According to a study commissioned by the Washington, DC-based Propane Education & Research Council (PERC), NOX emissions measured from propane autogas school buses were substantially lower than those measured from diesel school buses. The study was conducted in 2018 by West Virginia University’s Center of Alternatives Fuels, Engines, and Emissions (CAFEE).
The report states that the primary conclusions during the first round of testing in January and February of 2018 were:
- Distance specific NOX emissions measured from the diesel bus were significantly higher than those measured from the propane bus for both the Morgantown (180 to 175% difference) and Stop and Go (189% difference) routes.
- Low ambient temperatures may have exacerbated the NOX emissions from the diesel school bus in addition to the low speed, low load operation contained in these routes and the inherent difficulties to maintain adequate temperatures in the SCR system.
- Distance specific CO emissions measured from the propane bus were greater than those measured from the diesel bus for all routes with a percent difference ranging from 68% (Stop and Go) to 128% (Cold Morgantown). However, it is important to note that since 2010 there are no longer any non-attainment areas for CO in the United States.
- THC emissions followed a similar pattern, however the magnitude of THC emissions was lower than the CO emissions, particularly for the Morgantown routes.
- Average distance specific CO2 emissions were similar yet slightly higher for the propane school bus operated over the Morgantown route (Cold – 21% difference, Hot – 4% difference) however lower on average for the propane bus over the Stop and Go route versus the diesel school bus (15% difference).
Subsequent testing performed in July and August with newer model year and lower mileage propane and diesel school buses provided further confidence in many of the conclusions from the previous testing. The primary conclusions were:
- NOX emissions were an order of magnitude greater for the diesel school bus compared to the propane school bus during the Cold Morgantown and Stop and Go routes.
- Higher variation in the NOX emissions was observed during the Hot Morgantown route from the propane school bus due to a large NOX emissions spike during one of the tests with the propane powered school bus, that did not occur in the other tests. This resulted in an average of the three Hot Morgantown tests that was only 40% lower than the diesel-powered school bus.
- During the Stop and Go routes, OBD data revealed that the largest NOX production from the diesel school bus resulted during periods when the SCR thermal management strategy was not active or was not recently active.
- CO and THC distance specific emissions from the propane school bus were on average approximately double in magnitude compared to the diesel school bus over the Morgantown Route.
- Over the Stop and Go routes the trend reversed for CO emissions with the diesel school bus producing 20% more distance specific CO emissions (on a percent difference basis) over the Stop and Go routes. THC emissions were very similar and minimal for both vehicles.
- CO2 emissions shared a similar trend with the initial round of testing; the propane school bus exhibited higher distance specific CO2 emissions during the Morgantown routes (13% difference greater), yet lower CO2 emissions over the Stop and Go routes compared to the diesel school bus (21% difference lower).
- OBD data from the diesel school bus revealed higher CO2 emissions mass rates at idle when the SCR thermal management strategy is active, which led to a larger disparity in CO2 emissions during the 5-Minute Stop and Go test versus the 2-Minute Stop and Go test.
According to the report, the diesel school busses emitted less CO emissions, although there were exceptions during the Stop and Go route. However, as previously mentioned, since 2010 there are no longer any non- attainment areas for CO in the United States. THC emissions from both propane and diesel school buses were relatively minimal, but measurements from propane school buses exceeded those of the diesel school buses. CO2 emissions were lower for the diesel school busses during city and interstate driving, however, they were lower for the propane busses during the simulated stop and go testing. In general, NOX emissions were an order of magnitude lower from the propane school busses compared to the diesel school busses. This is significant due to the fact that NOX and Ozone is a predominant non-attainment concern for many areas in the United States.