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 speed.
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 load.
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.
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 are today.
For more information
Call 1-800-828-6344 x011 and enter the product code
Clutches from Horton (3): Product Code 4284
Clutches from Kysor (3): Product Code 4285