Hydrogen powered vehicles

Cars can be run on hydrogen in two ways:

1) as a fuel in an internal combustion engine ICE

2) as a fuel cell generating electricity that powers an electric motor

It seems unlikely hydrogen will ever be as efficient as batteries for vehicles.

 

Hydrogen in an internal combustion engine - ICE

Using hydrogen in an ICE is nothing new. In fact a mixture of H2 and CO was used before petrol/gasoline.

Hydrogen has very low ignition energy. The amount of energy needed to ignite hydrogen is about one order of magnitude less than that required for gasoline. This  enables hydrogen engines to ignite lean mixtures and ensures prompt ignition.  
Unfortunately, the low ignition energy means that hot gases and hot  spots on the  cylinder can serve as sources of ignition, creating problems of premature ignition and flashback.
Preventing this is one of the challenges associated with running an engine on hydrogen. The wide flammability range of hydrogen means that almost any mixture can be ignited by a  hot spot.

One of the benefits of hydrogen-powered ICEs is that they can run on pure hydrogen or a blend of hydrogen and compressed natural gas (CNG). That fuel flexibility is very attractive as a means of addressing the widespread lack of hydrogen fueling infrastructure in the near term. Hydrogen-powered ICEs also have many operating advantages. They perform well under all weather conditions, require no warm-up, have no cold-start issues (even at subzero temperatures), and are highly fuel efficient — up to 25% better than conventional spark-ignition engines.

Difference between a hydrogen ICE from a traditional gasoline engine could include hardened valves and valve seats, stronger connecting rods, non-platinum tipped spark plugs, higher voltage ignition coil, fuel injectors designed for a gas instead of a liquid, larger crankshaft damper, stronger head gasket material, modified (for supercharger) intake manifold, positive pressure supercharger, and a high temperature engine oil. All modifications would amount to about one point five times (1.5) the current cost of a gasoline engine.[8] These hydrogen engines burn fuel in the same manner that gasoline engines do.

The power output of a direct injected hydrogen engine vehicle is 20% more than for a gasoline engine vehicle and 42% more than a hydrogen engine vehicle using a carburetor.  Ref1 , Ref2  H2-ICE 

Hydrogen has a volume problem. The gas bags for cars produced during WW1 held towns gas, a mixture of CO, CH4 and H2. Or just methane. The cars had a range of about 80 KM. Pure hydrogen would need a far bigger gas bag.

This car ran on methane produced by heating straw. It was abandoned as impracticable

BMW Hydrogen engine ICE

BMW is developing a new hydrogen engine it claims will be as efficient as the diesel technology available today.

The engine being developed combines spark-ignition and diesel combustion technology to better exploit the favorable burning properties of hydrogen. The experimental system also involves direct-injection technology, with HOERBIGER ValveTec GmbH, one of the companies taking part in the research project, supplying high-pressure injectors that can push the hydrogen into the chamber at pressures of up to 300 bar.
Although still in the research phase, BMW claims the engine can match the combustion efficiency rating of 42 percent of modern turbo-diesels as well as deliver a power output of 100 kilowatts (about 134 hp) per liter of engine displacement.
BMW Hydrogen 7   More

 

How a hydrogen fuel cell works

The hydrogen fuel cell operates similar to a battery. It has two electrodes, an anode and a cathode, separated by a membrane. Oxygen passes over one electrode and hydrogen over the other.
The hydrogen reacts to a catalyst on the electrode anode that converts the hydrogen gas into negatively charged electrons (e-) and positively charged ions (H+).

The electrons flow out of the cell to be used as electrical energy. The hydrogen ions move through the electrolyte membrane to the cathode electrode where they combine with oxygen and the electrons to produce water. 

Efficiency of Hydrogen fuel cell vehicles

For a fuel cell car operated on gaseous hydrogen the following numbers are
representative: 
30% losses for water make-up and electrolysis: factor 0.70
10% losses for compression of hydrogen: factor 0.90
10% losses for distribution of gaseous hydrogen: factor 0.90
  3% losses for hydrogen transfer: factor 0.97
50% for conversion to electricity in fuel cells: factor 0.50
10% parasitic losses for the hydrogen fuel cell system: factor 0.90
10% electric losses in the drive-train between battery and wheels: factor 0.90
The "power-plant-to-wheel" efficiency of a fuel cell vehicle operated on compressed
gaseous hydrogen will be in the vicinity of 22%  

Source: http://www.efcf.com/reports/E04.pdf

Source: Bossel

The "power plant-to-wheel" efficiency of a fuel cell vehicle operated on liquid
hydrogen will be in the vicinity of 17%.
These numbers are certainly better than the drive cycle efficiency of yesterday's cars,
but they compete with the high efficiency of modern clean Diesel passenger cars and
commercial hybrid vehicles. Advanced Diesel-fuelled passenger vehicles now reach
HHV drive cycle efficiencies of over 25%. 
It is not the intent of this summary to present a precise analysis of all possible options
of hydrogen use for transportation, but the results suggests that the numbers used in
support of hydrogen programs should be checked carefully and corrected.
Finally, it might be useful to note that much higher efficiencies are obtained for hybrid
electric cars with Diesel-fuelled solid oxide fuel cells.
 

How much hydrogen would we need?

Hydrogen fuel cells are three times more efficient than a petrol/gasoline powered engine. About 40 million tonnes of hydrogen per year would be required to fuel about 100 million fuel cell-powered cars after full market penetration. 

 
http://www.hydrogen.co.uk/h2/hydrogen.htm