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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
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