5.2. Updates to Medium Heavy-Duty Emission Rates#
In EMFAC, medium heavy-duty vehicles are defined as vehicles having GVWR between 14,001 and 33,000 lbs. or U.S. commercial vehicle classes 4–7. These categories include all EMFAC T6 categories, as well as SBUS and OBUS. In EMFAC2021, medium heavy-duty vehicles were, for the first time, informed by emissions test data. EMFAC2025 updates build on this approach by including additional test data, while removing data from engines recalled through CARB’s in-use compliance program. Staff used test vehicles from CARB’s Truck and Bus Surveillance Program (Class 7) and Surveillance Program for Class 4–6 Heavy-Duty Vehicles (Class 4–6). Table 5.4 lists vehicle information available for medium heavy-duty NOx emission rate updates.
Vehicle ID |
Engine Make |
Model Year |
Odometer Mileage |
Emission Rate (g/mi) |
New Data in EMFAC2025 |
|---|---|---|---|---|---|
2S19H01-02 |
Cummins |
2015 |
58,475 |
0.01 |
No |
2S19H01-03 |
Cummins |
2015 |
92,914 |
0.05 |
No |
2S19H01-04 |
Cummins |
2015 |
155,537 |
0.02 |
No |
2S19H01-05 |
Hino |
2017 |
69,869 |
0.16 |
Yes |
2S23T01-02 |
Cummins |
2017 |
159,688 |
0.17 |
Yes |
2S23T01-04 |
Cummins |
2020 |
43,787 |
0.13 |
Yes |
J-1 |
Isuzu |
2013 |
96,562 |
0.46 |
No |
TBSP V13-1 |
Hino |
2017 |
151,150 |
1.05 |
No |
2S19H01-07 |
Cummins |
2019 |
24,078 |
0.41 |
Yes |
2S19H01-09 |
Cummins |
2019 |
47,745 |
0.04 |
Yes |
Unlike heavy heavy-duty vehicles, the sample size (\(N = 10\)) is not large enough to develop a robust deterioration function. Therefore, staff calculated a ratio between the average medium heavy-duty and heavy heavy-duty emission rates and applied that ratio to the heavy heavy-duty vehicle ZMR, which is the same approach used in EMFAC2021. The emission impact rate (EIR), which informs the deterioration function, of medium heavy-duty vehicles is assumed to be the same as heavy heavy-duty vehicles.
Initially, staff considered all engine model years 2013 and newer to develop this ratio. The estimated ratio was < 0.05 due to some heavy heavy-duty model years 2013–2015 having extremely high emission rates. To avoid underestimating medium heavy-duty vehicle emission rates, staff instead limited the ratio to engines with model year 2016 and newer. This ratio is 0.21 and was applied all engine model years 2013 and newer. The new ratio is smaller than the previous value of 0.273 used in EMFAC2021.
Figure 5.8 shows a comparison between the medium heavy-duty vehicle NOx emission rate for EMFAC2025 and EMFAC2021 (also listed in Table 5.5). Compared to EMFAC2021, NOx emission rates are 17% lower in EMFAC2025.
Figure 5.8: Medium Heavy-Duty Vehicle Emission Rates: EMFAC2025 vs. EMFAC2021#
Odometer Mileage |
EMFAC2021 Emission Rate (g/mi) |
EMFAC2025 Emission Rate (g/mi) |
|---|---|---|
0 |
0.152 |
0.126 |
50,000 |
0.274 |
0.228 |
100,000 |
0.309 |
0.258 |
150,000 |
0.335 |
0.279 |
200,000 |
0.355 |
0.296 |
250,000 |
0.372 |
0.310 |
300,000 |
0.387 |
0.323 |
350,000 |
0.401 |
0.334 |
400,000 |
0.413 |
0.344 |