Hybrid Not Always Greener

Testing of hybrid construction vehicles finds a reduction in fuel consumption, but an increase in harmful emissions

UC Riverside researchers stand on a hybrid bulldozer

UC Riverside researchers, from left, Kent Johnson, Eddie O'Neil, Don Pacocha and Sam Cao stand on a hybrid bulldozer.

RIVERSIDE, Calif. (www.ucr.edu) — Hybrid technology is not necessarily greener when it comes to comparing the latest hybrid and diesel construction equipment.

That’s the main finding of a first-of-its-kind study of hybrid diesel construction equipment by researchers at the University of California, Riverside’s Bourns College of Engineering Center for Environmental Research and Technology (CE-CERT).

Manufacturers reported the hybrid dozers and excavators reduced fuel consumption and greenhouse gas emissions by 20 percent and cut smog-forming oxides of nitrogen emissions (NOx) by 30 percent. But, UC Riverside researchers found fuel savings of between 7 percent and 28 percent and NOx emission increases of up to 21 percent when compared to conventional diesel equipment.

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 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,” said Johnson, who at CE-CERT has helped pioneer the use of portable emission measurement systems to test everything from off-road equipment to trucks, planes, trains and boats while they are being used.

He did add, however, that in the study the hybrid diesel construction equipment was compared to new conventional diesel equipment, which emits fewer harmful emissions than the older diesel equipment commonly used today.

In fact some of these newer units emit up to 90 percent 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 percent 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.

“It all comes down to testing when the equipment is being used,” Johnson said. “That is even more important with off-road equipment, such as the construction equipment we just tested, because applications are so widely different.”

The hybrid diesel construction equipment research was conducted after CE-CERT received a $2 million contract in 2011 from the California Air Resources Board to evaluate two models of commercialized hybrid construction equipment – the hybrid Caterpillar D7E bulldozer and hybrid Komatsu HB215LC-1 excavator.

The research complements efforts by the California Air Resources Board to expand the demonstration and deployment of hybrid and zero-emission technology to help meet the goals of AB 32, a 2006 state law that requires greenhouse gas emissions be reduced to 1990 levels by 2020. California Governor’s Executive Order S-#-05 also targets an 80 percent reduction by 2050.

Significant deployment of next generation hybrid and zero-emission technologies is also critical for California to meet the health-based federal eight-hour ozone standard. Researchers hope the findings in this first-of-its-kind study will help drive technology improvements to hybrids that lead to even cleaner construction equipment.

The $2 million was used for two purposes: as an incentive voucher to get public agencies and companies to purchase hybrid construction equipment and to fund the emissions and fuel use research on the equipment.

hybrid bulldozer pushing trash at landfill

A hybrid bulldozer equipped with testing equipment at the Waste Management El Sobrante Landfill in Corona.

The researchers facilitated the deployment of 10 hybrid Caterpillar bulldozers and six hybrid Komatsu excavators to six sites in four California counties: Riverside, San Diego, Orange and Sacramento counties.

The researchers used time lapse cameras, global positioning systems, engine control units and portable emission testing equipment during more than 2,000 hours of testing on both types of vehicles. They also designed a series of standardized tests, ranging from river bed clearing to trenching, for each vehicle.

The hybrid bulldozer was tested under six conditions, each with a different distance and load amount.

Fuel savings ranged from 7 percent to 28 percent. In general, the shorter the distance and lighter the load, the more fuel saved. The use-weighted average was a 14 percent fuel savings.

Tests for emissions of nitrogen oxide (NOx), a significant contributor to air pollution, showed a range from an increase of 21 percent to a decrease of 2 percent. The use-weighted average was a 13 percent increase in NOx.

The hybrid excavator was also tested under six scenarios, ranging from demolition, which usually means longer swings of the arms, to general construction work, which consists of more trenching and backfilling.

The fuel use ranged from a 28 percent savings to an increase of 1 percent. In general, more fuel was saved during the demolition test cycles. The use-weighted average was a 16 percent fuel savings.

For emissions, the researchers tested for NOx, particulate matter and several other pollutants. For NOx there was a range from an 18 percent reduction to an 11 percent increase, with an average of a 1 percent increase. For particulate matter, there was increase between 6 percent and 36 percent, with a use-weighted average of a 27 percent increase.

Johnson believes this study could help engine manufacturers consider their designs not only from a fuel economy and certification level, but also from an in-use basis. Researchers expect as the technology continues to mature that emissions performance will improve, much as it has with hybrid cars and light trucks.

In addition to Johnson, the team of UC Riverside researchers included: Tanjeng (Sam) Cao, a graduate student; Robert L. Russell, an assistant research engineer; and George Scora, an assistant development engineer. Andrew Burnette, an environmental consultant, also was part of the team.

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Kent Johnson
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E-mail: kjohnson@ucr.edu

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