The Why of Variable-speed Fan Drives

A great innovation that is only slowly catching on in North America is the variable-speed cooling fan drive. As innovative as it is, it surprises me that it isn’t being adopted more readily. Today, 85% of drives are still on/off drives instead of the newer two- or three-speed, or ECM-controlled variable-speed viscous drives.

Given the benefits of the new technology, I think anyone specifying machinery that requires a fan and cooling system would benefit from the variable-speed drive. It’s kinder to the drive belts, results in less thermal cycling of the engine, is generally a lot quieter, drags less dirt and other contaminants into the radiator, and is, in the case of the truck driver, kinder to the operator.

Oh yes, and it saves fuel. That’s why the simple on/off fan drive has virtually disappeared from Europe where fuel cost is approaching $8 a gallon.

Fleets that have spec’ed these fan drives to check out the performance report that a major plus is the enthusiastic reception from drivers. The most frustrating thing about an on/off fan drive is the way it engages the fan on a long grade. The 40 horsepower or so that was helping on the climb goes to the fan drive and the driver must grab a gear to pull the grade.

With a variable-speed fan drive – especially one controlled by the engine ECM – there’s no sense of the fan engaging. And with the ECM controlled fan, the logic can limit the amount of horsepower draw as well, further softening the consequences of the engagement on a long grade.

Another plus applauded by fleet specifiers is a safety, as well as a comfort, issue. When drivers are sleeping in the truck, especially during hot summer months, with the engine idling to power the air conditioning compressor, they’re not awoken by the sudden roaring of the fan as the A/C head pressure requires cooling of the condenser. Drivers actually report being more rested and alert after their time in the sleeper, a factor contributing to overall safety.

Another plus of the variable-speed drive is the decrease in the amount of garbage lodged in the radiator. The explanation from the manufacturers is that run time at full speed is extremely small, and then only in high-demand, low airflow situations. According to manufacturers, the average fan speed is 400 rpm less with a variable drive and there is a corresponding 9% reduction in the average power consumption by the fan. Particularly when maneuvering at a job site or when the truck is idling in a buggy environment, the fan is far less likely to pull debris up from the ground or out of the air and into the radiator.

Charge-air coolers need a lot less attention with the variable-speed drive, says heat exchanger manufacturer Behr. The on/off fan drive causes coolant temperature swings of 10 degrees F to 20 degrees F vs. 3 degrees or so when the variable drive is used. Those big swings result in much heavier thermal cycling of all the components in the cooling package, including the charge-air cooler.

Problems with cracking of this vital component become a thing of the past with the minimal thermal cycling of the variable-drive. And maintenance is further reduced from the softer engagement of the drive. This places a whole lot less load on the belts, so much so that they can last three years, even as fan diameters and blade numbers increase to deal with the higher heat loads of the latest lower emissions engines.