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Why are variable-speed fan drives taking over in trucking?


May 13, 2021

Pneumatic fan clutches have been standard on North American heavy-duty trucks since the industry was deregulated in the early 1980s. But a new wave of engine cooling technology is here, with implications well beyond simply regulating temperatures.

Variable-speed fan drives communicate directly with the engine ECU to regulate fan speed, providing the precise amount of cooling required — and only when necessary. This technology, which uses viscous oil between a pair of grooved plates inside the fan drive to increase and decrease friction and thus fan speed, is now standard on Peterbilt’s new Model 579 and the Kenworth Next Gen T680, and will likely be the main form of engine cooling tech introduced in the coming 10-20 years.

The question is — why?

More power to the wheels

The biggest reason for the shift is performance.

Simply put, pneumatic drives are either all or nothing. On/off fan clutches used in many long-haul applications are just that, fully on or fully off. Two-speed fan clutches, which modulate slightly but are still pneumatically controlled, are either off or on in “high-speed” mode or “low-speed” mode.

Variable-speed drives, meanwhile, provide the exact fan speed required. According to standard “fan laws” — equations that determine effects of change in air speed, fan diameter and system air density — power consumed by the fan goes up with the cube of the input speed. In simpler terms, if an engine uses twice the fan speed, it uses eight times the power.

Variable-speed fan drives do so much more than cool the engine

So meeting the exact amount of fan speed needed is crucial for fuel economy.

Turn down the volume

Noise output from the cooling system follows a very similar curve to fan power. We’ve all heard the abrasive noise when a fully-on fan kicks up in a truck or tractor.

Meanwhile, as urbanization accelerates across the globe, more stringent noise regulations are being passed. So when a machine needs cooling, rather than spinning it up to maximum fan speed and generating a lot of noise, a variable-speed drive allows it to run at varying percentages of full speed,

That doesn’t mean a fan running at 40 percent speed is 40 percent quieter, however. Going back to the fan laws, it’s orders of magnitude quieter than running at 100 percent speed.

Going green

By now, you might be able to recite the different tiers and stages of United States, California Air Resources Board and European Union emissions regulations in your sleep. What they all have in common is that they’re getting stricter.

This necessitates achieving the right engine temperature as soon as possible.

Imagine you’re driving down a road and trying to maintain speed by pushing all the way down on the throttle or all the way down on the brake. This would obviously be a lot harder than easing on and off the gas and brakes. Using a variable-speed fan drive is analogous to the latter; modulation of speed decreases strain and parasitic draw on the engine.

That means lower emissions and fuel savings in one fell swoop.

Conquering the cold

Conventional wisdom would tell you engine cooling systems are most important in warmer climates, right?


Even in very low temperatures — think a lumber operation in Canada during the dead of winter — internal combustion engines require heat rejection. But if they’re running with the fan fully on, that immediately leads to overcooling.

Understanding variable-speed fan drives

The implications are many. They include slower cab warm-up and more strain on an engine that’s already being pushed toward its limits by extremely low temperatures.

Variable-speed drives prevent putting an engine through these kind of non-optimal thermal cycles. And two in particular, Horton’s LCX and RCX Series, actually hold viscous fluid in the reservoir while the engine is off, which allows the vehicle to reach optimal temperature faster upon startup in the cold.

Increasing adoption

So there are several reasons viscous is the way of the future — technically, more like the present — for North American trucks. It’s a technology that’s been used in Europe for the past couple of decades, and until electrification fully permeates the industry (which could be decades), we’ll see more and more variable-speed drives in both OEM production and the aftermarket parts world.

One final benefit deals with that same parts business. Most variable-speed drives are preassembled and maintenance-free. So repairs will be as simple as popping in a new one, in most cases.

You can learn more about variable-speed fan drives at You can also watch back a recent webinar featuring Horton’s engine cooling experts discussing this technology.