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  #1  
Old 03-26-2006, 11:54 AM
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Default just GOT it - ITS TEMPERATURE

OK, i just got it - the reason that manufacturers like to use low compression pistons for turboed applications. WOW am i ever dense.

Temperature. Intake manifold temp. THAT is the main difference between a normally aspirated engine and a turbo engine - the IM TEMP is way higher and that by definition brings you closer to detonation.

for an experienced engine person this is obvious - for a duffus trying to put all the pieces together - it finally makes perfect sense. EVEN for just normalizing you can normalize the pressure but not the temp.

the higher the temp of the air/fuel goin into the engine - the closer to detonation and pre-ignition and destruction
(thank you again rv6guy - the 160 F just clicked in with its full meaning)
simple, simple, simple yet so hard to force it all into my skulllllll
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  #2  
Old 03-27-2006, 01:56 AM
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You know dust... I can tell you have the idea in your head, but you still didnt convey in your post WHAT the lower compression piston/cylinders do

As Dust said, its all about the heat....

Now, why does the lower compression matter? Well.... compression of a gas generates heat... this is the main way a turbocharger generates heat.

So, your turbo is compressing the air and heating it... so you now have hot air in your intake.

If you take that hot air and put it in a high compression cylinder with fuel it might push the temp past the ignition point for the fuel... which causes it to ignite prematurely, which can try to force the engine backwards or if it happens just right the pressure can peak as the piston, rod and crank are centered... which places tremendous strain on all the parts involved.

So by placing the lower compression cylinders and pistons on the engine you lower the chance of detonation.
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Old 03-27-2006, 10:22 AM
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Well JonC thanks for the added words. It's just that it finally makes complete sense to me now. turbo normalizing looks so benign. "just" keep Intake Manifold pressure the same - no boost - no harm.

But when you sit with the calcs like i just did for days and see the temps and then you find out that 160 or higher F is the dangerous temperature. whalla.

the higher you normalize - the higher the temps. if you start out at ground level boostin for power then it all happens at a lower altitude.

what i really liked about it was the effect that OAT has on the density of the air as you go up - really brings down the P/R and the sea level calcs just do not take that into consideration

Once i can get my spread sheet verified i'll clean it up and make it more user friendly
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  #4  
Old 03-31-2006, 11:40 PM
Don P-Factor Don P-Factor is offline
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Default Hot Air Engine

There once was a man named Smokey Yunik who besides being a famous race car driver was also one of the most innovative minds known to the practical world. He developed the hot air engine which using real world theories actually was tested by Hot Rod Magazine and produced 250 Hp from a 2.5ltr GM basically stock engine. The engine utilized a turbo/homoginizer that mixed the fuel and acted as a check valve to prevent the increasing pressure due to temperture from backing out of the intake stream. The temperature in the intqake manifold was 220degries F. This is below the Flash point of a properly atomized mixture. Upon entering the cumbustion chamber the mixture was compressed and increased to 440degrees F. There was no detonation or preignition. The car (A Pontiac Fiero) achieved 60MPG at 250hp.
This is very hard to comprehend when all our lives we are told that colder air is better. Unfortunately the efficiency of this type of thinking is a miserable 25%. The Hot air engine was at >75%. Proper control and engineering of systems can and will provide us with continuous advances in efficiency. Example using old formulas and thinking,How are the NASCAR 350CI engines producing 700HP with a maximum intake flow of 350CFM?
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Old 04-01-2006, 02:53 AM
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You are not a dufus at all Dust, I wish you would stop bad mouthing yourself

You are correct that low compression pistons reduce the cylinder air temperature which delays detonation.

Another good reason is that it reduces the forces exerted on the piston, rods, crank, head, etc. In fact, if you were to compare two similar engines, one turbo charged the other not, the turbo should have internals of a higher strength.

Useful links
Ford Zetec turbocharging guide
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  #6  
Old 04-01-2006, 08:36 AM
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You can turbocharge a high compression engine... but you need a fairly sophisticated computer control system with a knock sensor. I had a 89 Audi 5000 CS Turbo Quattro. It was unusual... a 5 cylinder turbocharged engine with 11:1 compression ratio. I always looked for 93 premium... and I always knew if the rating was correct 2 minutes after filling up.

