Probably a stupid car question...

[Rhydderch Hael]

... the higher the energy of the incident radiation, the more shielding required. It's not simply getting better reactions or smaller masses or using different methods to gather energy from standard radioactive decay ...

I agree with Lissa on this point. One can say "choose a better nuclear reaction", but nature really doesn't offer one. You can say "choose better shielding materials" but even the best possible still take pretty good thicknesses to work.

The Catch-22 is that if you use low energy reactions, you don't need as much shielding but then the pile has to be much larger to get a sustained reaction. If you use high energy reactions, you can make the pile smaller, but you then need more shielding. Optimizing just for weight, you still end up with something more suitable for railroad engines than cars or trucks. And that optimization involves using some nasty materials and insufficient containment for purely mechanical accidents.

On a (slightly) unrelated note, just how dense does the shielding have to be to impart Bremsstrahlung from Nitrogen-16 decay? I mean, just how energetic is that beta particle event?

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Drifting back to the original topic, really old gasoline engines could have this problem called 'dieselling', where some hot carbon deposit in the combustion chamber caused any residual fuel in the cylinder to burn, forcing the crankshaft around for yet another burn cycle in the engine. Bottom line— you'd turn off the ignition, but the engine continued to run for a few seconds because the hot carbon deposit acted as a sort of glow-plug that provided another source of ignition separate from the sparkplug.

This happened in old cars with carburetors because they usually had a mechanical fuel pump driven by an eccentric on the camshaft. So long as the engine continued to turn, more fuel was drawn into the engine. Modern engines use electric fuel pumps— turn off the key, and you turn off the fuel pump. No dieselling happens there because the carbon deposits have no more fuel to ignite.