sequential turbo operation?

[dnky]

I am currently underway for the Navy, and when I get back from this sixmonther, I will be putting twin turbo's on a 6MGE 3 litre. I am loking for help with how to run sequential turbo operation. I currently have a pair of ct12's, and don't know if they can be respooled, for different ranges, and if not what turbos I can get to spool where I need them.

 

[MikeHalloran]

On xV2000 and xV4000 and similar engines, the turbos are plumbed in parallel, but one has shutoff valves on the cold gas inlet and on the hot gas outlet. The valves are closed below ~1400 rpm.

The valves are simple flapper valves, actuated by small air cylinders. The one in the exhaust is made of semi- exotic high temperature materials, but, ahem, does not demonstrate an infinite life under all circumstances.

My understanding is that these boat engines are equipped with sequential turbos so that they can use a smaller turbo, effective at lower speeds. When the second turbo spools up, the noise level goes up, the boat surges forward, and the fuel flow goes way up, too.

When the owner is not in a great hurry, most of these boats cruise just below the transition point, on one turbo.

The sequential valves add a fair amount of bulk, weight and complexity. But if you've already got 12 or 16 cylinders, two or four litres per cylinder, the increment is not that great.

On a 3 litre engine, I'd wonder if it's worth the effort. What do you expect to gain that a wastegate can't give you?



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[dnky]

I understand the smaller/bigger turbo concept, and that is what I am trying to accomplish. I am looking to have one turbo spool from as close to bottom as possible, and the other to take over at around 3500 to the top. I want to maintain boost all the way across the rev range, and still maitain bottom end power. I am not sure as to how I can figure out what combo of turbo sizes I will need, I am trying to work up a formula for displacement to impeller, and compressor size, or just get lucky, and stumble onto the right combo.

 

[Warpspeed]

The basic principles are fairly straightforward. Start by selecting suitable compressor sizes based on airflow and pressure ratio. The exhaust wheel that "usually" comes with that compressor will have a suitable Rpm and flow capability to drive it. Then test it on the engine. Select a suitable exhaust a/r to get the required boost profile without creating excessive exhaust back pressure. You will quickly discover an optimum size where engine and turbo are happy together over the required speed range.

Garrett turbos might be a better choice than the Toyota turbos. Not that they are any better, but at least with the Garrett's getting your hands on some different a/r exhaust housings will be a lot less trouble.

Getting a smooth transition from small to large turbo may be your biggest problem. It may not run too well right at the point where both turbos are working simultaneously.

 

[dnky]

I will try the garret idea, and maybe I can contact them to find out what they recomend. I didn't clarify enough last time, I am going to try to get them to overlap, and maybe have the second turbo boost be what causes the first blow off valve to dump, this way I can get constant boost, and not blow my head off.

 

[Warpspeed]

Smooth changeover is going to be very difficult to achieve.

At low Rpm all the flow needs to go through the small turbine. But at some point you need to rob the small turbine in order to have something to drive the large one. As soon as some exhaust pressure is diverted away from the small turbine it will instantly fall off in speed. You probably will not have enough exhaust volume to drive both turbos to full boost right at the transition point.

I suppose it depends a lot on relative turbo sizing and how wide a boost curve you are seeking.

One possible way around all this is to place both turbos in series, that is the two compressors in series, large feeding small. And the two turbines in series, small feeding large. It would need a very large capacity external wastegate around the small turbine that could completely bypass it to effectively reduce its back pressure to near zero at high flows. That wastegate could be referenced to full combined boost pressure. As the big one spools up, the small one is progressively bypassed to hold overall boost constant. And a series arrangement may give a more seamless changeover. It also solves the compressor back-flow problem, and resulting compressor surge at low flows.

The small compressor housing may be restrictive at maximum top end airflow, but the big compressor should have pressure ratio to spare when absolutely flat out.

For a variety of reasons, I believe the series arrangement might be simpler to control and be far easier to get working than a parallel configuration.

Changeover, and compressor surge/back-flow will be the biggest hurdles to overcome with a parallel system.

 

[dnky]

So if I follow I am going to intrgrate the first turbo to the second, to spool it for power on the top end. What i am wondering is if I run sequential as the first turbo is spooling, and making pressure, can I have the exhaust already spinning the other turbo, and geting pressure built, for the change over, then when the first turbos is spun out the other turbo is already spooled, and pushing boost. If I can figurwe out what size impeller, and compressor combo i need to have the second turbo spooled and making boost at the right RPM, or is this just impossible? My biggest concern is getting the power at the bottom end, and still get power at the top end.

 

[Warpspeed]

To get the first turbo spooling early will require all of the exhaust flow.

If you are going to divert some of the flow away to the second turbo, the first is not going to be able to produce as much boost down low.

It is all very well to say have the big turbo already spooled up and ready to go, but it is not that simple.

While the small turbo might work fine by itself, and the large one might just be struggling up onto boost, I doubt if there will be enough exhaust flow to run both together at full boost simultaneously at the changeover point.

 

[dnky]

Ok so the problem is a lack of exhaust pressure to spool both turbo's at the same time. Then if I try to spool one then the other I will get a gap in pressure when I try to switch turbo's. Am I foloing here? So I need to paralell one turbo into the other in order to build boost for both. If I paralell them together will I still get the turbo lag on the botom end, or can I still get enough spool at the bottom to bulid boost?

 

[Warpspeed]

It will not be easy to get working no matter how you go about it.

I have had more success using a fairly large turbo in series with a roots blower. That certainly will work and be a lot easier to control than a pair of staged turbos.

 

[Buttersrq]

dnky your best bet is the get the twins from a 2jzgte (94-98 supra) then all you will have to come up with is an exhaust manifold to mount the ct26b's and run the vacuum lines, or if your really feeling daring you could get the twins from an 13b-rew (93-99 RX7). Biggest problem that you will run in to is both sets of turbos can kill a 6M I would think you better off finding a 7mgte with transmission or just using the 2jz with a aftermarket FMC / EMC. The 7mgte should bolt in with out much trouble but the 2jz may require a lot of fabrication to fit. Question what are you putting this in?

 

[dnky]

I have the ct12's alerady off of a 2JZTE, and I have a friend that does custom tig welded exhaust. I will be beefing the internals on the 6m with forged pistons and rods, with the whole things balanced, and blueprinted. I was considering running one ct12 and a ct26b, hopeing that the bigger ct26b would spool later than the ct12. It is going in my '83 supra. I have built twin turbo applications befoe, just never sequentials, so this is a whole new learning experience for me. I may go with the garret idea, because it seems that they are easy to respool.