Imagine a straight line across a piece of paper. All the cars that have ever been made exist somewhere on that line, including the 2011 Chevrolet Volt.
At the left endpoint, let's write "Gas" to represent vehicles powered by gasoline. Go ahead and add diesel, E85 or any other combustible fuel along with gasoline if you must. Point is, our left endpoint is not associated with batteries or electric motors.
That's because "Electricity" is the label we'll apply to the right endpoint of our line. This represents the pure electric vehicles — EVs for short. These have an electric motor, a battery pack and a place to sit. You plug them in and charge them up. When the battery runs out of juice, you're walking.
A hybrid, then, is any vehicle that lives between these two endpoints, any vehicle where motivation sometimes comes from electricity stored in batteries, sometimes comes from the combustion of fuel or from any simultaneous blending of the two. That's what hybrid means in the plain terms of automotive engineering.
By this simple definition, the 2011 Chevrolet Volt is a hybrid. It has a gas tank; it has batteries. That's two forms of energy. That's a hybrid.
Of course there are infinite waypoints between the endpoints of our line. And indeed there are all sorts of hybrids; it's a simple question of which endpoint they're closer to. The place a car takes on the line is determined by the strength of its electrical components — batteries and motor-generator(s).
Belt Alternator-Starter or BAS hybrids, like the 2009 Chevy Malibu Hybrid are the least powerful types, and they sit just off the left endpoint. Honda's IMA (Integrated Motor Assist) hybrids, such as the Civic Hybrid and the Insight, sit near the hash mark that's one quarter of the way along the line thanks to a somewhat larger motor and battery, plus a measurable range of electric-only driving.
The 2011 Toyota Prius has advanced to about the halfway mark on the line. Much of the reason lies in a regenerative braking system that works up to 0.6g of deceleration — enough to account for 90 percent of the stops you'll ever make without inconveniencing the pads and rotors. We've driven a 2011 Toyota Prius some 3 or 4 miles on electricity at 35 mph.
There are prototypes of the 2012 Prius Plug-In running around and, like the Volt, they can be plugged in for a usable amount of electric-only driving range before the engine comes on. We've experienced about 15 miles of EV operation per charge, which puts the Prius plug-in at the two-thirds marker along our imaginary line. This Prius still runs on gasoline much of the time, though.
Enter the Volt
But the 2011 Chevrolet Volt is supposed to offer 40 miles of electric-only driving range, GM claims — a big jump that potentially puts the Volt at the three-quarters point along the line and perhaps farther along than that.
GM seems to want to lay claim to the right-hand endpoint of the line, however. This is the Volt, a name synonymous with electricity, and GM has tried to reinforce its image by describing it as an extended-range electric vehicle. But we cannot ignore the fact that the Volt has a gasoline engine. The 2011 Chevy Volt is a plug-in hybrid. Not that there's anything wrong with that. In fact, there's a lot right with such a concept.
When GM first introduced the Volt, it told us that the car had two modes of operation. Its T-shaped lithium-ion battery pack would supply enough electricity to supply the electric motor for the first 40 miles. When the batteries ran down, the gas engine would come on and generate enough electricity to keep the electric motor happy, but not enough to recharge the battery. At this point the Volt would begin burning gasoline at an undisclosed rate until the next plug-in opportunity.
Because this explanation described no direct connection between the engine and transmission (the engine drove the generator, not the wheels), we put the Volt into the category of "series plug-in hybrid."
But now GM has admitted a few very important details. The Volt has two motor-generators, not one. They are connected through a planetary gearset. There are four modes of operation instead of two, and the gasoline engine is mechanically connected to the planetary drivetrain in one of those modes. According to the conventions of automotive engineering, the Volt is a series-parallel plug-in hybrid, similar to the plug-in Prius.
How It Fits Together
Compared to a Prius, the Volt's electric motors and gasoline engine connect to the planetary gearset at different points. The main traction motor (which also acts as a generator for regenerative braking under deceleration), connects to the central "sun" gear. The secondary motor-generator is sometimes clutched to the outer "ring" gear, sometimes clutched to the gas engine and, most important, sometimes clutched to both at once.
