Optimally, {HEAD ~ the residual engine} or exhaust heat, or heat from {TAIL ~ the catalyst}, may be used 
from
Optimally , { HEAD ~ the residual engine } or exhaust heat , or heat from { TAIL ~ the catalyst } , may be used
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH