## 4 Discussion
### Tailpipe Applications  
Interestingly, GTR 15 allows the use of FTIR for ethanol, formaldehyde, acetaldehyde, and N\({}_{2}\)O only from the dilution tunnel. This is because there is no exhaust gas measurement to determine the emissions from the tailpipe. Only NH\({}_{3}\) has to be measured from the tailpipe. At the moment, there is a limit only for heavy-duty engines (in ppm) in the EU regulation (not in GTR). Such specifications would need two FTIRs for the measurement of non-regulated pollutants (e.g., one for NH\({}_{3}\) at the tailpipe and one at the dilution tunnel for the other pollutants). Permitting measurement of all pollutants from the tailpipe would simplify the setup. Furthermore, FTIR could be used instead of other analyzers (NDIR for CO and CO\({}_{2}\)), CLD (for NO\({}_{\text{x}}\)), and possibly for hydrocarbons (instead of FID). Indeed, the use of FTIR at the tailpipe is a commonly accepted technique for research and development (see \"Introduction\"). Figure 4 gives an overview of the \"Results\" section summarizing the studies that FTIRs was compared with reference values: (i) calibration gases (\"Cylinder\"), (ii) reference instrument measuring in parallel with the FTIR at the dilution tunnel or at the tailpipe (\"Parallel\"), (iii) reference instrument at a different location (FTIR at the tailpipe versus reference at the dilution tunnel) (TP vs. CVS). Only cases where at least two studies were available were taken into account. It should be mentioned that the mean values of slopes or already averaged values do not give the complete scatter of the tests. Furthermore, a different number of tests in each case make any comparisons between different compounds doubtful. On the other hand, it has to be reminded that the results summarize 40 years of experience with a wide range of instruments manufacturers (and users). The mean differences from the reference values were \(\pm 2.5\%\) for CO\({}_{2}\), CO, NO\({}_{\text{x}}\), and NH\({}_{3}\), without any particular deviation when FTIR and reference instrument were at different locations. The variability (one standard deviation) of the means was \(5\%\) for CO\({}_{2}\) and \(10\%\) for the other three gases. Again no particularly higher variability of the \"TP vs. CVS\" cases. For HCs (THC, NMHC) the mean differences were 5-30% and the variability 30%. Smaller mean differences and variability (10%) was calculated for CH\({}_{4}\). The \"TP vs. CVS\" cases had similar means and variability. CH\({}_{2}\)O had closer to CH\({}_{4}\) behavior, while CH\({}_{3}\)CHO was closer to NH\({}_{3}\), but the number of tests was very limited to draw any conclusions. The results are reasonable, considering that the uncertainty of the different equipment to which FTIR was compared was not the same. As it was discussed in the respective sections, the reference instruments for THCs, NMHCs, and carbonyls have higher uncertainty than those for, e.g., CO\({}_{2}\) and CO. On the one hand, the THC measurement with FID is not specific, as different HCs can have different response factors in the flame; and on the other hand, THC estimate with the FTIR might be not exhaustive, as some HCs can be not quantified if they are not initially included in the calibration method. A proposal to bring closer the two methods was to use a group of HCs as a surrogate to NHMC and then combined it with ethanol, acetaldehyde, and formaldehyde to calculate NMOG [162]. Another way of research may be to apply chemometric tools, such as principal components regression (PCR) and partial least squares regression (PLSR), on the FTIR multivariate data to predict the THC estimated with FID. Either way, a higher uncertainty margin would be necessary when using FTIR to assess compliance to THCs standards. The analysis was repeated considering only the studies of the last ten years (i.e., 2011 and afterwards), in order to see whether there were any trends of improvements (i.e., closer agreement with the references).