A comparison of viscous drives and on/off fan drives for diesel engine cooling

Diesel Progress North American Edition, Sept, 1999 by Steve Clancey

A diesel engine fan drive is a device used to engage the radiator fan based on the need for cooling airflow through the radiator, and to reduce engine load and save fuel by disconnecting the fan when cooling airflow is not required. Fan clutches were not generally used on diesel engines before the early 1970s.

A sudden and dramatic increase in fuel costs at that time, fueled by the OPEC oil embargo, led engine manufacturers and truck fleet operators in North America to become more concerned with fuel economy issues. The drive to reduce environmental pollutants has also contributed to interest in more efficient diesel engine operation over the past 30 years.

For owners of light- and medium-duty diesel trucks, the issue of engine cooling translates into concerns over cab heating and cooling, fuel efficiency and conservation of power in heavy-load and steep-grade driving conditions.

A diesel engine operating without sufficient cooling air through the radiator may suffer damage to cylinder sleeves, pistons, bearings or valves. This overheat condition is most likely to occur under low road speed, high load and elevated outside temperature conditions, when ram air alone is not sufficient to remove engine heat.

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At the other extreme, overcooling of an engine caused by unnecessary fan operation wastes fuel, increases emissions and can result in driver discomfort because of poor cab heating. In addition, dust-laden air drawn through a radiator by unnecessary fan operation abrades the radiator and shortens its service life. The objective of radiator fan control is to provide optimal airflow through the radiator under wide-ranging conditions.

Today the on/off fan drive is a common and important component on medium and large diesel truck engines to help optimize temperature control, increase fuel efficiency and improve environmental performance. Fan drive technology has evolved over the past several decades along with other aspects of diesel engine design, and the latest fan drives make significant contributions to engine performance.

The earliest diesel engine on/off fan drives were adaptations of standard industrial clutches. While these units confirmed the value of modulated cooling fan control, they were not suited to the rigors of over-the-road vehicle operation.

Fan Drive Design

Before the availability of the on/off fan drive, diesel engine cooling fans were direct drive, with the fan engaged 100 percent of the time, regardless of engine temperature or operating conditions. Cooling fans on a diesel truck engine can consume from 16 hp to as much as 41 hp, depending on engine size and fan configuration. A direct fan drive contributes measurably to fuel consumption. Continuous radiator cooling resulted in wide variations in coolant temperature with changes in load, road conditions and weather. A properly operating fan drive minimizes fan "on" time and extends radiator life.

Diesel truck fan drive technology has evolved over the past several decades, from direct drive to viscous drives, then to on/off fan drives.

Viscous Fan Drives

Viscous fan drives are used to reduce the fan load when cooling is not required, and thereby lessen power requirements and related fuel demands. A viscous fan drive offers an improvement in efficiency over direct drive fan cooling. This hub-mounted, fluid-coupled device contains a hydraulic turbine using a viscous silicone fluid to transfer energy from driving element to driven element.

Typically, a bi-metallic element on the front of the viscous drive senses the temperature of air passing through the radiator and operates a valve to control the silicone transfer fluid, thus effecting fan engagement and disengagement. The fan drive is not controlled directly by engine temperature but indirectly by sensing the temperature of cooling air passing through the radiator.

The hydraulic nature of a viscous drive prevents it from achieving fully on or fully off operation. Consequently, in the disengaged mode, the fan continues to operate at speeds as high as 45 percent of the fully engaged rate, usually between 650 to 1000 rpm.

In the engaged mode the slippage (fluid shear) common to the viscous drive's liquid coupling restricts fan speed and associated cooling capacity by approximately 5 to 10 percent. Figure 1 shows the typical operating range of a viscous drive compared to the wider range of engine speeds, taking into account both parasitic drag (partial engagement) and fluid shear (slippage).

The partial engagement or parasitic drag of a viscous drive can contribute to engine overcooling, resulting in unnecessary fuel consumption. For example, with a truck engine speed of 1400 rpm, an average vehicle speed of 50 mph and yearly travel distance of 30,000 miles, a viscous drive consuming 0.5 hp (0.4 kW) in the off mode burns approximately 48 gal. of fuel per year.

The physical properties of the hydraulic fluid in a viscous fan drive may also change over time, and lead to decreased performance and further degradation in cooling performance.