Study shows air-quality benefits of biodiesel in city buses

By Mineta National Transit Research Consortium | October 29, 2014

Do transit buses operating on biodiesel emit more or less particulate matter and gases than those using conventional diesel? The Mineta National Transit Research Consortium has released its latest peer-reviewed study based on laboratory and field experiments. Among other test results, Combustion Chemistry of Biodiesel for Use in Urban Transport Buses: Experiment and Modeling found that using biodiesel could effectively reduce the mass of particulate matter released in both hot and cold idle modes. Reduction in amount of particulate matter, number of elements and elemental carbon was observed, and the reduction is considered beneficial to promoting the clean air and human health. Principal co-investigators were Ashok Kumar, a professor and chairman of Civil Engineering at The University of Toledo, and Dong-Shik Kim, associate professor of chemical and environmental engineering at University of Toledo, working with the research team of Hamid Omidvarborna and Sudheer Kumar Kuppili. The free report can be downloaded here

Biodiesel has many advantages over regular diesel even in a very low blend percentage. The benefits include low emissions of particulate matter, combustion elements (mainly sulfur), elemental carbon, and carbon monoxide. Comparing types of elements from field and laboratory experiments showed what types of elements are emitted only from the fuels.

“Physical properties of biodiesel blends are very important during engine combustion,” Kumar said. “Higher viscosity causes reduced fuel leakage during injection, which drives an advance in injection timing and an increase of mass injection rate. Density of the fuels affects the start of injection, injection pressure, fuel spray characteristics, etc. When the fuel temperature changes and enters an engine with different temperatures (hot or cold), fuel acts differently and the emissions are different.”

In sum, it is recommended that governments consider using blends of biodiesel in urban and commercial vehicles to enhance the quality of air and to promote healthy living.

Among the report’s findings:

-Combustion temperature and pressure of biodiesel blends are linearly correlated with the portion of oxygen in biodiesel fuels. This information can be used to make proper blends of biodiesel with regular diesel.

-The high oxygen content of biodiesel improves the oxidation of soot precursors and limits soot mass growth, resulting in less particulate matter formation.

-The results also confirmed that better combustion, with less emission of elements, occurred in hot idle mode (i.e., when engines were fully warmed) rather than in cold idle mode (i.e., at morning start-up).

-The results indicated that the use of biodiesel could effectively reduce elemental carbon, which is considered more hazardous than organic carbon.

-Source apportionments of detected elements are done by using laboratory experiments as well as field experiments.

-The neural network method along with the kinetic models is implemented to help us better understand the particulate matter formation mechanisms and come up with more efficient and effective operating conditions to reduce PM emissions.

-Higher flash point of biodiesel, suggests that it is safer than other fuels, and storage process is easier.

 Chapters cover five different parts including introduction, literature review, methodology, results and discussion, and finally conclusion and future works. The 92-page report includes 20 figures and 16 tables.

The full report is available for free, no-registration; download at


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