Start at 15 degrees after top dead centre and ends at 30 degi before bottom dead centre
It vary with different type/model of engines and depends on fuel injection timing & valve timing (closing and opening of valves) of the engine. But it always lies between some degree after TDC and some degree before BDC. In most of the tractor engine the fuel injection starts 05 to 10 degree before TDC and ends 05 to 10 degree after TDC, so the expansion of fuel starts 05 to 10 degree after TDC; And the exhaust valve remain closed 30 to 40 degree before BDC, so the expansion of fuel ends 30 to 40 degree before BDC.
The expansion of fuel in a four stroke cycle diesel engine Starts at 15° after top dead center and ends at 30° before bottom dead center.
Different engine have different piston and its top is also different so variation in angle also different. After compression expansion stroke happen, Diesel engine need high compression ratio about 14- 18. So while suction stroke need more air pressure for ignition. So overall it start means TDC Angle in between 10 degree to 25 degree (depend on engine design) and end at BDC 30-40 degree.