3.1. Emissions of Regulated Pollutants
distance-based fuel consumption levels as a function of speed for the different Euro standards and emission control technologies are depicted in Figure 3
. Singular emission values over speed were based on respective 500 m distance-integrated driving sequences data and these were then binned to 15 km/h-classes of average speed. In Figure 3
, dots represent the average NOx
levels for the (n) number of vehicles falling into this particular bin. The error-bar ranges correspond to the standard error limits (average ± standard error) of these (n) values. A monotonic decrease in emissions with speed is observed, indicating the clear effect of speed.
leads to some interesting observations. First, Euro VI appears by far the cleanest technology, even at very low urban speeds. Actually, it is much cleaner than the SCR-equipped Euro IV and V trucks, especially at the low-speed region, depicting a much better performance of late SCR systems compared to earlier ones. The second observation relates to the clear difference in emission performance depending on emission control technology. This obviously has to do with the overall emission control calibration. For example, in the Euro V case SCR vehicles appear as the best performers for most of the speed range while in Euro IV, SCR+EGR vehicles appear as the overall worst performers. Large differences also appear between different models with EGR systems. Actually, differences between EGR equipped vehicles within the same standard are larger than differences between Euro IV and V.
Comparing energy-based emission levels with corresponding emission limits indicated that all vehicle classes except Euro VI exceed the limit to a certain degree. Euro VI emission levels are on par with the limit, with slight variation according to speed bin considered. Such differences can be justified by the difference in driving conditions. The calculated emission levels in g/kWh for each vehicle group, along with the respective Euro standard limits, are presented in Table 3
. An interesting observation is that the Euro V (DOC, EGR) and (DPF, EGR) vehicles exhibit higher NOx
emission level ratios, i.e., ratio of the calculated emission levels over the respective Euro standard limits, than the respective Euro IV vehicles of similar after-treatment technologies. Overall, the results provide a useful insight into the significant deviations between real-world driving emissions and the legislative limits employed for the Euro certification of vehicles.
The respective energy-based emission levels, calculated as the median of the corresponding speed-bin values, and the associated Euro standard limits are presented in Figure 4
(n.a. denotes vehicle classes where no measurements/data were available). PM emission levels for Euro IV vehicles and the Euro V (DOC, SCR) one are above the respective regulatory limits (Euro IV: 0.03 g/kWh, Euro V: 0.03 g/kWh, Euro VI: 0.01 g/kWh). The Euro V (DPF, EGR) vehicle was measured below the limit. Euro IV (DOC, EGR) vehicles have the highest PM emission levels, whereas the Euro V (DPF, EGR) vehicles have the lowest ones, but very close to the PM emissions of Euro IV vehicles of similar technology. In both Euro classes, the DPF-equipped vehicles appear to have lower emissions than the respective DOC-equipped ones.
THC emission levels for all vehicle categories are below the respective regulatory limits for NMHC (Euro IV: 0.55 g/kWh, Euro V: 0.55 g/kWh Euro VI: 0.16 g/kWh). Euro IV (DPF, EGR) vehicles have the highest emission levels, whereas the Euro VI vehicle exhibits the lowest ones.
As regards CO emissions, an interesting observation is that in both the Euro IV and Euro V categories, the ranking of the groups in terms of CO emissions is similar, as follows (higher to lower emissions): (DOC, SCR), (DPF, EGR), and (DOC, EGR). CO emission levels for all vehicle categories, but Euro IV (DOC, EGR, SCR), are below the respective regulatory limit (Euro IV, V, and V: 4.00 g/kWh). The CO emissions-to-limit ratio range from 0.08 to 0.49 and only the emissions of the Euro IV (DOC, EGR, SCR) vehicles are 1.7 times as high as the Euro standard limit.
