Twin-Scroll Turbo System Design Explained

Interesting | January 2nd, 2010 by 12
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The second model in the U.S. 5 Series Gran Turismo line-up is due to be unveiled at the Detroit Auto Show in less than two …

The second model in the U.S. 5 Series Gran Turismo line-up is due to be unveiled at the Detroit Auto Show in less than two weeks. The launch of the BMW 535i GT also coincides with the introduction of a new BMW engine, internal codename: N55.

The new BMW Power Turbo uses a single twin scroll turbo instead of twin turbochargers as the N54 engine. The 2,979cc six-cylinder powerplant features for the first time High Precision Injection and variable VALVETRONIC valve management combined with the single twin-scroll turbocharger.

Even though BMW has not yet confirmed our suspicions, we believe the single turbocharger used in the 5 Series GT is the same as those fitted to the new 4.4 liter V8 engine in the X5M/X6M. Power and torque is similar to the twin turbo engine – 225 kW/306 hp at a speed of 5,800 rpm- but the 400Nm/295 lb-ft peak torque kicks in 1,000 rpm earlier at 1,200 rpm.

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Compared to the BMW turbo engine already well-known in the 335i, 535i and other models, the N55 engine offers even greater fuel economy combined with optimized CO2 management. The power unit of the BMW 535i Gran Turismo is the first turbocharged engine to feature a map-controlled oil pump which, taking up less energy, makes a further contribution to motoring efficiency.

Since the full details on BMW’s N55 twin-scroll engine are yet to be fully revealed, we believe you will find the article below quite interesting. Modified goes into some of the details related to a Twin-Scroll Turbo System design.

Twin-scroll turbo system design addresses many of the shortcomings of single-scroll turbo systems by separating those cylinders whose exhaust gas pulses interfere with each other. Similar in concept to pairing cylinders on race headers for normally aspirated engines, twin-scroll design pairs cylinders to one side of the turbine inlet such that the kinetic energy from the exhaust gases is recovered more efficiently by the turbine. For example, if a four-cylinder engine’s firing sequence is 1-3-4-2, cylinder 1 is ending its expansion stroke and opening its exhaust valves while cylinder 2 still has its exhaust valves open (while in its overlap period, where both the intake and exhaust valves are partially open at the same time). In a single-scroll or undivided manifold, the exhaust gas pressure pulse from cylinder 1 is therefore going to interfere with cylinder 2’s ability to expel its exhaust gases, rather than delivering it undisturbed to the turbo’s turbine the way a twin-scroll system allows.

The result of the superior scavenging effect from a twin-scroll design is better pressure distribution in the exhaust ports and more efficient delivery of exhaust gas energy to the turbocharger’s turbine. This in turn allows greater valve overlap, resulting in an improved quality and quantity of the air charge entering each cylinder. In fact, with more valve overlap, the scavenging effect of the exhaust flow can literally draw more air in on the intake side while drawing out the last of the low-pressure exhaust gases, helping pack each cylinder with a denser and purer air charge. And as we all know, a denser and purer air charge means stronger combustion and more power, and more power is good!

But the benefits of twin-scroll design don’t end there. 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.

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12 responses to “Twin-Scroll Turbo System Design Explained”

  1. Jordan says:

    The peak torque kicks in 100rpm sooner on the N55, not 1,000rpm.

    N54 – peak torque @ 1300rpm
    N55 – peak torque @ 1200rpm

    • Doug says:

      I think these “peak torque” rpm quotes are for 95% of torque, not peak torque, which should be up at 2500 or so. If I’m right, then this confusion could be comparing to different means of measuring and Horatiu could be right.

      However, I doubt that this is the same turbo — they’re probably buying from a company that makes turbos and has a whole line of them. Consider that the turbo is feeding 6 cylinders versus 4 for the V8. On the other hand, the N54/N55 has very low compression anyway, there’s probably more factors than cylinders and displacement.

      • Jordan says:

        As far as I know, I’m pretty sure it’s full torque they’re talking about.

        Here is a dyno chart on the N54 and it shows full torque at 1300rpm. Converting the power figures it gives, you get 306hp and 300 lb-ft which is what the engine is rated at.

        I remember seeing a 1,000rpm difference mentioned somewhere else as well. I think it was just a simple keyboard mistake that ended up getting copied. I think virtually all turbo engines are advertised with 100% torque since it’s available at low RPMs compared to N/A, and N/A would use a lower percentage rating to market the engine’s torque as best as possible.

        • Doug says:

          You know, I’ve seen that graph. It’s very strange because it looks like it’s been truncated exactly at 400nm; it’s just not a natural curve. Something artificial is going on, either with the test equipment/method or the ECU (or both).

          Take a look at this chart:

          • Jordan says:

            Ya that looks like it came right off a dyno. The one I showed I think is for BMW marketing and so it would be smoothed out. I think the squiggly lines from a real dyno chart might be interpreted by someone who isn’t familiar with dynos, to be a rough engine and therefore not a good marketing tool.

            However, that dyno shows the peak torque at about 2200-2300rpm… the only logical reason I can think of is that dyno was for power at the rear wheels and with frictional losses you get decreased power and it makes sense to me that it wouldn’t be able to produce maximum torque at the same rpm or “time” as it would be able to produce it at the crankshaft (BHP). there would be a “delay” because of frictional losses, etc.

            The dyno chart you showed would be an accurate representation of the power and torque at the rear wheels and the one I posted would be the crankshaft power and torque….. after BMW modified it to have less power than it actually does and then to smooth the lines out. this is just an educated guess on it and i don’t think BMW would necessarily “lie” about where the peak torque starts. if anything, they would say it starts at 1500rpm on a particular engine but in reality it starts earlier, at 1400rpm, at the crankshaft, which is better and i’m sure no one will complain!

          • Doug says:

            That’s a good point, mine is definitely at wheel. I guess metric “metrics” don’t specify which is which. But assuming that’s what’s going on, how do you interpret the difference between them… some theories might be:

            — the transmission increases in efficiency as engine rpm increases
            — without that transmission’s specific load, the engine produces up to some limit (limited by some component, or some safety limit via ECU)

            If it were a component limit, such as turbo blow-off, fuel pump, then the smoothing could be filtering out the fluctuations that should be occuring in that plateau area.

            They probably won’t lie, but …. there N54 is so good and has so much headroom that they may be holding it back for various reasons, not the least of which is competition with the M3’s engine. Or, possibly for reliability until its proven at those higher performance levels. The manner of “limiting” could show as a flat line.

  2. 100$ GUY says:

    Just unbelievable that as soon as you barely touch the accelerator, you have all the torque available!!!!!!!!!! Such a broad flat torque curve……where amazing happens LOL

  3. Doug says:

    Btw, speaking of twin-scroll turbos, is it true the some of the BMW diesels use variable-vane turbos?

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