[zck]

>...if you substitute fuel "burning" process with subsonic propagation used in every conventional engines with fuel "explosion" process with supersonic wavefront...

This fragment confused me, because it looks like there are three substitutions. There aren't; there's only one. Read it as:

If you substitute fuel burning (which has subsonic propagation, and is used in every conventional engine) with fuel explosion (which has a supersonic wavefront)...

The first and third "with" link a noun (the respective process) with a property (how fast it shoots gas out the back). The second "with" is the substitution.

English is hard! I'm a native speaker, and I had to take a look at a few webpages to understand just this part! And I'm still left with questions, like why subsonic is described as having "propagation", but supersonic is described as having a "wavefront". Is this a distinction with a difference? I don't know.

[numpad0]

I must admit that I'm in fact not a native English speaker and my sentence construction is still jarring at times... sorry for that.

That distinction was not intentional, but IIUC, chemical reactions propagating faster than the speed of sound(of surrounding material, not necessarily of air(which idk how is it even defined, ductile limit or something(of air!?))) has effect of consolidating what would normally be multiple consecutive waves of density changes following one another, into a single shockwave with higher peak pressure, and higher peak pressure in an engine is generally thought to lead to higher exhaust gas velocities and better efficiencies so long that the engine holds. I think this is what some comments mentioning P-V diagrams and Carnot efficiency is meaning to say, except mine is in uneducated terms.