As long as there have been vehicles, people have been trying to get one to run on some unusual fuel source. If you thought hydrogen or biodiesel were odd fuels, just wait until you see some of the things people have tried in the past.
The idea of compressed air conjures images of air dusters, tire pumps and CO2 canisters, and those images aren't far from the truth. Compressed air vehicles can run entirely on compressed air, or on a combined system of air and other hybrid fuel types. Compressed air engines use high-pressure air to drive pistons in the engine. When the air expands, it cools down, and that cool air can be used for air conditioning in the car. Compressed air tanks can be refilled at home or at a fuel station.
With no emissions beyond cold air, why aren't compressed air vehicles in common use today? They were invented in the 1920s, after all. Well, first of all, compressed air still requires electricity to create. Hand pumps and mechanical devices are too slow to be a viable option. Secondly, compressed air cooling can lead to iced up engines in humid climates. Finally, compressed air vehicles, while efficient in operation, are nowhere near as efficient as a Lithium-Ion battery powered vehicle. Still, the technology is in development, so perhaps an efficient compressed air vehicle is on the horizon.
Similar in function to compressed air, liquid nitrogen cars are a theoretical possibility at the moment. Liquid nitrogen is in many ways a more efficient fuel than simple compressed air, but the fuel has many of the same temperature issues. Liquid nitrogen is also inefficient to create with currently available tools, with much of it coming as a byproduct of the creation of liquid oxygen for other uses. Liquid nitrogen also has a few safety issues, including the faint possibility of a leaking nitrogen tank causing asphyxiation.
Yes, the steam engine was not limited to the rails. Modern vehicles are powered by internal combustion; a steam car was powered by external combustion. Burning wood, coal or other fuels generated heat, which heated water into steam, which itself generated power through a direct connection to the axle or to turbines in a steam engine.
Of course, a steam powered car required quite a bit of time to fire up. Once they got going, they could reach relatively high speeds -- some as high as 100mph -- on the power of steam alone. Of course, today, the steam engine is an outdated technology. The use of external combustion is generally not a good idea in modern automobiles.
Bridging the gap between a motorized bicycle and a Reliant Robin, pedal-powered motor vehicles run on a normal electric engine, with supplemental power from human pedal action for those steep hills or long journeys. As individual transport, they're comfortable little tricycles. For commuter cars, city driving machines or, really, any other purpose, they're little more than a novelty. They don't have the power or the storage capacity to take up a major role in the transportation business.
Step up a little larger and cut out pedal power, and the tricycle model becomes much more viable. Small vehicles such as those designed by Aptera or Elio Motors have more power, more range and more capacity. Of course, they're standard electric engines, not an esoteric fuel source fitting for this list.
Other, more conventional alternative fuels still exist, of course. Hybrids, compressed natural gas, biofuels, pure electricity; all of these are the high profile alternative fuels for a reason. These other old experiments, from steam engine cars to compressed air, all have critical problems that must be surpassed before the technology becomes viable. It doesn't matter how emissions-free a car is if it can't meet the needs of the average driver. That's why, when you look at a list of modern alternative fuel vehicles by Klosters, you see the typical hybrid/electric/diesel selection of fuels.
When it comes down to it, these alternative fuels are interest diversions from the path of history, but they aren't exactly viable modern technologies. Fossil fuels are on their way out, rightfully, and it's about time they joined the ranks alongside steam engines. It’s just a matter of which new fuel will succeed first.
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