was exceeded by more recent creations. Some were broken hearted, a couple of them were immediate colleagues who were inconsolable for weeks, others were downright furious with us, and few went back to work with a strengthened outlook for the "new" in their areas of knowledge and work. I hope you are in the last group...

Billski
 

PMD

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I am laughing at that, since I did some reciprocal "bad cop" work for a research agency when they were providing me with a subsidized engineer. Even people from senior levels in industrial companies thought that their "widget" just needed to be patented and they would be on easy street overnight. My job was to shoot down their enthusiasm with large doses of reality (that a few hours of simple research would have given them well in advance). Still remember many of the fallen faces as they left the room. Not a way to win a popularity contest, for sure. BTW: I found that a LOT of this wild enthusiasm (pre-internet days) was driven by patent lawyers ready to scalp their suckers...er...uh...CUSTOMERS.
 
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My home airport is at 5,000 MSL. I set my CR based on the lower atmospheric pressure. If I operate at lower altitudes I can simply limit the MP as suggested in the OP. However, I don't think that I would run the higher CR if I lived at sea level.
 
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Marc W

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My home airport is at 5,000 MSL. I set my CR based on the lower atmospheric pressure. If I operate at lower altitudes I can simply limit the MP as suggested in the OP. However, I don't think that I would run the higher CR if I lived at sea level.
Same here. Home drome is at 5200'. High compression pistons are a very common modification.
 

Pops

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The engine Bob Barrows built for the prototype Bearhawk LSA is a C-85 case and the normal 0-200 change over with special built pistons for a 10.5 to 1 CR. First used the C-85 pistons for the CR boost then to the special built pistons.
 

speedracer

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Any idea of how high folks are going in the compression ratio boost? I remember a fellow saying that he was using 10:1 compression ratio on an 0-200.
When I was racing my Long EZ I ran 11-1 compression. When I sold it there was 1,600 hours on that engine.
 

Martti Mattila

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I learned in recently from Ytube that in WW1 Germans use in their BMW amd Merchedes inline sixies a high c/r but trotle movement was restcricted to use wide open in lower altitudes. Only allowed in high altitude or maybe in emergency. Remember Germans obey the rules.
 

TFF

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I don’t know if they run naturally aspirated racers at Pikes Peak anymore, but the old times they ran crazy compression ratios. They couldn’t dyno them because of the altitude of the dyno. They we’re almost running diesel range, 16-17 to one. Then the Audi Quattro went there.
 

PMD

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I don’t know if they run naturally aspirated racers at Pikes Peak anymore, but the old times they ran crazy compression ratios. They couldn’t dyno them because of the altitude of the dyno. They we’re almost running diesel range, 16-17 to one. Then the Audi Quattro went there.
I think you meant "then Michelle Mouton humiliated the Good-ole-boys".
 

challenger_II

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I learned in recently from Ytube that in WW1 Germans use in their BMW amd Merchedes inline sixies a high c/r but trotle movement was restcricted to use wide open in lower altitudes. Only allowed in high altitude or maybe in emergency. Remember Germans obey the rules.
I believe it was the Mercedes L6 that had the crankcase section for the center two cylinder banks sealed from the rest of the crankcase, and used crankcase induction from the center section to boost the fuel/air mix to each of the cylinders, thus increasing altitude performance.
 

TFF

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She was up to the task. It also had been decades since a manufacturer campaigned a serious effort. The previous top cars had more in common with the front engine Indy and F1 cars with scabbed on wings, naturally aspirated. Unluckily it also changed the race forever.
 

PTAirco

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I believe it was the Mercedes L6 that had the crankcase section for the center two cylinder banks sealed from the rest of the crankcase, and used crankcase induction from the center section to boost the fuel/air mix to each of the cylinders, thus increasing altitude performance.
Dang, that was clever. I never cease to be amazed at how many smart idea people had about this stuff over a hundred years ago.
 
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