Almost 12 years ago, Nissan showed up at Detroit’s hometown auto show with a direct challenge to the Big Three: its first full-size truck, the Titan. Even if that truck barely registers as competition today—though it does have a pretty bitchin’ name—Nissan hasn’t been deterred from repeating the stunt with the second-generation truck. The 2016 Titan will make its debut at the 2015 Detroit auto show, according to a Nissan spokesman.
We already know that an optional turbo-diesel engine will make more than 300 horsepower and 500 lb-ft of torque. Now Cummins, the engine manufacturer, is detailing the tech that will force-feed the 5.0-liter V-8, revealing that it will be the sole light-duty vehicle sold in America with sequential turbochargers under the hood. Cummins calls it a Holset M2 Two-Stage System with Rotary Turbine Control. If that means absolutely nothing to you—and it shouldn’t—read on to learn what’s in the name.
We’ll get the easy stuff out of the way first. Holset is an English engineering firm that was purchased by Cummins in 1973 and renamed Cummins Turbo Technologies in 2006. But because Holset—we’re guessing here—means something to the people that buy and drive big diesel rigs, every Cummins turbo is also a Holset. Remember the time Mazda tried to drop “Miata” so that its iconic roadster was simply MX-5, but everyone called it Miata anyways? We’re thinking it’s kind of like that.
The two-stage bit means there are two turbochargers plumbed in sequence, so that they both feed pressurized intake air to all eight cylinders. Don’t call it a twin-turbo setup, though, because as with all sequential arrangements, these turbochargers are significantly different in size. (Twin turbos are the same size, with each one feeding half the engine’s cylinders.) The smaller of the two turbos provides boost at low engines speeds, where the lighter compressor and turbine spin up to speed quicker to reduce lag. However, at higher revs and engine load, when the engine is pumping more air, the smaller turbine and compressor begin to reach their limits. Instead of increasing power, they become restrictions in both the intake and exhaust streams, choking the intake supply and increasing backpressure in the exhaust. This is when the larger turbocharger takes over, raising the overall boost pressure and peak power output.
Still with us? You should know that nothing we’ve described thus far is especially novel. Sequential turbo systems, while not exactly common, have been around for decades. In Europe, Audi’s A6 and A7 are available with a sequentially turbocharged diesel engine that is more powerful than our 240-hp TDI. In the U.S., sequentially boosted engines have previously been used in performance heroes like the Porsche 959 and the third-generation Mazda RX-7. But sequential turbo systems are costly and complex, and getting the controls right to blend the two turbochargers is particularly tricky. And that’s where a Cummins innovation at play here becomes noteworthy.
Now we’re talking about the Rotary Turbine Control, which is the marketers’ term for a single electrically controlled rotary valve that directs the exhaust flow to either of the two turbochargers or a wastegate depending on the engine load and rpm. At low engine speeds, the valve directs all of the exhaust gas through the smaller turbine. As air flow increases with engine speed and load, the valve rotates to favor the larger turbocharger, although some exhaust still spins the small, low-pressure turbo. To regulate the boost pressure, the rotary valve turns to open a path between the exhaust stream and the wastegate, venting waste gas around the turbines.
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The rotary valve can also act as a throttle in the exhaust stream, raising the backpressure to act as an exhaust brake when the driver lifts off the accelerator. This throttle mode can also increase the work done in the cylinder, in turn elevating exhaust-gas temperatures to burn off the soot that’s been captured in the diesel particulate filter. The fact that this is all controlled by a single, rotating valve means that redirecting the exhaust stream can be done with smooth, supposedly seamless, transitions.
How well does it actually work? Based on the Detroit auto show debut in January 2015, we’re guessing we’ll get our first chance behind the wheel sometime in the middle of next year.
from Car and Driver Blog http://ift.tt/1wJ5YYi
via Agya