The reasons for some exceedances of CO limits and some comparatively higher THC emission levels from some Euro IV vehicles are largely unknown and can only be hypothesized. Exceedances of limits for these pollutants are not necessarily associated with a malfunctioning emission control system, such systems are mostly there for NOx and PM reductions and not so much for CO or THC. In general, high CO and THC emissions on a diesel vehicle without DPF like one of the Euro IV categories showing high levels of these emissions may be caused by spray formation (e.g., worn or partly blocked injector) issues or faulty EGR operation and air metering. For DPF-equipped vehicles, emissions of such pollutants may also increase by high backpressure caused by the DPF. Some of these issues may be individual vehicle-specific, as vehicles age and wear out. This may also explain the high variability of emissions, especially for the Euro IV SCR-equipped vehicles.
presents the comparison of the emission levels measured in this study to the ones of similar studies. Euro IV HDDTs in HK appear to be, more or less, at similar levels, except CO2
emissions, compared to the levels reported by Yao et al. [19
] and Huo et al. [10
] (Beijing and Jinan, China). All emissions of the Euro V vehicles in this study were measured to be lower than those reported by Fontaras et al. [11
] (Milan Italy). This is possibly explained due to the significantly different speeds during testing (47 km/h in HK compared to 7 km/h in the tests at Milan). With regards to the Euro VI vehicles, the emissions of HDDTs in the current study appear to be at higher levels, except HC emissions, compared to the levels reported by Grigoratos et al. [13
]. The comparison with the work of Heinje et al. [20
] indicated that the NOx
emission levels of Euro VI vehicles in the current study were higher, and CO and THC emissions were significantly lower. As the experimental results of this kind of studies heavily depend on the driving, operating and ambient conditions during testing, one has to be aware of the respective experimental configurations when interpreting the results in a comparative manner.
3.2. Variance of Emission Levels
The estimated variance of the mean emission level within each speed bin was calculated based on the unweighted mean, statistical approach (unweighted sum of squares—USS estimators) z to address differences among the sample sizes of the vehicles included in each vehicle class. The coefficient of variance (CoV) of the NOx emissions within each speed bin was calculated for the three vehicles classes which comprised two vehicles or more. The median values of the respective speed-bin CoV were 9%, 11%, and 9% for the Euro IV (DOC, EGR), Euro IV (DOC, EGR, SCR), and Euro V (DPF, EGR) vehicle classes, ranging from 3% to 19% for the different bins.
A random-effects analysis-of-variance (ANOVA) procedure was employed [21
] to assess the impact of factor individual vehicle on emissions, i.e., to investigate the contribution of variance between vehicles and variance within vehicles to the calculated uncertainty of the emission factors. The analysis was applied to Euro IV (DOC, EGR) and Euro V (DPF, EGR) vehicle classes as these classes included five and 11 vehicles respectively, thus providing a satisfactory sample size to extract some reliable results on the effect of individual vehicles—within a given vehicle class—on emission factors. The results revealed that the impact of the individual vehicle factor on emissions is statistically significant (p
-value= ~0.000 to 0.002) for both vehicle classes that were examined. The average contribution of the between-vehicles variance to the total variance within a speed bin, most commonly named as interclass ratio [23
], was 8.7% (2.3% to 16%) and 15% (7.9% to 16%), respectively, for the two groups of HDDTs. As a reference value for comparative reasons, the respective values for CO2
emission levels were 7.3% (1.3% to 9.4%) and 6.4% (2.7% to 11.9%), respectively, lower than the corresponding NOx
ones, indicating a more robust behavior of the different vehicles in CO2
performance. The within-vehicle variance due to factors (e.g., driving conditions) other than the vehicle differences, as well as randomness, seem to be the most important component of the total variability of the estimated emissions factors.
3.3. Emissions of Non-Regulated Pollutants
The available FTIR dataset included NH3
O measurements for three vehicles, i.e., one Euro IV (DOC/EGR/SCR), one Euro V (DOC/SCR), and one Euro VI (DPF/SCR) vehicle. The instantaneous NH3
O mean exhaust contents (ppm) and the respective 500 m driving sequences mean distance-based emissions levels (mg/km) along with the corresponding minimum and maximum values are presented in Table 5
The instantaneous NH3
exhaust content for the Euro IV vehicle was above the respective Euro VI provisions for heavy-duty engines, i.e., an ammonia concentration limit of 10 ppm [25
]. Average NH3
exhaust contents for the Euro V vehicle were marginally higher than this limit, whereas the emissions for the Euro VI vehicle were significantly lower than the regulatory limit and mostly below the detection limit of the instruments.
