I doubt it would be economically feasible to convert existing cars to hydrogen. If you want to use a hydrogen fuel cell, you'd have to replace the gas engine with a hydrogen fuel cell and electric motor, then mate that to the existing transmission (though with an electric motor, things like a clutch and six gears wouldn't be very useful). Then you'd need another electric motor to run the accessory belt(s) for things like the power steering pump and the air conditioning compressor. If you want to warm the interior of the car, you'd have to replace the heater core with some sort of heat pump or resistive heating element.
If you want to modify the gasoline engine to run on hydrogen, you'd need to replace the entire top end and many components in the bottom such as the connecting rods. You'd also need different spark plugs, a higher voltage ignition coil, different fuel injectors, a stronger head gasket, and a modified ECU. Burning hydrogen at stoichiometric ratios causes lots of nitric oxide, so you'd probably have to run lean to get emissions down. This means the engine won't be as powerful.[1]
There are other modifications you'd have to do in either case. You'd have to replace the fuel tank with a hydrogen tank, replace the fuel lines, and replace the gas cap with a high pressure nozzle. Hydrogen is an odorless invisible gas, so you'd need to add some hydrogen sensors to the car to warn about leaks.
After solving all of these problems, your car would have less range and fewer ways to refuel.
If you want to modify the gasoline engine to run on hydrogen, you'd need to replace the entire top end and many components in the bottom such as the connecting rods. You'd also need different spark plugs, a higher voltage ignition coil, different fuel injectors, a stronger head gasket, and a modified ECU. Burning hydrogen at stoichiometric ratios causes lots of nitric oxide, so you'd probably have to run lean to get emissions down. This means the engine won't be as powerful.[1]
There are other modifications you'd have to do in either case. You'd have to replace the fuel tank with a hydrogen tank, replace the fuel lines, and replace the gas cap with a high pressure nozzle. Hydrogen is an odorless invisible gas, so you'd need to add some hydrogen sensors to the car to warn about leaks.
After solving all of these problems, your car would have less range and fewer ways to refuel.
1. See page 25 of https://web.archive.org/web/20110905043908/http://www1.eere....