Return-Path: Sender: "Marvin Kaye" To: lml@lancaironline.net Date: Sat, 22 Jan 2005 14:09:24 -0500 Message-ID: X-Original-Return-Path: Received: from wind.imbris.com ([216.18.130.7] verified) by logan.com (CommuniGate Pro SMTP 4.2.8) with ESMTP-TLS id 617627 for lml@lancaironline.net; Sat, 22 Jan 2005 13:54:12 -0500 Received-SPF: none receiver=logan.com; client-ip=216.18.130.7; envelope-from=brent@regandesigns.com Received: from [192.168.1.100] (wireless-216-18-135-19.imbris.com [216.18.135.19]) (authenticated bits=0) by wind.imbris.com (8.12.11/8.12.11) with ESMTP id j0MIrVaH061134 for ; Sat, 22 Jan 2005 10:53:32 -0800 (PST) (envelope-from brent@regandesigns.com) X-Original-Message-ID: <41F2A122.5070509@regandesigns.com> Disposition-Notification-To: Brent Regan X-Original-Date: Sat, 22 Jan 2005 10:53:22 -0800 From: Brent Regan User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.2) Gecko/20040804 Netscape/7.2 (ax) X-Accept-Language: en-us, en MIME-Version: 1.0 X-Original-To: Lancair Subject: Re: IV-P continental detonati.. Content-Type: multipart/alternative; boundary="------------010600040305080605090701" This is a multi-part message in MIME format. --------------010600040305080605090701 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Charlie writes: >>I guess it's debatable whether or not it's compressor or turbine. Results are the same. Compressor quits. As I understand it, and the fix was to give the turbocharger more exhaust air volume at lower engine rpm.<< In turbosupercharger (aka turbocharger) technical terms it is called "Compressor Stall" and is a component of compressor surge (like a stall is part of a spin) and occurs at a threshold where the air mass flow is insufficient for the desired pressure ratio or where the pressure ratio is too high for the mass flow. It is a non-linear event that frequently is accompanied by a loud noise or, in turbine engines, a flameout. Ever notice how a fan will speed up when the discharge is blocked? Similar effect but less dramatic. The delineation of the area on the compressor performance map where stall will occur is called the Surge Line. There is a very good book about turbochargers by Hugh MacInnes called "Turbochargers", printed by HP books and available at may performance auto stores and, I am sure, off the web. Craig writes: <> You have to think in terms of power (torque times RPM) and not just RPM. Turbocharger speed is determined by compressor load and the A/R ratio of the turbine. A/R ratio is defined as the area of the turbine inlet divided by the radial distance from the inlet to the turbine axis of rotation. The A/R ratio of a turbine housing is frequently stamped or cast into the housing at the factory. The basic A/R ration can be "trimmed" with an orifice or nozzle on the turbine discharge but this is a "bandaid' fix for a poor turbo selection. Turbocharger selection is a tricky business. So much so that when it came time to select turbochargers for my Lycoming installation I hired a retired Garret engineer to help. This was the same guy who designed the Malibu installation (which was the precursor to the IV-P Continental). Money well spent as he understood the issues with the Continental engine and was able to make a good recommendation for a compressor that had better aerodynamics. Cabin leaks will NOT have a significant effect on engine performance. Air for pressurization is tapped off of the deck pressure (after turbos and intercoolers but before the butterfly) using a sonic venture. A sonic venturi is nothing more than a hole with known flow characteristics. The "venturi" part of the name refers to the tapered bore which produces a vena constricta that is close to the physical diameter of the orifice. This makes the performance of the sonic venturi more consistent and predictable as compared to a sharp edge orifice. The "sonic" part refers to the fact that the velocity of the air throughout he orifice is the local speed of sound, the fastest rate that a pressure wave can propagate in the medium. Sonic venturis regulate flow regardless of changes in the down stream conditions so cabin leaks have a negligible effect on pressurization air flow rates. Think of it as a bucket with a hole in the bottom. Fill the bucket with water and the water will flow through the hole at some rate. How far the water falls after it exits the hole has little effect on that flow rate. Similarly, the pressure difference across the sonic venturi has little effect (in the range of pressures we are interested in) on the flow rate through the sonic venturi. Flow rates into the cabin of a IV-P should be about 40 SCFM (FAA says 10 SCFM per passenger) which translated into a sonic venturi orifice diameter of about 0.55". Turbochargers and Sonic Venturis are simple devices with complex dynamics and a detailed discussion of those dynamics is available in the literature. I have simplified some elements for the purpose of clarity and encourage you to research this topic, if interested. A good source of compressor maps can be found at http://www.turbocharged.com/main.htm. Regards Brent Regan --------------010600040305080605090701 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit
Charlie writes:
>>I guess it's debatable whether or not it's compressor or turbine. Results are the same.  Compressor quits. As I understand it, and the fix was to give the turbocharger more exhaust air volume at lower engine rpm.<<

