29 August 2011
By John Donovan, Low-Power Design for Drive for Innovation
There are a lot of reasons for thinking of buying a hybrid electric car—ecological, economic, political, and just getting cheesed off at seeing all those hybrids with one passenger whiz by you in the Diamond/HOV lane. Besides, admit it—the technology is cool. So just what is the technology inside the Chevrolet Volt?
You Want Gas with That?
There are two basic types of hybrid drivetrains: series and parallel. Series hybrids (left) have a gas engine that turns a generator that charges a battery bank that powers an electric motor that powers the car; the engine is not connected to the drivetrain. The Chevy Volt—which GM refers to as “an extended range electric vehicle (EREV)”— is essentially a series hybrid, though with a twist that we’ll describe in a moment.
In parallel hybrids (below) both the electric motor and the gas engine are connected to the transmission through clutches that enable one or the other to power the vehicle.
Then of course there's the series/parallel hybrid. In this configuration the two power sources are joined in a planetary gear set that enables either the motor or the engine to power the vehicle, or to share the burden as needs be.
Despite being primarily an electric vehicle, the Volt actually falls into this category. When a driver needs rapid acceleration, the engine works in parallel with the motor until you let up on the accelerator. Also, the gas engine takes over from the motor when you exceed 70 mph. That's an appropriate place for the motor – which has its greatest torque at low rpm – to hand control over to the engine, which generates maximum torque at high rpm. Besides, at 80 mph, you’ve ceased being an ecopurist and are just in a hurry.
Both the Volt and the Prius are essentially series/parallel hybrids. The main difference is that, on the open, road the Volt relies more on electrical power and the Prius more on its engine. The Volt as a result, has a considerably larger battery bank: 16 kWh vs. 5.2 kWh for the Prius. Not surprisingly the Prius has a larger gas engine: a 1.8 liter/98 hp engine vs. the Volt’s 1.4 liter/80 hp engine. Furthermore, the Volt’s 111 kW (149 hp) electric motor can generate 273 lb-ft of torque, considerably more than the Prius’ 80 hp, 153 lb-ft motor. You might think of the Prius as a gas/electric hybrid and the Volt as an electric/gas hybrid.
Looking at Table 1, the Volt has about the same power as my Mazda 3, though it gets >3x better gas mileage—and infinitely more for trips under 35 miles, where it’s purely electric. It’s also a lot quieter and more fun to drive.
The Train is Leaving the Station
In late 2010, GM formally introduced the Voltec powertrain upon which the Volt is based–though its roots go back to 2007. The basic design combines a small gas engine and a large electric motor that drives the vehicle, but they can work smoothly in tandem when it makes sense to do so. The large lithium-ion battery bank is designed to be recharged at home overnight—in 10 hours from a 110 VAC source or four hours from 220 VAC.
Table 2 shows the basic specifications for the 2011 Chevy Volt. GM has announced plans to use the Voltec powertrain in other cars, SUVs, and even trucks, bringing down the cost by using the platform across a much larger base of vehicles. Expect the Volt specs to scale for SUVs and trucks. Even Porsche is getting into the act; toying with the idea of an electric 911 (though not the Turbo GT2).
Maybe drivers won’t miss the roar of a big engine so much while they’re quietly zipping past yet another filling station advertising gas for $4/gallon.