The approach for defining "continuous monitoring" should be consistent with paragraph 2.4.5 of the NOx Technical Code, which states that NOx monitoring should take into account both steady-state and transitional operations. This is significant because most operation in ports, where emissions control is most important, will be transient. Drawing samples infrequently and only during steady-state conditions may also encourage operators to draw samples at the points at which the engine is operating most efficiently, and may also be inconsistent with the spirit of the NOx Technical Code, which provides in paragraph 2.1.2.5 that these devices be used only as an alternative to periodical and intermediate surveys, which are intended to demonstrate continuous compliance with the Regulation 13 NOx limits. It is also important to monitoring NOx levels while the engine is operation to determine compliance with the standards for engine using additional NOx reduction methods such as SCR, emulsified fuel, and water injection systems.

　

The United States recommends that the overall goal of a NOx monitoring and recording device should be to continuously and reliably measure emissions over a broad range of operating conditions, to ensure that the engine remains in compliance with the NOx limits specified in Regulation 13 of MARPOL Annex VI and there has been no engine malfunction or tampering. The methodology should draw on existing technology, and should be simple so that the monitoring can be performed without sophisticated training for the crews. With today's advanced computer technology, this monitoring can be largely or completely automated. At least one emission sampling equipment manufacturer builds systems that can be zeroed and spanned with the push of a single button.

　

With these goals in mind, the United States recommends that "continuous" should be understood in its narrow sense and samples should be taken more frequently while the engine is in operation. More frequent sampling will permit emissions monitoring over the full range of engine operations and not just at steady-state where emissions are likely to be at their lowest. Also, by requiring frequent sampling, ship operators will be discouraged from drawing samples only when the engine is operating at its most efficient point for emissions.

　

At the same time, the guidelines should provide for discontinuing monitoring for an appropriate period of time whenever calibration or maintenance is necessary.

　

　

If the definition of "continuous" is adopted, as proposed by the United States above, it will be necessary to determine how to compare the measurement data with the Regulation 13 emission limits. This is because the emission limits are based on a weighted average of emission results over a range of test points. However, a particular engine may not operate at that combination of loads and engine speeds (rpm). Also, actual engine operating conditions may affect emissions (e.g., temperature, humidity, and fuel). These variables make it unreasonable to compare any discrete sample result from a continuous monitoring device to the Regulation 13 limits.

　

In lieu of developing conversion factors for ambient conditions (air quality and ambient temperature) and fuel characteristics, the United States recommends an alternative approach that would rely on an adjusted compliance determination. Instead of correlating the engine operating data to the initial test by using conversion factors, a decision rule can be adopted that accommodates variables in engine operation, temperature, humidity, and fuel. For example, the emissions results drawn every 15 minutes can be averaged over some period of time (one or several days) and that average emission result can be compared to the highest modal value of the test points measured in the initial certification test. Comparison to the highest modal value should provide an adequate cushion to accommodate differences in operations as well as operating conditions, since it is unlikely an engine will be operating at its worst modal point under the worst operating conditions over the entire averaging period.

　

The above compliance determination approach is less stringent than comparing the average emission to the Regulation 13 NOx limits, however, it is consistent with the on-board simplified measurement approach. Both of these approaches seek to identify high emitters and discourage engine tampering. This compliance determination approach also acknowledges that not all engines are operated the same way, under the same conditions. Also, by adopting this compliance determination approach, the results will be more consistent with the requirements of the initial test since it will reflect operations over several modal points. Also, only an engine that is consistently operating outside its NOx limits will have average emissions higher than the emissions at its highest modal test point.

　

The main advantage of this approach it that it is based on a reliable method of continuous emission monitoring that seeks to obtain accurate data on the engine's emissions.