The separate gas engine was Caterpillar's method. IH used the method described above, with little chambers off the cylinders that were opened to reduce the compression for starting.LouNY - I'd heard about these gas/diesel engines, but never knew how it worked. I'd heard it was a separate gas engine that became the "starter" for the diesel. Your explanation makes more sense.
We had a "new-fangled" TD-18 on the farm in the later years, but mainly used TD-40's for farming when I was a kid. They were hand-cranked, so the compression release was necessary for cranking, as well as to allow use of gasoline to warm up.