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 0,19 kWh of energy per gallon (or 0,05 kWh per liter), whereas say gasoline has 33,41 kWh per gallon (8,825 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.
A makeshift solution that could still allow you to use compressed air to "some degree" could still be practical though. 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 hereby 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 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. An additional switch would be added to allow the alternator to be disconnected by it (electrically, not mechanically !). The latter switch would hence reduce the mechanical resistance of the alternator, conveyed unto the engine. The switch would be used when driving on the compressed air (as in this instance, no spark ignition is needed anyway, and any devices that need power can just obtain it from the battery; the battery won't be recharged by the alternator any more, but since the engine can only run limited time on compressed air, it wont be long enough for the battery to get exhausted). Optionally, a compressor can be added to the system, which can be made to run only when the engine runs on its regular fuel. 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 somewhat impractical. The main problem is the storage of the fuel itself: to store hydrogen effectively we need either extreme pressure (3600-10000 psi) or extreme cooling (to − 252.87 °C). We need such pressure or cooling to make the hydrogen more dense, because at no pressure, ambient temperature (gas-form) the energy it contains per liter is just 0,01136 kWh/gallon/bar or hence 0,003 kWh/liter/bar (and with that little energy, you can't power an internal combustion engine).
When applying the cooling technique to increase energy density, 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.
When applying extreme pressure, it is possible to use it in a internal combustion engine effectively as you can use CNG tanks which can take 3600 psi of pressure (special H2 tanks that can handle up to 10000 psi are also available but far too costly). However the range you can drive with CNG tanks is quite low. Basically, to store a same amount of energy as in 1 gallon of gasoline (33,41 kWh/gallon), you need to use a 12 gallon CNG tank and store the hydrogen at 3600 psi (=248 bar).
Calculation: 12 gallon x 0,01136 kWh/gallon/bar x 248 bar = 33,80 kWh / 33,41 kWh = 101 %
or in metric: 45,42 liter x 0,003 kWh/liter/bar x 248 bar = 33,80 kWh / 33,41 kWh = 101 %
As everyone knows, with a regular car (fuel consumption being 2,6 gallon / 100 miles) you can drive about 38,46 miles (61,89 km) with the energy in 1 gallon of gasoline (gasoline has 33,41 kWh of energy /gallon) so that is suitable for city use but little else. Also, fitting in such a big tank would be quite difficult in a regular car (the 3 gallon tank format is probably much easier to fit but then range is even more limited: just 9,61 miles or 15,46 km). Adding to this issue is that, compared to biogas, hydrogen is also far more dangerous. So, using hydrogen in pressurized tanks, connected to an internal combustion engine (which runs hot and effectively combusts the gas in the engine) isn't a very safe or practical solution.
You also need to consider that you will still emit some CO² per kWh (so far less then natural gas but more than biogas, ethanol, ...). This, because the energy that was needed to make the hydrogen has had to come from somewhere, and this electricity that was produced prior to make the hydrogen comes with its own CO2 emissions (amounts differ, depending on the source):
If coal was used to generate the electricity: 1 kg CO² per consumed kWh was emitted.
If it came from PV solar panels: 0,1 kg CO² per consumed kWh was emitted.
If it came from nuclear power plants: 0,006 kg CO² per consumed kWh was emitted.
Besides these initial figures, there are also the inefficiency losses when converting the electricity to hydrogen. We can safely assume a 50% inefficiency loss for the conversion (electrolyser) alone -not counting the energy losses of the compressor which might be even higher, depending on the compression level required-. So you need to multiply the amounts listed above by at least a factor 2 (perhaps as high as 4 if counting the compressor losses too).
Are there any fuels that are similar to hydrogen (emissionless and not even causing any air pollution at all), but which also don't have the disadvantages hydrogen have (storage problems) ?
Yes. There is formic acid aka "hydrozine" (CH2O2). Unlike hydrogen, formic acid is a liquid and thus very dense (much denser/higher in energy than hydrogen). CH2O2 contains 6,81 kWh/gallon (1,8 kWh/liter) which compares to H2 pressured at 599 bar (6,81/0,01136 =599).
It can be used in combination with an internal combustion engine in a car by converting it first to hydrogen (at 250 bar) which is then combusted in the engine. By doing so, you increase the range of the hydrogen vehicle greatly, while still being able to use CNG-equipment and a CNG-tank for storing the hydrogen temporarily; the formic acid is stored in a plastic tank, which by the way doesn't need to withstand any pressure at all). You can also make the entire system a lot safer (because unlike hydrogen, formic acid will not burn/explode), and as a result of this you can also make the tank as big as you want (because unlike with compressed hydrogen tanks, making the formic acid tank bigger doesn't pose an increased safety risk). A system that converts the formic acid to hydrogen and CO2 (called a catalyst) is then required.
Formic acid is typically used in a formic acid or hydrogen fuel cell (because it is more efficient to use then) but obviously if you use this approach then you can only convert electric cars. The fuel cells (DFAFC's or HFC's -combined with a catalyst-) are also still expensive, and difficult to acquire.
There is also methane. This is similar to hydrogen in that it too is a near-emissionless fuel (if it's made using the Sabatier process, so from hydrogen), and it causes some air pollution when burned. It stores easily (regular tanks also used in CNG conversion kits would do), and it can even make cars act as (temporary) carbon sinks. The same emissions as mentioned at the hydrogen-section (electricity production emissions prior to making the fuel) should be taken into account.
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. It's also high in energy (containing 3x more energy than in hydrogen, namely 0,034 kWh/gallon or 0,009 kWh/l per bar of pressure)
Hybrid vehicles are not mentioned either ?
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 (or freight 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.
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