Twin Scroll or Twin Turbo?

[JimD]

I found the description of the new N55 engine with a single twin scroll turbo interesting but was having trouble picturing the benefits versus the N54 twin turbos. The N55 has other benefits but the twin scroll is given as a benefit and it was not clear to me if that was or was not true versus two turbos. I think it is not but the explanation gets a bit involved.

A twin scroll turbo has two paths for exhaust to enter the turbo so that pressure pulses in the exhaust stream from one cylinder does not impede the passage of exhaust from another cylinder into the turbo. So my question could be summarized as "how is two passages into one turbo different from two turbos?"

I looked up the firing order for BMW's 6 cylinders and found it is 153624. We know that it is a four cycle engine meaning it has a power stroke (180 degrees crankshaft rotation) followed by an exhaust stroke (also 180 degrees), followed by an intake stroke (180), followed by a compression stroke (final 180). So in two revolutions we get all four strokes. I googled a bit to get an idea how long the exhaust valve is open and found something suggesting a practical maximum is 280 degrees. That may not be true for a variable system like we have but I used it. So obviously the exhaust valve is open the 180 degrees of the exhaust stroke but it also has to be open at the end of the power stroke and the start of the intake stroke to be open 280 degrees. If you assume these amounts are equal and set up an example where TDC of #1 at the end of compression/start of power is your zero rotation point, you have the exhaust open from 130 degrees to 410 degrees of crankshaft rotation. We also know #5 cylinder will be 120 degrees rotated from #1, #3 will be 240 degrees rotated relative to #1, etc..

I set up a couple examples to show that if we assume BMW has plumbed the N54 dual turbo such that 1, 2, and 3 go into one turbo and 4, 5, and 6 go into the other, you do not have multiple cylinders with their exhaust valves open at the same time feeding the same turbo. For my example with #1 at TDC, cylinders 3 and 6 have their exhaust valve open, the others are closed. I looked at 90 degrees of rotation from this and found #5 and #3 open with the rest closed. So unless I messed something up (possible as I have visably proven), there does not seem to be a benefit from the standpoint of exhaust flow from a twin scroll versus a twin turbo. Weight is lower and other things may be better but exhaust flow should be the same.

Maybe twin scroll is just a way to get this benefit of twin turbos at lower cost and weight?

Comments?

Jim

[PrimoM3]

"Check out the big brain on Brad..."



It's definitely a cost thing for BMW, no question. It might prove to make the ///M models more exclusive though as they seem to all be going twin turbo 6, 8, 10 cylinder. Personally, I'll take the twin-turbo N54 over the standard N55 twin scroll. We'll see what they do with the ///M version though.

[amdmaxx]

twin turbo N55 is the rumored engine for M1.
Best of world world..

My Guess that future M3 will have some super variation of that TT N55..

[scs]

So what makes this twin-scroll turbo so special?

After searching for a long, in depth article on the subject of twin-scroll turbos, the best one was found over at Modified magazine written by David Pratte titled "Twin Scroll Turbo System Design - Divide And Conquer?"

For those who don’t know what a turbocharger is, it is a supercharger driven by the exhaust gases of the engine. The more air you can get into an engine, the better the combustion, power and so forth. The Turbocharger is the most efficient supercharging system because it uses the exhaust (waste) gases created by combustion.

The key to having an efficient motor, turbocharged or naturally aspirated, the exhaust gases need to leave the combustion chamber completely and smoothly. In a perfect system, the exhaust gas leaving the exhaust manifold will leave a low pressure pulse. The exhaust gases are being pushed out of the combustion chamber is in a state of high pressure, and is attracted to this low pressure area. This is known as exhaust scavenging.

In a single-scroll turbo system, especially one with an unequal length exhaust manifold, all cylinders exhaust gases interact with the flow of each other. Just as the exhaust gases have left one cylinder, it is just leaving another. The low pressure pulse isn’t there.

This can cause the opposite of scavenging called reversion, where the exhaust gases go back into the combustion chamber which creates higher temperatures, less power, and more NOx emissions. To remedy this manufacturers have the exhaust valve open as long as possible before reversion occurs.

The a twin-turbo setup, especially on an inline six like the N54, has a better scavenging effect because the six cylinder exhaust system is split in two, with three cylinders for every turbo. This split creates the opportunity for low pressure pulse to occur in the exhaust manifold. But twin-turbo systems are complex and have a higher weight than a single-turbo system.

A twin-scroll turbo allows for the benefits of the twin-turbo system’s exhaust without the weight and complexity. The exhaust housing of the twin-scroll turbo is split in two, one path for three of the N55’s cylinders, allowing exhaust scavenging to happen. The exhaust is also coming in twice as often than a twin turbo setup, which had to be split up. This then spins the compressor side more consistently.

According to Pratte, “With its greater volumetric efficiency and stronger scavenging effect, higher ignition delay can be used, which helps keep peak temperature in the cylinders down. Since cooler cylinder temperatures and lower exhaust gas temperatures allows for a leaner air/fuel ratio, twin-scroll turbo design has been shown to increase turbine efficiency by 7-8 percent and result in fuel efficiency improvements as high as 5 percent.”