This would be too complex to try modifying an aircraft engine though. But Dust, what kind of temperature increase are you looking at? At 25000 feet I would expect -40 OAT, and even with less efficient cooling in thin air, I would still think you would have the cooling capacity.
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Old 04-01-2006, 10:49 AM
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Quote:
Originally Posted by Don P-Factor
There once was a man named Smokey Yunik who besides being a famous race car driver was also one of the most innovative minds known to the practical world. He developed the hot air engine which using real world theories actually was tested by Hot Rod Magazine and produced 250 Hp from a 2.5ltr GM basically stock engine. The engine utilized a turbo/homoginizer that mixed the fuel and acted as a check valve to prevent the increasing pressure due to temperture from backing out of the intake stream. The temperature in the intqake manifold was 220degries F. This is below the Flash point of a properly atomized mixture. Upon entering the cumbustion chamber the mixture was compressed and increased to 440degrees F. There was no detonation or preignition. The car (A Pontiac Fiero) achieved 60MPG at 250hp.
This is very hard to comprehend when all our lives we are told that colder air is better. Unfortunately the efficiency of this type of thinking is a miserable 25%. The Hot air engine was at >75%. Proper control and engineering of systems can and will provide us with continuous advances in efficiency. Example using old formulas and thinking,How are the NASCAR 350CI engines producing 700HP with a maximum intake flow of 350CFM?

Car and Driver mag did an evaluation of Smokey Yunick's engine and found that it did not have anywhere near the power stated, got little better economy than the unmodified engine and that it detonated almost constantly. The editors questioned the lifespan of the powerplant. It is not with us today of course. This contradicts Hot Rod's findings completely. I'd put my faith more in C&D in exposing truths about engine/ car performance than Hot Rod in that era. They blew up many a "bulletproof" engine from the sales brochures in short order. Remember the original Callaway Corvettes?

Obviously increasing the operating cycle Delta T could improve thermal efficiency to the point of materials failure. Something most of us are not interested in on our aircraft.

NASCAR engines are flowing a lot more than 350 CFM. This is only what the restrictor plate or carbs are rated at at a given depression which has nothing to do with what it flows attached to the engine. Camshaft design, 15+ to 1 CRs, thermal coatings etc. help these engines squeeze every hp out of their highly restricted airflow.
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Old 04-01-2006, 11:32 AM
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MT - I'm lookin at a temp rise of 326.85 F, just to normalize that is before the intercooler, add to that the -30 and you still get temps that are way too high
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Old 04-01-2006, 01:43 PM
Marc Zeitlin Marc Zeitlin is offline
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Quote:
Originally Posted by rv6ejguy
Car and Driver mag did an evaluation of Smokey Yunick's engine and found that it did not have anywhere near the power stated, got little better economy than the unmodified engine and that it detonated almost constantly.
Thank you for pointing out that the laws of thermodynamics are immutable, and you can't get 75% efficiency out of a Carnot Cycle type engine without achieving temperatures that engine materials cannot reach. Whenever someone makes astounding claims of MPG or efficiency (or any other claim that violates the 2nd law of thermodynamics), there are only two choices:

1) they're sincere, and fooling themselves because they don't understand how to measure what they're trying to do, or:

2) it's a scam, and they KNOW that they're full of it

Given his previous accomplishments, I'd put Yunick into category (1).

