Google Foundation has a program, RechargeIT, to promote plug-in hybrids. This is very exciting because a plug-in hybrid can have radically less impact than a regular hybrid. In a nutshell, a regular hybrid is just a standard gasoline car with some doodads to make it more efficient. But a plug-in hybrid is a real electric car, that can also use gasoline so you don't get stranded on the road.
And now for the details. First, although hybrid sometimes means an engine that can burn natural gas or ethanol in addition to gasoline, here we're talking only about gasoline/electric hybrids.
Cars burn gasoline to move you around. That is, they spray gasoline into cylindrical chambers of an internal combustion (e.g., "inside-burning") engine, mix it with air, and then explode the mixture with a spark. The hydrocarbons—molecules of hydrogen plus carbon—in the gas combine with the oxygen in the air to produce carbon dioxide and hydrogen dioxide—water— plus some energy. The resulting hot exhaust gas wants to be much bigger than the space at the top of the cylinder where we detonated the fuel/air mixture, so it pushes a piston in the cylinder. The motion of the pistons spin a shaft, and that drive shaft in turn connects to the wheels via the transmission, moving the car forward. But only about 35% of the energy from the gasoline makes it even as far as pushing a piston. The rest just heats up the engine block, which heats up the coolant fluid, which heats up the air passing through the radiator. Unless you live in Alaska and hence diverted a bit of that heat to warming up the inside of the car, it's purely wasted energy. And of the fraction of initial chemical energy that did go into pushing the pistons, almost half will be wasted in friction along the path between the driveshaft and road/rubber interface, so that only about 20% of the energy in the gasoline actually moves the car forward. At best.
The exhaust gas, meanwhile, has some impurities, such as incompletely burned carbon (carbon monoxide instead of dioxide, the former being far more toxic) and even raw gasoline. It travels through a device which catalyzes the incomplete chemical reactions into finishing (hence, a "catalytic converter"), various other devices to capture other impurities such as sulfur from the gasoline, a muffler, and out the tailpipe. If all of that equipment is modern and in good working order, the exhaust gas is actually fairly free of toxins and smog-producing chemicals. It does, however, have plenty of carbon dioxide, which of course is a greenhouse gas the emission of which will possibly turn Miami into another New Orleans by 2050.
A diesel is much the same except that it burns at a temperature so high it doesn't need a spark; because of the higher combustion temperature, a modern diesel engine can be more efficient than a gasoline engine.
So what's a gas/electric hybrid? Well, it's a gasoline engine car with a set of electric motors and really big batteries. Big as in hundreds of pounds of batteries. What's the point? Well, gasoline engines aren't especially flexible. They like to run at a certain speed. Even with a transmission—a set of gears that let the engine shaft turn at a different rate than the wheels—cars are finicky. My old Toyota MR2 had an unusually wide "power band", from about 2000 rpm to 7000, but even it didn't develop much power at 1000 rpm. And to get a range that wide, the engine computer has to play tricks like spraying in extra gasoline that won't get burnt (called a "rich" mix), or extra air (a "lean" mix). I hope you will not be surprised to learn that a rich mix is not good for your mileage, or, for that matter, your catalytic converter. I'll skip over the details of Carnot efficiency and stoichiometric ratios; what you need to know is that any particular gasoline-burning engine can be made very clean and very efficient, but only in a very narrow range of speed and power. The engine block design that got 30 miles per gallon in my 2500-pound MR2 at 60 miles per hour (or would have, if I hadn't driven it with a 21-year-old right foot) will not be as efficient in a two-ton truck. In fact, it won't even be as efficient in the MR2 at 30 miles per hour. And that's where hybrids come in.
The thing with energy is that it can't be created or destroyed, only transformed. Power plants don't create energy; they just transform the chemical energy in coal or oil or natural gas into the kinetic energy of a spinning turbine, and then into electrical energy. Dams and windmills, well, you can probably figure them out yourself.
The other thing with energy is that it's hard to store. So almost all of the energy that we generate—by generate I mean liberate from coal, or steal from rivers and breezes, etc—gets used right way. And I don't mean right away as in "your call is important to us" right away. I mean that if you flip a switch to turn on the light on the ceiling, at the same instant a turbine in a big industrial building somewhere shudders microscopically.
