From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of REHBINC@aol.com
Sent: Tuesday, March 22, 2005 11:11 AM
To: Lancair Mailing List
Subject: [LML] Re: superchargers

George,

You've got me there. I don't have any maps for "aircraft compressors". I do however have many for general applications and it is not uncommon for them to peak in the upper 80s to 90% efficiency. I only assume that aircraft would use equipment with similar efficiency.

 

Why wouldn't an aircraft compressor operate at or near its optimum efficiency nearly all of its life? Airplanes are routinely flown at roughly the same altitudes and power settings for the majority of there lives.<<

 

 

Well,  as it happens,   I have on my desk  the compressor map for what is probably  the best compressor Garrett has ever made for small general aviation piston engines.

The sweet spot  for a mass flow of about 30 lb/min and a pressure ration of 2.0 to 2.2  has an efficiency of  76%.

It rapidly deteriorates down to 60% when you get off of that design point.    

 

Thus, at 5000 feet MSL,  at a pressure ratio of  ~ 1.25 (36-38” Hg MP or there about) the efficiency would be down around 60 to 65%.

Airplanes,  unlike cars,  operate from a baseline 30” Hg ambient at sea level - - to a baseline ambient of only 12-13” Hg ,    with pressure ratios varying from  1.2  up to 2.8 to 3.0.

Thus, at constant RPM,  the compressor would spend most of its life  well off  of any “optimum”  pre-determined design point.

 

The variable speed turbo-supercharger compressor helps to mitigate this problem to a substantial degree.

 

 

 

>>If all you want to do is get away from detonation, cut the compression ratio by a quarter point. If you want to make more power, ad an intercooler.<<

 

Rob,  I think that is not the optimal approach.  However,  I think that is close to  what  a lot of  automotive background people would suggest when first considering the issues. 

 

Take another look at it.

 

Assuming the  engine is already operating  at or near the engine’s  detonation limited  maximum BMEP - -  (many aircraft engines are) - - then in order to improve the detonation margin, without reducing power (reduction in CR goes the wrong direction on the power and the efficiency issues)  or in order to increase the  BMEP - - and remain free of detonation,  one will have to do one or more of the following:

 

1)       Reduce the CHTs;

2)       Increase the fuel octane;

3)       Reduce the induction air temperature.   

 

Cars with liquid cooled engines can’t readily reduce the CHTs a lot.

 

Air cooled aircraft engines can improve the engine cooling with some amazingly simple revisions to the baffling systems.  That helps a lot.  You can “buy”  about 5 octane points of detonation protection by dropping the CHTs by 50d F.

 

Fuel octane is difficult to mess around with for aircraft.

 

That leaves - - - intercoolers.  

 

Reductions in IAT from the intercooler will buy one  LARGE improvements in the  detonation limited BMEP (ie, more horsepower becomes usefully available).

 

 

>>However, when you make the blanket statement that a mechanically driven supercharger will result in a slower airplane, we have to part company.<<

 

I didn’t say that.  I said the data I had seen revealed that one particular aircraft using one particular configuration of belt driven compressor ended up, based on actual test,  going no faster with the belt driven compressor than it did without.

 

Beech 18s have mechanically driven compressors and they do go faster with those than the would without.  They go even faster at altitude  with higher gear ratios on the compressors when at altitude,  but at the penalty of substantial  losses in low level single engine performance.

 

 

Regards,  George