## 3 Results
### Carbon Monoxide (CO) and Carbon Dioxide (CO\({}_{2}\))  
CO and CO\({}_{2}\) are typically measured with NDIR (Non-Dispersive Infrared) absorption type detectors. NDIR is one of the IR methods, which is not spectroscopic analysis but uses non-dispersive infrared light. In NDIR, the region of wavelengths used for analyzing the target component is selected by using an optical filter (multilayer interference filter) or a gas filter (a cell enclosing interfering gases). Other gases that show absorption in the same wavelength region will contribute to the measurement result of the target component. For example, when analyzing CO in the engine exhaust gas, CO\({}_{2}\) and H\({}_{2}\)O also included in engine exhaust gas are likely to interfere with the measured CO concentration [131]. The CO NDIR analyzer may require a sample conditioning column containing CaSO4, or indicating silica gel to remove water vapor, and containing ascarite to remove carbon dioxide from the CO analysis stream (USA EPA Title 40, Chapter I, Subchapter C, Part 86, Subpart B, SS86.111-94). Many analyzers use coolers. The regulation requires interference checks of the CO analyzer with CO\({}_{2}\) and H\({}_{2}\)O at concentrations expected during the tests. The CO response has to be within 2% or \(\pm\)50 ppm (whichever is larger). For analyzers measuring from the dilution tunnel, the allowed interference is between \(-\)1 ppm to 3 ppm. Regarding CO\({}_{2}\) measurements of diluted exhaust, the regulation requires that the CO\({}_{2}\) concentration in the dilute exhaust sample bag is \(<\)3% for gasoline and diesel engines, \(<\)2.2% for LPG engines, and \(<\)1.5% for natural gas and biomethane engines. When the concentration of water is \(<\)3% H\({}_{2}\)O the error is \(<\)1% [132].  
For tailpipe sampling, in order to minimize the interference for non-diluted exhaust, additional detectors sensitive only to the interfering component can be added [133], or the H\({}_{2}\)O can be removed (e.g., by a cooler) [134]. Errors after corrections with additional H\({}_{2}\)O detector, the main interfering component in the exhaust gas, are \(<\)2% for H\({}_{2}\)O concentration up to 12% [133]. Removal of the water needs a dry-to-wet correction, which has a low error, as long as no condensation takes place at the sampling lines, e.g., at cold start. In such cases errors of up to 10% have been reported during the cold start period [135].  
Table 1 summarizes the studies assessing FTIR CO\({}_{2}\) readings with calibration cylinders or NDIR analyzers. Table 2 summarizes for CO. Regarding CO\({}_{2}\), in most cases, the differences were within \(\pm\)5% (slope 0.95-1.05), with only a few exceptions that the differences were around 10%. There were no particular differences between gasoline or diesel vehicles. The correlation coefficient was high (\(>\)0.95). The only exception (\(R^{2}\) = 0.70, FTIR at tailpipe vs. dilution tunnel) was when a calculated from the intake air exhaust flow was used instead of measured with exhaust flow meter. Higher differences (\(-\)21%) were reported when the response of the FTIR was slow [34].  
Regarding CO, the mean differences were within \(\pm\) 5% of the reference analyzers, but the scatter was very high in some cases, exceeding 50%. The slopes were within 0.8 and 1.2 with no indications of higher slopes when the FTIR connected at the tailpipe was compared to the dilution tunnel or bags. The worse performance of the FTIR CO compared to the CO\({}_{2}\) might have to do with the higher interference of water and CO\({}_{2}\), and the higher concentration range of CO compared to the CO\({}_{2}\).