Therefore, when {HEAD ~ the temperature} of the recirculated exhaust gas is increased in order to make 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., {TAIL ~ the amount} of NOx discharged from the engine 1 is held substantially constant 
Therefore , when { HEAD ~ the temperature } of the recirculated exhaust gas is increased in order to make 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. , { TAIL ~ the amount } of NOx discharged from the engine 1 is held substantially constant
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD 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 TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH