20 replies to this topic

[Christian L]

I have a project where I am researching exhaust manifold design for a turbocharged race enigne spinning upwards of 18K rpm. While looking through old designs from various teams in various forms of motorsport I always see a trend of having the individual pipes collect right before the turbine, this is the same on the inline 4 cylinder 1.5l F1 turbo engines, and on the v6's we see 3 pipes on each side meet right before the turbo- almost as if that ~2inches of straight pipe is just so a wastegate fits.

I thought a 4-2-1 design (much like on tuned NA enignes) would be best for the harmonics of the exhaust pulses but why do these other teams not do it that way? I would think that you would want a nice straight shot into the turbo and not have 3-4 pipes meet and have the directional vector pointing in 3-4 separate directions if that makes sense.
The only idea I can come up with is that those engineers wanted the longest, uniform diameter pipe possible so the gases would not have to expand and contract.

I would greatly appreciate any information on this

[12.9:1]

Christian,

Why do you doubt the consensus of what works? Perhaps you are limited to extremely low boost ? If not just do as the professionals do, 4 into 1, and as short as "packaging" allows. 18000 rpm !?

[Christian L]

Thank you for your reply- I would like to do 4-1 but our chief engineer/engine lead and others think that a tuned 4-2-1 would be better, or atleast, would not harm performance. If I want to go with a 4-1, as I do, I will have to present the reason to the other engineers and back up my reasoning in detail, and later there will be much stricter scrutineering on our design.
Boost levels will be around 1 bar with short periods of 1.1 bar- at this level we are developing enough power and more boost would just take away engine reliability. The rpm's are not a typo, it is just a high revving package.

[12.9:1]

Christian

Ok that's not a lot of boost, In any case if you figure a system for a 'NA' engine with a peak torque at around 16k your primary tube length would be in the 25cm to 35cm range, and so will be as short as "packaging" allows - more or less.

[shaun979]

May I ask what engine this is that's spinning upwards of 18,000 RPM at 1 bar of boost?

[Christian L]

They are small displacement motorcycle engines, we have a few that have been running with good reliability at these rpm and boost values.

[J. Edlund]

Back in the days of the turbocharged formula one engine there was basicly two configurations used; a V6 engine with two turbochargers and three cylinders per turbocharger; an inline four using a 4-2 exhaust system and a twin scroll turbocharger.

Turbochargers are also a bit different when it comes to exhaust tuning, for a turbocharged engine the use off correct pipe lengths are not as imortant, there are other more important issues like getting the turbocharger spooled.

I would also recommend that you get a book that describes the basic principles of exhausts used with turbochargers; consant pressure, pulse pressure and pulse converter.

Also, you usually don't want to connect more than three cylinders per turbocharger inlet to avoid overlappning exhaust valve timings while less than three cylinders should be avoided as there then would be periods when there are no exhaust flow at all.

[Christian L]

Thank you for your reply. Which book would you recommend that deals with turbocharged exhausts? I ordered a book last night from the SAE that covers design of engine manifolds but I am unsure if it covers turbocharged applications as well.
We must tie all four exhaust pipes to one turbo as the engine displacement is so small that running duals would not be possible (such turbo's do not exist from our target suppliers -Garrett and IHI).

[12.9:1]

Try the old, Search BB - The Technical Forum - turbo - any date

Good reading and book recommendations.

[Christian L]

I tried seraching through old topics, the lack of applicable data led me to post my question because I did not see the specifics I asked for in any thread. Many people dance around the topic of 4-1 or 4-2-1 headers on a boosted engine because they just copy the designs of others and move on, and that is not an option here. Books like "Turbochargers" or "Maximum Boost" are not technical and are setup more like picture books and contain just plain wrong information because so many of the formulas carry with them false assumptions that are defying laws of thermodynamics.

[Greg Locock]

It's been a long time since I've done turbos.

One thing I thought of - the turbine speeds up and slows down by ~50% during one cycle, as the pulses hit it. By making the pulses stronger you may be amplifying that effect, which I suspect is not a good move.

Secondly, a lot of the energy in the exhaust is heat, so by running longer headers you will be wasting that energy (as the temperature drops your product (P*vdot) will drop.

