The present invention discloses a special measurement 
system for concurrently measuring 
ammonia gas and 
nitrogen oxide in motor 
tail gas. The special measurement 
system comprises a motor 
exhaust gas sampling unit, a 
NOx / NH3 analysis unit and a 
computer control and 
management unit. According to the present invention, sample gas and air are subjected to 
dilution mixing to obtain diluted 
discharge gas, wherein the air is filtered, and then enters a CFV1 for metering 
dilution air flow; the diluted 
discharge gas passes through a CFV2 to meter a 
dilution discharge gas flow; the resulting gas is divided into two parts according to the flow path, wherein one part of the gas passes through an 
ammonia gas 
oxygen catalysis conversion device, and enters a CLD1 to obtain the total NO measurement value containing NH3 concentration, and the other part of the gas directly enters a CLD2 to obtain a NO measurement value, wherein the NO measurement value does not contain NH3; and concentration values of NH3 and 
NOx are obtained based on the difference between the two measurement values ??of the two parts of the gas. The special measurement 
system of the present invention has the following advantages that: requirements on engine 
transient state operating condition power measurement and NH3 measurement by the national stage IV can be met, NH3 / 
NOx component concentration can be measured simultaneously, 
measurement precision is high, and the like. In addition, with the present invention, NOx conversion efficiency (SCR 
catalysis reduction efficiency) can be calculated according to the measurement results.