EarthTalk is a weekly installment from E/The Environmental Magazine.

Dear EarthTalk: Should we expect to see “plug-in” hybrid cars anytime soon? I’ve been hearing they are on the horizon but I wonder if that means in one year or 10.

Gasoline-electric hybrids now, like Toyota’s popular Prius, don’t need to plug in—you just fill their tanks with gasoline and the battery keeps charged by the internal combustion engine and by energy generated from the wheels when braking (a feature known as “regenerative braking”). The battery then powers the electric motor when it is called into service during idling, backing-up, crawling in gridlock, maintaining speed while cruising, and for extra uphill power when needed. As such, the electric motor is essentially a back-up engine while the hybrid relies mainly on the gasoline engine.

Plug-in hybrids take the concept further by plugging into a regular electric outlet to enable the vehicle to operate solely on its electric motor for ranges of 40-50 miles or more on a single charge. This has profound implications for commuters who need only drive short distances to and from work every day and who may be able to do so solely on electric power. The gasoline engine then becomes the supplemental one for when the car needs to travel farther than the electric engine can take it.

According to researchers at the University of California Davis, the electricity cost for powering a plug-in hybrid is only about one-quarter of the cost of powering a like-sized gasoline vehicle. Other benefits include far fewer fill-ups at gas stations and the convenience of recharging at home.

Toyota, currently the world’s largest producer of hybrid vehicles by far thanks to the success of its Prius, announced that it expects to have a commercially viable plug-in hybrid available to consumers as early as 2010 and is now testing prototype versions of plug-in hybrids at two California universities.

Felix Kramer of the California Cars Initiative (CCI), a non-profit dedicated to promoting plug-ins, called Toyota’s announcement “stunning and very welcome,” and says that these vehicles will be the cleanest practical cars on the road in a world where gas stations dot just about every intersection. The promise of such cars, says CCI on its website, is that drivers will have a “cleaner, cheaper, quieter car for local travel, and the gas tank is always there should you need to drive longer distances.”

U.S. automakers are also jumping onto the plug-in bandwagon. General Motors says that it will have mass-market plug-in hybrids—modifications of its Saturn Vue and Chevrolet Volt—on the road by 2010. Ford has also developed a small fleet of plug-ins, but is not yet ready to offer them to the public. Fisker, a U.S. start-up focusing on the creation of high performance, energy efficient vehicles, plans to sell an $80,000 plug-in hybrid sports car by late 2009. Chrysler’s Sprinter van was the first plug-in from a major U.S. manufacturer, but it is only presently available to a limited number of institutions as a fleet vehicle.

Plug-ins have also caught on elsewhere. Chinese carmaker BYD plans to sell a plug-in hybrid sedan in the U.S. within five years. And Volkswagen hopes to have a plug-in hybrid Golf ready to roll by 2010.

CONTACTS: California Cars Initiative, www.calcars.org; BYD, www.byd.com; General Motors; Fisker

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The following is a guest post from reader Jaimie Scott, who was kind enough to write an article for The Good Human. Thanks Jaimie!

Something occurred to me while I was riding my bike today. I bet not many of the design engineers at the major auto makers ride bikes. I came to this conclusion when I stopped at an intersection alongside a car. As the car took off I was engulfed in a cloud of hot exhaust. So I started looking at cars with this in mind and noticed that the side of the car that the exhaust comes out on and the direction it points is different on nearly every make and model. Of course for cyclists, the worst combination is the passenger side exhaust pipe that comes out the side of the vehicle, followed closely by the passenger side pipe that comes out the back of the vehicle. It will sure be nice when a non-polluting form of transportation replaces the car!

By “cloud” I mean a thermal cloud, not a visual one. It has been very warm in Sacramento this week and the wildfires still burning in Northern, CA are not helping matters. Even though it was almost 90 degrees out at 11am when this happened, I had no problem feeling the hot exhaust hit my face. It made me wonder how hot those car exhaust gases are. I figured they must be at least 50 degrees higher than the ambient air in order to be that easily noticed. Then I thought to myself, “I wonder if the temperature of those hot exhaust gases significantly contributes to global warming?” We all know that greenhouse gases such as CO2 play a role in trapping heat near the earth that results in global temperatures rising. But what about the energy in the form of heat that we create when we drive. Does this play a role and if it does, to what extent? I don’t see how it could not with the exhaust gas temperature being much higher than the air temperature.

So as I always do when I have a question I turned to the web. At first I tried to find out the temperature of those exhaust gases. I couldn’t find much information, but from what I was able to find most cars’ exhaust temperature after it passes through the catalytic converter is in the hundreds of degrees, like somewhere between 300F and 500F. That would explain why I could feel it so powerfully ten feet away on a 90 degree day. This is an incredible amount of heat we are releasing into the atmosphere. I think it’s easy to forget that many thousands of explosions take place every second in your car’s engine. That heat has to go somewhere. Cars are very inefficient at converting that energy into power to drive the wheels. Exhaust gases are only one way in which heat is transferred from our car engines to the atmosphere. Engines produce a great deal of heat that radiates directly from the block and cylinder head, not to mention the heat that the coolant removes which gets transferred to the air by the radiator. Much to my surprise, when I started looking for articles on the contribution of the heat generated by our cars to global warming I was unable to find a single one!

It seems like the calculation should not be too difficult. I mean it’s easy enough to figure out how much heat all of the cars in the world are putting out in their exhaust if we make assumptions for the average exhaust temperature and the volume of exhaust generated per gallon of gas burned. Burning a gallon of gasoline creates about 18 pounds of C02. But is that the only contribution to global warming we should be concerned about with all the driving we do? The difficult part of the calculation for me comes from assessing the impact of that heat on the planet. I’m not knowledgeable enough about thermodynamics to figure out that part of it.

Does anybody reading this have the knowledge or data at their disposal to complete the calculation? Have you read about this topic anywhere else? Do you know a physicist that is well-informed in the ways of auto exhaust and heat transfer that you can forward this too so we can figure it out?

Many thanks to Jaimie Scott for this guest post!