June 2018 | VehicleServicePros.com 21
the vehicle won’t start the next morning because
the battery bank hasn’t had time to recharge.
An oversized alternator would cut down on the
recharge rate.”
Additionally, local delivery trucks can benefi t
from a higher-output alternator. Th e constant
starting/stopping, deep discharges and high-cycle
demand can lead to batteries not getting charged
fast enough.
This is why Steve
Rosenbloom, a compliance
engineer at C.E. Niehoff,
says fleets should not
completely rely upon nameplate
output ratings.
“You should compare the
alternator capacity at high
ambient temperature to the
expected speed range of the
vehicle engine in your specific
application,” Rosenbloom
says. “Th en, compare that
capacity to the expected
worst-case vehicle load while
also allowing for excess alternator
capacity. Th is helps to
ensure that the batteries fully
recharge during a typical
drive cycle.”
C.E. Niehoff designs and
manufactures heavy duty
brushless alternators.
Speed range and temperature
are two influential
variables.
With respect to speed
range, average engine cruising
speed, for example, is
more important to long-haul
trucks. Average engine low
idle speed, on the other hand,
is more important to in-city
delivery vehicles.
Temperature factors in as
well. An alternator’s nameplate
output rating is taken
at a shaft speed of 6,000 rpm
and with 22 degrees C (about
72 degrees F) ambient cooling
air per SAE J56 and ISO 8554.
However, the reality is that
alternator output at typical
ambient engine air temperature
is less than at ambient
room temperature.
“This effect is referred
to as derating and is due
primarily to the increase in
alternator internal copper
resistance with increasing
temperature,” says Ciaran
Patterson, chief engineer
at C.E. Niehoff . “So a fl eet
manager should look at
alternator capacity curves
over the entire speed range—
while also remembering that
the published 22-degree
C curves will need to be
reduced by up to 15 percent
to account for this eff ect. Understanding
alternator output curve will help you
select an alternator with adequate
» Delco Remy’s 39MT Heavy Duty Gear
Reduction Starter is designed for heavy
duty truck and off-highway applications.
Photo courtesy of BorgWarner
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