Post turbo exhaust tuning important? 1

[Viper488]

I've been searching the internet and can't any info on choosing pipe diameter, flow requirement calculations etc for a turbocharged car. Plenty of theory for n/a cars of course.

People in the car forums I hang out in say it 'doesn't matter'. Could be I suppose. With the turbo blocking so much of the exhaust's strength, maybe post turbo exhaust details aren't so important?

 

[turbomotor]

Hello Viper488,

I have some experience that you may find interesting:

1. In the '80s, I was involved in altitude chamber testing of an aircraft engine that was turbo normalized. We achieved a test point with 30"HgAbs intake manifold pressure and 18"HgAbs pre-turbo system pressure (the turbo system was a two-stage system). The high positive delta P across the engine helped achieve a BSFC of 0.36 lb/bhp-hr, which is relatively good economy for a spark ignition avgas engine. However, the engine/turbo system was designed for one operating point, and transient response was not good. There is a lesson, though. Consider the whole four stroke cycle and the integrated PdV for the whole cycle when modeling the indicated work.

2. A little later, I was involved with the application of a VNT turbo to a passenger car engine. We were testing in the lab with a simulated exhaust restriction when we ran into a problem and had to remove the exhaust restriction. Engine bhp rose from 175 to 225 (with the same intake MAP and fuel flow) when we ran without the production (cat and muffler) restriction level. Some production exhaust systems can be very restrictive.

3. If you look at the turbo installation in large stationary engines, you will note exhaust diffusers that diffuse the turbine exit velocity prior to an atmospheric dump. This is done to reduce the turbine out static pressure, because it has a significant effect on engine system efficiency.

The bottom line is that careful turbocharger exhaust design can improve overall system power output amd thermal efficiency. However, this is rarely done in passenger cars (including performance automobiles) for two reasons:

1. exhaust diffusion requires space and can be bulky

2. transient response demands usually require the turbine expander to be undersized for most of the engine operating points and a lot of entropy is generated by the mass bypass control (wastegate) anyway, so why bother perfecting the exhaust?

I hope this wasn't too wordy.

PS. The many comments regarding turbine work are good but not quite complete. Turbine work is related to the mass flow times the actual enthalpy change across the stage. So it is related to mass flow, available pressure ratio, inlet temperature, gas composition, and turbine efficiency. The turbine expander work conditions are easy to quantitatively understand in a steady-state test rig, but can be more complicated in the dynamic environment of a recip engine exhaust stream.

 

[patprimmer]

Rob

Why do you think there is a total lack of participation in this thread by the almost all the established respected professional members of this forum.

I'm surprised it's still here. I guess it survived because despite the poor science exhibited, everyone has behaved in a respectful manner, so others have just ignored it.

I guess if someone reacts emotionally to this post, it might then get red flagged and go.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[jbthiel]

viper488 " I tend to think of course post turbo exhaust component selection IS just as important as in a n/a car. At least in a performance turbo car; street or not."
--I agree, thats what I was trying to say.

My "TURBO street car comment" was aimed at PRE-turbo exhaust system tuning. In street cars (or street cars modified for race use) there is a significant space and dollar restriction on the installation of an all-out equal lenth header feeding a turbo. Especially when you consider the minimal power increase for all the pain involved. In street cars you can just up the boost to make up for the losses. But if you consider the case of F1 or Indy they have to take every opportunity possible, and this includes proper headers. But they have the benefit of time, money, fewer interferences, etc.

As for pipe diameter, POST-turbo, I would think that could be estimated from the total CFM your combination is using/expelling. But I still agree with "Hemi" on the overall design philosophy. Smooth transitions, etc.

I also found a rule of thumb that says " 1 inch Hg backpressure = 1 HP lost" ((For reference, we have the following conversions factors: 1 ATM = 14.7 PSI = 76 cm of Hg = 29.921 inches of Hg = 1.013 bar)) Use at your own peril

 

[jbthiel]

I finally found that info I was looking for. Check out

http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html