The Quasiturbine engine can be run on compressed air, and in transportation applications, it should (http://qtusa.promci.qc.ca/QTEfficaciteComparative.html http

The Quasiturbine engine can be run on compressed air, and in transportation applications, it should (http://qtusa.promci.qc.ca/QTEfficaciteComparative.html http://qtusa.promci.qc.ca/QTEfficaciteComparative.html) have better than 50% efficiency. But great power density and awful energy density sounds like something an engineer should work with a little before declaring viability: A hybrid design with some other power source having high energy density, even if it can't deliver it very quickly.