In this planetary gearset, the teeth are arrayed around the inside of the ring. Five perfectly sized "planet" gears are held apart at equal intervals around the sun by a carrier, such that each simultaneously meshes with the sun and the inward-facing teeth of the outer ring gear. The spin of the planets is unrestricted and unimportant, but their spin causes them to orbit the sun, and that orbit speed is harnessed by the carrier, which is connected to the differential, the axles and the tires.
Four Modes of Operation
Let's walk through the four modes in which the 2011 Chevrolet Volt works.
Mode 1: Low-speed electric-only mode up to 70 mph. The main traction motor-generator is turning, but it can only go so fast. Here the ring gear is locked so it can't turn. The second motor-generator and the engine are disconnected and out of the picture. The main traction motor is left alone to drive the sun gear from battery power, which sets the planets in orbit. The orbit speed is sent out through the carrier and is proportional to vehicle speed.
Mode 2: High-speed electric-only mode up to top speed (100 mph). The engine remains dormant. Now the ring gear unlocks and is instead clutched to the second motor-generator, which is acting as a motor to set the ring in motion. With the sun already spinning, any rotation of the ring in the same direction will increase the overall orbit speed of the planets, which in turn increases vehicle speed beyond 70 mph.
Mode 3: Low-speed series hybrid mode up to 70 mph. Gasoline is now the base fuel that's propelling the Volt in series-hybrid fashion. The battery has run down as far as the control system will allow. The ring gear is unlatched from the second motor-generator and locked in place as it was in Mode 1, and the main traction motor once again is solely responsible for making the planets orbit. But the electricity to do that now comes from the gas engine, which has been clutched to the second motor-generator, now in generator mode.
Mode 4: High-speed series-parallel hybrid mode up to top speed. This is classic gasoline-powered series-parallel operation. You can't have both electric motors driving the car at high speed like we saw in Mode 2 because the battery is discharged, meaning that the second motor-generator must continue to be a generator driven by the engine. This is where the engine begins to directly drive the ring gear. The engine is already clutched to the second motor-generator, so a straight-through mechanical connection is established when the ring's motor-generator clutch is engaged. Compared to Mode 3, the engine works harder here because it is simultaneously driving the ring gear and the shaft of the generator.
Getting the Terms Right
GM points out that the different planetary layout used by the Toyota Prius Plug-In makes its engine come on in Mode 2. On the other hand, GM's Voltec system has three clutches, while the Prius HSD system has exactly zero.
GM also reminds us that the Volt's gasoline engine (premium fuel required, by the way) never directly drives the car without the help of an electric motor. That's true, even in Mode 4 where the sun is still being turned by the main traction motor. But the same is true of every Ford, Lexus, Nissan and Toyota hybrid produced to date. None of their gasoline engines can move them an inch without an electric motor applying torque to one of their planetary elements.
Including the Volt, all these hybrids burn gas some of the time while they're moving. That's what makes them hybrids. The Volt is no different except for the 40-mile head start GM claims it offers before the engine fires up. The next closest is the Toyota Prius Plug-in, which offers 15 miles of electric-only range.
The Sawtooth Principle
Turns out, the right endpoint may not be where you want to be. EVs come with a leash. You can't forget to recharge them or trip a circuit breaker while you sleep. You have to plot every move and strike the words "spontaneity" and "road trip" from your personal vocabulary. You'll need a second car.
As it turns out, we might have been looking at our line in the wrong way. Imagine it as an incline that swiftly rises as it progresses toward the right-hand side of the paper. The Volt is close to the summit in practical utility, but electric vehicles at the far right fall off the cliff of practicality because of an absolute limit to driving range, with a long recharge time waiting for you at the end. At some point battery technology, quick-charging and a fully developed electric infrastructure will remove most or all of the range limitations associated with electric cars, but we're not there, not by a long shot.
Plug-in hybrids like the 2011 Chevrolet Volt might actually be near the sweet spot along our line. We wonder why GM is so anxious to make everyone think the 2011 Chevrolet Volt is an EV when they might be sitting on something much better.