Courtesy UCR

Testing equipment sits atop a hybrid dozer at Waste Management's El Sobrante Landfill in Corona, Calif.

Scientists at the University of California, Riverside, have found that while hybrid construction machines reduce fuel consumption and greenhouse-gas emissions, they produce more smog-forming gases than their non-hybrid counterparts.

The school's two-year, $2-million study, funded by the California Air Resources Board, evaluated the Caterpillar D7E dozer and the Komatsu HB215LC-1 hydraulic excavator in various field trials. In the tests, the machines demonstrated savings of up to 28% in fuel while emitting up to 21% more nitrogen oxides, or NOx, compared to traditional, non-hybrid equipment.

While the mixed results could be viewed as a setback for the manufacturers, UCR researchers say the hybrid machines use older versions of clean-diesel engines, which are less sophisticated than the latest ones that incorporate NOx scrubbers in the tailpipe.

The higher NOx numbers also reflect the challenge of tuning engines for better fuel economy and performance while cutting engine-out emissions, say UCR researchers.

"The reason we see increased NOx in this first-generation technology is simple: Getting the best fuel economy is going to sell the equipment," says Kent Johnson, the study lead.

The full report notes that while the hybrid and non-hybrid engines were evenly matched (Interim Tier 4 on the dozer and Tier 3 on the excavator), the hybrid machines operated within a narrower speed range that perhaps contributed to higher concentrations of NOx.