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The electronically controlled all-wheel-drive system that is standard on the new Volvo XC90 is one of the most technically sophisticated systems available, characterized by its swift response and intelligent activation that delivers just the right power to the right drive wheels automatically.
In normal driving situations with good traction, the XC90 delivers most of the engine's power to the front wheels. The system only operates when it detects the front wheels have lost traction. The highly sophisticated system is able to divert between five and 65 per cent of the power the rear wheels, quickly and smoothly. In most instances, the driver is never aware when the system has intervened.
Here's how Volvo's new AWD system works:
100 times per second, the electronic control system makes an 'intelligent' assessment of information from a number of different data sources.
The system has three key parts: a hydraulic pump which is actuated by differences in speed between the axles, a 'wet' multi-disc coupling, and a control valve with electronic activation.
The whole coupling can be regarded as a hydraulic pump in which the pump housing and its (ring-shaped) piston are connected to one axle, while the piston control unit is connected to the other axle. When both axles are rotating at the same speed, no pumping takes place. As soon as a difference in speed between the two axles arises (the result of a driven wheel losing traction), the system starts pumping oil. Because the pump is a reciprocating design, its action is virtually instantaneous, avoiding the delay inherent in a slower type of pump.
The oil is pumped to a coupling piston, which compresses the multi-disc portion of the coupling, thereby 'connecting' the front axle with the rear and reducing the speed difference. From here the oil is returned to a reservoir via an adjustable check valve that controls the oil pressure and therefore the force acting on the coupling discs.
Electronic control means that the coupling can respond quickly and with variable power transfer suitable to a whole range of driving situations.
Managing the distribution of power between right and left is Volvo's Traction Control System (TRACS). TRACS operates by applying a braking action to a wheel that is spinning to increase the relative power to the wheel with the best traction. This means that the AWD system, working in conjunction with TRACS, can distribute power to the wheels that have the best traction at any given time.
On good surfaces, the electronic AWD system further enhances the car-like qualities of the Volvo XC90. The steering precision is excellent even when accelerating hard: torque steer is eliminated by the rapid response of the new system in combination with the precision steering gear from ZF.
The AWD system is controlled during parking to prevent the front and rear axles from 'competing' for power at steering angles up to full lock, ensuring easy manoeuvring for the driver. Under braking, the system is deactivated so that the brake and ABS systems can function effectively, for high stability and short braking distances.
The AWD system is also deactivated by the Dynamic Stability and Traction Control system (DSTC) when using braking intervention to counteract a skid.
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Volvo Cars of Canada Ltd.
MacDonald & Co.
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