Another question is, how much of a reflective pulse do you get off a turbo? if the inertia matching is correct you might not see any, in which case you don't need to worry about tuning. (This is the killer I think)

In principle I agree that properly tuned headers would be beneficial, but perhaps the above, and friction, outweigh the slight (?) advantage in scavenging.


Anyway, guessing that this is FSAE, why not build both and find out. Hell, it might even get you a few design points!

[12.9:1]

Christian, This may be the book your looking for; Also have a look at Concepts NREC

For a less technical yet authoritative book try - "Maximum Boost"by Corky Bell



____________________________________

Fundamentals of Turbocharging
Nicholas C. Baines
$95.00

Available NOW!


This book is the first comprehensive treatment of turbochargers and turbocharging to be made widely available in the last twenty years. It is intended to serve as both an introduction to the turbocharger itself, and to the problems of matching a turbocharger with an internal combustion engine. The turbocharger is a highly sophisticated device, which has been described as aerospace gas turbine engineering allied to mass production techniques. Undoubtedly the key to commercial success lies in achieving the correct compromise between performance, life, and cost, and this runs as a continuous thread through the book.

The operation of turbomachines is fundamentally different from that of reciprocating machines, so that the turbocharged engine has many complex characteristics, not all of them desirable. The means by which the advantageous characteristics are exploited to the full, and the technology required to overcome disadvantageous, are fully explained. The latter includes modern developments such as variable geometry, turbocompounding, and electric assist.

Contents:

*
Introduction to Turbochargers and Turbocharging
*
The Centrifugal Compressor
*
The Turbine
*
Turbine testing
*
Mechanical Design of Turbochargers
*
Matching the Engine and Turbocharger
*
Turbocharging System Developments
*
Pulse Flow Performance of Radial Turbines

[voice_of_reason]

The standard reference text on the subject is:

Watson, N., and M. S. Janota. Turbocharging the Internal Combustion Engine. New York: John Wiley & Sons, 1982. ISBN 0471870722

"An extensive and excellent professional reference text on turbochargers, and turbocharged engine performance".


It is a complete treatment of the theory and practical engineering of turbochargers and their installation.

It is not a lightweight, and not suited to a "back street tuner".

[J. Edlund]

"Fundamentals of Turbocharging" is probably not as complete as "Turbocharging the Internal Combustion Engine", but it's newer. Both are probably worth to look into. Books such as "Maximum Boost" are sadly not that correct...

No info about exhaust manifolds but still worth a look
http://users.du.se/~...ohed/870701.pdf
http://users.du.se/~...ohed/890877.pdf

Several turbocharger manufacturers does also have small twin scroll turbochargers availible, they allow the cylinders to be separated but with only one turbo.

[Halfwitt]

You might find something interesting by looking at the turbo engines in Le Mans / ALMS and of course, Indy / Champ Car / whatever it is called nowadays.

If I remember correctly, the Cadillac LMP car had a V8 turbo, with only three headers of each bank driving the turbo which, similar to your application is low boost. Their reckoning was that they could spin the turbo with three pipes, therefore the other cylinder on each bank didn't have to suffer the unwanted back-pressure caused by the turbo. Therefore the engine made more power.

I think the Cosworth (and probably most of the others) used turbochargers with variable inlet guide vanes on their Indy engines.

[J. Edlund]

Originally posted by Halfwitt
You might find something interesting by looking at the turbo engines in Le Mans / ALMS and of course, Indy / Champ Car / whatever it is called nowadays.

If I remember correctly, the Cadillac LMP car had a V8 turbo, with only three headers of each bank driving the turbo which, similar to your application is low boost. Their reckoning was that they could spin the turbo with three pipes, therefore the other cylinder on each bank didn't have to suffer the unwanted back-pressure caused by the turbo. Therefore the engine made more power.

I think the Cosworth (and probably most of the others) used turbochargers with variable inlet guide vanes on their Indy engines.

The Cadillac uses two Garrett TR30R turbochargers fed by four cylinders each.



The type of turbocharging (assymentric) you mentioned have however been used, and a Saab V6 turbo is the only engine I know that have used it. It used a turbocharger on one of the banks which then supplied all six cylinders with air. Such an engine is very difficult to make, airmassflow though one bank will be higher than through the other which the fuel injection must compensate for. There was even different hardware on the turbo and non turbo side. The boost in that case was very low (no wastegate was used) and the engine had less top end power than the NA version.

