When two deuterons fuse conventional wisdom says they create a highly excited helium 4 nucleus. That excess energy can be released in three ways, emit a photon, emit a proton and become tritium, or emit a neutron and become helium 3. The helium 3 has the highest probability of occuring.
Both forms of helium are considered stable isotopes. It appears no one ever considered the possibility of fission involving stable isotopes. Well, nearly everyone. The people who built Joseph Papp's Noble Gas Engine either stumbled into it or had an inkling.
Here's how it works:
When the compression stroke finishes and the power stroke begins the spark excites enough He4 atoms to cause enough energetic neutrons to be produced to cause a large pressure increase and drive the piston. These neutrons, however, are quickly absorbed by the other Noble gasses. So the pressure drops. Also the various gasses become neutral. In particular the just created He3 atoms acquire electrons and since He3 is a refrigerant they suck up some of the heat of the reaction. So, the engine puts out mechanical power with little heat output.
If the calorimetry guys got this as a black box, they'd go nuts trying to figure out how the efficiency could be so high.
What we have here is a device combining both nuclear and chemical processes. Well, that's engineering.
