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why are turbocharged engines designed with lower compression ratios?


RobertE

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On 2/21/2017 at 9:14 PM, RobertE said:

To put the question succinctly, why is it more efficient to have lower compression but higher boost than higher compression and lower boost?  

Key words that are questioned in this discussion...

 

TC'd engines traditionally got lower mechanical compression ratios.

- improved volume of cylinder to stuff more fuel mixture into...  efficiency (like mpg) isn't the best doing it this way.

- improved engine safety by avoiding preignition... allowing more fuel to be burned each stroke.

 

The TN'd engines are technically more efficient as their mechanical CR is higher...

- intercoolers are required to lower the air temperature that is being introduced into the engine.

 

Sometimes the word efficiency means different things to different people.  Thermodynamics is probably one of those cases...

the ability to burn more fuel in a smaller, lighter engine is pretty efficient!

the TC'd engine gets pretty good mpg at high altitudes because the air resistance is much lower up there...

the 10:1 CR of my LT1 firebird produced 300hp compared to the lower CR of the previous firebird.  The L98 engine had 220hp in the same 350cui.  Same engine block, more efficient output...

Now for fun add timing to the discussion... 25°BTDC improves power output compared to the 20°BTDC.  More fuel is burned inside the cylinder to produce more pressure to push the piston...  the risk of pre-ignition has also increased some at the same time...

How is that?

Best regards,

-a-

Edited by carusoam
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7 hours ago, carusoam said:

Robert,

JL has written a bunch regarding engine ops of his TC'd 231.  

He supplies enough technical detail in a well written format, you feel like you are flying right seat with him...

Best regards,

-a-

Thank you gentlemen, but it has just been alot to learn since I got the aircraft.  I wish I had known then what I know now. Just trying to pass on what I have learned the hard way, so someone else does not have to.  I guess that's my curse, 67 and still have lessons to learn.

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The large bore size of a typical aircraft piston engine does negatively impact detonation resistance. It's not unusual for modern small bore automotive engines such as the Ford Ecoboost, to operate at a 10 to 1 compression ratio. And they do so on low octane unleaded and high boost levels, as high as 20PSI. Certainly direct injection is a big help, but it's not the only reason automotive engines can operate with high compression ratios and high boost levels. Active knock sensors, higher RPM and small bores are also important. 

These technologies may translate to an aviation application. But for now, it's really hard to beat the reliable efficiency and power to weight of air cooled aircraft engines. One thing many pilots and aircraft owners are unaware of, is the superb BSFC (brake specific fuel consumption) of modern piston aircraft engines. In fact, until recently, there were exactly zero automotive engines that match the BSFC of an injected Lycoming. That changed recently with the 2016 Prius engine. Which only just matches an IO-360 Lyc in cruise flight. 

In addition, air cooling can have lower cooling drag, due to higher temperature differential for rapid heat transfer. Alternative engines such as the Rotax 4 cylinder aviation engines, while thrifty, don't match injected Lycoming efficiency. With the notable exception of the FADEC controlled Rotax 912iS, which just matches a 1970 Lycoming IO-360's BSFC. 

But in the end, the things that increase aircraft engine efficiency, the large bore sizes, the low RPM and high operating temperatures significantly reduce detonation margin on aircraft engines. The turbocharger adds additional stress and often requires lower compression ratios. 

Edited by cujet
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Detonation happens from a combination of temperature and pressure. The combustion chambers of water cooled auto engines rarely get above 200F while ours are around 400F. Automotive ECUs can detect the onset of detonation and retard the spark to control maximum cylinder pressure. There comes a point of diminishing returns when you delay combustion to the point where there is excess unburied fuel in the exhaust. 

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