3.2.1. On-Road Emissions from the Gasoline Vehicles
The two studied gasoline vehicles presented relatively low emissions of NO and NO
2 emissions during the on-road tests (< 60 mg NO
x/km). The exception being GV1 during the dynamic test, which resulted in 195 mg NO/km and 10 mg NO
2/km. These high NO
x emissions were attributed to a possible regeneration that may have taken place during the rural section (see emission profiles in
Figure 3). The vehicle was retested along the same route under dynamic driving, using just the PEMS as the analytic system. In the second test, NO
x emissions were consistently lower. For more details on the regulated emissions of this vehicle in the laboratory and on-road please see [
41]. NO emissions from the gasoline vehicles were also substantially lower than those measured from the diesel vehicles tested (up to 69 mg/km lower during the Route-1 tests; see
Table 3 and
Table 4).
Emissions of N
2O and HCHO from the gasoline vehicles were very low, presenting an emission peak above 1 ppm only during the vehicles’ cold-start (see
Figure 3). In gasoline vehicles, N
2O is formed on the three-way-catalyst during light-off [
9]. Emissions from gasoline passenger cars have been reported to be low, often below 1 mg N
2O/km during the type-approval cycles [
32,
42], which is in line with the emissions measured from GV1 and GV2. In line with [
32], the emissions of N
2O from gasoline vehicles were lower than those measured from the tested diesel vehicles (from 4 to 27 mg/km lower for gasoline than for diesel vehicles; see
Table 3 and
Table 4). On the other hand, HCHO emissions, which result from incomplete combustion of the fuel [
43], were emitted before the catalyst light-off, and then dropped below limit of detection of the instrument for both vehicles (see
Figure 2). Emission factors during the urban section were in line with those reported for Euro 6 gasoline vehicles tested with E5 gasoline over the Worldwide Harmonized Light-Duty Vehicles Test Cycle (WLTC) [
44] and also with other spark ignition vehicles tested with E5 over the WLTC and whose HCHO emissions were measured by means of FTIR [
45] and other techniques [
46].
On-road CH
4 emission factors from gasoline vehicles were 2 mg/km during moderate driving and 4 mg/km for the dynamic test performed with GV1. Interestingly, the highest emissions were measured during the motorway section of the test (7 mg CH
4/km). CH
4 emissions were lower in all cases than the limit currently in forc in the USA (18.8 mg/km over the Federal Test Procedure (FTP)). The similar emission CO and CH
4 emission profiles (see
Figure 3) suggests that the TWC was not able to fully control CH
4 emissions during transient operation as a result of temporary fuel mixture enrichment after catalyst light-off. Nonetheless, CH
4 emissions were relatively low during all sections of the on-road tests.
NH
3 emissions from the gasoline cars varied from 21 mg NH
3/km (GV2) to 49 mg NH
3/km (GV1). Both vehicles presented emissions along all three sections of the routes. NH
3 emission factors were highest on the motorway section for both vehicles and under the two driving styles. The emissions during the motorway section reached 85 mg NH
3/km for GV1 and 46 mg NH
3/km for GV2. On the other hand, NH
3 emissions during the urban section were 38 and 32 mg NH
3/km for GV1 (for the normal and dynamic test respectively) and 11 mg NH
3/km for GV2. Staying with the results presented by Link et al. [
28], these NH
3 emissions when reacted with HNO
3 present in the atmosphere would result in approximately 38 mg PM
2.5/km for GV1 and approximately 11 mg PM
2.5/km for GV2. It should be noted that tailpipe PM limit for passenger vehicles in the EU is 4.5 mg/km. Hence, these vehicles may lead to the formation of more secondary inorganic aerosols than the PM primary emissions allowed in the laboratory test. In a recent study Simonen et al. [
47] showed that the dominant part of aged aerosol mass from two gasoline vehicles was inorganic, including ammonium nitrate and sulphate.
The obtained NH
3 emissions factors were in line with those recently reported in tunnel and laboratory studies worldwide [
23,
48,
49]. However, they were 2.6 to 9.8 times higher than the on-road NH
3 emissions from a Euro 6b gasoline direct injection (GDI) vehicle presented in a previous work [
31].
The two tested gasoline vehicles presented higher molar-based NH
3 emissions (mol/km units) than NO
x emissions. This result supports previous studies that have arrived at the same conclusion by measuring NH
3 emissions from gasoline vehicles using remote sensing [
16] and chassis dynamometer testing [
8]. If one considers that the atom of nitrogen contained in the NH
3 emitted by these vehicles comes from the engine-out NO (NO is the precursor of NH
3 in gasoline cars, see [
50]), and calculate total NO
x emissions including NH
3’s nitrogen, the GV1 and the GV2 would have presented 168 mg NO
x/km and 92 mg NO
x/km on the Route-1.
