Start with the idea of a six stroke engine (like the one Leonard H. Dyer invented in 1915).
For those who are unfamiliar with the six stroke concept, the six strokes are as follows: air intake, air compression/combustion, combustion/power, air exhaust, water injection/boiling/steam expansion, steam
Create two sets of exhaust valves: one set for air exhaust, and the other for steam exhaust. Also, there should be two exhaust manifolds, one for steam, and one for air.
After pressurizing water from the reservoir (or from the steam condenser), heat it using the hot air in the engine's air exhaust manifold. This very hot water is what's injected into the engine's cylinders for the fifth stroke of the cycle.
Since the water is very hot, (unlike the original six stroke engine concept, and unlike Bruce Crower's six stroke engine), very little thermal shock occurs when the hot water sprays into the cylinder and hits the hot cylinder walls and the hot piston.
This reduction in thermal shock is probably the most important aspect of this idea ... so if I'm wrong about this part, then I'll probably delete the idea as a whole.
Furthermore, due to the water's higher starting temperature when entering the cylinder, it takes a smaller amount of the engine block's waste heat, per unit mass of water, to boil that water. This is because more of the engine block cooling occurs due to the water's latent heat of vaporization, and less occurs due to heating cool water up to it's boiling temperature.
This means that a greater mass of water must be injected in order to remove the same amount of the engine block's waste heat.
Obviously, this means that a greater mass of steam is produced per cycle than if the water hadn't been preheated. Less obviously, this steam also has a greater temperature and pressure (both at the beginning of and at the end of the steam expansion stroke).
To take maximum advantage of the increased quantity and pressure of the steam produced, a steam turbine should be placed between the steam exhaust manifold and the steam condenser.
The increased mass and pressure of steam in the cylinder directly produces more engine torque than if the water hadn't been preheated.
I'm not sure of the best way to use the extra power produced by the steam turbine... if this is a hybrid car, we can simply hook it up to an extra alternator, and make electricity with it. Or, we could use it to power an air intake compressor.
Gearing it down, and using it to provide extra power to the driveshaft probably isn't a good idea, since that would be just too much added complexity.