It seems that the low efficiency when using hydrogen in an internal combustion engine, combined with it's low energy density, makes it unlikely to be viable for mainstream vehicles.

https://evcentral.com.au/why-burning-hydrogen-in-an-ice-is-a...

The U.S. unlike the U.K. never adopted LPG or natural gas vehicles in any types of scale; That's with natural gas going directly to many houses.

If LPG/Natural gas ice conversions had taken off in the 80s/90s THEN evidence would be more in Hydrogen's favor.

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.

1. See page 25 of https://web.archive.org/web/20110905043908/http://www1.eere....

What would "convert existing cars" get you?

A BEV is so much simpler to build and maintain.

Why would you want to? Existing cars suck to work on compared to an electric. Ice needs too many parts it's inefficient for moving vehicles. Legit ditch all that running gear and give me hub motors on suspension arms. That's it. Don't need anything else.

Ice vehicles gonna get squeezed out by economics. Why pay some Muppet a few k to pull a block out to do gaskets when for little more than removing a Tyre I could replace the entire hub motor. Maybe a few hundred in labor compared to a few thousand. Hell for some car models like my subi I basically have to remove the entire front end or pull the block out just to do the head gaskets.

Not sure who's pocket you're in but you need to get your facts straight. I can convert anything from a Model T to a modern vehicle from ICE to EV with nothing more complex than a conversion kit, and those come in flavors. Want to keep your original transmission and drive train? They make EV kits in a form factor to replace just the motor portion of a power train. Not married to having a stick shift? Toss the entire drive train, drop in a solid rear axle with hub motors. Fact is you can convert a vehicle from ICE to EV in a weekend and most of the work involves removing unnecessary deadweight.

unfortunately no one's going to be converting existing cars to anything other than scrap metal no matter which technology proves to be the most promising

How is it better? It seems worse than electric by just about every measure