Edited by Kenneth J. Korane
Belt-driven fans have been the most common method of cooling vehicle engines for more than 100 years because they are simple and reliable. After being aligned by the engine manufacturer, belt drives give years of troublefree service. However, as emission standards become more demanding, vehicle designers need innovative ways to cool engines.
First, they must develop more-efficient cooling systems because reducing the power required for cooling also reduces the engine’s fuel consumption and makes more power available to do work. Second, diesel-engine temperature affects both fuel consumption and emissions. So maintaining engine temperature within a narrow range has become a priority.
Benefits of belts
Belt drives are durable and, from an initial cost standpoint, the most economical means of power transmission. The simplest drives have one or more belts and a set of sheaves (pulleys). Once installed in a vehicle, belts will withstand thousands of hours of operation (or tens of thousands of miles) and the sheaves usually last the life of the engine.
Belt drives are also efficient. At their peak, they transmit more than 90% of the power from the driving shaft to the driven. However, the belt drive itself doesn’t provide cooling — the fan does. And a rotating fan uses a substantial amount of power whether the engines is idling or running at full speed.
The problem is, belt-driven fans pull air through the radiator all the time, regardless of how much or how little cooling the engine needs. This is because belt drives operate at a fixed speed ratio to the engine speed. Whenever the engine is running, so is the fan. And the faster the engine runs, the faster the fan.
This is not the most-effective means of cooling because the belt drive consumes engine power whether cooling is needed or not. An engine running at full speed may not necessarily be operating at full load. Likewise, an engine under full load at moderate speed may overheat if the fan is also running at moderate speed. The faster the fan spins, the more air it pulls through the radiator and the more engine power — and fuel -– it consumes.
With so much attention placed on efficiency and emissions, except for the least-demanding applications, cooling requirements for today’s engines are beyond the capabilities of belt drives. That’s why car manufacturers have switched from belt-driven to motor-driven fan drives. Efficient cooling is even more important for off and onhighway equipment because it requires more power for adequate cooling. And as mentioned above, increasingly stringent emissions standards require tighter control of engine temperature.
A more-effective alternative
Hydraulic fan drives are more effective at engine cooling because cooling demand, not engine speed, determines fan speed. The hydraulic system drives the fan at the precise speed required to provide only the amount of airflow needed. This means the fan is not wasting energy rotating faster than necessary, just because the engine is running at high speed.
A basic drive has a hydraulic gear pump transmitting power (flow and pressure) to a hydraulic gearmotor, which drives the fan. An electronic-control unit (ECU) monitors operating conditions and commands an electrohydraulic pressure-control valve that regulates power to the hydraulic motor. The ECU typically communicates with electronic-engine controls to ensure the vehicle operates at peak efficiency.
The ECU can receive inputs from sensors monitoring ambient air temperature, coolant temperature, fan speed, and other parameters. (Typically, the ECU only monitors water temperature to control fan speed.) Software developed for specific vehicles and applications lets the ECU generate signals to control the electrohydraulic valve. This proportional valve adjusts hydraulic pressure across the motor based on a proportional command signal, controlling fan speed anywhere within its operating range. Higher pressure equates to higher available torque to turn the fan faster.
A hydraulic drive does not waste power turning the fan when cooling is not needed, so the engine burns less fuel and releases fewer emissions. For example, when a vehicle’s engine first starts, the fan is typically not needed. This lets the engine reach operating temperature faster than if the fan was pulling cold air through the radiator, which is normally the case with belt-driven fans.
On the other hand, when the engine requires peak cooling and maximum fan speed (typically only about 1% of the time), the ECU commands the control valve to supply maximum pressure. Hydraulic-motor torque is proportional to the fluid pressure across it, so regulating hydraulic pressure accurately controls fan speed. And because the ECU adjusts fan speed based on coolant temperature and other key parameters, the end result is more-precise control of engine temperature than is possible with a belt drive.
