Not many people know that the Empire Cinema in Neath was one of the very few cinemas that had its own stand-by diesel engine - similar to the one pictured. In fact this supplied power to the dance hall and for the Christie organ.
It was a single cylinder machine with a large flywheel, which was started by compressed air stored in a high pressure bottle.
When starting the engine the first task was to recharge the compressed air bottle from a compressor driven by the unit. There was also a facility to charge the bottle by a mains driven compressor.
A motor alternator set (like the one pictured) converted the generated DC to an AC supply to power the motors of the film projectors in the operating box - the projection room. This must have come from America because the output was 60 cycles per second - instead of the UK mains 50 cps. This meant the projectors would run a little faster when the diesel engine was running, with the result that films would finish a few minutes earlier than normal.
Although the mains supply was normally used, the engine would be run occasionally to ensure that it was kept in working order.