{HEAD ~ The timing} can be changed from {TAIL ~ the exterior} of the engine on the basis of factors such as the quantity and nature of pollutants in the exhaust gases, the rate of revolution of the engine and the load on the engine 
changed from
{ HEAD ~ The timing } can be changed from { TAIL ~ the exterior } of the engine on the basis of factors such as the quantity and nature of pollutants in the exhaust gases , the rate of revolution of the engine and the load on the engine
HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH