Thursday, May 26, 2016

Volvo’s Latest “Twin Engine” Plug-In Hybrid Powertrain Explained

Volvo T5 Twin Engine CMA platform

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Volvo’s Twin Engine plug-in hybrid branding might not be completely accurate—so far, each iteration of the theme has incorporated only a single engine, as well as an attendant electric motor or two—but the newest version is quite novel nonetheless. While the “T8” Twin Engine setup in the XC90 SUV carries two electric motors, one fitted between the engine and the transmission and another powering the rear axle, the “T5” variant destined for Volvo’s new compact 40-series cars has only one electric motor aiding a turbocharged three-cylinder engine in a crafty way.

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Volvo T5 Twin Engine CMA platform

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As is the case for most hybrids, the transmission is among the most critical components in Volvo’s setup. The seven-speed dual-clutch automatic is bolted to the end of Volvo’s all-new turbocharged 1.5-liter three-cylinder engine, just as it would be in a non-hybrid application, and its output shaft turns a differential and two unequal-length driveshafts that spin the front wheels. So where’s the electric motor? Unlike pretty much every other front-wheel-drive, transverse-engine hybrid out there, the Twin Engine T5 system hangs the electric motor off the top of the transmission—it’s mounted externally—and delivers its torque to the transmission’s output shaft via one of the dual-clutch’s two input shafts.

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Volvo T5 Twin Engine CMA platform

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In case we’ve lost a few of you, here’s a quick refresher on dual-clutch automatics, so that you have an idea of what we’re talking about when describing those input shafts. A dual-clutch transmission is almost like two manual transmissions in one, carrying its even-numbered gears on one input shaft and its odd-numbered gears on another; the two shafts are concentric (meaning one spins around the other—think one pipe slipped over another), and the shafts are alternately engaged via separate clutches (hence the “dual-clutch” moniker). Starting from rest, the transmission selects first gear, and the clutch between that input shaft and the engine is engaged; meanwhile, second gear is pre-selected so when it’s time to upshift, the clutch on the odd-numbered gears’ input shaft disengages while the clutch on the even-numbered gears’ input shaft engages simultaneously. There is no efficiency-robbing fluid coupling, like with the torque converter of a conventional automatic, therefore dual-clutch autos can be more efficient than their planetary cousins.

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The Twin Engine T5 setup takes advantage of the dual-clutch automatic’s layout by linking the electric motor to one of the transmission’s input shafts (the one for second, fourth, sixth, and reverse gears) after the clutch. Put another way, both of the transmission’s input shafts spin with the output shaft all the time, regardless of whether the clutches are open or closed; the clutches simply control which shaft receives engine torque depending on which gear is engaged. According to Volvo’s engineers, when the electric motor contributes power in hybrid mode, it does so either through second or fourth gear, even when the engine torque is directed to first, third, fifth, or seventh gears on the other input shaft. For example, when the transmission is in third gear—as in, the clutch for the input shaft handling first, third, fifth, and seventh gears is engaged and fed power from the gas engine—the electric motor adds power to the other input shaft with second gear engaged.

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When running on electric power alone, with the engine off, the electric motor sends power to the drive wheels via second or fourth gear; since reverse is on the same input shaft, Volvo’s system allows for pure electric travel backward. (In pure electric mode, neither of the dual-clutch automatic’s two primary clutches are engaged.) When coasting or decelerating, the electric motor captures energy to store in the onboard lithium-ion battery pack.

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Volvo’s setup isn’t new—Honda employs a similar layout for the front half of the Acura RLX Sport Hybrid’s V-6/electric motor/dual-clutch automatic powertrain—but it’s no less clever for it. For now, it seems the Twin Engine T5 powertrain is designed specifically for the new CMA-platform “40-series” Volvos, meaning the brand’s smallest cars. (The larger 60- and 90-series Volvos ride on a different platform dubbed “SPA.”) Volvo has yet to decide which body style the plug-in hybrid setup would appear in (crossover/XC40, sedan/S40, or hatch/V40), but designed it to fit in any CMA car. The battery is positioned lengthwise down the center of the chassis, where the transmission tunnel in a rear-drive vehicle would be, and the engine and transmission are fitted transversely, as you’d expect.

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Volvo T5 Twin Engine CMA platform

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We extracted a few preliminary details about the Twin Engine powertrain, including the engine’s output (180 horsepower), the electric motor’s output (55 kW, or 74 horsepower), total system output (250 horsepower), and battery capacity (9.7 kWh). While final specifications could change, Volvo is adamant that the Twin Engine T5 powerplant will be capable of  31 miles of pure-electric driving range and class-leading CO2 emissions in Europe. Those figures not only place the T5 powertrain on equal or better footing in terms of output against Audi’s A3 sedan, BMW’s four-cylinder 2-series, and Mercedes-Benz’s CLA250, but its hybrid and electric-only capability should deliver competitive fuel economy.

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