Twin turbo problems - Printable Version

Twin turbo problems - Printable Version

+- STD (https://www.superturbodiesel.com/std)
+-- Forum: Tuning (https://www.superturbodiesel.com/std/forumdisplay.php?fid=6)
+--- Forum: Engine (https://www.superturbodiesel.com/std/forumdisplay.php?fid=7)
+--- Thread: Twin turbo problems (/showthread.php?tid=1449)

Twin turbo problems - muuris - 06-20-2010

Earlier I put He351VE into OM606. Got max tq at 3200rpm, but I wanted it more lower down without sacrificing max power... so I put in another, a smaller turbo from a 325tds beemer. It's a Garrett TB25 unit with 41mm compressor ind and 42mm turbine exd.

They have been basically arranged this way:

[Image: ttt.png]

So both turbos are on the manifold, as well as external wastegate (which was already with He351, as a "safety" feature to control exhaust manifold pressure.. so not driven by boost). I put a butterfly valve between the small turbo and its downpipe, so I could bypass it while Holset is on full charge. There is also a throttle butterfly from a Volvo 960 on the cold side. While it's closed, air must go from the Holset through Garrett and into the cooler. While on full load, this opens so there should be no pressure differential over the small turbo, so it should not make boost.

There is a branch on the inlet of small turbo. This has a one-way check valve and a separate air filter. This is because before I had this, it made vacuum between the turbos!

In real life the system is mostly hidden, you can barely see the small turbo and its tubing:

[Image: horrett2.jpg]

The problem is... it won't build boost! While set on small turbo alone (cold side butterfly closed, hot side open, VGT vanes closed) it barely gets 0,1bar of boost. While on VGT alone (cold side butterfly open and hot side closed) it will boost, but not as well as it did before the twins. I understand that even though the butterfly in the dp of the Garrett should almost "eliminate" it, the is some flow which weakens the respose of the Holset. To ensure the butterfly won't stuck, I made it 2mm smaller in diameter than the pipe it is in.

Haven't adjusted the changeover-period settings, since neither of the two modes this system has works properly yet. Any ideas, what am I missing?

RE: Twin turbo problems - E300TSC - 06-20-2010

At first glance, it would seem that closing the vanes on the VGT does not create sufficient flow to the smaller turbo.

Try this as a suggestion:

[Image: ttt.png]

RE: Twin turbo problems - ForcedInduction - 06-20-2010

You're trying to run it as a purely sequential system, that won't work very well with two vastly different sized turbos. As you experienced, it will make spoolup time for the big turbo much longer and the small turbo won't be able to keep up with airflow demand. The small turbo is consuming exhaust energy that would otherwise be put into spinning the large turbo.

I believe you don't make much boost for two reasons;
1, you're running the small turbo in its choke range. 0.1bar or 1.0bar, that turbo is on its choke line on your engine above 2500rpm for what T25 maps I can find. You need something in the T3 range.
2, you're consuming exhaust trying to spool up the big turbo as well.

You need something closer to a compound or modulated two stage system.

Here is how BMW and Cummins do it.
[Image: attachment.php?aid=1607]

*Note VNT+Mech. Compressor producing more low-end power.
**Also note its Mean Pressure, not boost.
[Image: attachment.php?aid=1592]

RE: Twin turbo problems - muuris - 06-21-2010

(06-20-2010, 04:22 PM)E300TSC At first glance, it would seem that closing the vanes on the VGT does not create sufficient flow to the smaller turbo.

This is what I believe and am afraid of now, since many things have been ruled out and I have not many ideas left. Still looking for a clear fault, as the big one's performance is so weak compared to it being as single. Not much exhaust energy should flow through the small turbo when butterfly closed.

RE: Twin turbo problems - Tymbrymi - 06-23-2010

(06-21-2010, 11:13 AM)muuris This is what I believe and am afraid of now, since many things have been ruled out and I have not many ideas left. Still looking for a clear fault, as the big one's performance is so weak compared to it being as single. Not much exhaust energy should flow through the small turbo when butterfly closed.

Have you tried making the butterfly valve closer to the size of the pipe instead of 2mm smaller? I can't think of anything else besides the VGT not providing enough restriction. Maybe the crappy GT37 would provide enough Wink

RE: Twin turbo problems - muuris - 06-23-2010

Today locked VGT vanes closed with a proper bolt. Seems I can get around 30psi of EMP at ~2500rpm while nearly (practically) all gases go through the Garrett. So maybe the hot side is adequate. Tested the boost pipes for leaks with compressed air. Found one where I didn't suspect; where the boost pipe connects the Garrett. Now hoping that sealing it will make a big difference. We'll see tomorrow.

There's really not much space, so I couldn't connect boost pipe to Garrett with a silicone. Instead, I had to make a steel pipe that connects directly to the compressor housing and is bolted to it using stock wastegate mounting points. I intended to seal it before putting things together, but seems that I forgot to. Very annoying since it very unreachable now, got to disassemble quite a bit.