hybrids — indeed, most preferred
the hybrid machine over the conventional equipment. On average, more
positive comments about the excavator were received than about the
bulldozer. Some fleets commented
that the D7E controls were rougher than a conventional dozer and a
landfill operator commented that the
machine was too light relative to the
conventional D8 dozer.
The study found that while the
hybrid dozers and excavators showed
an overall fuel consumption reduction compared to the equivalent conventional machinery, some emissions
Compared to the D6T conventional
dozer, the hybrid dozer produced 7
to 21% more NOx emissions — still
within regulatory standards, Mitchell
noted. No benefit or drawback could
be quantified for PM, CO, and THC
due to the low emissions levels from
the aftertreatment systems used on
both the D7E and D6T dozers.
The excavator test results showed
that the hybrids provided an average
fuel savings of 16%, but NOx emissions
increased by 1% and particulate matter
increased by an average of 27%.
“If I had a large fleet and I wanted
to replace my older, retiring fleet, most
likely I would replace a D8 with a D7E,”
Johnson said. “If that’s the case, I show
the benefit of replacing an older unit. It
was more than 90% (improvement) on
PM, CO and hydrocarbons and a 70%
reduction in NOx and higher fuel con-
sumption. So it’s a great change.
“Now if they replaced it with a conventional (dozer), it would be the same
great change with a little bit more NOx.”
Excavator testing included more than
15 different modes including trenching,
dressing, lifting, holding, hammering
and demolition. The Komatsu hybrid
employs a 4. 5 L Komatsu 4D10 7E- 1
Tier 3 diesel engine rated 148 hp at
2000 rpm. The engine has an electric
motor/generator at its flywheel and
an electric swing motor/generator to
operate the superstructure. The swing
motor system generates energy as the
upper structure rotation is slowed and
that energy is stored.
The conventional PC200 excavator
incorporates a 6. 7 L 6D10 7E- 1 diesel
engine rated at 155 hp at 2000 rpm.
The hybrid system allows the engine
to operate at a lower idle speed of 700
rpm while the conventional machine
idled at between 950 and 1050 rpm.
The difference in NOx emissions
varied greatly depending on application. NOx emissions from the hybrid
excavator were reduced 18% during
demolition work, but increased 11%
University of California, Riverside researchers (from left) Kent Johnson, Eddie O’Neil, Don
Pacocha and Sam Cao on a hybrid dozer used in their two-year study. The researchers
deployed 10 hybrid Caterpillar dozers and six hybrid Komatsu excavators to six sites in
California as part of their research.
during general construction work, the
The study also found that by purchasing an HB215 instead of the
conventional PC200, the typical excavator owner could expect to save
from 13 to 23% in fuel, but would emit
from 26 to 27% more PM. Again, a lot
depended on what type of work the
machine was doing.
In a written response to the study,
Caterpillar stated that variations in NOx
production between the D6T and the
D7E “as reported, are minimal, within
normal variability and most importantly,
below the NOx standard.” Therefore,
the company said the report’s finding
that the D7E has an overall NOx emissions deficit when compared to the
D6T are misleading.
Johnson’s report offered several suggestions to improve the hybrid technology, including working to stabilize
the speed variation during the excavator operation to reduce PM emissions
and possibly NOx emissions as well.
According to the study, the PM increase
for the hybrid excavators may be a
result of the difference in engine speed
during operation. Changing the ECM
fuel injection timing could help reduce
NOx emissions in both hybrid machines,
“My goal with the study was to talk
about the benefit of hybridizing and
maybe that distinction is getting the
wrong press,” Johnson said. “From
the environment’s benefit, if a company does decide to replace an older
machine because they see a fuel
economy improvement, there is going
to be an emissions improvement.
“But the question gets back into
what the hybrid is really replacing.
We had these discussions with CARB
over and over and we basically said
we’d focus on what our original ques-
tion was — did the hybrid provide the
While there was some variability,
the answer, it seems, is no. dp