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Engine power output: turbo vs normally aspirated

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Engine power output: turbo vs normally aspirated

Old 31st Oct 2014, 13:06
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Engine power output: turbo vs normally aspirated


Few months ago I've opened a thread about the turbo auto waste-gate operation. Now I've up to something a little different. I'm sure that someone here will be able to shed light on this.

Speaking of Continental IO-520-R, I see that both turbo and non-turbo are rated [email protected]
Now, if the turbo version outputs 310hp at [email protected], what about the non-turbo? Is it able to output the same amount of power at [email protected] on takeoff?
If so, what's the difference in the mechanics of the engine? At this point, it seems that the difference is not only about the turbo per-se.

Thank you!
nico87 is offline  
Old 31st Oct 2014, 13:49
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I see the point you're making, but the engine models are not lining up exactly for me. The first thing is to assure you have the engine models correct, as per the TCDS for each engine (which will also give you the limitations, so you can compare "apples to apples". Thereafter, a review of the performance charts for each engine might point you toward an answer.

But, there is a little bit of marketing "black magic" still latent in some of these numbers....
9 lives is offline  
Old 31st Oct 2014, 14:18
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What they have probably done, is reduce the engine compression ratio without turbocharging so that, at sea level, it produces the same power with turbocharging as the normally aspirated version produces without. The advantage of this, is that the turbocharged engine will then be able to continue producing the same power up to high altitide by simply maintaining the boost pressure, wheres the normally aspirated engine would loose approx 1" of manifold pressure with every 1000' gained.

So. They will both develop the same power on take off at sea level, but wheres the normally aspirated engine will loose power as altitude is gained, the turbocharged version will maintain it's power. This not only enables climb perfomance to be maintained to a much higher altitude, but also improves high density altitude takeoffs by maintaining the engine's rated power at high density altitudes.

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Old 31st Oct 2014, 14:29
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As MJ says, I am pretty sure turbo engines have a lower compression ratio
meaning lower power
Camargue is offline  
Old 31st Oct 2014, 15:28
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Thank you all. Now I understand.
Lowering the compression ratio in the turbocharged model, saying to 7,5:1 instead of 8,5:1, at 36MAP will provide the same power.
I didn't think about this at first but it was pretty simple.

Thank you!
nico87 is offline  
Old 31st Oct 2014, 15:54
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I can't speak specifically to the details of these two engines but......

If you simply turbo(or super)charge a normally aspirated engine without lowering the compression ratio and changing the ignition timing and fuel ratio, the engine will almost certainly detonate (pre-ignite) itself into oblivion in short order. The increased cylinder pressures generated by the "overfilling" of the combustion space will cause the fuel charge to explode spontaneously as the piston rises and compresses the gas. This "pre-ignition" (sometimes referred to as pinking in car engines) will melt holes through the piston, destroy the valves et al.

Seen it so many times on turbocharged race-car engines..........
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Old 31st Oct 2014, 15:57
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This is what's called a turbo-normalised engine.

From personal experience - engines that have a naturally low compression ratio and rely on boost to get the compression up, so to speak, have less fuel economy than an engine that's higher compression to start with.
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Old 31st Oct 2014, 16:00
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Join Date: May 2014
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A good place to go for more info on Those power breathing turbos is John Deakin's series.

The way I understand it, the basic engine has a max power rating, limited by lots of structural and thermal factors inherent in the whole engine design. If you plonk a turbo or supercharger (or both) on it, all those other factors will remain (or even be exacerbated), so the max power rating does not change. Therefore, most systems will either have a bypass system to limit Sea level boost pressure or another system to limit overall power at sea level - to avoid the thing 'blowing up'. At the end, Take-off power at sea level therefore does not change.

However, what does change is that you can maintain that power throughout a climb and cruise - as long as your turbo/SC has enough puff, the engine will in essence continue to see sea level conditions, and therefore put out sea-level power levels.

Check out John's series - they are well worth a read. (And then come back and correct what I wrote - I*m sure I got something wrong : )

Baikonour is offline  
Old 31st Oct 2014, 16:32
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Thanks to everyone.
Baikonur, the link you posted is really interesting.
nico87 is offline  
Old 2nd Nov 2014, 01:24
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I agree this is a turbo-normalised engine and does not us the turbo to increase power, just correct for altitude and compensate for a little intake pressure drop. With an automatic waste gate, no danger of over boost.

This can be done to almost any engine with no internal changes best with an intercooler after the turbo to cool the compressed air and keep cylinder head temperatures down.

Believe the there are many STC to fit these in GA aircraft in the US.
horizon flyer is offline  
Old 2nd Nov 2014, 13:16
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Just to add physics to answers - to burn fuel you need two thingies, one is fuel, the other oxygen. Fuel is almost uneffected of the turbo aspiration, but the crucial point is air/oxygen. At sea level it does not really matter, as there is enough oxygen to burn fuel, but if you are getting higher and higher you will notice that you run out of oxygen first, not fuel ;-) - remember? If you now have the same engine, one normal one turbo, you will find out that the turbo is shuffling more compressed oxygen to you fuel2noise conversion unit. So, the higher you get, the more advantage you get in comparison by using turbo.
ChickenHouse is offline  

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