• California Air Resources Board

Study yields curious NOx results from new technology diesel engines running RD/BD blends


The California Air Resources Board contracted with the University of California at Riverside Bourns College of Engineering-Center for Environmental Research and Technology to evaluate oxides of nitrogen (NOx) and particulate matter emissions from the use of renewable diesel/biodiesel blends in one on-road and one off-road new technology diesel engine (NTDE) with selective catalytic reduction (SCR) and diesel particulate filter (DPF) exhaust aftertreatment systems, and one off-road non-NTDE (legacy engine) without DPF and SCR.


Previous studies using diesel engines with experimental or first-generation SCR indicated that SCR fully controls NOx emissions from biodiesel (i.e., NOx emissions due to biodiesel do not exceed those from conventional diesel fuel in the same engine). However, more recent studies with newer, more mature original engine manufacturer SCR systems indicate that NOx emissions from biodiesel may not be fully controlled in NTDEs. To further investigate NOx emissions from NTDEs, CARB contracted CE-CERT to evaluate NOx and particulate-matter emissions from the use of renewable diesel/biodiesel blends in NTDEs with mature, modern emissions control systems.


Testing Results Summary

In both NTDEs tested, excess NOx emissions from biodiesel were shown to be not fully controlled relative to reference CARB diesel. Excess NOx is any additional NOx produced from the use of biodiesel in NTDEs relative to conventional CARB diesel. In both NTDEs tested, blending of renewable diesel and biodiesel was shown it did not reduce excess NOx emissions from biodiesel. This differs from legacy engines, where renewable diesel reduces excess NOx from biodiesel. In the NTDEs tested, particulate-matter emissions were shown to be very low and near background levels for all blends and there were no statistically significant differences. In the non-NTDE (legacy) engine tested, results were shown to be within the range of prior studies. RD100, or pure renewable diesel, reduced NOx emissions by about 5 percent and particulate matter by about 30 percent. In the legacy engine tested, renewable diesel was shown to also reduce excess NOx from biodiesel in the renewable diesel/biodiesel blends. In the legacy engine tested, for particulate matter, the greater the biodiesel concentration in the renewable diesel/biodiesel blends, the greater the observed particulate matter emissions benefits.


Staff Summary

CARB contracted CE-CERT to evaluate NOx and particulate-matter emissions from the use of renewable diesel/biodiesel blends in NTDEs. Results show that NOx emissions in renewable diesel/biodiesel blends were shown to be higher in the NTDEs tested than observed in some previous studies, and that NOx emissions from RD100 were shown to be no different than conventional diesel.


The results of this study raise additional questions, for example:


  • How do these results align with previous studies on biodiesel and renewable diesel emissions performance in legacy engines and NTDEs? Are there additional data or testing results that CARB should be aware of to further evaluate biodiesel and renewable diesel emissions performance?


  • How representative of NTDEs used in California are the engines used in this study?


  • How representative are the test cycles in this study to in-use activity cycles?


  • Would the addition of a low-load test cycle contribute to the accuracy of calculating in-use NOx excess emissions from NTDEs?


  • How might these findings impact NOx emissions estimates from biodiesel and renewable diesel fuel use?


Next Steps

CARB staff has identified several questions about the study results that require further discussion and evaluation. Staff invites stakeholders to evaluate the questions above and offer feedback on what additional considerations or other data we should include in our evaluation. Feedback on this report and the questions above will be accepted until Jan. 31, 2022, and should be sent by email to adf@arb.ca.gov. Feedback received will be considered public information and may be posted.


To view the full report, click here.

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