defining {HEAD ~ the model variable} for the air mass flow into the cylinder as {TAIL ~ a linear function} of the inlet pipe pressure; and 
as
defining { HEAD ~ the model variable } for the air mass flow into the cylinder as { TAIL ~ a linear function } of the inlet pipe pressure ; and
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH