Scania buses are internationally recognized for their high fuel economy, reliability, and low impact on the environment. However, one system in particular has been subject to renewed scrutiny at Scania AB, which is based in Södertälje, Sweden. Few passengers will ever know it, but a new standard fan drive system provided by Sauer-Danfoss has a central role in keeping up Scania’s reputation for top performance on the road.
The fan drive’s integrated electric controls eliminate the need for a thermostatic hydraulic valve, and the characteristic compactness of components allows designing systems that can be neatly tucked away as best suits each bus design.
Low energy, high control
To meet Scania’s requirements for energy savings, easier control, and quieter operation, Sauer-Danfoss has brought together its Turolla SHU2 22 cm3 open circuit gear motors with integrated check valves and a series 45 KRL 38 cm3 variable-displacement axial-piston pump with integrated electronic control valve. The Turolla open circuit gear motors are fully compliant with Sauer-Danfoss’ PLUS+1 system for easy, seamless integration into electronic control systems. The special shape and surface finish of the gear teeth provides quieter operation than with standard gear motors.
Mounted to the vehicle’s diesel engine, the pump is controlled by a signal sent from the engine’s integrated control unit to the electronic control valve. Fan motor speed for drives of this size is usually regulated though system pressure. Because a fan is a centrifugal load, its driven speed is an exponential function of system pressure. So regulating hydraulic pressure differential at the motor provides a simple and accurate method of controlling fan speed. The variable pump pressure enables the gear motor to drive the fan at speeds ranging from 500 to 2200 rpm.
Scania is the first company in Europe to use this type of system on buses, and the benefits are clear. Compared to a conventional fixed-displacement control or belt-driven system, efficiency is increased substantially, with low pressure loss and reduced pump noise. Using an electronic valve instead of a load-sensing or bypass valve further rules out the need for a thermostatic hydraulic valve, which is often adversely affected by extremes of hot or cold. The end result is a more energy-efficient control over a far wider temperature range. Moreover, eliminating the thermostatic valve and associated hoses contributes to making the system more compact and easier to assemble.
For one of Scania’s larger bus models, where the fan drive pump is driven through a power take-off (PTO), Sauer-Danfoss has developed a customized version of the axial-piston pump with its own PTO. This allows the pump to be mounted in the position normally occupied by the fuel pump. With all the engine PTO outlets in use for other bus functions, the fuel pump can be connected to the PTO on the axial piston pump instead — allowing Scania to continue using a standard engine.
Scania’s road tests routinely challenge buses with the cold climates of northern Sweden to the high temperatures and steep gradients of Spain. Operational performance and passenger comfort have proven exceptional under any condition. In that, the small but effective Sauer-Danfoss fan drive plays a part that defies its size.
Sauer-Danfoss’ gear pumps, gear motors, and hydraulic fan drives are available through its Turolla Open Circuit Gear division. Visit www.turollaocg.com for full details, including local sources. For information on piston pumps and motors and other products, visit www.sauer-danfoss.com.