L . Wesley includes this passage in his Air Guns and Air Pistols book.

“ I once designed a gadget that could be fitted to any underlever air-rifle, which would produce this diesel action continuously without any harmful effects on the rifle.
It consisted of a small cylinder and piston and a tiny reservoir which held about a thimbleful of thin oil. There were also two very fine tubes with the ends set at right angles to one another in a similar way to the nozzle of a scent spray. It was arranged in such a way that the action of cocking the rifle caused the small piston to blow air drawn down one of these tubes and oil to rise in the other one. When this oil was met by the air it was atomised and injected directly into the cylinder of the air rifle. Best results were obtained when only one drop of oil was injected for each cocking of the air rifle. Various kinds of oil were tried, and maximum power was found to be nearly three times the normal performance. The device was made and fitted purely for experimental use and is never likely to be commercially made as it would produce power in excess of allowed standards”.

So, not much new under the sun.

Regards

Brian