Ether lowers the ignition point of the fuel and enables the mineral based kerosene to be combined into the organic based castor oil.
(Also provides a lower % calorific rating and cooling - kerosene packs quite a few BTU's and some designs do not like the extra thump or heat)

Do a search on HCCI engines (homogenious charge compression ignition) and you will soon see that model engines are not real diesels.

P.S. If you find a suitable replacement for ether then please let us know!