NH
3 emissions from spark ignition vehicles have been indicated to be correlated with CO emissions, as the two pollutants are emitted during fuel rich events [
6,
51]. In this line, GV2, which resulted in lower NH
3 emissions than GV1, also presented lower CO emissions (533 mg/km) than GV1 (1522 mg/km). Nonetheless, while NH
3 emissions from GV1 during moderate driving (Route-1) were comparable to those measured during dynamic driving, CO emissions were 1.8 times higher during dynamic driving (2666 mg/km) compared to moderate driving. Both vehicles always presented the highest emissions of CO during motorway operation. This suggests that gasoline vehicles equipped with TWC can result in high emissions of CO not only during cold start but also after catalyst light-off. High CO emissions from gasoline vehicles during dynamic driving have recently been reported during on-road and on-dyno RDE tests [
52,
53]. Moreover, Kelly and Groblicki already reported this operation in the U.S. in 1993 [
54]. Currently, the U.S. code of federal regulation limits vehicle enrichment [U.S. 40 CFR 86. 1811-17] [
26].
3.2.2. On-Road Emissions from the Diesel Vehicles
Table 4 summarizes the emission factors obtained from the two diesel vehicles (DV1 and DV2) tested on moderate and dynamic driving along the two RDE compliant routes. Aiming at reducing its NO
x emissions, the DV1 was equipped with an exhaust gas recirculation (EGR) + DOC + SCR system. The DV2 was equipped with a combination of EGR + DOC + LNT + SCR for this purpose.
NO
x emissions from the two Euro 6d-TEMP diesel vehicles, DV1 and DV2, during the RDE-compliant test (Route-1) were below Euro 6d-TEMP on-road emission requirement (80 mg/km × 2.1 conformity factor), for the complete and the urban sections (see
Table 4). The highest NO
x emission factors were registered on the motorway section for the two diesel vehicles under the two studied conditions.
In the dynamic tests, NO
x emissions were ~3.5 times higher than in the RDE compliant tests. Higher emissions during dynamic operation compared to “normal” driving have previously been reported for Euro 6b diesel vehicles equipped with SCR and LNT systems [
55]. Although NO
x emissions from vehicles type-approved using RDE have been improved overall, the relatively high emissions measured from the two Euro 6d-TEMP diesel vehicles during dynamic driving and on the motorway section illustrate that there are still operating conditions that could be improved.
NH
3 emissions from DV1 were higher (2 mg NH
3/km on Route-2 driven dynamically and 17 mg NH
3/km on Route-1, which was RDE compliant) than those measured from DV2 (max. 1 mg NH
3/km). The DV1’s NH
3 emissions during Route-1 (17 mg/km) were comparable to those measured from GV2 over the same route (21 mg/km). NH
3 emissions from DV1 on Route-1 were highest on the urban section of the test, reaching 32 mg NH
3/km and lowest (3 mg NH
3/km) on the motorway section. This indicates that new diesel vehicles could become a new source of NH
3 in urban areas, which until now have been dominated by gasoline vehicles [
21,
22,
23,
24,
25]. Owing to the low NH
3 emissions measured from DV2 on all tested conditions, it is assumed that, in addition to LNT and SCR, it was also equipped with an ammonia slip catalyst (ASC). Similar efficiency at removing NH
3 using ASC was recently shown for a Euro 5 and a Euro 6 diesel vehicle retrofitted with a solid ammonia reduction system [
55,
56].
NH
3 emissions from DV1 on the RDE-compliant Route-1 were 2.4–5.7 times higher than those reported from an SCR-equipped Euro 6b diesel vehicle tested on-road on an RDE compliant route [
31]. Nonetheless, DV1 resulted in 4.3–5 times lower NO
x emissions than the Euro 6b vehicle (389–448 mg NO
x/km). Hence, although NO
x emissions from DV1 are lower than those commonly reported for Euro 6b vehicles [
57], its NH
3 emissions are higher. This indicates that diesel vehicles could become an important source of NH
3 in urban areas.
Interestingly, NH
3 emissions from DV1 during a regeneration event reached concentrations above 2000 ppm (see
Figure S3 in the supplementary material). This concentration was one order of magnitude higher than the maximum concentration measured during the other tests performed with the same vehicle.