This is especially important with diesel engines because operating temperature has a twofold effect on engine performance. First, a diesel engine’s power-tofuel- consumption ratio peaks within a relatively narrow range of temperatures. Second, emissions released per pound of fuel used are lowest within another narrow range of temperatures. These two operating regions overlap within an even tighter band, so keeping the engine within this narrow temperature range provides the best fuel economy and lowest emissions.
Hydraulic fan drives can offer advantages beyond fuel savings and reduced emissions. Adding a flow-reversing valve to the hydraulic circuit lets the hydraulic motor turn in the opposite direction and reverse airflow. Pushing air out through the front of the radiator purges dirt and debris that reduces cooling efficiency as much as 50%. This is especially useful for vehicles operating in dirty environments because it reduces the need for screens, which add cost and size to the radiator and can restrict airflow. The reversing feature can also blow water out of the radiator to prevent freeze damage in cold climates. All of this eliminates the need for someone to periodically clean the radiator. The operator can reverse the fan with a switch, or the ECU can be programmed to reverse it at regular intervals.
Another advantage is design flexibility. Because the hydraulic fan motor connects to a pair of hoses rather than a belt attached to the engine, the radiator and fan can mount anywhere on the vehicle. Installing the radiator and fan outside the engine compartment makes space available for other components — such as emissionsreduction hardware required to meet Tier IV and Stage IV regulations.
Plus, the engine compartment may not be the best place for a radiator because ambient air drawn in by the fan may be hot and dirty. Putting the fan in another location may let it draw in cooler and cleaner air. This flexibility also lends itself to distributed designs, with coolers strategically sized and located throughout the vehicle for better performance and efficiency.
Hydraulic fan drives can also increase power to the engine for short periods. When the machine needs quicker acceleration or more muscle for work, the ECU has the valves shift the pump and motor to a neutral position, so the fan drive consumes no power.
This may let equipment designers use a smaller engine. This is important because increasingly strict regulations have led to new engine designs which demand more power for emissions control, instead of for work or propulsion. Consequently, larger engines often are needed. However, a power boost from the fan drive may sufficiently offset power requirements to avoid using a larger engine.
Moreover, hydraulic fan drives can be integrated with other vehicle systems. For example, excess capacity from the fan pump can provide hydraulic power to assist auxiliary functions. And by incorporating the fan drive into a vehicle’s braking system, the hydraulic fan circuit can help decelerate the drivetrain by using the fan motor to dissipate excess power. This relieves some of the load on the brakes, helping extend brake life.
Hydraulic fan drives also help OEMs meet more-stringent noise legislation, because the fan in proportional drives is tested at only 70% of maximum speed, compared with full speed for belt drives. Noise is not proportional to fan speed, so noise reduction exceeds 30%.
Finally, the ECU constantly monitors operating conditions and instantly alerts the operator to any malfunctions. In fact, the controller calculates and can display power draw of the fan drive at any time, helping maximize performance and efficiency.
Selecting fan drives
Given that hydraulic fan drives hold several advantages over belt drives, engineers might be tempted to assemble one from off-the-shelf components. This isn’t necessarily the best approach.
Cooling experts generally recommend selecting and sizing fan-drive components to match specific applications. A mining truck has different cooling requirements from an excavator, which has different requirements from a harvester. So it’s advisable to choose suppliers well versed in hydraulic fan drives to ensure a system best suits the equipment and application. A capable fan-drive manufacturer should have:
- Engineers and technicians experienced in hydraulics, electronic control, cooling systems, and Tier and Euro emissions standards.
- Pumps, motors, valves, and controls designed and manufactured specifically for hydraulic fan drives.
- Engineering tools that help experts design fan drives that deliver the highest performance and efficiency for the quickest return on investment.
Hydraulic fan drives may not be a viable alternative in every case. OEMs simply looking for the lowest initial cost will still find belt drives the power transmission method of choice. But in most cases, hydraulic fan drives offer vehicle manufacturers and users higher productivity, better fuel economy, and lower emissions — as well as significant savings over the life of a vehicle.