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Dissecting Chevy's 2nd-Gen Moonshot - Technologue

Motor Trend logo Motor Trend 4/22/2015 Frank Markus

When Chevy unveiled the thoroughly revamped Volt at the 2015 Detroit auto show, it kept the geekosphere in suspense about the nitty-gritty details of the upgraded powertrain except to say it involved two traction motor/generators and a new double planetary gearset. Well, now the SAE papers are published, and they describe a technological leapfrogging on the order of Gemini Titan over Mercury Redstone.

To improve performance, sustainability, efficiency, and cost, two very different types of permanent magnet motors were engineered with differing, complementary load/speed "sweet spots." To eliminate rare earth metals, the smaller motor (A) uses simple ferrite magnets. Arranging them in what look like a dozen little four-row amphitheaters around the circumference of the rotor optimizes both magnetic and reluctance torque so that these cheap iron-oxide magnets can pull more like their fancy rare earth cousins, with peak efficiency at low torque in the 5,000-9,000-rpm range. On the larger motor (B), novel metallurgy helps concentrate the heaviest dysprosium rare earth material near the corners of the magnets where it does the most good, and arranging these magnets in a dozen double-vee cavities around the rotor likewise optimizes their efficiency. This motor's sweet spot is at higher torque from about 2,000-4,000 rpm. The system uses 80 percent less total rare earth metals with 50 percent less of the heavy stuff than the first-gen.

Chart LocationColorEngineering NicknameC1 StateB1 StateDescription
BottomRedE Boost ModeClosedClosedIn E Boost Mode, Motor/Generator "A" will be locked to case and not contribute to torque. Motor/Generator "B" will be able to electrically boost or regenerate the vehicle torque in parallel with the engine torque
Second From BottomYellowRange Extend HighClosedOpenIn Range Extended High, the online optimizer will balance engine, motor/generator A and motor/generator B torques to achieve the most efficient operations at higher vehicle speeds.
Third From BottomGreenRange Extend LowOpenClosedIn Range Extended Low, the online optimizer will balance engine, motor/generator A and motor/generator B torques to achieve the most efficient operations at lower vehicle speeds.
TopBlueEV DriveOpenClosedIn EV Drive, Motor/Generator A can "Lever" against the "One Way Clutch 1" to produce vehicle torque in parallel with motor/generator B. Motor/Generator B can "Lever" against B1 to produce or absorb vehicle torque.

Dissecting Chevy's 2nd-Gen Moonshot - Technologue

2016 Chevrolet Volt EV Technologue© Provided by MotorTrend 2016 Chevrolet Volt EV Technologue Here's the über-geek gearset detail in one paragraph: Each motor connects to a sun gear. The planet carriers connect directly to the wheels. One clutch locks the B motor's ring gear to the case, another can lock the A motor sun to the B motor's ring gear, and the engine connects to the A motor's ring gear while a one-way clutch prevents the engine from ever turning backward, essentially locking it when the engine is off.

This more efficient Volt nowgoes 50 miles on electricity and accelerates much harder.

The extra motors and gears provide two modes of electric and three modes of range-extended driving. The EV modes are B-motor only (light loads and lower speeds, through an 8.1:1 ratio) and A and B pulling together. (The A ratio is 7.6:1.) In both cases that B ring clutch stays locked. The three modes of gas-powered operation are low, fixed-ratio, and high. In fixed-ratio both clutches are locked, grounding the A motor to the case so that the engine drives the wheels directly through a 1.5:1 gear ratio. (The axle ratio is 2.64:1.) The B motor remains free to contribute or recapture energy as conditions demand in this mode. Low mode locks the B ring, and high mode locks motor A and the B-ring to the case. In each of these modes, the two motors can spin in either direction with infinitely variable gear ratios, adding torque or returning electrons to the battery as needed, such that the total net axle torque meets the driver's demands.

A system optimizer selects between these modes 100 times per second. In extended-range (engine-on) mode, one-motor EV operation accounts for 73 percent of the city-cycle operation, and high mode handles 75 percent of highway operation. Fixed-ratio operation accounts for just 9 and 12 percent of the city and highway driving cycle operation. But in all cases, the transmission keeps the new 1.5-liter four operating at peak efficiency under most conditions, which usually means at lower (quieter) speeds.

The new battery packs 12 percent more energy and weighs 21 pounds less -- total vehicle mass was reduced nearly 250 pounds. The more efficient Volt goes 50 claimed miles on electricity and accelerates much harder. Even the onboard charger is 7 percent more efficient. It's the most advanced plug-in hybrid on Earth. Another leapfrog of this magnitude would take Voltec 3.0 to Apollo Saturn V status -- a moonshot, indeed.

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