The on-board NOx monitoring method is one of the three ways for the periodical survey, which demonstrates compliance by means of the measurement of the NOx emission value on in-service conditions. In this study of the NOx emission measurement, made on the same two-stroke engine, and with the same gas analyzer, was carried out on three occasions. That is a shop test, an at sea trial and during in-service. The effects on the NOx emission value are discussed with regard to the derivation methods of the exhaust gas flow rate. They are gas concentrations in UOB (universal oxygen balance) and UCB (universal carbon balance) method, compositions of the sampled fuel, stability of engine output power in operation, and the temperature & humidity correction factor K
Hdies. The coefficient of humidity term, for the factor K
Hajes, should be larger than that of K
Hdiesas specified in the NOx technical code. This correlates the NOx emission values measured at sea trial and during in-service. The temperature and humidity should be measured in the engine room rather than in the steering house. The stability of the engine power can be evaluated alternatively by using the C.O.V. of the oxygen concentrations. The NOx emission values on an actual vessel do not agree to those at a test-bed due to the effect of the derived exhaust gas flow rate and the temperature of the intercooler outlet air.
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