When people who know little about cars and even less about basic mechanics (the sort that describes motion and energy, not fiddly bits in engines) try to think of an environmentally friendly car, the usually only focus on the propulsion system rather than addressing the more obvious areas for improvement. This invariably leads to environmentally pointless cars, like the Toyota Prius. While I am not saying that Toyota are clueless (I think at this point it is probably thoroughly proven that they know more about making cars than any other company on the planet), the markets are.
Of course, the Prius isn't entirely pointless. It represents a step in the right direction with regards to inspiring people to think about energy efficiency and it goes to show that you can actually put forward-thinking cars into production. The car industry has a habit of building all these neat prototypes, but never putting them into production. Toyota demonstrated willingness to do this. The Prius raises awareness and signals willingness to stray from the beaten path.
This is why the Prius is important although it isn't going to save the planet.
But it is ugly, it is slow and equivalent energy efficiency can easily be achieved by actually learning how to drive and sticking to cars that are light, reasonably aerodynamic and most importantly: suitable for the sort of driving scenarios it will mainly be used for.
Still, the Prius doesn't address the fundamental issue of fuel efficiency: weight. And this is my main beef with the car industry: to save fuel you must reduce the amount of energy needed, and to save the amount of energy needed you must save weight and reduce friction. Saving weight is the shortest path to reducing emissions. And there are several reasons for this.
First off, if you bump the weight of a car down from, say, 1.5 tons (which is a typical C-class car) to 500kg (which is doable) you have one ton less dead weight to accelerate. This means that you need significantly less energy to power the car so even with traditional engines, this has a big impact on emissions. It even reduces the amount of frictional force you need to accelerate the car -- including lateral acceleration (ie. turning), which translates to less tyre and road wear and heat dissipation (guess what tyres are made from). It might not be obvious why reducing road wear is important, but if you live in a city like Trondheim, where road-dust is a major problem, you'd quickly get the idea.
Another reason this is important has to to with energy efficiency. Petroleum is pretty much the best mobile energy source we have. It is fairly easy to deal with and it has extremely high energy density. Alternative fuels have lower energy density so you will actually have to expend a lot of energy for dragging your source of energy around with you. In the short term, cars with alternative energy sources will not deliver performance, convenience and cost comparable to gas powered engines.
People are going to want roughly the same driving characteristics as they enjoy now, so you have to make up for the energy deficit somewhere else. For "normal" speeds, this means lower weight.
For higher speeds this means a combination of low weight and improved aerodynamics (possibly even non-static aero devices). The sad truth is that at high speeds, good aero becomes all important since the equations are dominated by the square of speed and unfortunately, lowering the scaling factor (drag coefficient) significantly is Very Hard.
Now don't expect aerodynamics to improve all that much. Most well-designed cars today have fairly decent drag coefficients and we will see further improvements in this area as simulation software improves, I wouldn't expect overly dramatic results. (If you want to know what I mean, pick up a book on automotive aerodynamics, read it and then ponder the problem of getting the marketing department on board :-) ).
The most dramatic improvements in the short term (next 5 years) will be when we shed weight. This is low-hanging fruit from an engineering point of view.
We actually do have the materials and the engineering knowledge to make light cars. We've had them for quite a while. However, it is expensive to build cars from what is today considered exotic materials. The production process is too expensive and labor intensive. But as for most high-tech things: productions costs are coming down and they are coming down more rapidly than the industry had anticipated. In addition, the materials are steadily improving.
This is why I think the Volkswagen One Litre concept car is important. Volkswagen are going to put it into limited production in 2010 and I really do hope they manage to get the cost down so they can start volume production of it. The car's dry weight is 290kg and the drag coefficient is a mere 0.159. If you compare this to a Prius, the Prius is almost a whole ton heavier and it has a drag coefficient of 0.26 -- which isn't bad, but not really anything to get too excited about. (Image from Wikimedia Commons).
Of course, the Prius is a much roomier car and can take 5 people, whereas the One Litre can only take two people. The Prius has 5 times the luggage capacity of the One Litre.
Also, some of the metals used in the One Litre prototype aren't going to get any cheaper, so I would expect the production version to make compromises and end up significantly heavier.
It has all the safety features you'd expect in a modern car (crumple zones, airbags, ABS, stability control etc) and it also features an electronically controlled manual gearbox.
If you used this as a commuter you'd probably use about 1/10 of the fuel that an ordinary car in the US consumes (that's an order of a magnitude, for you engineering types) with moderate driving.
Since a lot of people drive alone to work, or with only one passenger, I think this car would make a lot more sense as a commuter than a Prius.
Okay, so it seems the car industry is slowly catching on and starting to make a bit of sense. I hope that the companies that produce mass-transit systems are going to take a step out of the 17'th century and start thinking about lightweight solutions as well. Those who do mass-transit systems seem to have some problems thinking outside the box, so perhaps the next revolution will come from the sort of people who build modern rollercoasters. Who knows. :-)