i would like to state that thermo syphoning can take place with as little as one degree of temperature differential.
so without a thermostat and useing a water tank that is not closely matched to the exact load, ambient temperature(which would have to be constant) there is no way to get the engine up to proper operating temp with any consistancy.
for instance
say a minipetter was put together with a thermosyphon system using a 5 gallon tank and carrying a 1.5 kwatt load on a 90 degree day(ambient) it might come up to around 190 degree's F.
now using the same system, and moving the ambient temp lower by 40 degree's, and reducing the load to .5 kwatt, the operating temp of the engine will fall below 190 degree's and likely far below 190.
the tstat control would maintain the temp up to 190 or so, reguardless of load or ambient temperature.
even modern designed diesel engine's will emit a haze or gray smoke when running in cold ambient temps and no tstat, especially at no or low load conditions.
also if you want the engine to live a long life you want it to run up to around 180 to 195 degree's.
other consideration are when running an engine at low temp's it is hard to get enough heat rise in the crankcase to boil off
condensation in the lube oil, causing acidity to build up and other problems that shorten engine life.
i have seen too many diesel engines that are poorly maintained, with either missing or failed tstats, stuck fan clutches or fan controlls worn out far too soon. and they all emit smoke!
i really don't know why a petter or a lister would be any different.
bob g