One of the more challenging side effects of today's engine emissions technology is that the new engines generate more heat than their predecessors. That trend will accelerate in Tier 3 and later Tier 4.

There are also several additional complications. Engines must operate in a relatively narrow temperature range to meet the emissions levels, yet off-highway engines typically operate in the broadest range of climates imaginable, from desert heat to arctic cold. Along with basic engine cooling, there is also a growing number of other cooling loads to be dealt with--engine and hydraulic oil cooling, air conditioning refrigerant, charge-air cooling, transmission cooling, etc. These increased cooling loads must be accommodated in the same, or often less, space in the engine compartment. Finally, there are the traditional requirements of cost and reliability that must at the very least be maintained.

To address this list of challenges, one technology that has received increasing attention has been hydraulic fan drive systems. An alternative to traditional engine-mounted, belt-driven fans, hydraulic fan drive systems have evolved over the last decade from relatively simple on/off systems into sophisticated, digitally controlled units that provide several operating and packing advantages.

Perhaps the primary benefit of hydraulic cooling systems is their ability to control fan speed independent of engine speed, which allows the fan to be operated at the precise rpm needed to accommodate the thermal load at any given part of the operating cycle. This is critical in terms of overall cooling efficiency and engine fuel economy, as typical belt-drive systems can consume as much as 10% of engine horsepower.

"Cooling system designers are beginning to recognize that the major advantage of de-coupling the cooling fan from the engine shaft or pulley system is not the ability to place the radiator virtually anywhere, or move the cooling pack away from the engine to gain easy access," said Gary Gotting, global industry market manager, fan drives for Parker Hannifin's Mobile Division. "The critical facts are that a de-coupled system is the only way to achieve higher cooling rates at low engine speeds and the only practical way to achieve cooling on demand which, during 'ram air' operation saves fuel, cuts both emissions and noise generation and flees up engine horsepower to increase productivity.

"We are seeing today that a single multi-path radiator very often is used to cool multiple fluids," Gotting added. "A traditional direct-drive fan system lacks the flexibility and controllability to accomplish all of these tasks efficiently, because its performance is almost entirely determined by engine speed. And with the traditional approach, engine speed and cooling load are not generally well coordinated, with the result that a lot of fuel is wasted turning a fan at much higher speeds than necessary.

"For example, if more cooling is required at idle due to high ambient temperatures, the only way to obtain it is to increase engine speed. But, running the engine faster only adds to the thermal load on the system while increasing fuel consumption and exhaust emissions. At the other end of the spectrum, no fan-induced airflow may be needed when a vehicle is in motion, but the direct-drive fan will continue to consume engine power and waste fuel.'

The fan drive systems produced by Parker Hannifin's Mobile Division for large and midsize diesels used in both on- and off-highway applications utilize a digital controller to monitor system conditions and adjust fan speed accordingly. At the heart of the system is a Parker IQAN controller in one of two basic configurations.

Depending on application requirements, an IQAN TOCB may be used to communicate with other on-board systems via an industry standard J1939 communications Bus and integrate multiple functions such as engine and transmission temperature control. Or a simpler IQAN MDM or MDL model may be used to monitor inputs from temperature sensors in the various cooling loops and intelligently adjust fan speed as necessary to keep all systems within programmable limits.

Parker also offers other controllers, both digital and simple analog, designed to give users flexibility for retrofit or vehicle upgrades. Some of these can be directly connected to engine electronic control units and provide options including purge or fan reverse and visual diagnostics with data logging. These controllers are encapsulated in a flameproof epoxy block and ruggedized for mounting within the engine compartment or other exposed location. They are EMC compliant to EN-12895-2000 (Industrial Truck) and EN-50081-1 and EN-50082-2 (Heavy Industrial) standards, fully electrically isolated, and protected against damage from I/O shorts, opens and reverse connections, the company said.

Because fan operation is now fully programmable, a broad range of features intended to improve efficiency can be included. Examples are accelerated warm-up with the fan deactivated; "on delays," which remove tan load during hot starts; and automated and/or on-demand purge or reverse airflow through the radiator to blow out dirt and debris.