Turbochargers with variable inlet guide vanes aren't used on spark ignition engines since they can't handle the temperatures. For example Garretts VNT turbochargers can handle a maximum of 825 degC and given that even the methanol fueled champcar engines reached 900 degC...
In any case, variable geometry is also prohibited in champcar aswell as in WRC and LeMans.

Champcar and WRC only allows a single turbocharger.

[McGuire]

In CART this setup was known as SST or single-side turbo. The theory: at relatively low boost levels as were then mandated by the rules, and with a single turbo of fixed size as also mandated by the rules, it could be more efficient to drive the exhaust turbine with just one bank of the engine, with the intake compressor feeding both banks. In practice it did not prove out. In order to fully exploit the assymmetry it must be chased all through the engine, in cam timing and everything else, and even then the potential gains amount to some very fine whittling.

As Edlund noted, the Cadillac LeMans effort used two turbochargers, one on each bank. That program was performed by McLaren Performance Development in Detroit (no longer any relation to the Brit F1 company).

The SST is a sort of engineering freak, inspired by the peculiarity of the rules. In normal practice there is no reason on earth for such a thing. Interesting if inane factoid: if you track back through the regulations and their origins, the CART turbocharger spec actually had its roots in an Allis-Chalmers farm tractor unit. The latest word I hear on the next Champ Car engine is Cosworth is looking hard at a twin-turbo -- cheaper these days, and allows superior packaging. The current single turbo is bulkier than it needs to be for the boost levels involved and the only good place for it is tucked into the gearbox area, which creates a number of problems. In a spec car working from a clean sheet of paper, there is almost certainly a better way. Provided the initial investment in the change can be supported, of course.

When all is said and done, I believe there is no real advantage in either a 4-2-1 or common collector header design. The overriding issue is packaging.

[zac510]

I've made interesting observations by observing hot turbo exhaust manifolds on engine or chassis dynos. Obviously the tight angles appear far hotter than the smoothly curved sections. I know it doesn't answer your question about pulse tuning, sorry.

I was even fortunate enough to observe my own self-designed and built manifold (4 cylinder) heat up significantly on a chassis dyno when we experienced a timing problem that retarted the timing advance a lot. I wasn't quick enough to take a photo, unfortunately.

I did a quick google search and came up with a couple of examples:

http://home.comcast....kysrt/turbo.jpg - Indy engine, DFX?

http://greenstuff.de...mages/turbo.jpg - Engine unknown, perhaps 4G63.

http://www.lotusespr.....e glowing.jpg - Lotus V8

Can anyone elaborate on this type of self-analysis?

[Fat Boy]

Originally posted by McGuire
In CART...The SST is a sort of engineering freak

And it was too damn loud.

If I remember right it actually performed well on ovals where the lag was less of an issue.

[Ross Stonefeld]

From the fan side thats where I remember it showing up. I want to say first public memory is Motegi 00 in a Montoya-Lola-Toyota? Loud as hell, and it seemed to me like the experiment didnt last long? Then again everything broke on Montoya's car that year except maybe the driver so who knows what the R&D follow through was like over there.

but yeah...loud

[Powersteer]

From what I know it is important to have a good exhaust header for turbo engines. It doesn't really matter how much lag if there are more important issues to adress. When boost builds up, there is just too much exhaust going out that it builds so much pressure (known as back pressure) that tuners are known to do their best in both exhaust house tuning and wastegate design to minimise this. In relation with this is a good exhaust header design.

I think a 4 into 1 because it does not slow down the exhaust gas until the collector and just shoots the exhaust through the turbine. A 4-2-1 slows the gas down sooner after the first collector because the second stage usually has a larger diameter pipe. It is better to evenly distribute the exhaust pulse because a turbo behaves in some ways like a turbine engine where the more constant the flow the more efficient it works, just the way a turbine engine becomes more efficient with rpm. Probably get less lost gasses too.

Also, with a more efficient extraction of the exhaust more air/charge gets to pass the combustion chamber which naturally means more cooling for the whole chamber. I would not mind on a laggy turbo set-up if it does not keep too much heat. I wonder how much tuning turbo experts do to their exhaust blade attack angle. The steeper the more efficient for the turbine but more harsh towards flow while a shallow attack angle will help exhaust flow better but you get more lag. Maybe variable blades is the answer to the future of boost control. Helicopter blade style.