lml@lancaironline.net Mailing List Archive

I've been reading this thread with interest - and difficulty. For some reason only emails from this mail list (I'm in the digest mode) have been coming through garbled. My son the IT expert says it is being sent as http instead of text, but why only from this source and why only since a few days ago? Anyway, something Brent said got me to thinking. He said the compressor inlet pressure drop had a significant effect on the critical altitude, especially the air filter pressure drop. So I went through some calculations. A typical air filter drop at sea level might be 4 inches of water (makes my math easier :-), or 1% of an atmosphere. As you go up in altitude the inlet density drops, but at the same power the mass air flow remains roughly the same. With turbulent flow through an orifice (the air filter) it turns out that the pressure drop is inversely proportional to the density. The density at the altitudes we're talking about might be 1/3 that at sea level. So the pressure drop is 3 TIMES the seal level pressure drop. The compressor operates at a given pressure RATIO at critical altitude (waste gate closed) - in this case the pressure ratio is about 3:1. Bottom line? Instead of a 1% power reduction at sea level, the same air filter would create a 9% reduction at altitudes over 25K. Which says that anything you can do to reduce the compressor inlet restriction will make a big difference in critical altitude. The same argument can be made for exhaust restriction after the turbine, but this is less of an issue because the exhaust restriction is already very low. Other considerations: If the divergent geometry of the compressor discharge could be extended past the compressor, more pressure recovery could be realized. The same is true of a divergent section after the turbine. Often the flow "refinements" in a turbo design are neglected, as I suppose the idea is that as long as boost can be maintained by closing the wastegate there is no problem. In reality, any flow improvements can make a big difference, mostly in lower engine inlet temperatures, lower exhaust back pressure, and yes, higher critical altitude. At "normal" altitudes these improvements will result in more power for a given fuel flow. Gary Casey From: "Rick Titsworth" <rtitsworth@mindspring.com> To: "" <lml@lancaironline.net> <html> <head> <style>  </style> </head> <body bgcolor="white" lang="EN-US" link="blue" vlink="purple"> <div class="Section1"> <p class="MsoNormal"><font size="2" color="navy" face="Arial"><span style=' font-size:10.0pt;font-family:Arial;color:navy;'>Technically, the compressor inlet pressure is normally ALWAYS below ambient