2010 Volvo XC60 T6 AWD: Twin Scroll, No Waiting -- A Turbo Walkaround
January 04, 2011
By now you know that our longterm 2010 Volvo XC60 has a lot of motor under the hood, even if you can't really see it when you pop the clamshell. The mill is torque-rich at low engine speeds and hustles the big wagon around with surprising authority. In short? It's got sauce.
For whence the sauce came, click the jump.
Let's begin at the beginnning. The XC60 T6 has a turbocharged 3.0-liter (actually 2953 cc) inline six that bears the inelegant engine code B6304T2. A single turbo huffs 8.7 psi into the mill, which develops 282 horsepower and 295 lb-ft of torque at its peak, which is actually not a peak at all.
And that, friends, is the beauty of boost -- the pointy heads in Goteburg can dial in with precision the amount of airflow the turbo provides at a given engine speed, and it is largely airflow through the engine that dictates torque. Provided there is adequate exhaust energy present to drive the turbine to drive the compressor, torque output can look, well, like the tabletop you see in the above dyno chart of the B6304T2. You'll never get this kind of area under the curve with a normally aspirated or even a supercharged engine.
The area of greatest focus by engineers of modern turbocharged passenger car engines is not maximum output or efficiency or emissions. It is transient response, or the amount of time after you press your Chuck Taylor down that the engine actually delivers the torque being commanded.
This is particularly difficult for turbo engines when the engine speed is low and you're pootling around at part-throttle. You give the throttle a little squeeze, expecting a little more torque, and you have to wait for that little extra fart of exhaust gas to drive the turbine to drive the compressor to pump a little extra fresh air, to which the fuel system adds a bit more fuel to make bigger farts. Producing this flatulence takes time, and the longer it takes the less happy the Converse-wearing dude is.
To help speed the process along, Volvo employs a twin-scroll turbo. That is, the housing that surrounds the turbine (exhaust side) wheel is split into two independent exhaust paths. These two paths, or volutes, dictate that the exhaust side of the engine be grouped into two bunches.
See here. Cylinders 1, 2, and 3 feed one of the volutes (the snail-looking parts of the housing), and Cyls 4, 5, and 6 exhale into the other volute. In this manner the exhaust manifold is essentially a pair of manifolds that each contains the exhaust of three cylinders.
This achieves two major objectives -- it delivers more pulse energy to the turbine wheel, and it keeps cylinders from contaminating each other with exhaust gas.
Here's how. Because this is a six-cylinder engine, a cylinder fires every 120 degrees of crank rotation. This means that when, say, Cyl 1 is puking a bunch of hot, high-pressure exhaust gas into the manifold, Cyl 4 is on its intake stroke, part of which involves having its exhaust valves briefly open at the same time as its intake valves are open. This is called valve overlap.
Picture a conventional inline-six exhaust manifold. The angry exhaust pulse from Cyl 1 travels down its runner and into the common collector that joins all six exhaust runners. Here, the pulse "sees" the nice low-pressure region of Cyl 4's overlap and really, really wants to be there. So much so that part of the exhaust pulse from Cyl 1 travels backwards up the Cyl 4's exhaust runner.
Now picture the divided twin-scroll manifold. Here, Cyl 4's overlap period is completely isolated from Cyl 1's pissed-off exhaust pulse, so instead of that pulse energy being drained off into Cyl 4, it is delivered to the turbine wheel instead where it can do some useful work. And so on for the remaining pairs of cylinders. Furthermore, because the collector regions of a twin-scroll manifold are each inherently lower-volume than the big single one of a conventional manifold, less energy is wasted in an unnecessary expansion.
The exhaust pulse recovery aspect of twin-scroll turbos is the primary reason for their existence. They simply build boost in a more linear and responsive fashion than do single-scroll turbos, improving time to boost and time to torque, every time you dip the throttle.
There's an additional benefit in dividing the manifold and turbine housing, though. Remember the exhaust pulse that traveled up the Cyl 4 runner in our conventional manifold? Not only did this bleed off pulse energy but also a bit of exhaust flow that instead is now taking up space in Cyl 4 that could otherwise be filled by fresh, oxygen-rich intake air. This phenomenon has many names, among them reversion, cylinder contamination, impaired volumetric efficiency, etc. Whatever you call it, it's bad, and it also heats up that cylinder somewhat, adversely affecting its knock resistance. Double bad.
With a twin-scroll arrangement, there is more freedom for the engine designers to employ more aggressive valve overlap too, since they're not forced to mix farts into their flowers.
The Volvo's turbo setup is not unique. Inline-six diesel engines have been doing this for decades but it has taken advances in metallurgy to allow twin scrolls to be used with the high exhaust gas temperatures of a gasoline engine. Particularly one running relatively lean air-fuel ratios at full load like this Volvo. When that divider wall between the scrolls gets really hot, it wants to taco and fracture radially, which isn't the best for durability. Modern high-nickel austenitic stainless steels have made this far less of an issue.
BMW's N54 engine achieves the same goal of enhanced pulse recovery as the Volvo but does it with two conventional turbos rather than one twin-scroll unit. However, the company's recent twin-scroll N55 suggests they're getting the same job done without the complexity (and cost) of the N54's two turbos and associated plumbing, hardware and sensors.
The now-defunct Cadillac SRX Turbo uses a similar arrangement as the Volvo, only with cylinders arranged in a vee rather than in a row. That one's sort of an odd bird as it's a hassle to plumb two banks of a V6 to a single turbo. It's simpler to use two turbos here (see Ford Ecoboost V6).
Anyway, a little insight into what makes the XC60 a torquey and responsive steed.
Jason Kavanagh, Engineering Editor