Post by Töuge-MR on May 31, 2005 16:01:41 GMT -5
I've been fascinated by the science of turbo matching and wanted to share some of the info I learned from an SCC article from July 2002. It was written by Jason Kavanagh, a turbo engineer with Garrett and titled Performance Dictionary, Turbine Efficiency. I will paraphrase and reduce it to the main points.
-it's "the ratio of useful energy to the energy supplied to it as delivered by a dynamic system which converts kinetic and thermodynamic energy into mechanical power by means of blades arrayed about the circumference of a wheel". You could also say it's a measure of the turbine's ability to use the exhaust's heat and flow to turn the shaft that spins the compressor wheel.
-he claims that turbine efficiency is a much bigger contributor to a turbo's effectiveness than compressor efficiency.
The turbine struggles to turn the compressor wheel at a speed that is required to maintain any given boost level. Turbo lag is when the turbine is unable to meet that demand in a timely fashion.
-The ability of a turbine to do this work is determined by exhaust flow, exhaust temperature, turbine eff., and expansion ratio (turbine inlet pressure minus pressure in the exhaust downstream of the turbine). So expansion ratio is optimized when there is a minimum of resistance in the turbine housing, downpipe, and exhaust. No surprise then why larger dia. downpipes, no cats (or free flow cats), free flow exhausts, and improving turbine housing flows on the CT26 and TD06 are preferred. They all improve the expansion ratio...and the reduced back pressure also improves the engine's volumetric efficiency! We also know that we can increase 'exhaust flow' with cams, intakes, valves, displacement increases, etc. 'Exhaust temp' can be affected with cam gear settings and ignition timing tuning. Refer to RickyB's recent thread "How much would you like to boost today" for a variety of ways to impact turbine efficiency by altering flow and temps.
He discusses how some turbos mate compressor wheels with much smaller turbine wheels and how this will be restrictive as flow increases (ie rpm rises), and cause spool up to be worse. He uses an example of a 76mm comp. wheel mated to a 53.85mm turbine wheel (ie. HKS GT2540 and E.B. GT25) and compares it's efficiency to a 60mm/53.85mm combo (HKS GT2530 and GT28RS). The larger compressor wheel reduces turbine efficiency by 8-15% which slows spool-up by roughly 1500 rpm! "An engine that requires a 76mm compressor wheel would be much better served with a larger diameter turbine wheel". Factoring in inertia of larger wheels, he recommends a compressor to turbine wheel diameter ratio of 1.1-1.25:1. (Note that the GT28RS has a ratio of 1.1:1, ATP GT2871R 1.32:1, GT3071R 1.18:1, ATP GT3076WRG/HKS GT2540 1.41:1.). Okay, so we need to use the compressor map then to determine the compressor wheel size to match our engine's flow...and then match the turbine wheel to that compressor wheel using the wheel ratio guide...piece of cake!
He also quickly points out the adverse affect on turbine eff. when "clipping" wheels, thicker ceramic wheels, and turbine housings bored out for larger wheels; and the benefits of smoothing the casting bumps and lumps with a die grinder and Extrude honing the turbine housing.
While this article used some GT turbos as examples, I'm sure the info applies equally to all makes and models. I encourage the experts to add to this info as we don't usually discuss turbine efficiency directly, while some of our mods obviously have an impact on it.
Edit: He did also show a couple of turbine eff. maps to illustrate what he was saying, but noted a full discussion would come later. I don't know if that was done, and would welcome anyone to add info on using these maps. Garrett shows their maps on their website.
-------------------------
www.sportcompactcarweb.com
www.mr2oc.com/showthread.php?t=108366
-it's "the ratio of useful energy to the energy supplied to it as delivered by a dynamic system which converts kinetic and thermodynamic energy into mechanical power by means of blades arrayed about the circumference of a wheel". You could also say it's a measure of the turbine's ability to use the exhaust's heat and flow to turn the shaft that spins the compressor wheel.
-he claims that turbine efficiency is a much bigger contributor to a turbo's effectiveness than compressor efficiency.
The turbine struggles to turn the compressor wheel at a speed that is required to maintain any given boost level. Turbo lag is when the turbine is unable to meet that demand in a timely fashion.
-The ability of a turbine to do this work is determined by exhaust flow, exhaust temperature, turbine eff., and expansion ratio (turbine inlet pressure minus pressure in the exhaust downstream of the turbine). So expansion ratio is optimized when there is a minimum of resistance in the turbine housing, downpipe, and exhaust. No surprise then why larger dia. downpipes, no cats (or free flow cats), free flow exhausts, and improving turbine housing flows on the CT26 and TD06 are preferred. They all improve the expansion ratio...and the reduced back pressure also improves the engine's volumetric efficiency! We also know that we can increase 'exhaust flow' with cams, intakes, valves, displacement increases, etc. 'Exhaust temp' can be affected with cam gear settings and ignition timing tuning. Refer to RickyB's recent thread "How much would you like to boost today" for a variety of ways to impact turbine efficiency by altering flow and temps.
He discusses how some turbos mate compressor wheels with much smaller turbine wheels and how this will be restrictive as flow increases (ie rpm rises), and cause spool up to be worse. He uses an example of a 76mm comp. wheel mated to a 53.85mm turbine wheel (ie. HKS GT2540 and E.B. GT25) and compares it's efficiency to a 60mm/53.85mm combo (HKS GT2530 and GT28RS). The larger compressor wheel reduces turbine efficiency by 8-15% which slows spool-up by roughly 1500 rpm! "An engine that requires a 76mm compressor wheel would be much better served with a larger diameter turbine wheel". Factoring in inertia of larger wheels, he recommends a compressor to turbine wheel diameter ratio of 1.1-1.25:1. (Note that the GT28RS has a ratio of 1.1:1, ATP GT2871R 1.32:1, GT3071R 1.18:1, ATP GT3076WRG/HKS GT2540 1.41:1.). Okay, so we need to use the compressor map then to determine the compressor wheel size to match our engine's flow...and then match the turbine wheel to that compressor wheel using the wheel ratio guide...piece of cake!
He also quickly points out the adverse affect on turbine eff. when "clipping" wheels, thicker ceramic wheels, and turbine housings bored out for larger wheels; and the benefits of smoothing the casting bumps and lumps with a die grinder and Extrude honing the turbine housing.
While this article used some GT turbos as examples, I'm sure the info applies equally to all makes and models. I encourage the experts to add to this info as we don't usually discuss turbine efficiency directly, while some of our mods obviously have an impact on it.
Edit: He did also show a couple of turbine eff. maps to illustrate what he was saying, but noted a full discussion would come later. I don't know if that was done, and would welcome anyone to add info on using these maps. Garrett shows their maps on their website.
-------------------------
www.sportcompactcarweb.com
www.mr2oc.com/showthread.php?t=108366