With the inner workings of the twin-scroll turbo revealed, the more adventurous might want to venture over to the article linked below.

source link http://www.bmwmonitor.com/blog/10410...l-system-works

[JimD]

I think you might have wanted to link this article:

http://www.modified.com/tech/modp-09...ign/index.html

The N55 has valvetronic which is a good thing and a little less mass, the article on the new 5 series wagon thing says 4.4kg less for N55 versus N54, but I don't see the gas flow advantage over 2 turbos.

You get more total exhaust into the twin scroll but you also have to spin a bigger turbo. That part get complicated because increased mass is not the only effect, there is also worse gas flow around the greater surface area of the twin turbos. But there should be shorter exhaust plumbing to the two turbos.

But simplistically two turbos have two inlets as does a twin scroll single turbo. So how different can they be?

Jim

[BroDoze]

Twin-scroll turbos have been huge in the Subaru world for many years.
The JDM STi's all have them and our 2.5L's don't. Much more efficient turbos capable of a much broader boost curve.

[Revelate]

I don't have experience with the detailed mechanics of engines, turbos and air flow, but I have yet to see the answer to a question no one has apparently asked.

Is there a significant advantage of a twin scroll turbo over a *twin* turbo setup?

All the comparisons I've seen for a twin scroll turbo are with the idea of a single turbo application, even in BMW's own marketing for the N55. It appears obvious that the twin scroll turbo is a huge benefit over a traditional single scroll turbo.

For the N54 parallel twin turbo, 3 cylinders are feeding each turbo so the exhaust pulse collision is reduced just the same as in the twin scroll, no? It has two paths to the turbo, just happens to be two physical turbos instead of split piping to one. There might be some reprocussions in the exhaust path after the turbos causing backpressure issues, but that's just a guess.

As for the N55 twin scroll application, I understand the benefit of a more efficient exhaust flow, with all 6 cylinders spooling one bigger turbo and allowing for the lower RPM torque. But is that a significant difference over a twin turbo, especially for performance? Peak output doesn't improve, and is anyone going to notice the 100rpm sooner torque curve, especially if you shift in the meat of the power band(4-7k)? Seat of the pants? 0-60? 1/4 mile time/trap speed? Can it really be more than thousands of a second difference?

As for a broader powerband claimed by the N55, is that solely based on the extra 100rpm on the torque curve? Does anyone believe that the twin scroll turbo compressor map will reveal that it's capable of more airflow than the combined N54 turbos? Is that a general assumption of twin scroll advantage vs twin turbo?

I'm out of questions... for now.

[Nicky G]

I love this forum, great thread gang! I learn so much on here.

And the Pulp Fiction reference literally made me crack up out loud -- humor and edumacation, what's not to love about 1addicts?!?!

[and 1]

definitely one of the best threads i've read lately

[KingOfJericho]

I find it interesting that the new 335is has the N54 with the Performance Power kit (larger fan and extra cooler) while the regular 335i gets the N55. Sport model gets the twins turbos, regular model gets the twin scrolls.

[JimD]

I think there is a new engine announced for the Z4 which is a power increased N54 also.

I don't know whether BMW wants to use the N55 awhile before increasing the boost or what. It is causing some speculation on what will be in the M1.

[530i-2002]

Originally Posted by scs

With the inner workings of the twin-scroll turbo revealed, the more adventurous might want to venture over to the article linked below.

source link http://www.bmwmonitor.com/blog/10410...l-system-works

I am puzzled by the following quote from the linked article - "The current production N54 makes 335HP and 332 ft.lbs of torque. The N55 makes the 306 HP and 295 ft.lbs of torque, the same amount the N54 made when introduced."

Are these 'unofficial' N54 specs?? BMW still lists 300 HP/300 ftlb for N54, where do these numbers come from? Thanks.

[KingOfJericho]

I think it's their way of helping to lower our insurance premiums.

[Ash83]

Originally Posted by 530i-2002

I am puzzled by the following quote from the linked article - "The current production N54 makes 335HP and 332 ft.lbs of torque. The N55 makes the 306 HP and 295 ft.lbs of torque, the same amount the N54 made when introduced."

Are these 'unofficial' N54 specs?? BMW still lists 300 HP/300 ftlb for N54, where do these numbers come from? Thanks.

good catch 335HP is a lot compared to the official 300HP..

[southlight]

Originally Posted by 530i-2002

I am puzzled by the following quote from the linked article - "The current production N54 makes 335HP and 332 ft.lbs of torque. The N55 makes the 306 HP and 295 ft.lbs of torque, the same amount the N54 made when introduced."

Are these 'unofficial' N54 specs?? BMW still lists 300 HP/300 ftlb for N54, where do these numbers come from? Thanks.

These are the specs of the Z4sDrive35iS, the most powerful of the three models still having the N54.


