Hydraulic Fan Drive Systems

By Tom Eystad


In the internal combustion engine, much of the energy is lost to inefficiencies such as heat. Only about 30% of the input energy is actually converted to mechanical power.

For large vehicles, engine cooling systems require a significant amount of horsepower and greatly increase the 1416DFC3-AEDE-41A9-879D-27BA54CEB5CE.pngnoise level of the machine. With Tier 4 engines, the cooling requirements have increased and the demand for more efficient systems has multiplied. There is a greater push for quieter operation and greater machine efficiency than ever before.

Conventional fan drive systems overcool under most conditions in order to insure adequate cooling during the more demanding high heat operation condition. With this system, much of the power used to drive the fan is wasted. The temperature-activated, electronically-controlled, hydrostatic fan drive system can offer finer control and reduce the fan speed during times of low cooling demand. Therefore, the fan drive system can use less power. The power saved can be used to increase fuel efficiency and the overall operating efficiency of the machine.

Variable and Alternating Hydraulic Fan Drives

lternating, or on/off fan drive systems use a relief valve with a control that is either on, or off. It is the simplest and lowest cost option. The pump volume and relief setting determine the maximum fan speed. When engine temperature reaches a preset level, a signal is provided to turn the fan on by closing the vented system. When engine temperature drops below a certain level, the signal is removed to allow the system to vent and the fan is allowed to either stop or run at a low RPM. When the fan is on, it is at maximum speed and noise level.

Variable fan speed control using a proportional relief valve is the most cost-effective solution, providing very high operating efficiency. The fan speed is varied as a function of the pressure set by the proportional relief valve. Maximum fan speed is attained when the proportional pressure control valve is at its highest setting. The relief valve will "failsafe" to high pressure so that losing the electrical signal causes the fan to operate at maximum speed for maximum engine cooling if electrical power is lost.

Variable fan speed using a variable displacement pump is the most efficient and offers the quietest operation, however the initial cost of this type of system is relatively high, and therefore its use is limited. In this system the pump drives the fan but also provides other functions. When the other functions require a higher pressure than the fan, a reducing valve can be used to limit pressure to the fan. A proportional flow control is used instead of a pressure relief valve. The pump could include a load-sense control which would require a sensing line downstream of the flow control. In applications where the pump is completely dedicated to the fan drive, the pressure and flow control valves would typically be an integral part of the bolt-on pump control.


Advantages of Hydraulic Variable-Speed Fan Drive

Precise Control of Coolant Temperature — The curve below shows a typical result for fan speed vs. coolant temperature. The difference between the switch-on curve and the switch-off curve (hysteresis) is very small, approximately 1°C. This permits precise control of the coolant temperature with minimal temperature fluctuations, even when there are extreme changes in the engine load.


Reduced Engine Wear — Allowing engine coolant to get too hot breaks down engine oil properties and reduces lubrication. Cylinder and bearing wear can be reduced considerably when the operating temperature is reached quickly and maintained.

High Flexibility — The vehicle designer is allowed to optimize the arrangement of the engine and cooling system components. A considerable advantage can be achieved here in regards to noise insulation of the engine.

Speed Modulation — Provides constant regulation from minimum to maximum fan speeds over a temperature range of approximately 10°C. This prevents abrupt speed changes which can lead to heavy loads on drive parts and high noise levels. The bandwidth of 10°C permits average cooling output without unnecessary switching between minimum and maximum fan speeds.

Maximum Speed Limitation — The proportional pressure control valve also acts to limit maximum fan speed to control power consumption and noise level of the impeller.

Energy Savings — Under normal cooling requirements, the fan will operate at about 50% to 80% of maximum RPM and can go as low as 10% to 20% in cold weather climates. This results in power savings and a drop in fuel consumption. The curve above shows a typical power requirement curve for a hydrostatic fan with proportional control. When the fan is running between 50% and 80% of maximum speed, the input horsepower is reduced to approximately 40% to 80% of maximum. In cold weather climates this could be as low as 15% to 20% of the maximum horsepower. The input horsepower is the sum of the fan horsepower plus the mechanical and hydraulic losses of the system.

Electronic Control Systems and Components

Hydrostatic fan drive systems can utilize electronic controls ranging from simple single-sensor circuits to complicated multi-sensor circuits with fan speed feedback. The type of electronics used in a given application is determined by the level of accuracy and control required.

In a simple two-speed control there is a temperature sensor or switch and a proportional relief valve. In this system the fan will run at maximum speed when cooling demand is high, and at an intermediate speed when cooling demand is lower. The electronic control is set to wait until a signal is generated by the temperature sensor or switch before starting the fan. This allows the engine to come up to temperature before the fan starts operating.

The electronic control has a built-in ramp function which will bring the fan speed up to maximum at a controlled rate of acceleration. This reduces the startup shock loading on the fan drive components. When the coolant temperature decreases below the set point, the fan RPM is reduced to the intermediate speed. The fan will then cycle between the two speeds with a ramped acceleration from intermediate to maximum speed.

The proportional systems illustrated are more sophisticated, offering more precise control of fan speed. These systems can employ a separate fan module or use a signal from the engine control module (ECM). These systems can provide precise engine temperature control. Fan speed is constantly modulating over approximately 10°C with a hysteresis of about 1°C.


The major factors to consider when applying fan drive systems are:
• Fan specifications
• Pump/motor sizing
• Proportional valve selection
• Electronic control requirements

The fan specifications will typically establish airflow at a given RPM which will result in a horsepower and torque requirement. From this specification, the pump/motor can be selected. This determines flow and pressure requirements which will determine the proportional relief valve to be used. The electronics can then be selected and programmed based on the system requirements for the inputs and final control parameters.


For the machine designer, the hydrostatic fan drive offers flexibility of installation along with reduced noise and high efficiency. The reliability of the fan drive system is improved because the components operate at a lower duty cycle with less shock loading.