The timing can be changed from the exterior of the engine on {HEAD ~ 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 {TAIL ~ the engine} 
The timing can be changed from the exterior of the engine on { HEAD ~ 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 { TAIL ~ the engine }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD 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 TAIL TAIL TAIL TAIL TAIL TAIL