Vehicle conversion FAQ
Compressed air is not mentioned at opengarages. Why is that ? Air, compressed at 30 MPa (4,500 psi) contains but a mere 0,050 kWh of energy per liter, whereas say gasoline has 8,9 kWh per liter. So, a vehicle that could drive say 700 km with its given gasoline tank (most have about a 50 liter tank (1) -and a consumption of about 15 to 21 km/liter (2)-), could drive only 3,9 km.
Besides this, a conversion would require implementing tanks that can withstand a high pressure (which again are expensive) and would also require swapping the internal combustion engine with a compressed air engine.
A makeshift solution that could still allow you to use compressed air to "some degree" does exist though and could be practical. The method to do so would be by using a conversion kit that would convert the internal combustion engine in your vehicle to allow it to run on either regular fuel or compressed air (so, basically making your vehicle into a flexible fuel vehicle).
The internal combustion engine can be made to run on compressed air in the city (where there's a lot of starting/stopping). Compressed air tanks hold very little energy, but it might be enough for running in the city (where the tanks can be refilled fast at regular filling stations -filling stations tend to have an compressor service too, ie for inflating tyres, ...)-. The idea here is that a cheap compressed air tank could be added and an electric switch too would be added near the driver, allowing him to switch between the fuels. This idea is a variant on a similar idea of Leroy K. Rogers
Hydrogen (used as fuel on itself -so not as a booster) is not mentioned at opengarages. Why ? Hydrogen is often hyped as the fuel of the future, but it still has many problems making its use in vehicles today impractical and not at all ecologic. The main problem is the storage of the fuel itself: to store hydrogen we need either extreme pressure (5000-10000 psi) or extreme cooling (to − 252.87 °C).
So, when applying the cooling technique, we need to have a refrigeration system on-board the vehicle that can cool it to this temperature. This hence requires huge amounts of electricity, which itself needs to be derived from the power the engine (running on hydrogen) needs to provide. If we consider that the efficiency of the engine itself isn't all that great (35% when using the hydrogen in an internal combustion engine), it quickly becomes clear the whole thing is very inefficient (and not to mention costly, as such extreme coolers can't be acquired cheaply).
When applying extreme pressure, it may be possible to use it in a internal combustion engine effectively, but tanks that can withstand that kind of pressure can't be obtained easily, and are again, very costly even if you do find them.
Using the fuel in a fuel cell is much more efficient (50 to 85% efficient compared to 35% for a internal combustion engine), but these fuel cells generate electricity, so you need a vehicle with an electric engine for that. Most cars sold today come with an internal combustion engine, so -if you have a vehicle with an internal combustion engine- that would mean you'd need to replace the engine as well, making the conversion even more costly (remember you also need to calculate in the cost of the hydrogen tanks). If you allready have an electric vehicle, you would get away with just buying the tanks.
If hydrogen tanks do become cheap enough, we also need to consider that you might still emit some CO² per kWh. This, depending on where the electricity you use to recharge your batteries came from: If coal was used to generate the electricity: you'll emit 1 kg CO² per consumed kWh. If it came from PV solar panels, you'll emit 0,1 kg CO² per consumed kWh. If it came from nuclear power plants, you'll emit 0,006 kg CO² per consumed kWh.
Ethanol is listed as a useful fuel, but what about methanol ? This too is a biofuel, and it's practical. However it is very toxic and can be absorbed by the skin. If absorbed, it could cause blindness. So, we rather avoid it.
What's the difference between CNG and the "biogas" you keep mentioning ? CNG is a compressed gas. More importantly, it is "natural gas", meaning gas that is derived from cavities under the the soil where it has been locked away for millions of years. It is as such a fossil fuel and contributes to global warming (quite considerably even, it contains 0,6 kg CO²/liter). Biogas is created artificially, using anaerobic digesters, and contains 0 kg CO²/liter.
Hybrid vehicles aren not mentioned neither ? The reason why hybrid vehicles (3) tend to have a greater fuel economy (km/l) is because people tend to use their vehicles in congested areas (cities, ...). This, as in congested areas, vehicles are stopped and started a lot, and often need to run idle for a considerable length of time. Internal combustion engines consume more under these conditions, but electric engines/batteries do not. A hybrid vehicle can switch to its electric propulsion when in congested areas, and switch to its internal combustion engine when on open roads. So, people that use their vehicle both in the city as on open roads will find them much more fuel economic than non-hybrid vehicles.
That said, few people will actually need vehicles that need to be able to do both (as for each situation, you can just opt for a different vehicle, say a bicycle in the city, and a car for travelling to further away locations). So, the amount of people that really need a hybrid vehicle is probably not very high.
In addition, there are very few kits available on the market that can convert a internal combustion engine vehicle to a hybrid. The kits that are available tend to be quite expensive, and are difficult to install. The high purchase price often means that they don't allow the user to regain their investment.
1: a 50 liter tank is about a 13 gallon tank 2: 15 to 21 km/liter is about 35-50 mpg 3: the "hybrids" discussed here are hybrid vehicles combining a internal combustion engine with a electric engine