N2O is a powerful greenhouse gas with a high 100-years global warming potential (GWP100, N2O = 265), albeit not regulated thus far by the EU. In this study, two of the vehicles, i.e., the Euro IV and Euro V trucks, could be characterized as high N2O emitters, as the respective CO2 equivalent emissions of N2O contributes approximately an additional 11% and 14%, respectively, to the corresponding total greenhouse gases’ levels. The Euro VI vehicle appears to be a low emitter with the respective contribution limited to 2.3%.
The three vehicles belong to three different Euro technologies, but they are all equipped with an SCR system. Despite the differences in their particular characteristics, and although NH3 and N2O are not yet regulated pollutants in the context of Euro standards, the results provide some evidence on the reduction of both NH3 and N2O emission levels for more recent technologies. This could be considered as an indirect index of the improvement of the respective SCR technologies over the years.
emission mass rates and the exhaust gas temperature profiles of a trip excerpt example for a Euro IV and a Euro V HGV are illustrated in Figure 5
In the top panel (Euro IV), we observe that there is a region of low NOX emissions where NH3 and N2O levels are high and also the temperature is high. This is shown to be assisted by the rather simplified urea injection strategy in this vehicle. Urea slip seems to be proportional to steady-state temperature and no ammonia slip is observed during transients, leading to peaking NOx emissions. This means that limited ammonia storage is possible on the particular catalyst, and this results in high ammonia slip and N2O formation.
On the Euro V vehicle (bottom panel), the performance is somehow different as N2O is mostly formed during accelerations and there is much less ammonia slip. It is obvious that a different ammonia injection strategy is used in this vehicle, seems as there is much more ammonia storage (ammonia profile does not follow speed profile), and this also affects N2O emissions. By comparing the two vehicles, the formation of these two components seems to be very much specific on catalyst type and SCR calibration.
3.4. DPF Regenerations’ Characteristics
A number of DPF regenerations were observed during the HDDTs’ testing campaign, originating from nine different vehicles. Table 6
presents the main characteristics for 19 representative regeneration cases (events that started and were completed during a single trip). THC emissions and exhaust temperature were the two quantities that appeared to be most affected by these events, exhibiting some extremely high levels during the regenerations, as presented in Figure 6
(regeneration events correspond to three different vehicles).
To obtain a view of the effect of regenerations on pollutant emissions, we calculated—for each regeneration and pollutant—the ratio of the distance-based emission level of the pollutant during the regeneration event over the respective emission level before the start of the event, i.e., at no regeneration conditions, and at the same average speed. As the respective distribution of the ratios (per pollutant) was significantly skewed for some of the pollutants, e.g., CO, including both very low and very high values, Figure 7
presents the median value of these ratios along with the respective 10th and 90th percentiles. THC levels during the regeneration were 32 times as high as the levels under no regeneration conditions. However, the effect of DPF regeneration on NOx
and CO is less significant or less clear, respectively, compared to THC emissions. NOx
emissions during the events were found to be higher than the respective ones under no regeneration conditions by a factor of 1.9. CO emissions exhibit a mixed performance as both increasing and decreasing trends were observed, CO emissions during regenerations were measured to be at similar levels as before regenerations (median = 1), but also a few high ratio values were observed. PM emissions during regeneration were measured to be 2.8 times as high as the respective levels before regeneration. DPF regeneration seems to impact the rather low-level THC emissions and also PM emissions.
