The NOx Technical Code under the MARPOL 73/78 Convention ANNEX 6 specifies the NOx emission limits and has determined the direct measurement and monitoring method for the on-board NOx verification procedure. But the monitoring method might not have been established yet, since it is reported that some results of NOx emission values measured on board exceed the 15% allowance even in the case of engines with certification. Since there are several constraints in the measurements on board, the uncertainty is enhanced comparing to the measurements at a shop-test. In this report an alternative index is proposed to reduce the uncertainty and simplify the on-board measurement. The index is named as NOx 13 that is normalized with oxygen concentrations and contains the correction factor for humidity and temperature. The NOx emission value, NOx [g/kWh], is theoretically correlated to the NOx13 as in the following expression and the correlation is experimentally confirmed.
NOx [g/kWh] = α1×α2× be [g/kWh] × N0x13 [ppm]
α1, α2; constant, be [g/kWh] ; specific fuel consumption
