Powerelectronics 897 Evolutionary Powertrain200 120 0

The Chevy Volt: A big step in the right direction

Nov. 1, 2010
Whether you call it a series hybrid, an electric vehicle, a plug-in, or just a plain old sedan, the Volt’s powertrain embodies creative engineering that lowers mpg.

The Chevy Volt is in the headlines, and with good reason. The most intriguing feature of the 2011 Chevrolet Volt five-door sedan is its powertrain. It includes two electric motors, three hydraulic clutches, and a planetary gear set, according to Chevy. But some might say it also includes a 1.4-liter engine, which has emerged a point of controversy. More on that later.

From a driver’s perspective, the Volt decides which motor(s) to use, when to rely on the battery pack, and when to switch the engine on or off. The Volt always tries to use the battery first until it is depleted, because this is the least expensive way to power the vehicle. And if you drive the car only 40 miles per day, you may never need to take it to a gas station. But if you want to travel farther — up to 310 miles farther — the Volt comfortably handles that with its gas engine.

A 350-mile drive

Assuming the car starts with a full charge and a full tank (9.3 gallons) of premium gas, it completes the first 40 miles or so on battery power. An inverter turns battery dc into ac, which powers a three-phase traction motor. The motor cranks out almost 150 hp (111 kW) and 273 lb-ft of low-speed torque. That’s enough to get the Volt going from 0 to 60 mph in nine seconds and through a quarter mile in less than 17 seconds. The Volt’s top speed, which is governed, is 100 mph.

Total range on just battery power varies between 25 and 50 miles, depending on temperature, terrain, number of passengers, and driving style. And using the car’s heat or air conditioning, as well as the stereo and other electronic devices, also reduces the range.

When the battery drains down to its minimum charge level, the 84-hp gas engine starts up and spins a 54-kW generator/motor. Electricity from the generator/motor goes to the inverter for conversion to ac, which powers the traction motor. This electricity never recharges the battery -- the car is designed to be recharged strictly from household current, which is always less expensive than recharging via the gas engine. The engine extends the range of the Volt by 310 miles, which varies with driving style and conditions.

There’s more mileage available if the regenerative brakes send enough electricity to the battery pack during Regen Power Recovery mode. Regen kicks in whenever the driver lets off the accelerator to brake or coast. During those times, the traction motor becomes a generator, slowing the wheels slightly and recharging the battery pack. Braking recoups up to 0.2 g of stopping power to generate electricity.

If the driver accelerates while driving at highway speeds, some of the torque from the engine is sent to the wheels, bypassing the generator, a fact which has sparked some controversy. In several earlier press releases and statements, Chevy indicated the Volt’s wheels would always be powered by an electric motor. But during development, engineers discovered that at high speeds, sending more torque into the generator from the gas engine was less efficient than sending that torque directly to the wheels.

If the driver insists on driving after travelling more than 350 miles and eventually runs out of gas, Chevy engineers designed a little mercy into the Volt; it gives you five more miles on battery only. However, doing so runs the battery past its lower charge limit may shorten the life of the battery pack.

The battery pack

The 435-lb lithium-ion battery pack plays a major role in the Volt’s powertrain. GM, along with partner LG Chem, perfected the 16-kW pack at GM’s Battery Lab in Warren, Mich.. It’s now being built by a GM subsidiary in Brownstone Township, Mich. The T-shaped pack measures 5.5-ft long and runs between the driver and passenger seats and under the rear passenger seats. In fact, the size and shape of the battery dictated that the rear seats be two buckets rather than a bench.

The pack consists of 288 prismatic cells, each measuring about 5 × 7 × 0.25 in. and weighing about a pound. The pack also includes electronics for battery management, diagnostics, and thermal controls.

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The Volt caries a sticker price of $41,000, but this is cushioned by a federal rebate of $7,500. This still makes the car rather pricey. But Chevy says the first production run of Volts has already sold out.

Thermal control is critical to battery life, so the pack has its own liquid heating and cooling subsystem, which functions whenever the car runs or recharges. This lets the Volt operate in temperatures ranging from -13° to 122°F.

Another critical factor in battery life is charge level. Engine management circuitry ensures the battery pack never fully charges or fully depletes, which shorten its life. Instead, the Volt uses the middle 65% of the battery charge, leaving about a 17% buffer on the top and bottom. Chevy says this approach is conservative in that designers were willing to sacrifice a bit of battery-only range for longer battery life.

