Machinedesign 2840 Volt Sm 0

Chevrolet Volt Combines Electric Motors and Gas Engines

Nov. 18, 2010
Chances are, the new Chevy Volt could reduce the number of stops you make at the local gas station.
Chevrolet Volt,

Chevy engineers pulled out all the stops when designing the 2011 Volt. They came up with an entirely new powertrain, complete with a patented battery pack and innovative transmission. They uncovered new ways to cut weight and reduce drag in an effort to boost range. They even had new tires designed that add range. And they didn’t ignore safety along the way. But most importantly, they focused on making a car — whether you call it a series hybrid or an all-electric vehicle — that could help much of the population use less gas. In fact, if you drive less than 40 miles a day, you might never need to stop at a gas station again. However, if you want to take a weekend road trip, the Volt can handle that as well.

Batteries and an engine
The five-door Volt carries a 435-lb T-shaped battery pack in the center tunnel running between the driver and passenger seats. The top of the T sits just under the rear two seats. The pack’s shape, in fact, forced Chevy to turn the normally bench-style rear seats into a pair of bucket seats.

The 16-kW-hr lithium-ion battery pack was developed by GM and partner LG Chem, a Korean firm, and will be manufactured in GM’s Brownstone facility near Detroit. The designers chose lithium-ion chemistry over metal hydride because it packs two to three times the power in a smaller package, sees little loss of charge when not in use, and isn’t as prone to losing storage capacity after repeated rechargings and partial discharges.

The pack consists of 288 prismatic cells grouped into nine modules. Every 10 sec, diagnostic circuity on the pack runs 500 tests, with 85% of those tests ensuring the battery is working safely and the other 15% tracking performance and life. The pack also contains thermal-management circuitry and a liquid-cooling subsystem that heats or chills the battery, depending on ambient conditions. This lets the battery operate efficiently in temperatures from –13 to 122°F, and extends its life. In fact, there are three liquid-cooling subsystems on the Volt. One for the battery; one for the traction, generator motors, and electronics; and one for the gasoline engine.

The battery sends electricity through an inverter to a 149-hp traction motor to power the Volt for about 40 miles. This range can vary between 25 and 50 miles, depending on road conditions, driving style, and the number of passengers. Once the battery is down to a certain level, an 84-hp, 1.4-liter gas engine switches on to spin a 54-kW generator, which powers the traction motor. Regenerative braking lets the traction motor act as a generator to recoup up to 0.2 g of braking force and convert it to electricity. The traction motor also switches to a generator when the Volt coasts, slowing the car a bit to generate more electricity.

Because the car is so quiet, engineers worked with the American Federation of the Blind to add a second honk mode to the horn. Pulling up on the turn signal makes the horn bleat rather than blare to gently alert pedestrians in intersections.

Battery-management controls restrict the Volt to using the middle 65% of the battery packs’ power. Never fully charging or fully draining the battery extends the life of the battery. That’s one reason the battery pack caries an eight-year/100,000-mile warranty. In the future, after studying data on how consumers actually use their Volts, Chevy might widen this zone to give drivers more range on battery power.

The Volt comes with a 22-ft cord that plugs into household current. It recharges the battery pack in 10 to 12 hr. A $400 (optional) 240-V charging station does the job in 4 hr. The Volt can also be instructed via the console or a smartphone to recharge itself when utility rates are lowest.

The Volt travels 310 miles on a full (9.3-gallon) fuel tank after the battery pack is depleted. And although Chevy officials have termed the Volt an “all-electrically driven vehicle,” when accelerating at highway speeds, some engine power is mechanically coupled to the wheels. Chevy engineers explain that this was done because past a certain rpm level, throwing more torque into the generator is less efficient than sending that torque directly to the wheels.

Trimming weight and drag
Chevy engineers knew that every pound they could eliminate meant a bit more range for the Volt. So they designed 17-in. forged aluminum wheels that weigh 17.8 lb, compared to conventional 17-in. wheels weighing 22 lb. And the rear torsion-beam suspension is 10% lighter than conventional versions, thanks to thin-walled trailing arms and lighter springs. The suspension and rear axle are also more compact, letting Chevy engineers mount them lower for a better center of gravity and fit the gas tank between the rear wheels, which means the rear seats can be lower.

Even the Bose audio system is 40% lighter than systems with comparable power. The system also needs 50% less energy, a plus for extending the Volt’s driving range.

Weight is important, but so too are aerodynamics. For comparison, every 400 lb cut from a car’s weight yields another 1 mpg. But every 20 counts or 0.002 shaved off a vehicle’s coefficient of drag results in another 1.1 mpg. So Chevy smoothed off many of the rough edges on the Volt concept car. They rounded the front fascia and made it flush with the rest of the front body work, tapered the corners, and increased the rake of the front windshield and back glass. A crease curving down the back was eliminated and another 5 mm added to the rear spoiler. And the side-view mirrors went from having patch mounts to post mounts. Changes to the mirror and spoiler alone cut drag by 10 counts. The Volt ended up with a 0.28 coefficient of drag.

About 80% of the car’s structure consists of advanced, high-strength steels. These include boron, martensite, dual-phase, and low-alloy steels that combine strength and light weight. For example, boron steel stretches from the front pillar to the rear pillar for better side-impact protection and rollover support. And for more rollover protection and upper-body stiffness, a shear plate of high-strength steel is MG welded to the upper part of the center pillar. To protect the battery pack, ultrahigh-strength martensite support rails run down the center of the floor line. The battery pack itself adds stiffness to the chassis while letting engineers lower the car’s center of gravity. This helps in handling and performance, as well as safety.

Volt Trivia
• Viridian Joule Tricoat, the name for one of Volt’s six exterior colors, was chosen in a national contest.
• The 240-V charging station will call on your smartphone when your Volt is fully recharged.
• The Volt’s lithium-ion battery pack holds the same 16 kW-hr as the lead-acid battery developed for the EV1 electric vehicle, but is one-third the size and weight.
• A fully charged Volt battery pack could power an iPod Nano for 112 years of continuous use.
• The Volt carries a 60-Gbyte hard drive with 30-Gbyte dedicated to storing music.

© 2010 Penton Media, Inc.

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