i think the engine is not main kulprit.
when engine is ON, alternator is also ON. and whole car / bike 's body gets negative charge due to body earth.
and all petrol pumps also has enough earthings on all filling station / pumps. but all earthing pits are being checked twice per year as per rules. so, if there are any leakage of earth or loose contact with earthing in any section of pump, it may cause a spark when the nozzle comes in contact with vehicle.
I konw this coz, i was automation incharge of a IOC (indian oil corp ) for 3 years. we ware used to fill about 350 tankers of diesel and petrol in 8 hrs.