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