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