Therefore, when the temperature of the recirculated exhaust gas is increased in {HEAD ~ order} to make {TAIL ~ the temperature} of the filter 28 rise, simultaneously the opening degree of the exhaust gas recirculation control valve 4 b is increased such that the exhaust gas recirculation rate, i.e., the amount of NOx discharged from the engine 1 is held substantially constant 
to make
Therefore , when the temperature of the recirculated exhaust gas is increased in { HEAD ~ order } to make { TAIL ~ the temperature } of the filter 28 rise , simultaneously the opening degree of the exhaust gas recirculation control valve 4 b is increased such that the exhaust gas recirculation rate , i.e. , the amount of NOx discharged from the engine 1 is held substantially constant
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH