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From: "Ed Anderson" <eanderson@carolina.rr.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-2603684@logan.com>
Subject: Re: [FlyRotary] Re: Supercharging
Date: Tue, 18 Dec 2007 11:55:03 -0500
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> wrjjrs@aol.com wrote:
>> Greg,
>> Superchargers are sometimes simpler to implement, but there are other 
>> reasons they aren't as popular on aircraft. First they are a power 
>> robbing device. There is a net gain in power of course, but the charger 
>> can require 10-15 HP to drive it at WOT and it makes the engine less 
>> efficient. The other problem is you must be sure the unit is up to 
>> continous use in the boost RPM range. Lastly the mount must be solid 
>> enough to hold up to continous use as well.
>> Bill Jepson
> Doesn't the same apply to all intake charging devices?
>
 Yes and No, Ernest.

Yes, they both consume fuel energy to produce boost.  The Superchargers are 
driving off the crankshaft power - meaning after the fuel's combustion 
processes and conversion to mechanical energy (with their losses) has 
already taken place - so this is power ready to drive your prop that you are 
using to drive the supercharger.  The turbocharger on the other hand is 
driven off exhaust gases (unconverted to mechanical energy) which is 
generally "waste" energy. vs the crankshaft power as "useful" energy. 
Depending on the compressor type used on the supercharger your efficiency 
can drop down to below 50% (some of the older roots type expellers) , the 
centrifugal compressors (which can be on super or turbochargers) are 
generally higher efficiency (can approach 75%)  in converting input energy 
into high pressure air. So a bit more efficient.

There are certainly advantages to a supercharger in some applications - and 
to a turbo in others.  The immediate boost response of a roots type blower 
is about the only reason they are the bedrock of the top end drag racers. 
Unlike a turbo which needs some time to spool up, the roots type 
superchargers produce boost much quicker.  There are several types on 
impellers designed to increase the efficiency of the root types blowers, but 
I am not aware of any that approach the centrifugal impeller in efficiency.


While the turbo uses the "free" exhaust flow energy,   you do get back 
pressure with the turbocharger that effects (decreases) engine power so that 
has to be considered also.  But, overall, its my understanding that the 
turbo is a bit more efficient and also does not decrease crankshaft power 
(at least not as much) as a supercharger.  However, there may be other 
reasons (like not having to have all that heated exhaust pipes routing 
around to and from a turbocharger) as well as cost for using one vs the 
other.

Gee, its great to be back {:>)

my 0.02

Ed