snippage.....

As stated, propane is a fuel. A fuel which has a much higher flash point than diesel and a higher expansion rate.
Now fuel is injected into an engine at an exact moment. That's what we all know is timing. Inject too early and you have un atomized fuel being compressed and going off at or before top dead center. Resulting in a windowed block. Same goes for too much timing. Now. Why on Gods green earth would you inject a fuel that spikes cylinder pressures to an unsafe level without controlling when the fuel is injected? People do it. Some get lucky and it lasts a little while longer than others. Eventually though it'll bite em.
First post, researching these engines as a possible swap into my yota Xcab 4x4 when this caught my eye. Can't speak to the expansion rate, but the flash point statement is wrong if an SAE publication is to be believed. "Alternative Fuels Guidebook" (R. Bechtold, 1997 SAE; ISBN 0-7680-0052-1) pg. 66 has a listing of Propane and diesel fuel properties side by side. Flash point for diesel is 165*f; flash point for Propane is -156*f

Perhaps flash point was confused with Auto-Ignition temperature? Diesel's Auto-Ignition temperature is listed as being 600*f while Propane's is listed as being 855*f. Assuming that the combustion chamber temperature never reached 855*f & nothing in the chamber (head surface, valves, etc.) was at or above that temperature then the propane would not ignite. It would be there waiting for the diesel injection to commence combustion. This variability in when induction induced propane ignites could explain why propane caused catastrophes are random rather than exactly predictable.

Combustion commencing before TDC isn't uncommon, it intentionally happens very frequently. Witness that timing advance is always before TDC and poor running results from late timing. Some of that is to offset the "inertia" of components in the system, but not all of it. Combustion starting TOO early is altogether a different thing and is a problem.