Heavy Duty Hybrid Vehicle Testing:

Particulate Matter (PM) Emissions

How does particulate matter form?

Particulate matter (PM) from internal combustion engines is composed of a combination of carbon particles, on the surface of which organic compounds are adsorbed. If there is sulfur in the fuel, sulfur compounds will also be present in the particulate along with some metals from the fuel, lubricating oil and wear products. While sulfur emissions are a concern, it is the adsorbed organic fraction that poses the largest toxic risk associated with the particulate. Because the carbon particles are generally less than 2.5 microns (greater than 90 percent, by mass, are less than 1 micron), they typically remain airborne and can be inhaled into the lungs where the adsorbed organic compounds can potentially cause damage.

All fuels produce carbon particles as a result of incomplete combustion. The organic fraction is dependent upon the fuel combusted, its combustion residence time, combustion temperature, engine lubricant, and whether an oxidation catalyst or regenerative particulate trap is installed. Several things can initiate the formation of carbon particulate emissions, either separately or in combination, including incomplete combustion from engine over fueling, engine misfiring, lubricant combustion and impurities in the fuel.

Overview of Results

PM emissions from the hybrid vehicles were generally 50 to 70 percent lower than emissions from a conventional diesel. In several cases, the actual reduction could not be quantified, as the measurement equipment did not have the sensitivity to quantify the mass emissions from the hybrids.

Several systems on the hybrid buses are responsible for these PM reductions: regenerative braking, less transient engine management, and regenerative particulate trap control.

The graph below provides a comparison of the PM emissions of each bus type tested during this project. CNG buses, powered by DDC Series 50G engines, had PM emissions at around 80 to 90 percent lower than a conventional diesel bus.

The CNG and hybrid buses had comparable PM performance on each cycle when the hybrids were operated on very low sulfur fuels. However, when the Orion-LMCS hybrid was operated on conventional diesel fuel (300-ppm sulfur), CNG bus PM levels were 50 to 80 percent lower than the hybridÕs levels.

The Nova-Allison bus exhibited PM emission rates consistently lower than CNG buses as this bus was operated exclusively on very low sulfur diesel. With the Nova-Allison hybrid buses regenerative braking disabled, hybrid PM emissions increased slightly giving CNG a small advantage. More on PM and sulfur.

More on heavy-duty hybrids:

Introduction to Heavy-Duty Hybrid Buses

What is a Hybrid Bus?

Hybrid Bus Demonstrations

Certification of Hybrid Buses