Best regards,
south

[530i-2002]

Originally Posted by southlight

These are the specs of the Z4sDrive35iS, the most powerful of the three models still having the N54.


Best regards,
south

Is that the tuned "IS" engine that 2011 3 series is getting? I thought BMW official position was the max amount of torque the 6speed MT could handle was 315 ft/lbs (at least when the Performance kit came out)...

[mapezzul]

Originally Posted by 530i-2002

Is that the tuned "IS" engine that 2011 3 series is getting? I thought BMW official position was the max amount of torque the 6speed MT could handle was 315 ft/lbs (at least when the Performance kit came out)...

"is" models will not have manuals (period).

The other thing is that the N54 has been around since MY2007 and has been vetted and engineering has been completed on the performance upgrades that is why it remains in the "is" models. It is a simple life cycle, much like cars- BMW has done this before with NA motors so I am not sure why everyone is so surprised by this, the N52 in the 325,328,330,X3si are all essentially the same. They design the base motor then they design the upgrades. The 740 in the ROW was the first upgrade, then the BMW Performance kit and finally the Z435is. The 3 series and Z will not be the same for the "is" models.

The N55 will also get a performance boost in the next year with a similar life cycle and roll out as the N54.

The valvetronic is a huge upgrade in the N55 and the dual scroll is comparable to the twins. Initially it will cost more to recoup the redesign of the Valvetronic (costs more than a turbo) and the dual scroll costs more than a single but over the long haul it will save BMW build money while providing the same performance and greater efficiency...

oh and I am sure this will come up- there is a HPFP with the N55 as it is DI, BMW is in Rev. 3 of it.... let us hope that the issues do not get worse. It is still attributed to high ethanol (some fuel is at 20% even though it states 10.. bad mixing to save costs).

[530i-2002]

There's been some talk of upgraded (more efficient?) 6MT in the 2011MY 1 series. I wonder if one reason they are upgrading it is so that it can handle more torque than the current version (max 315 ft/lbs)? I am still trying to decide between 2010 and 2011 MY of the 1 series (2010 is on order but I pushed the production date as far as I could hoping some additional information comes out in the mean time)...

[ChrisK]

My biggest question regarding the N54 vs N55 debate is what the hell is BMW thinking? Why keep the N54 for some and not others...very confusing right now.

[BMW269]

The N54 is the "best" engine, look how many awards it one, but the N55 is better. The N54 is a simple Twin-Turbo engine, but with low boost an small turbochargers to reduce lag. The N55 has one Turbo, but is technically more advanced than the N54, and is better. But that BMW makes the N54 yet, specially in the U.S., is because the Americans simply prefere "Twin-Turbo" over "Single-Turbo", yet without knowing anything on the N55. When it will be out and tested by the buyers, then it will change. The other reason for going Single-Turbo over Twin-Turbo is for making the next M3 more exclusive, special and impossible to be beaten by a 335i. You know a TT L6 is equal to a NA V8. And some 335i are faster than M3. This will not be possible with the next 335i and M3 F30 generation, as the 335i should have Single-Turbo and the M3 Twin-Turbo. And to make the 335i faster than the M3 is not worth the price and the work. So this will push all those not liking Single-Turbo buy the M3 with Twin-Turbo. I think the M3 F30 will be a succes in the States, Japan and Europe, specially if BMW M will be able to keep redline at 8.000 rpm, because the M3 will be more daily drivable, more fuel efficient, lighter and easely tunable compared to NA engine.

[UR2SLOW1]

I was just reading in my Road & Track from a few months back that BMW is going to the single turbo set-up purely for emissions reasons - seems that the single turbo motor heats things up quicker and reduces emissions. The article went on to say that the twin turbo set-up would not meet new US emissions requirements. TM2C

[KingOfJericho]

Originally Posted by Levi

The N54 is the "best" engine, look how many awards it one, but the N55 is better. The N54 is a simple Twin-Turbo engine, but with low boost an small turbochargers to reduce lag. The N55 has one Turbo, but is technically more advanced than the N54, and is better. But that BMW makes the N54 yet, specially in the U.S., is because the Americans simply prefere "Twin-Turbo" over "Single-Turbo", yet without knowing anything on the N55. When it will be out and tested by the buyers, then it will change. The other reason for going Single-Turbo over Twin-Turbo is for making the next M3 more exclusive, special and impossible to be beaten by a 335i. You know a TT L6 is equal to a NA V8. And some 335i are faster than M3. This will not be possible with the next 335i and M3 F30 generation, as the 335i should have Single-Turbo and the M3 Twin-Turbo. And to make the 335i faster than the M3 is not worth the price and the work. So this will push all those not liking Single-Turbo buy the M3 with Twin-Turbo. I think the M3 F30 will be a succes in the States, Japan and Europe, specially if BMW M will be able to keep redline at 8.000 rpm, because the M3 will be more daily drivable, more fuel efficient, lighter and easely tunable compared to NA engine.

The only problem with this logic is that the TT N54 has been out for 3 years so if anyone wants to buy a twin turbo car they can easily do so.