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