If Yunick's engine did what is claimed, and since the patents have run out, one would expect to be seeing this engine showing up all over the place. But it isn't. There's a reason for that.
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Old 04-01-2006, 01:52 PM
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Quote:
MT - I'm lookin at a temp rise of 326.85 F, just to normalize that is before the intercooler, add to that the -30 and you still get temps that are way too high
... but then you have a much higher delta T at the intercooler at 25k then at ground level, so what are you using for post intercooler intake air temp? (ignoring that I think 326 seems high)
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Old 04-01-2006, 05:02 PM
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Well - its all computation right - you are welcome to look at the spread sheet and review them. when you compress the gas 2.13 to one it takes on allot of temp and i am estimating an extra temp rise over and above the stated compressor efficiency of .75. i add an additional 30 %

I am HOPING for 50% inter-cooler efficiency - its all hopes - when i test things i will do it gradually and see what the results are, plot them to see what the actuals are to the computed and i should be able to tell what the next increase in altitude will do before i get there.

It looks so benign - just keep the power and normalize - what harm can that do
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Old 04-01-2006, 11:21 PM
Don P-Factor Don P-Factor is offline
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How do the 350ci engines in GM cars get 28-30mpg? They brought the coolant temps UP to 225 Degrees F. Not down with 180 thermostats. In 1985 a GSXR motorcycle was the first bike to achieve the unbelievable 10hp per cc. Today a 750cc engine is putting out 20hp per cc. Increased atomization of fuel mixture. Proper intake pulse flow and a host of other remarkable engineering theroies that are now proven. It does work. There was also a Top perfomance challenge sponsered by Car Craft a few years ago that tested Dragstrip, Slalom times and Efficiency for street cars. The winner sometime around 10 years ago was a Buick Gran Sport turbocharged that did 12.5s in the 1/4mile and got 45mpg through a 100mile course around So Cal. Also Craig Vetter had his own motorcycle challenge that had the contestants ending up after a few years with well over 500mpg. The point is that when you build your powerplant every part must compliment each other, when ths is done the most efficiency can be had. Good luck. Don.
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Old 04-02-2006, 12:01 PM
Marc Zeitlin Marc Zeitlin is offline
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Quote:
Originally Posted by Don P-Factor
How do the 350ci engines in GM cars get 28-30mpg? They brought the coolant temps UP to 225 Degrees F. Not down with 180 thermostats.
Yes, the hotter you let the ENGINE material get, the more efficient the engine will be. However, hot AIR in the intake will lead to detonation and NOT help efficiency, as RV6ejguy pointed out.

Quote:
Originally Posted by Don P-Factor
In 1985 a GSXR motorcycle was the first bike to achieve the unbelievable 10hp per cc. Today a 750cc engine is putting out 20hp per cc.
I believe you mean 20 hp/100cc - if they were getting 20 hp/cc, motorcycle engines would be putting out 20,000 HP. They haven't quite got to that level, no matter what magical claims some folks might make.
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Old 04-02-2006, 12:50 PM
Lynn Erickson Lynn Erickson is offline
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Quote:
Originally Posted by Don P-Factor
How do the 350ci engines in GM cars get 28-30mpg? They brought the coolant temps UP to 225 Degrees F. Not down with 180 thermostats.
They don't!
And if they can make the temp. of ice lower the eskimos would buy more? They get that kind of mileage out of the 350's by turning off 4 of the cylinders. and they have been running the coolant temps at 230 to 240 for the last 20 years.
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Old 04-02-2006, 03:48 PM
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With a turbo engine, it behoves us to get induction temps as low as possible for max power. Lower temps equals higher mass flow equals higher power. It is crazy talk to suggest otherwise.

Thermal efficiency on a given Otto cycle engine is mainly affected by overall compression of the charge. Whether this is achieved by CR alone or a combo of CR and forced induction is relatively unimportant.

Detonation and pre-ignition cannot be tolerated in aircraft applications therefore CRs are usually restricted somewhat on turbocharged engines. Induction temperature, CR, manifold pressure, fuel octane and spark timing play major roles in keeping these at bay.

You can raise the CR on low boost turbocharged engines running good fuel, you just need to be careful. Flying Tiger used 12 to 1 CR with a turbo however they did have some failures until they sorted it all out.

The safest bet is probably to leave the Conti CR where the factory engineers put it using factory parts. Fit an efficient intercooler system and keep all temps within factory limits. To do otherwise may open more cans of worms and increase the likelihood of failure or draw out the development and test period substantially.
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