It's not economically viable for the car companies to build them.
Sure they may be more expensive to purchase initially but gas costs a lot more than distilled water does. Run your car off of rain water??? I think I'm going to get into the cistern business...lol.
electrolysis puts more energy into getting the hydrogen out of the water than you get out of the hydrogen so it is not a good way to make mass quantities of hydrogen. tecniques using natural gas are far more efficient but it still takes time to set up a refuelling infrastructure. There is a hydrogen fuelling station being built in LA right now and honda has been designing units that run off of natural gas for your home that provide heat, hot water and fuel for your car. so you will see hydrogen celled vehicles but it will take time for them to be available or economical everywhere
Answering this question cannot be done with a purely technical-level response. Neither do I believe that the reason "we" don't have a hydrogen cell onboard electrolysis commercially produced vehicle is because "they" don't want it. I do believe there are market considerations, but there are also considerations of scientific paradigms. Hydroxy gas is complicated and overpowering cells with higher voltage and/or amperage produces a lower quality gas, but conventional science doesn't know much about why this is. Technically, the standard piston engine isn't ideal for fast-burning hydrogen fuel; a Wankel engine might be better. I also believe recent advances in ceramic coating of metal engine parts utilizing the engine oil as the carrier--i.e. Cerma and Ceramizer--would assist in building a hydrogen engine.
Search General Questions