18, 1998 Design News
BRAKES AND CLUTCHES
Fan clutches scrimp on energy use
These on/off devices for
diesel engines deliver cooling only when needed
by Karen Auguston Field, Managing Editor
Given that the price of gasoline is hovering around
$1 per gallon here in the U.S., fuel economy is hardly
a national fixation these days. Just consider the burgeoning
fleet of ever-bigger sport utility vehicles on the road,
whose average fuel consumption is only slightly better
than that of a military tank.
In direct contrast to consumer behavior today, initiatives
like the United Nations environmental conference in
Kyoto, Japan, last fall are reminders that there remain
very real concerns over fuel emissions, air pollution,
and the depletion of energy resources. As a consequence,
efforts continue apace in the auto industry to improve
vehicle fuel efficiency, reduce emissions, and develop
alternative propulsion technologies.
Articles about electric cars are staples of the daily
newspaper. But one technology virtually unknown outside
of the transportation industry is quietly making a significant
impact on fuel consumption. It's the diesel engine on/off
fan clutch, a thermostatic control device originally
developed for the trucking industry following the OPEC
oil crisis of the 1970s. Today, these devices are ubiquitous
on Class 7 and Class 8 heavy-duty trucks.
On/off products under development will feature lighter
weight, higher torque capabilities, and higher operating
speeds. The advancements are being driven by a better
understanding of friction materials and component packaging
to get the best performance.
The design premise of the device is engagingly simple:
The clutch engages the fan only when augmented airflow
is required to cool the radiator, significantly reducing
the huge variations in operating temperature of a diesel
engine--which can range as high as 150F with unregulated,
direct-drive cooling. The upshot? Lower fuel consumption.
"Cooling fans on large diesel engines consume
as much as 75 hp, which makes them real fuel guzzlers.
Since fan on-time with our on/off clutches typically
amounts to less than 25% of engine run time for over-the-road
operation, there is a significant savings," says
Stephen M. Clancey, product manager for engine cooling
systems, at Horton, Inc. (Minneapolis, MN).
A mechanical engineer with 14 years of experience in
engine technology, Clancey is involved in the design
of next-generation fan clutches. "Along with other
aspects of engine design, fan clutch technology has
evolved over the years, and the latest clutches make
significant contributions to diesel engine performance,
reducing engine wear, and minimizing abrasion of radiator
surfaces by dust-laden air," he explains.
An early approach to reducing the load on diesel engine
fans was the application of a viscous fan drive, a technology
in use on many automobiles today. Essentially a fluid-coupled
device that operates much like a variable torque converter,
it consists of a hydraulic turbine with silicone fluid
that operates through centrifugal force. A valve, which
is controlled by a bi-metallic element that senses the
temperature of air passing through the radiator, controls
the amount of fluid-coupling slippage. The less slip,
the more efficient the clutch, and the higher the fan
A shortcoming of this technology is that it can never
be fully engaged or disengaged, leading to over-cooling
and high engine load or ineffective fan operation. While
this does not impact the performance of smaller cars
and trucks, it is an issue with cooling fans on large
trucks that consume up to 75 hp.
Electric fan drives are not really an option, given
the fact that a Class 8 truck would require a large
electric motor of impractical size. But since most trucks
have compressed air on board to operate the brakes,
pneumatically actuated mechanical friction clutches
appeared to be a natural fit.
Early developers discovered that these clutches, which
may be designed for either spring-engaged and air-disengaged
(normally on), or air-engaged and spring-disengaged
(normally off), substantially improved not only overall
diesel engine fuel economy (see chart on next page),
but reduced the stresses on engine components and prolonged
diesel engine life.
The technology has been evolving ever since. Today's
clutches are available with either pneumatic or electromagnetic
actuation. The advantage of belt-driven electromagnetic
clutches is that they are compact, yet extremely efficient.
However, the tradeoff is lower torque capability, which
makes the light-to medium-duty truck market a better
target, explains Clancey.
Rather than having discrete sensors with wires and
connectors, the latest generation of on/off fan clutches
is controlled by input from a diesel engine's electronic
control module (ECM). This microprocessor-based control
monitors parameters such as ambient temperature, engine
coolant temperature, charge air temperature, and engine
The newest approach to fan control includes no clutch
at all. It combines electronic controls and engine/running
gear inputs with a variable pitch fan drive. "This
type of design offers us a powerful opportunity to effectively
integrate information from multiple inputs, giving us
the ability to provide the precise airflow required
to maintain optimum engine temperature under varying
conditions," says Clancey.
With fan power now in the range of 75 hp,
on/off clutch manufacturers estimate that the
fuel savings would be more than double what was
reported in 1986 (shown above). The tradeoff?
While this technology is more costly, Horton hopes
that the ability to more precisely control critical
engine temperatures will offset the higher price.
Will electronic controls supplant mechanical clutches?
Clancey doesn't think so. "We think there's a market
for both types of technology. The variable pitch drive
has a lot of complexity to it and of course a higher
cost. It is going to be used in applications that require
a greater level of control--say a bus stopping at every
corner, as opposed to a long-haul diesel truck operating
at constant speed going across country."
Given the substantial benefits in fuel savings, one
interesting question is whether the on/off clutch has
any future place in the auto industry. Although automobiles
have other alternatives, such as electric fan drives,
it's a possibility, Clancey cautiously ventures. But
first, consumers clearly are going to have to get a
whole lot more excited about fuel efficiency than they
How one vendor tests its fan clutches
by Stephen M. Clancey,
product manager for engine cooling systems
For years, engineers at Horton, Inc., a manufacturer
of fan clutches and related system components for diesel
engine temperature control, have performed environmental
and accelerated durability testing of diesel engine
cooling systems. These laboratory and field tests have
been important to continuing product development, but
neither could reveal the moment-by-moment demands placed
on engine cooling components during highway operation.
Recently, we have developed an over-the-road monitoring
capability for truck and engine cooling systems, which
allows us to monitor the performance of our on/off fan
clutch and other products under actual operating conditions,
and with daily or on-demand downloading of results.
The system combines on-board data collection with global
positioning and wireless data communication. Each test
vehicle has an onboard computer system that collects,
processes, stores, and forwards data concerning the
vehicle's engine load and ambient operating conditions,
as well as its physical location and progress. A Global
Positioning Satellite (GPS) Receiver on each vehicle
provides position in longitude, latitude, and altitude,
and records vehicle speed and direction. This information
is collected in a database that is used by Horton design
engineers and field service personnel, and shared with
engine OEMs and truck fleet operators.
Engineers at Horton designed this remote
vehicle test system to measure key operating parameters
during field testing and periodically transmit
data back to them for analysis.
The on-board data collection system monitors engine
data from as many as 16 different points, ranging from
thermocouples and pressure sensors to analog voltage
and frequency output devices. Monitored engine parameters
include fan drive air pressure, coolant temperature,
charge air outlet temperature, air temperature around
the fan drive, ambient outside air temperature, engine
speed, fan speed, air conditioning clutch on/off and
fan drive on/off. The GPS unit yields vehicle speed,
altitude, and latitude/longitude records for the recorded
period. Remote data sampling can also be triggered separately,
such as when the fan drive turns on, or the engine exceeds
any preset operating parameter.
All of this vehicle operation and location information
is stored in the truck's on-board computer. Data samples
are transmitted daily by means of the Remote Communications
Unit, which polls the computer's hard drive for new
data and then forwards it to Horton via a cell phone
or other wireless link. Horton engineers can also initiate
communications from Minneapolis to any remote unit to
upload or download information.