The DV1 resulted in 327 mg NO
x/km on the dynamic test performed on Route-2, which in turn emitted 2 mg NH
3/km. Moreover, for the DV1 (equipped only with SCR) the highest NH
3 emissions were obtained along the lowest NO
x emissions (see
Table 4). A similar trend (higher NO
x emissions on dynamic tests compared to RDE-compliant test) and similar emission factors were obtained for DV2. In both cases, these NO
x emissions were achieved with low emissions of NH
3 (max. 1 mg NH
3/km).
The two diesel vehicles, fueled with B7 diesel, presented very low HCHO emissions that were measured by the portable FTIR only at the vehicles’ cold start (see
Figure 4). In line with the usual behavior observed for diesel vehicles, CO emissions were also very low on all the tests performed. The highest CO emission factors for DV1 (30 mg CO/km) and DV2 (5 mg CO/km) were measured on the urban section of Route-1. Emissions of CO were mainly concentrated at the cold start. The DV2, equipped with DOC, SCR and LNT systems also present some CO peaks typical of the LNT regeneration episodes (see
Figure 4). During those LNT regeneration events, other pollutants such as CH
4 and N
2O were emitted. CH
4 emissions from the DV2 ranged from 8 mg CH
4/km on the dynamically driven Route-2 and 21 mg CH
4/km on Route-1. Hence, CH
4 emissions from the DV2 on Route-1 were higher than those measured from the gasoline vehicles (see
Table 3 and
Table 4) and slightly higher than the limit allowed in the USA under the FTP (18.8 mg/km).
The emissions of N
2O ranged from 19 mg N
2O/km (Route-2-dynamic) to 27 mg N
2O/km (Route-1) throughout the entire tests. These emissions were 3 to 4 times higher than the current USA limit (6.3 mg/km over the FTP) and 1.5 times higher than the China 6 standard (20 mg/km over the WLTC). These on-road emission factors measured on the RDE-compliant route (Route-1) were higher than those measured from the GV1, GV2 and CNG-LCV. They were also higher than those measured from an LNT-equipped Euro 6b diesel vehicle (12 ± 3 mg N
2O/km and 14 ± 2 mg CH
4/km) tested over the WLTC at 23 °C [
58].
Since CH4 and N2O are green-house gases, it is possible to calculate their CO2 emission factor equivalency (g CO2 eqv/km) using their emissions factors and their global warming potential (GWP). CH4 is a green-house gas with a short lifetime. Therefore, we use the CH4 GWP over 20 years (84–87). Hence, CO2 g equivalent/km from DV2 related to CH4 emissions ranged from 0.8 to 1.8 g CO2 eqv/km. Those related to N2O emissions (calculated using N2O’s GWP over a 100-year timescale 265–298) ranged from 5 to 8 g CO2 eqv/km.
Therefore, although the after-treatment system used by the DV2 (DOC+LNT+SCR and possibly an ASC) allow the vehicle to meet the NOx requirements under real-world driving conditions, it resulted in emissions of up to 9.8 g CO2 eqv/km in the form of GHGs that are not regulated with respect to vehicle emissions in Europe.
The DV1, equipped with a DOC and SCR, presented very low CH
4 emissions (see
Table 4) and the emissions of N
2O were lower than those measured from the DV2, ranging from 4 mg N
2O/km (1–1.2 g CO
2 eqv/km) to 5 mg N
2O/km (1.3–1.5 g CO
2 eqv/km). The N
2O emissions from DV1 were lower than the China 6 and USA limits. These N
2O and CH
4 emissions were in line with those from a SCR-equipped Euro 6b diesel vehicle tested over the WLTC at 23 °C (1 mg CH
4/km and 7 mg N
2O/km) [
58].
In summary, NO
x emissions from the two Euro 6d-TEMP diesel vehicles, DV1 and DV2, during the RDE-compliant tests (Route-1) were below the Euro 6d-TEMP on-road emission requirement (80 mg/km × 2.1 conformity factor) for the complete route and for the urban sections. The DV1 met the NO
x requirements using a DOC + SCR and the DV2 using a combination of DOC + LNT + SCR and possibly an ASC. Due to the intensive use of the SCR, and the high production of NH
3, to reduce the NO
x emissions, and in absence of an ammonia oxidation catalyst, the DV1 emitted 17 mg NH
3/km (32 mg NH
3/km in the urban section). In turn, these NH
3 emissions will potentially react in the atmosphere to produce approximately 17 mg PM
2.5/km [
28].