may be used towards the oxidization of the particles trapped inside the DPF. The median value of the ratios of the NO2
emission levels during the regeneration process over the respective levels before the regeneration is also presented in Figure 7
levels during the regeneration are similar (median = 1.0) to the levels before the process begins, which is suggestive of mostly O2
-led regeneration rather than NO2
3.5. The Impact of Road Grade on Emissions and Fuel Consumption
Based on the 500 m integrated data, Figure 8
illustrates the impact of road grade on NOx
emissions and fuel consumption levels. Three generic classes of road grade were examined, namely (i) flat road driving (road grade between −0.5% and +0.5%), (ii) uphill driving (road grade > +0.5% and up to +6%), and (iii) downhill driving (road grade < −0.5% and down to −6%). NOx
emissions and FC for the uphill and downhill driving modes are expressed as ratios over flat driving (reference level). As emissions and FC were initially calculated per speed bin to eliminate the impact of speed on the outcomes, Figure 8
depicts the median value of the respective speed bin ratios.
A monotonical increase of NOx emissions and FC with road grade is observed for uphill driving, with a few exceptions mostly due to small sample sizes, particularly for the Euro VI category (one vehicle was tested). Similarly, a monotonical decrease was observed for the downhill driving mode. Interestingly, the impact of road grade on NOx emissions and FC appears to be quite consistent among the different vehicle categories. The average uphill driving NOx emissions may range from 1.07 (road grade: 0.5–1%) to 3.35 (road grade: 5–6%) times the respective flat road levels, whereas FC may range from 1.23 (road grade: 0.5–1%) to 2.57 (road grade: 5–6%) times the respective flat road levels. The impact of road grade on FC is more consistent than the one on NOx emissions. No clear conclusions can be drawn with respect to CO, THC, and PM emissions in terms of the systematic impact of road grade on these pollutants.
The different NOx pattern of the Euro VI vehicles, compared to previous technologies, as road gradient increases may have to do with their SCR operation. The Euro VI vehicle was equipped with an SCR device. In general, the SCR efficiency drops above the optimum range of 250–350 °C. The average temperature at the tailpipe was at 260 °C over these high gradient driving conditions, suggestive of temperatures that can exceed 350 °C at the SCR. Therefore, high engine-out NOx emissions at such high load conditions and the compromised performance of the SCR due to high temperatures seem probable reasons for this significant increase in NOx only at this highest road gradient case.
3.6. The Impact of Loading on Fuel Consumption and Emissions
Ten of the vehicles were measured under different weight loadings, i.e., 30%, 50% (half truckload, HTL), and 100% (full truckload, FTL) of their payload (average payload ~ 6t). A fixed route was selected for each truck and all measurements were conducted on this specific route to ensure similar testing conditions for the trips of different loadings. The 30% loading setting was considered as the reference loading, thus the average FC (L/100 km) and the average distance-based NOx
and PM emissions (g/km) for the HTL and FTL cases were expressed as a ratio over the corresponding 30% reference values. The one-way repeated-measures ANOVA was employed to check for the statistical significance of the effect of the factor weight loading on emissions and FC (a = 5%). The bar plots of Figure 9
present the average emissions and FC ratio of all ten vehicles for the different loading classes. The error bars correspond to the respective ± one standard deviation limits.
The impact of loading found to be statistically significant for NOx emissions and FC (p-value = 0.002 and <0.001). The effect of loading on PM emissions found to be statistically non-significant (p-value = 0.111). The notable differences in PM emissions that were observed between the sample vehicles, as also indicated by the respective error bars, seem to affect these outcomes.
The results reveal a positive correlation of loading with fuel consumption and emissions. NOx emissions were observed to increase by 7.8% on average for the HTL setting (ratio range from 0.93 to 1.28) and by 23% on average for the FTL setting (ratio range from 1.13 to 1.42), compared to the respective 30% reference loading values. PM emissions were observed to increase by 16% on average under the HTL setting (ratio range from 0.37 to 1.83) and by 29% on average under the FTL one (ratio range from 0.78 to 1.77), also compared to the 30% loading setting. Fuel consumption measured to be 6.3% higher, on average, under the 50% loading (ratio range from 0.99 to 1.12) and 15% higher, on average, under the 100% loading (ratio range from 1.04 to 1.24) compared to the respective 30% loading values.