In turbosupercharger (aka turbocharger) technical terms it is called "Compressor Stall" and is a component of compressor surge (like a stall is part of a spin) and occurs at a threshold where the air mass flow is insufficient for the desired pressure ratio or where the pressure ratio is too high for the mass flow. It is a non-linear event that frequently is accompanied by a loud noise or, in turbine engines, a flameout. Ever notice how a fan will speed up when the discharge is blocked? Similar effect but less dramatic. The delineation of the area on the compressor performance map where stall will occur is called the Surge Line.

There is a very good book about turbochargers by Hugh MacInnes called "Turbochargers", printed by HP books and available at may performance auto stores and, I am sure, off the web.
 
Craig writes:
<<I do not know much about turbos, but since all of the exhaust gas goes thru the turbine housing, the only way to generate more torque to the turbine wheel would be to reduce the turbine inlet size, increasing the gas back pressure, and therefore increasing torque to the turbine wheel. This seems to me it would work up to the point the gas was approaching super sonic speed at which time the flow would greatly diminish. I'm curious...  I wonder if some planes exhibit turbine stall due to cabin leaks and therefore higher demand on the turbos.>>
 
You have to think in terms of power (torque times RPM) and not just RPM. Turbocharger speed is determined by compressor load and the A/R ratio of the turbine. A/R ratio is defined as the area of the turbine inlet divided by the radial distance from the inlet to the turbine axis of rotation. The A/R ratio of a turbine housing is frequently stamped or cast into the housing at the factory. The basic A/R ration can be "trimmed" with an orifice or nozzle on the turbine discharge but this is a "bandaid' fix for a poor turbo selection. Turbocharger selection is a tricky business. So much so that when it came time to select turbochargers for my Lycoming installation I hired a retired Garret engineer to help. This was the same guy who designed the Malibu installation (which was the precursor to the IV-P Continental). Money well spent as he understood the issues with the Continental engine and was able to make a good recommendation for a compressor that had better aerodynamics.

Cabin leaks will NOT have a significant effect on engine performance. Air for pressurization is tapped off of the deck pressure (after turbos and intercoolers but before the butterfly) using a sonic venture. A sonic venturi is nothing more than a hole with known flow characteristics. The "venturi" part of the name refers to the tapered bore which produces a vena constricta that is close to the physical diameter of the orifice. This makes the performance of the sonic venturi more consistent and predictable as compared to a sharp edge orifice. The "sonic" part refers to the fact that the velocity of the air throughout he orifice is the local speed of sound, the fastest rate that a pressure wave can propagate in the medium.

Sonic venturis regulate flow regardless of changes in the down stream conditions so cabin leaks have a negligible effect on pressurization air flow rates. Think of it as a bucket with a hole in the bottom. Fill the bucket with water and the water will flow through the hole at some rate. How far the water falls after it exits the hole has little effect on that flow rate. Similarly, the pressure difference across the sonic venturi has little effect (in the range of pressures we are interested in) on the flow rate through the sonic venturi. Flow rates into the cabin of a IV-P should be about 40 SCFM (FAA says 10 SCFM per passenger) which translated into a sonic venturi orifice diameter of about 0.55".

Turbochargers and Sonic Venturis are simple devices with complex dynamics and a detailed discussion of those dynamics is available in the literature. I have simplified some elements for the purpose of clarity and encourage you to research this topic, if interested. A good source of compressor maps can be found at http://www.turbocharged.com/main.htm.

Regards
Brent Regan
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