On the other hand, the DV2 did not result in high NH3 emissions (max. 1 mg NH3/km), possibly thanks to the use of an ASC. However, the intensive use of the LNT to reduce the NOx emissions and the presence other catalyzed after-treatments (SCR, DOC and ASC) lead to the emissions of up 9.8 g CO2 eqv/km in the form of GHGs that are not regulated with respect to vehicle emissions in Europe.
3.2.3. On-Road Emissions from the CNG Light-Commercial Vehicle
The CNG-LCV’s emission factors obtained on an RDE-compliant test (Route-3) and on the RDE route driven dynamically (Route-2) are summarized in
Table 5. The emissions of HCHO and N
2O from the CNG-LCV were low. These emissions were only present until the catalyst light-off (see
Figure 5). HCHO from the CNG-LCV were comparable to the light-duty vehicles tested (both gasoline and diesel). N
2O emissions were comparable to the GV1 and GV2, and were much lower than those measured from the diesel vehicles (see
Table 3,
Table 4 and
Table 5). Although the CNG-LCV and the gasoline vehicles used different fuels, their N
2O emission profiles were also similar. These similarities with the gasoline vehicles come from use of the same after-treatment system, the TWC. Rašić et al. [
59] recently reported the possibility high and frequent emissions of N
2O from a dual-fuel CNG/gasoline Euro 5 light duty vehicle tested on CNG on the road. This work highlighted the possibility of a cross-interference between N
2O and CO in the nondispersive infrared (NDIR) that was used for the measurement of CO. The measurements of N
2O and CO performed with the portable FTIR compared with the measurement of CO with the NDIR installed in our PEMS indicate that there was no cross-interference in the PEMS system used in our study (
Figure S4 supplementary material).
CO emissions ranged from 399 mg CO/km on the dynamic test (Route-2) to 418 mg CO/km on the RDE-compliant test. These emissions are approximately 3.5 times higher than those measured on-road from a series of Euro 6b CNG light commercial vehicles [
24].
NO
x emissions were high on the two tests performed. They ranged from 386 mg NO
x/km during the dynamic test, to 424 mg NO
x/km during the RDE-compliant test. In both cases, the highest NO
x emissions were measured during the urban section (up to 837 mg NO
x/km). High NO
x emissions were in fact presented by Jahirul et al. [
60] as the main concern when using CNG. High NO
x emissions from a CNG-fueled engine have been explained to result from high cylinder temperature, and lean air-to-fuel ratios [
59]. However, the engine used in the CNG-LCV should have been a stoichiometric engine, otherwise TWC would not work. The efficiency of the TWC is more sensitive to the air fuel ratio when running on CNG relative to gasoline, as there are far fewer products of incomplete combustion—H
2, NMHC and CO, needed in the TWC to reduce NO
x—with CNG than with gasoline. Therefore, the high NO
x emissions could be attributed to sub-standard engineering, not reflecting the best available technology.
High emissions of NO
x from a series of Euro 6b CNG light commercial vehicles were also reported in a previous on-road study [
24]. Nonetheless, the CNG-LCV’s NO
x emissions obtained during Route-3 and Route-2 were two times higher than those reported in Vojtíšek-Lom et al. [
35].
The CNG-LCV also resulted in high NH
3 emissions. These emissions ranged from 62 mg NH
3/km (Route-2 dynamic) to 69 mg NH
3/km (Route-3). The highest NH
3 emissions on the RDE-compliant route were found during the rural section (90 mg NH
3/km) and in the dynamic test during the motorway section (82 mg NH
3/km). Nonetheless, NH
3 emissions were also high during the urban section (46 and 38 mg NH
3/km on Route-3 and Route-2, respectively). With respect to gasoline vehicles, NH
3 is formed on the TWC present in the CNG-LCV. These emissions are six times higher than those previously reported for from a series of Euro 6b dual fuel (CNG/gasoline) light commercial vehicles tested on-road [
35].
Finally, CH
4 emissions from the CNG-LCV ranged from 72 mg CH
4/km (Route-2 dynamic) to 75 mg CH
4/km (Route-3 RDE-compliant). Hence, in terms of CO
2 equivalent emissions, the emissions of CH
4 from the CNG-LCV represent up to 6.5 g CO
2 eqv/km (GWP over 20 years 84–87). The CNG-LCV resulted in the highest CH
4 emissions on the motorway section (126 and 127 mg CH
4/km on Route-3 and Route-2, respectively). The CH
4 emissions were 4 times higher than those previously reported for a series of dual fuel (CNG/gasoline) LCV [
35].