The total power output of the entire Western United States power grid at any moment equals the total power consumption. (Almost. About seven percent of the output is wasted in transmission, turning into heat and that eerie hum you hear around power lines.)
The power grid notwithstanding, it is actually possible store power on a smaller scale. And that's what a hybrid car does. Each wheel has its own electric motor, and they all connect to a big battery. The big battery in turn connects to the engine. It tries to keep the engine running at its optimal speed, and when that speed produces more power than the car needs to move, the extra power at the drive shaft is converted, via an alternator, into electricity stored in the battery. When the driver asks for more power than the engine can efficiently produce, the battery sends electricity to the wheel motors to supplement the power coming from the transmission. Electric motors are more efficient over a much wider range of speeds and powers, especially in starting from zero, than gas engines attached to mechanical transmissions, so this can work out quite well. In particular, hybrids will often use only battery power at low speeds, with the gas engine helping push the car only at faster or freeway speeds.
With all of this extra gear, you can play some extra tricks. The coolest one is regenerative braking. Electrical motors can easily reverse; that is, they can turn motion into electricity instead of vice versa. So when you tap the brakes in a hybrid, the car does not need to push brake pads into rotors, thus converting your precious energy of motion into hot brakes. Instead, the motors turn into little generators, with your car wheels playing the role of the rushing river. The wheels get slowed and the battery gets topped off. If everything were perfectly efficient, you could start at the top of the Grapevine with a dead battery and empty tank of gas, roll several miles down the hill, brake to a halt, enjoy an In-n-Out burger, then turn around and roll all the way back up on battery power. (This would work better if the Earth didn't have any air to cause wind resistance, but then the milkshakes probably wouldn't taste right.)
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Regenerative braking is unique to hybrids, because you need both the electric motors and big battery. But the other trick hybrids can play is simply to turn the gas engine off whenever it's not needed. In particular, an idling engine always gets zero miles per gallon regardless of how efficient it might be. (Similarly, a really expensive Lamborghini which is lost and going in circles isn't any faster than a Hyundai, though it's much funnier to watch go past you the second time around.) This is something that most new gasoline cars are supposed to start doing Real Soon Now, since improvements in gas engine technology mean that starting the engine no longer wastes a minute worth of gas, the way it used to when Eisenhower was president.
The upshot of all of this technology and cleverness is that the current batch of hybrid gas/electric cars, most famously the Toyota Prius but also plenty of Hondas, Fords, and so forth, get maybe a quarter more energy out of a gallon of gas. So the Prius gets about 45 mpg. By the way, there's all sorts of controversy about how the US government calculates mileage. One of the things RechargeIT is doing is driving some Priuses around with lots of instrumentation; among other things, they are averaging 44.6 mpg over the last year. That's pretty good, but it's barely better than a diesel Jetta, which doesn't have the hundreds of pounds and thousands of dollars of extra equipment. And VW claims to have 60 mpg Jettas on sale in the US in months.
To make sense of this apparently poor performance, you have to remember is where a gas/electric hybrid gets its energy from. Yes, it has an electric battery, but what charges that battery? The Toyota Prius doesn't plug into anything. The only way that you add energy to the car is by pumping gasoline into the tank. The battery charges only when the engine is running. So all that fancy technology merely makes the car a really, really efficient gas-powered car. But gasoline engines have efficiency limits; the hybrid system lets the car spend more time at those limits, but cannot exceed them. The bottom line is that current hybrids are still 100% gasoline-powered.
