Abstract: There are many types and large quantities of in-use fuel-fired construction machinery,which emit large amounts of NO_x during fuel combustion,but there are currently no established methods for detecting NO_x or setting emission limits for in-use machinery.To explore rapid NO_x detection methods for such machinery,a Portable Emission Measurement System (PEMS) was used to conduct NOx emission tests on six excavators and loaders,and typical operating points that characterize the average emission level and are suitable for rapid detection were identified.Comparison experiments of NO_x emissions from PEMS and rapid detection equipment were carried out at these typical operating points to quantitatively evaluate the deviation rate and the representativeness of emission levels,and to propose a idea for the rapid detection of NO_x in construction machinery.The results show that the operation of excavators and loaders is greatly affected by working conditions and loads,leading to substantial emission variability and transient discontinuities.The NO_x emission levels of typical excavators and loaders were 362 μmol/mol and 172 μmol/mol,respectively.Idling,high idling and free acceleration demonstrated high representativeness of emission levels,with the proportion of their NO_x emissions in the range of ±60% of the mean value being 82%,65% and 50%,respectively.These conditions also offer good identifiability and are suitable for rapid detection.Considering the deviation rates and correlations under different conditions,a weighted average method (with weights of 0.25 for idling,0.23 for high-idle,and 0.52 for free acceleration) is recommended as the standard determination method for rapid detection.