Mike Everman wrote:the designs seem to expect that the hot exhaust of one chamber will compress fuel mix in the other without touching it off prematurely. Seems to me there should be a plug of non-fueled air as a buffer so that the compression can take place before ignition, or fuel introduced at the appropriate time with special attention paid to instantaneous mixing.
OK, I guess there's nothing else for me to do but put the teacher's cap back on.
It is fallacy to view ignition as something instantaneous. Ordinary combustion in a fuel-air mixture is slow. I don’t have numbers at my fingertips right now, but believe me, it’s slow for our purposes.
There’s a lot of work to be done before fuel will ignite, especially liquid fuel, in which the tiny droplets have to evaporate before it will even think of combusting. The mixture has to be heated up to a certain temperature. If the fuel is in complex molecules, they have to dissociate into simpler ones. Oxygen has to get close enough for reaction to start. Other fuel and oxygen molecules have to be close enough so that the energy the combustion gives off will set off the process in the neighboring molecules etc. The lower the pressure, the slower and less effective the process because the distance between the molecules is greater.
When the hot blast hits the cold mixture, it will NOT ignite it. That happy event if relatively far in its future. What happens is the formation of the so-called hot-cold interface. It is a physical barrier that acts in many ways like a piston face. As heat exchange and pressure exchange take place, the interface gradually disintegrates, but the process takes time. While time passes, the interface moves and compresses the cold mixture.
If you filmed the hot-cold interface with a very high speed camera, you would see tiny tendrils of ignition gradually moving from the hot side towards the cold, getting longer, more tortuous and more numerous and developing new branches until the entire cold side is completely filled. That’s what happens in real life and, as I said, it takes time.
That is why the hot blast can and will compress the cold mixture and why it will effectively ignite it only when the mixture is already compressed as far as it will go.