I wrote about this when I first drove a hybrid Honda in Seattle's Flexcar fleet, five years ago. I wrote then that "by putting electric technology into a non-masochistic package (unlike the EV1 or earlier Honda Insight) that will actually sell tens of thousands of units, familiarizing consumers and generating real-world trial experience, it's a medium-sized technological step towards true renewable-resource cars." Well, Toyota's sold a million Priuses, gas has passed US$4/gallon, and the next step is at hand. The next step is plug-in hybrids. The difference between a regular hybrid and a plug-in is $10,000 worth of extra equipment and lots more batteries. Enough batteries for the car to hold forty miles worth of electrical energy. And instead of filling the batteries by burning gasoline, you can plug the car into regular 120-volt outlets. Suddenly, that entire second drivetrain, previously slave to the gas tank, is liberated. To be sure, it's still going to be slave to oil, natural gas, and coal-fired power plants, but the frying pan is a better place to be than the fire: even coal plants can be well over 40% efficient, and an electrical drivetrain is much more efficient than a mechanical transmission. Of course, from a climate change perspective we probably want to look at pounds of CO2 emitted mile traveled, which is a research topic for another day, but remember that you can use zero-emission wind, solar, and nuclear power as well. So a plug-in hybrid is the best of both worlds: you can use clean electrical power for shorter trips, plug in anywhere to recharge, and if you don't have time to recharge or want to go more than 40 miles, you've got a gasoline motor and gas tank that give you all the freedom of a very efficient regular car.
How well does it work in the real world? Again, RechargeIT has equipped a fleet of cars with sensors, in this case four cars, and they end up getting 66.2 mpg. Since every single trip is recorded down to the second, you can start getting at the why of the numbers. Here's a trip from yesterday:
Speed (mph):
Engine RPM
Battery (%)
Even though the trip was only 3.6 miles and never broke 40 mph, it looks like the gas engine still kicked in 11 times, almost once per minute. And of course on a long freeway trip, you're going to completely deplete the pre-charged battery and then all of your energy will come from the gas tank. So I guess there's still a ways to go. Even so, over the last year the plug-ins' mileage averages about 50% better than the stock hybrids, and the greenhouse gas emissions are 29% lower per mile. I think the GHG numbers are based on the Googleplex using all solar power, in which case the plug-in charge is emission-free. I would want to know if they take into account drivers' plugging in at home or elsewhere, in which case you'd want to charge the plug-ins with the GHG emission per Wh of power sources for the Bay Area power company. And of course a true lifecycle analysis would take into account the GHG emitted building the cars, shipping them to Mountain View, etc etc. Even so, plug-ins are almost certainly another step in the right direction.
Oh, and one other thing. Remember before when we were talking about the power grid in the US, and how it doesn't have any ability to store power? Well, what if we attached a bunch of batteries to the grid, and charged them up at night, when there is surplus (and more efficient) capacity, and then dumped it back into the grid at noon, when demand peaks and the old, less economical, heavily polluting power-plants have to be maxed out to prevent brownouts? A fleet of plug-in hybrids, of course, could do that. Apparently, if you play your cards right and all the regulation falls into place, your plug-in hybrid could be a profit center.
One thing on the RechargeIT blog really surprised me:
Not the Lexus hybrid; I've even seen them on the street here in Singapore. But I had assumed that when a hybrid motor is put into a luxury car, producing only a 1-2 MPG efficiency improvement, it was pretty much a farce, a bit of greenwashing. But what this graph points out:
is that an improvement from 10 MPG to 12 MPG is worth as much as an improvement from 30 to 60. So maybe those hybrid limos and SUVs aren't completely ridiculous. But don't get carried away; 30 is still much than 12.
Meanwhile, California's regulators, CARB, seem to have done a decent job over the last few decades in standing up to car manufacturers to force them to improve the environmental impact of their cars. But the terminology can get confusing: LEV, for Low-Emission Vehicle, ULEV for ultra-low, and on and on to the latest: AT-PZEV, "Advanced Technology Partial Zero Emission Vehicle". This refers to different levels of pollution coming out of the car, like sulfer dioxide and carbon monoxide. The confusing thing is that, when they use "emission", they aren't talking about carbon emissions. So a car can be ATPZEV and still be pumping out the CO2 that will ultimately put Miami and Bangladesh underwater. Perhaps one day CARB will start regulating greenhouse gases as well, though for all I know that's tied up in a lawsuit or something. Apparently RechargeIT is lobbying CARB on the issue.
By the way, the head of the EPA, which is federal as opposed to the California state CARB, has been going to absurd, and possibly illegal, lengths to stall on doing anything about GHGs. Watch for yourself: