Just an idea I've had kicking around in my head for some time now I'd like to throw out for some debate. A compound diesel engine that re-uses the hot exhaust gas in a second piston to recycle the heat and utilize it to make steam. Here's how it works.

Following the diagram below. The piston on the left is a traditional diesle engine. Numerically, the traditional 4-stroke combustion process is followed.
1, intake
2, compression
[2.5, injection]
3, power
4, exhaust

The exhaust gasses exiting the combustion chamber do so at aprox. 800*F. While the expansive properties of the exhaust can be reclaimed via a turbocharger, the heat energy is completely lost.

But if that heat could be used to make steam in a compounding cylinder, well, that's a game changer. The 1234 combustion cycle is repeated in the 5678 as shown, only this time, a tiny droplet of water is injected instead of diesel fuel. Otherwise, the 4-stroke process is the same.

The crank cycle[s] would be timed so as the primary piston is on it's exhaust stroke, the compounding cylinder would be on it's intake, thereby creating a scavenging effect, increasing the efficiency of the primary.

The gas entering the secondary would do so at aprox 500*F accounting for the expansion, but the compression of the secondary would then raise the temperature of that air-charge to well over 1000*F, providing a perfect environment for an injected droplet of water to flash to steam.

When water [instantaneously] flashes to steam, it does two things. First, it absorbs a TREMENDOUS amount of heat, which would mean the exhaust from the secondary would exit at about 600*F, i.e. less chance of melting exhaust valves. The second thing it does is it CONTINUES to expand. The net result would be a much cooler and much larger air charge hitting the turbo. This would reduce thermal stress on the turbo while at the same time making it more efficient at compressing air to be fed back into the primary cyl. for the whole process to begin again.

The pro's would be:
More heat converted to energy instead of lost
Less thermal stress on components
Increased efficiency
Increased economy

The con's would be:
Increased cost of the secondary systems
Lower RPM range to accommodate thermal expansion of steam
Access to two fuels for the same engine
Un-tested technology

What are your thoughts?