LO 11-7 Identify and describe the types, features, and applications of cooling fans

1. Fans and Fan Drives

Fans and Fan Drives

Engine fans are designed to increase airflow across the radiator, which improves the effectiveness of heat removal from coolant by the radiator, air charge cooler, A/C condenser, and other accessory coolers located in front of the radiator. The number of blades, as well as the fan speed and fan pitch, determines the volume of air moved by the fan Figure 60. Mechanical engine power coupled through drive belts is used to rotate fans, but electric fans can also be used Figure 61. For most on-highway vehicles, engine fans pull air through the radiator in the same direction as airflow when traveling in a forward direction. When the vehicle is traveling, its movement usually creates enough airflow, and the fan is not required. At low-speed operation and at idle, air movement is often inadequate, and the fan is required by the cooling system and to move air across the A/C condenser and any other front-mounted heat exchangers. To reduce the parasitic loss of power from the engine, which is often more than 25 horsepower (hp; 19 kilowatts [kW]) on some HD fans, a fan drive mechanism is used to decouple the fan from the engine or to regulate its speed when high fan airflow is not required.

FIGURE 11-60: A conventional fiberglass engine fan and variable-speed, viscous-type fan clutch. Fins on the fan clutch absorb heat from the radiator to make the fan drive more temperature sensitive.

FIGURE 11-61: Without an engine, electric cooling fans are needed by this battery electric vehicle to increase airflow across the radiator.

1.1. Viscous Fan Clutches

Viscous Fan Clutches

One of several common methods of controlling fan operation is through a temperature-controlled, modulating fan clutch. These fans are coupled to the drive through a highly viscous silicone fluid. The amount of fan clutch slippage is determined by a temperature-sensitive, bimetal control valve that regulates the quantity of silicone fluid in the clutch coupling mechanism. When the clutch is cool, little silicone fluid enters the clutch and the fan freewheels. As the temperature of the air in the fan clutch area increases, more fluid enters the clutch drive mechanism, creating more drag or silicone fluid shear within the coupling mechanism Figure 62. The fan speed is slower than engine speed and is adjusted according to the heat load across the radiator Figure 63. These clutches are quieter because fan speed is kept proportional to heat load. Because their operational speed is proportional to engine temperature, they conserve energy and extract significantly less energy from an engine than a directly coupled fan.

FIGURE 11-62: A variable-speed engine fan connects the inner and outer hub by shearing silicone fluid between discs.

FIGURE 11-63: This silicone cooling fan hub freewheels when the hub is cool. As more heat is absorbed by the hub, internal drag causes the fan speed to increase, moving it closer to the driven speed of the hub. Note the bimetal spring in the center of the hub, which is connected to a valve that controls fluid movement.

1.2. Variable-Speed Engine Fan

Variable-Speed Engine Fan To reduce parasitic loss caused by an operating cooling fan, many late-model MD and HD engines use a variable-speed cooling fan Figure 64. Operation of the fan is controlled by the engine ECU using a PWM signal to the clutch coupling mechanism to control the fan speed. An electrically operated control valve is used instead of the bimetallic, heat-sensing spring used in conventional viscous cooling fans. Slower fan speed also translates into less engine noise.

FIGURE 11-64: An electrically controlled, variable-speed engine fan reduces parasitic power loss and provides optimal control of fan speed.

1.3. Hydraulic Fan Drive

Hydraulic Fan Drive

A hydraulic fan drive system enables variable fan speed to be independent of the engine speed. These fan drive mechanisms are commonly found in off-road equipment and transit buses, where cooling demands can exceed the ability of a conventional fan drive system to move air through a radiator and other heat exchangers. A hydraulic pump and an electronically controlled pressure control valve supply hydraulic pressure to a hydraulic motor that drives the cooling fan. Because airflow across radiators in these applications is not provided by forward vehicle movement due to a rear- or side-of- vehicle location, the fans can actually be driven at much higher speeds than an engine could provide at idle, high-load, or low-speed engine operation. Like electronically controlled, variable-speed fans using air or silicone fluid, cooling speeds can be ramped to avoid shock-loading of the engine or belt drive system. Electronically controlled fan speed yields significant power and fuel savings Figure 65 and Figure 66. The direction of the suction-type fan can even be reversed to blow out debris from the radiator.

FIGURE 11-65: Electronic control of fan speed using a hydraulic-pressurized fan drive system enables fan speeds in excess of engine speed.

FIGURE 11-66: The hydraulic fan drive system of a skid steer loader with a radiator located at the rear of the machine.

1.4. On/Off Fan Hubs

On/Off Fan Hubs

A more traditional fan engagement mechanism is an on/off fan hub, which uses air pressure to either engage or release a spring-loaded friction clutch Figure 67 and Figure 68. The air supplied to the fan hub is electrically signalled through an electric over-air control valve. Either the ECU provides an electrical signal to open or close the air supply solenoid or a temperature-sensitive switch in the cooling system activates the solenoid. If the refrigerant pressures in the condenser become too high, an A/C high-pressure switch supplies an input signal to the fan, which then pulls air across the condenser Figure 69. Typically, a timer is connected to the circuit to keep the fan engaged for 90–120 seconds to prevent excessive cycling between on and off. Longer and fewer fan cycles prevent premature wear of the friction materials in the fan clutch and remove more heat when the A/C refrigerant pressure is excessive.

FIGURE 11-67: An expanded view of an on/off fan hub.

FIGURE 11-68: A schematic of an electrical circuit controlling an on/off fan hub.

FIGURE 11-69: The engine fan pulls air across multiple heat exchangers to increase the effectiveness of heat removal from the heat exchangers.

The coolant temperature at which the fan engages is 10–15° F (−12.2 to −9.4° C) above the normal engine operating temperature, which allows vehicle movement to first move air across the radiator and ATAAC and only engages the fan if airflow provided by vehicle speed is insufficient. Because engine fans consume a lot of engine power—easily 25 hp (19 kW) on HD engines—the fan is often activated with the engine brake to increase the engine’s resistance to driveline rotation. During engine warm-up operation, the fan may also engage to apply a load to the engine when the transmission is in neutral. A two-speed fan hub is also available, which provides a combined variable-speed operating mechanism and spring-engaged, on/off operation.

1.5. Radiator Fan Shroud

Radiator Fan Shroud

Fan shrouds seal the gap around the fan and radiator to ensure that cooling air runs through the radiator rather than leaking around its sides or past the headlights. Recirculating air that has already passed through the radiator decreases cooling efficiency. Fan shrouds minimize the likelihood of air passing through the fan and back into the radiator. Fans for on-highway vehicles are usually pulling fans, which means they pull in air in the direction the vehicle is traveling. Off-road machines, transit buses, and highway coaches with side-mounted radiators, particularly side-mounted A/C condensers, use pusher fans instead.