A term used to describe a smaller gasoline (petrol) powered engine that's used to start a big diesel, Tony... One would start it up, usually by an electric starter, and then engage it with a lever to the diesel to get it to revolving... especially helpful in cold weather. They were around along time before the Mustang or Pinto.
Problem is an electric starter would have to be very large and use an inordinate amount of amperage to get the diesel to start...
(Apologies to Connemmara for the mini-hijack of the thread).