Ah, Zone 2 diesels, my favourite. Air-operated starter, flame trap, spark arrestor, Chalwyn valve air-velocity overspeed cutout, plus exhaust and oil pressure sensors that were pilot trips, and if there were any electrics they are EXd rated.
I used to do a load of mobilisation inspections, and investigate failures returned by hirers. Nearly every time we had one returned it was because they had just run them till the flame traps blocked up and they stalled.