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