...and why hasn’t this engine caught on?
Seems to have more than a few potential stress points that may fail.
I think the near future, using existing technology, is Nat GAS. It works. If Auto Makers get behind a “Passenger car” version, you’ll be able to swap out a tank (just like your gas grill).
Demand vs. Supply, the chicken and egg paradox of building the vehicle before having the proper filling stations.