As engineers gather more data on the Volt and how most customers use it, they could shrink the buffers and give the car more range in battery-only mode. As proof of the company’s confidence in the battery pack, Chevy covers it and the entire powertrain with an eight year/100,000- mile warranty.

There are several ways to recharge the battery pack. Every Volt comes with a 20-ft, 120-V cord that plugs into household current. It takes 10 to 12 hours using the cord to get a full recharge. Chevy says most recharges should cost about $1.50, but this varies with local electrical rates. For about $400, owners can get a 240-V charging station that delivers a full charge in only four hours.

Owners can specify one of three charging modes on the car’s control panel or a smartphone app (OnStar Mylink Mobile App). In Immediate Mode, charging starts once the Volt is hooked up to an electrical outlet. A timer on one of the Volt’s two touchscreens shows an estimate of when the Volt will be fully charged. In Delayed Departure Time Mode, the driver enters an estimated departure time, and the Volt initiates charging so that the battery pack is fully charged when the driver is ready to roll. And in Delayed Rate and Departure Time Mode, the Volt takes into account the lest expensive time to buy electricity (set by the local utility) when deciding when to recharge the battery. For those using the smartphone app, the car sends an email or text message when the car is fully charged.

Taking the Volt for a spin

Editor Leland Teschler and I were fortunate enough to get a dayand- a half-long test drive in a Volt, courtesy of Chevrolet. We put over 190 miles on it as we trekked back and forth across Detroit and out into the surrounding countryside.

On the first day, we were challenged to see how much mileage we could squeeze out of the battery-only mode on a 43-mile trip. To help keep us fuel-conscious, a display on the dashboard screen acted as a reminder. It displays a rotating green ball floating midway up the screen. If you pull away from a stop too fast, it climbs up the display and turns yellow. And when you brake too hard, it sinks to the bottom and turns yellow.

Despite babying the car a bit and being almost overwhelmed by the various dashboard displays and controls, we found the car handles well. Steering is precise, thanks to electric power steering. It features dual pinions, one for normal driving, one for assist at slow speeds. The ride is comfortable and the brakes, whether regenerative, friction, or a combination of both, provide plenty of stopping power. We finished that first ride with only an estimated mile to spare, so we used absolutely no gas. And that was with three good-sized passengers, luggage, and camera equipment.

On the next day, we were free to drive as we wanted during a 150-mile trip and the car behaved just as one might expect of a small, upscale sedan. It is solidly built, not a tinny econobox by any means. And acceleration was good, as was cornering. Visibility forward and aft was better than average, and the controls were easy to get use to.

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I did experience a few firsts in the car. For example, the navigation system (optional) linked to a traffic service so that roads with congestion were outlined in red, while roads with traffic moving freely had a green outline. For example, early-morning traffic showed up clearly on several roadways on the nav screen. In one instance, the touchscreen display showed a danger sign. I touched it (Leland was driving), and a note appeared warning of an accident on the right side of the road in 0.7 miles. Sure enough, 0.7 miles later we passed an abandoned, burned-out wreck. The nav subsystem also links to OnStar, which will download turn-byturn directions to a destination directly to your car on request. The car’s dash and center console screens also show more than the usual dials and gauges. One in particular indicated whether you were on battery only or in extended-range mode with the engine on. When the switchover came, we tried to hear or feel the engine kick in. We could do neither. The center console also played an animation that showed the current source of power and what route it was taking. There’s even a library of tips on how to drive with fuel mileage in mind.

The five-door sedan has plenty of room up front, and the rear two seats are usable, but headroom in the back is a bit tight. The trunk, with 10.6 ft3 of cargo space, is roomy enough to hold a fair amount of luggage or groceries. But there is no spare, which is increasingly common on new cars to save space. Instead, the car carries a 300-psi air compressor and a can of tire sealant.

The tires themselves are Goodyear Fuel Max 17-in tires. These low-rolling-resistance tires were developed by Goodyear specifically for electric vehicles to help boost range. They mount on forged aluminum wheels which weigh 17.8 lb, significantly less than conventional 17-in wheels that weigh about 22 lb. The only downside to the tires is that they can create noise when travelling over certain road surfaces.

In general, one of the most amazing aspects of the car is that it drives so much like a conventional car. And it does so using considerably less gas, especially if you have access to the optional charger. During our time in the car, we recharged at night, and for about two hours the next day while we had lunch. In all we used 2.25 gal. of gas to go 190 miles. Not bad.

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