see also http://www.liquidpiston.com
Wonderful! Now only if we had a supply of hydrogen.
Since very little free H2 exists in nature, it must be generated from our old friends the hydrocarbons - plus a large amount of energy to separate out the H2. So hydrogen is not actually an energy source. It is, in effect, a way to store energy for later use (in the Starfire engine for example).
Even if you’re willing to spend all that money and energy to create the H2, you would still have to confront the fact that high pressure hydrogen tanks in motor vehicles would create a hell of a problem.
Other than that, hydrogen energy has a rosy future.