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UCR CE-CERT testing of hybrid construction equipment finds reduction in fuel consumption, but increase in NOx emissions

Summary of comparison of NOx emissions and fuel consumption of hybrid (D7E) and conventional (D6) dozers. Negative values mean hybrid benefit and positive values mean dis-benefit. PM, THC, and CO emission rates were very low for both the D7E and D6T and thus were not able to be quantified as a hybrid benefit or dis-benefit. Idle emissions showed a similar trend, with the hybrid emitting less CO2 (-15%) and more NOx (+15%). Johnson et al. Click to enlarge.

A study of hybrid excavators and dozers in use in public and private fleets found that while the use of hybrid equipment could decrease fuel consumption and CO2 emissions by up to 28%, depending upon the type of work being done, NOx emissions could increase by up to 21%.

In the study, researchers at the University of California, Riverside’s Bourns College of Engineering Center for Environmental Research and Technology (CE-CERT) completed activity characterization; sequence of operations/test cycle development; and in-use emissions testing on three hybrid Komatsu HB215-LC-1 excavators and three hybrid Caterpillar D7E dozers.

The report was prepared for the California Air Resources Board (ARB) as part of the FY 2010-11 Air Quality Improvement Project Hybrid Off-Road Equipment Pilot Project. CE-CERT completed the project testing element in April 2013, in consultation with ARB staff.

Hybrid PC2015 NOx and FE benefit for typical excavator vocations. Negative values mean hybrid benefit and positive values mean dis-benefit. CO was mixed and ranged from -5% to +21%. THC was lower and ranged from -48% to -73%. The hybrid idle emissions showed a lower CO2, PM, CO and THC emissions of -43%, -85%, -66%, and -75% respectively and no difference in NOx emissions (0%). Johnson et al. Click to enlarge.

To characterize the typical operation of different units, the UCR team made activity measurements on 6 hybrid and 4 comparable conventional construction equipment; and obtained activity data using interviews, historical records, and in-use activity measurements that included time-lapse video, real-time engine control module (ECM) broadcast data, and real-time GPS data. The researchers used the activity measurements to develop duty cycles that were representative and also repeatable in terms of getting good comparisons between the different equipment.

The emissions and fuel consumption performance evaluations were primarily based on the mass of emissions per ton of earth moved. Idle, travel, and non-earth moving actives were factored into the overall emissions comparison on a grams per hour basis with a weighting function derived in the study. The researchers also evaluated emissions on a per brake horse power hour, per fuel use, and per yard basis.

Among the overall findings were:

  • Dozer. The hybrid dozer provided a 2% percent increase to up to 28% decrease in CO2 emissions, and a NOx increase of between 7 and 21%, with lighter loads providing the greatest efficiencies and fewest NOx dis-benefits.

    The use-weighted average for fuel consumption was a 14% reduction; the use-weighted average for NOx emissions was a 13% increase.

    No benefit or dis-benefit could be quantified for PM, CO, and THC due to the low emission levels from the aftertreatment system (ATS) equipped engines on both the D7E and D6T units.

  • Excavator. The Komatsu excavator provided between a 1% increase to a 28% decrease in CO2 emissions, with demolition work providing a 23% decrease and general construction a 13% decrease in CO2 emissions. The NOx impact of the hybrid Komatsu excavator ranged from an 18% decrease for demolition work to an 11% increase for general construction.

    The use-weighted average for fuel consumption was a 16% reduction; the use weighted average for NOx emissions was a 1% increase.

    However, PM dis-benefit for the hybrid excavator was around 27% for all types of work and ranged from 6% for travel to 36% for backfill.

The data, the researcher said, suggests that the next generation of hybrid construction equipment will need additional technological advances to ensure it achieves substantial greenhouse gas benefits while also delivering NOx emission reductions across all duty cycles.

The increase in NOx emissions illustrates the difficulty in tuning engines to simultaneously reduce both NOx and greenhouse gas emissions. Most modern diesel engines are tuned to balance the need for both NOx and greenhouse gas emission reductions, and utilize sophisticated after-treatment systems to minimize NOx before it leaves the tailpipe, said Dr. Kent Johnson, an assistant research engineer at CE-CERT and the principal investigator on the project.

The reason we see increased NOx in this first generation technology is simple—getting the best fuel economy is going to sell the equipment.

—Kent Johnson

In the study the hybrid diesel construction equipment was compared to new conventional diesel equipment, which emits fewer emissions than the older diesel equipment commonly used today. Some of these newer units emit up to 90% less emissions compared to older units, thus the benefit of replacing old units with hybrids is a great benefit to the environment. Both hybrid diesel construction equipment and new conventional diesel construction equipment are much cleaner than old diesel equipment. The drawback is that the hybrids cost 20% more than new conventional diesel.

Other UC Riverside studies of today’s first generation heavy-duty hybrid vehicles and equipment have found varying results relating to emissions and fuel economy. Hybrid marine vessels performed well, while on-road heavy duty hybrids, such as tractor trailers, had mixed results.



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