Validation of nitrogen oxides and PM correction factors for heavy duty diesel engines

Considering the contribution of oxides of nitrogen (NOx) from on-road heavy duty diesel engines to the atmospheric inventory, their accurate measurement is important. According to the Code of Federal Regulations Title 40 Part 86 Subpart N, NOx measured at a given temperature and relative humidity is corrected to standard conditions using a humidity correction factor. This factor depends on the intake air absolute humidity. However, this correction factor was developed in the 1970's and may not be applicable to the existing engine technologies. Also, after the 1998 Consent Decrees, in-use testing has become a part of engine certification and the measurement of the on-road NOx emissions is required. As varying ambient conditions are found during in-use operation compared to engine dynamometer certification, the use of a correction factor accounting for in-use ambient testing conditions become significantly more important. The aim of this research was to examine the humidity correction factor specified in 40 CFR Part 86 Subpart N, observe any deviations in emissions after correction and also modify the empirical relationship for humidity correction factor for a range of conditions allowed for testing a heavy-duty engine on an engine dynamometer. Tests were conducted on 1992 Detroit Diesel Corporation Series 60, 1999 Cummins ISM370 and 2004 Cummins ISM370 engines to obtain emissions results for various intake conditions for a range of engine technologies. The results demonstrate that the range of conditions allowed by the CFR for engine dynamometer testing may not be corrected to the reference condition and thus may produce inconsistent results. However, the variation in NOx emissions was observed to be different from engine to engine indicating that engine technology influenced these emissions along with humidity. After regression analysis of the available data, two correction factors were developed for each engine. One based on absolute humidity and the second correction factor was based on temperature and absolute humidity. A cumulative correction factor was also formed based on all the three engines. However, no single correction factor could correct NOx of all the three engines to a proper baseline. The correction factor dependent on temperature only absolute based on each engine corrected the respective NOx emissions within 1% of the baseline (77°F/50% RH) NOx emissions. PM correction factors were also developed in similar lines and compared with the ISO PM correction factor used for marine heavy-duty engines and the with correction factor developed by Southwest Research Institute. Again, a cumulative correction factor developed based the available PM data did not conclusively reduce any variability in data.