Hydraulic fan drives have caught on as the most effective way to cool diesel engines. A diesel’s operating efficiency (fuel consumption) and emissions output are closely related to operating temperature. A diesel consumes the least amount of fuel per horsepower produced within a relatively narrow band of temperatures. Likewise, a diesel puts out the least amount of emissions per horsepower within a similar temperature range. So operating the engine within the narrow range of temperatures where the two overlap derives the most power from a diesel engine while minimizing fuel consumption and emissions.

Traditional, belt-driven fans produce air flow that is dependent upon engine speed, not cooling demand. This means belt-driven fans usually deliver too high or too low an air flow, which not only causes wide fluctuations in engine temperature, but wastes power. Maximum power is lost when the engine (fan) runs at high speed, even though little cooling may be needed. Moreover, an idling engine may need very little air flow, but, again, the fan continues to run at an unnecessarily high speed. Unlike traditional systems, hydraulic fan drives operate with variable speed. This means air flow through the radiator can be closely matched to the cooling requirements of the engine.

Multiple benefits

In its simplest form, a hydraulic fan drive consists of a pump, pressure control valve, motor, sensors, electronic control and, of course, all the hydraulic fan drives also allow mounting the radiator in a location that may be more practical than adjacent to the engine. Placing the radiator in a location away from major sources of dirt and contamination means it will stay cleaner longer. Furthermore, fan direction can be reversed when accumulated debris needs to be blown out of the radiator. These characteristics improve cooling by keeping the radiator operating more efficiently. Furthermore, the radiator and fan can be mounted in a location that directs heat away from the vehicle, which can save additional energy by reducing the cooling load on the operator cabin.

A design to suit the application

The simplest and least-expensive hydraulic fan drive uses a fixed displacement pump driving a fixed-displacement motor, with a pressure-control valve to regulate motor torque (fan speed). These systems react quickly and offer high repeatability to small speed trimming commands, typically using an inverse-acting proportional pressurecontrol valve. This setup ensures that the fan defaults to full speed if the controller loses power or some other malfunction occurs. With a conventional drive, a broken belt can cause the engine to overheat or shut down.

When high power is needed for cooling — typically 50 hp and greater — a system driven by a variable- displacement pump becomes more practical because its ability to produce virtually zero flow means even higher system efficiency. This, in turn, produces a quick payback on its higher initial cost. Some of these systems control fan speed directly by varying flow from the pump instead of controlling pressure. This type of pump control typically consists of a small pilot version of the inverse acting proportional valve to control the pump’s pressure compensator. The same controller can be used for either type of system because a proportional valve serves as the interface between the hydraulics and the electronic control. Furthermore, if a variable-displacement pump has overcenter (reverse-flow) capability, the fan can be reverse driven without having to use a directional valve.

Hydraulic fan controllers from High Country Tek (HCT) use discrete temperature thermistors. These are readily available from the automotive industry and are an inexpensive method of measuring temperature. The controllers can operate very well as stand-alone systems, making them easy to retrofit or upgrade existing systems, especially on older equipment that is being repowered or refurbished. However, they also readily interface with application status monitoring systems.

The latest generation of fan-system controllers for new-equipment designs is more reliable and easier to use by minimizing external connections and doing away with external sensors. Instead, they rely on the established J1939 communications BUS standard for the temperature data needed for the fan speed control. This BUS is also used to send system and controller status messages to in-cab instrument clusters to inform the operator in real time of operating conditions and if any action should be taken.