Hello all,
Let's have a think about what is happening to oil as it flows thru the system of something like our old Lister(oid). We will need to have warmed it enough to get it fairly fluid prior to the injector pump drawing it in and pushing it up the line, so some preheater/exchanger will be needed. If this is warmed by the coolant, it will only become effective when the coolant is warm enough to ensure reasonable fluidity, my guess is around 60*c , say 130*f, and it will take some time to warm the cold body of the pump, which on a Lister is somewhat isolated from head and block. A bit of lagging round it may help to reduce heat loss, but the fuel volume passing thru the pump is small, so it will take a while. When the pump body has reached the above temp range it will be capable of handling the still slightly viscous oil, but you need to retain this heat just by INSULATING the injector line with foam tape etc, and then when the oil reaches the metal mass of the injector body now surrounded by a warm head, it will pick up further heat, and as it arrives at the nozzle which is in close thermal contact with the combustion chamber via its copper seating washer, the tiny shot of fuel will be hot enough to atomise at the nozzle tip. The final injection temperature of the oil will mainly be determined by the temperature of the lower end of the injector body and nozzle tip. You can pre heat it to hell and it will still be injected at nozzle temperature. So change-over from warmup fuel depends on having the pump at least warm to the touch, as these old low speed pumps will handle stuff somewhat thicker than diesel, and there is not a lot of restriction for small fuel flows till it arrives at the nozzle tip, where it should be very close to head temperature. So for good atomisation and clean running I feel that switching to oil should not happen till the head is not too far below its normal loaded running temp range. We can complicate things with line heaters etc, but as long as we can retain most of the heat applied before and around the pump, the heat from the head and injector body will take care of the rest. Just my observations on the subject, Regards, Combustor.