Most of us know by now that a plug-in hybrid is a car with a hybrid drivetrain that can have its batteries recharged from the power grid. It's turning out, though, that such cars may come in more flavors than Starbucks has syrups.
Case in point: We drove a prototype version of the plug-in Prius, or PHEV, that Toyota will offer corporate and fleet customers starting in late 2009. Our experience was a short tour of downtown Portland, Ore., and for the most part, it was what you'd expect: It drove just like a Prius, but one that runs less often on its engine, more often (and at somewhat higher speeds) in pure electric mode.
More striking is the way this PHEV (plug-in hybrid electric vehicle) operates distinct from what we are hearing about the Chevrolet Volt that General Motors will offer in late 2010. Toyota's approach is to start with a Prius and add plug-in rechargability. So the car retains the firm's hybrid-synergy drive, using what Toyota calls a series/parallel arrangement between the gas engine and electric motor.
In the PHEV version, there's a button (in the prototype, it was on the dash to the left of the steering column) labeled "EV mode" that lets the driver choose to run the car only on electricity. If you've recharged the batteries by plugging in to the electric grid and then select this mode, the prototype will deliver six or seven miles of pure electric driving, burning no gas. The target for the production model is 10 miles of such range, enough for a lot of the trips many drivers make.
After that--or if you demand more performance in the short term than the small motor or the battery-pack state of charge can offer--the engine kicks in. As with any Prius, the engine drives the wheels through the transmission when you need more power or range.
During our drive, when we'd been moving around in pure EV mode for several blocks, we wanted to get out from behind a diesel city bus as we climbed one of the many bridges over the Willamette River. Booting the accelerator for an uphill surge from less than 25 mph to more than 40, we saw a dashboard gauge indicating that we were demanding more energy than was available from the batteries. The engine kicked in briefly until we let up on the pedal, even though we selected EV mode. After the acceleration, the car almost immediately returned to pure EV mode.
Pushing the EV-mode button again, to turn it off, returned the car to what Prius owners would regard as "normal" operation. A perhaps crucial distinction is that when the engine's power was not needed to move the car, the control systems are set up to charge the pack of nickel-metal-hydride batteries. In theory, after a period of such use, the batteries would be replenished and the owner could again press the EV-mode button and go another several miles without gasoline.
A side note: Our first response to being stuck behind the bus was to hit the climate-control button to recirculate the air inside, rather than taking in not-fresh air from outside. (We were using neither heating nor A/C). The Toyota rep riding in the back seat was eager to have us turn off the climate control, especially the recirc mode --it runs the ventilation fan at a high speed, a drain on the battery.
PHEV drivers would have to judge for themselves, with feedback from the gauges, whether it is better to draw down the battery in this way or by accelerating to pass. You probably could learn to modulate the pedal up to the borderline just before the engine kicks in, though we wouldn't be eager to do that on the open road, where the higher priority is safety, not consuming a little gasoline. Regardless, the experience was too short, and so was the indicated remaining pure-EV range of less than three miles, to allow such experimentation.
From what we're hearing about the Volt, GM is taking a radically different approach. Think of the Volt as an electric car with a range-extending gasoline generator. The Prius, by comparison, would be more like a normal car with an efficiency-enhancing electric supplement.
The Volt will be a series hybrid, meaning its engine only powers a generator to create electricity. The engine won't drive the wheels at all. With an advanced (and more expensive) pack of lithium-ion cells, the Volt should be capable of 40 miles of driving in pure electric mode after a full charge. After that, the engine will kick in to extend the range. Reports vary, but what we've heard most recently is that the Volt is not likely to use its engine to recharge the batteries, ever. Recharging will come only by plugging in the car.
After the charge is depleted, the engine will spin a generator that drives a bigger, stronger electric motor than in the Prius, one big enough to give all the performance the driver needs. In this phase, the Volt would operate in the same manner as a diesel locomotive, with the generator's output going straight to the motor, which then drives the wheels.
The upside is that a Volt-like series hybrid places fewer demands on its engine, which can be sized, engineered and controlled to operate in a narrow range of rpm and peak efficiency. One downside might be that on a long trip, after using up the 40 miles of EV operation, you couldn't return to that mode until you could plug it in. Stop at a hotel without easy access to an outlet, and you might have to begin the next day with depleted batteries. Still, if the efficiency benefits of electric drive are realized, you could be getting 60 mpg or better in that mode.
GM is refining the control systems and operational modes, so it's too early to be certain how it will all work out in the end, or how driving a Volt will differ from driving a Prius PHEV. What is clear is that a plug-in hybrid from GM will be a much different animal from one made by Toyota.
Part of what's going on in this field is the interplay between today's preferred battery, the NiMH cell, and the li-ion units coming to market. Toyota is preparing to make lithium-ion cells for the day when it begins offering plug-ins to the public. The newer lithium batteries are more suited to the deep draw-down and recharge cycles expected of plug-in hybrids or pure electric cars, while the NiMH cells are more tolerant of rapid switching between discharging and recharging on the go. This speaks to one reason GM might want its batteries to recharge only from the grid.
There is chatter from other companies that some plug-in hybrids might use both kinds of cells-the more-expensive lithium-ion variety to recharge from the grid for extended EV range, and the less-expensive NiMHs for extended-range operation, resembling what we have come to regard as "traditional" hybrids.
That neither variety is yet perfectly suited to the role of automotive propulsion may be inferred from the creation of a new group within Toyota's battery-engineering division only three months ago. Its mission is to determine what comes after lithium ion.
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