Optionally, an EGR valve recirculates {HEAD ~ a portion} of the exhaust back into {TAIL ~ the intake manifold} 22 
back into
Optionally , an EGR valve recirculates { HEAD ~ a portion } of the exhaust back into { TAIL ~ the intake manifold } 22
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH