The additive can be introduced into {HEAD ~ a combustion chamber} as {TAIL ~ part} of the fuel 
as
The additive can be introduced into { HEAD ~ a combustion chamber } as { TAIL ~ part } of the fuel
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH