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cantly and the actual benefits depend
on the real-world application.
The deployment of hybrid technology in construction machines, which
have unique and diverse duty cycles,
is in the very early stages and hasn’t
been studied closely, Johnson said.
To find out what environmen-
tal benefits hybrids might offer in
construction equipment, Johnson’s
study looked at two commercially
available hybrid off-road construction
machines, the Caterpillar D7E dozer
and the Komatsu HB215 excavator.
The D7E was compared with a Cat
D6T dozer and the Komatsu was
compared to Komatsu’s PC200 exca-
vator. The researchers deployed time-
lapse cameras, and gaseous and par-
ticulate matter emissions and engine
operating parameters were monitored
by an onboard portable emissions
measurement system (PEMS).
Getting an apples-to-apples com-
parison wasn’t easy with the dozers,
Johnson said. Caterpillar has posi-
tioned the D7E as a replacement for
the D8T, but Johnson’s study chose
the D6T because it is powered by an
engine from the same engine family,
with the same displacement and the
same load characteristics. The D8T
is rated about 150 hp — 40% higher
— than the D7E and is powered by a
different engine family, Johnson said.
The D7E is equipped with a 9. 3 L
C9.3 ACERT Tier 4 interim engine
rated 235 hp. The electric drive
system enables the use of a narrow
rpm range between 1500 and 1800
rpm, instead of the 1600 and 2200
rpm for the conventional system.
Because of this narrow speed range,
the power shift transmission doesn’t
need to rev as high or lug down as
low, Johnson said. Additionally the
dozer has significantly fewer mov-
ing parts with the replacement of its
mechanical gear system.
In order to characterize the typical
operation of different units, activity
measurements were made on a sub-
set of six hybrid and four comparable
conventional construction machines.
Activity data was obtained through
interviews, historical records and in-
use activity measurements that includ-
ed time-lapse video, real-time engine
control module (ECM) broadcast data
and real-time GPS information. The
activity measurements were used to
develop duty cycles that were repre-
sentative and also repeatable in terms
of getting good comparisons between
the different equipment, Johnson said.
Johnson noted that the opera-
tors had a positive impression of the
When compared with Komatsu’s diesel PC220 excavator, the Komatsu HB215 hybrid
excavator produced slightly more NOx on average, along with a 27% increase in particulate
matter emissions, according to the University of California, Riverside study.
continued on page 44