It _is_ practical for cars, but a similar technology is easier and better: Exhaust Gas Recirculation. The problem of water injection is engine corrosion and sy

It _is_ practical for cars, but a similar technology is easier and better: Exhaust Gas Recirculation. The problem of water injection is engine corrosion and system maintenance. EGR further improves the exhaust gas composition by burning the CO and similar not fully oxidized parts. The main effect is the same, i.e. providing some inert material that expands after detonation using up the heat.