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.