By Jim Kerr
Hyundai has just introduced the 2007 Santa Fe and one of the features available on this mid-size SUV is all-wheel drive. It is a new system for Hyundai and is a good example of how electronics are improving all aspects of driving.
All-wheel drive systems are confusing for many people. I must admit to wondering myself to what type of system is being described in the sales literature. Let’s see if we can simplify it. All-wheel drive provides power to all the wheels, as opposed to front-wheel drive or rear-wheel drive systems that only provide power to one end of the vehicle. That’s clear enough, but some systems are also called four-wheel drive. Those systems also drive all the wheels, but are not designed to operate in 4wd mode all the time. The driver has to select 4wd when traction conditions are poor. But operate in 4wd mode on hard pavement and you will soon be paying for expensive driveline repairs.
All-wheel drive however, can be operated on hard road surfaces. Some systems drive all the wheels all the time. A viscous coupling or a variable clutch inside the transfer case controls the rate of slip between the front and rear axles. Subaru and Audi are examples of great all-wheel-drive vehicles that use these controls.
Many of the compact and mid-size SUVs such as the 2007 Santa Fe use a front-wheel drive system with an auxiliary rear-wheel drive to provide all-wheel drive. This is better than it sounds. Modern control systems allow the vehicle to operate with front-drive only for most driving to optimize fuel economy, but engage the rear-wheel drive as soon as additional traction is needed.
Hyundai’s new system uses a computer-controlled clutch mechanism mounted in front of the rear axle to engage the drive. It is a Borg Warner system that can provide up to 99% of the torque to the front wheels, but automatically diverts up to 50% of the torque to the rear wheels when needed. The driver can push a button on the dash as an input to the computer, commanding it to “lock” the torque transfer at 50/50 for getting out of slippery parking spots in winter or ploughing through some soft sand. While there is no low range in the Santa Fe all-wheel drive system, it is more than capable of handling many off-road excursions.
Because the torque transfer to the rear wheels is variable, a dependable, durable clutch mechanism is needed that can be instantly engaged. To do this, the computer monitors wheel speed, accelerator pedal movement and steering inputs. When 4% or more front wheel slip is detected, the rear axle starts to engage. It can also anticipate the need for additional traction and engage the AWD system when the driver accelerates the vehicle. Another feature is it can disengage the rear axle during ABS events to optimize ABS stopping.
The computer controls a large solenoid coil in the clutch housing. When energized, the solenoid pushes against a multi-plate clutch, which in turn holds a washer-like plate from turning. Ramps and balls between this plate and a second plate cause the two plates to be forced apart, placing pressure on a second larger multi-plate clutch that connects the driveshaft to the rear axle. The path of torque is complete and the rear wheels drive.
A button on the dash can lock the clutch to provide 50% torque to the rear wheels, but this only occurs below 35 kph. Above that speed, the computer pulses the solenoid to disengage the clutch mechanism, but it will automatically engage it again when vehicle speed lowers.
Finally, the system monitors steering wheel angle. Turn the steering wheel, such as when parallel parking or turning a tight corner and the computer will decrease the torque applied to the rear wheels to there is no driveline binding during the turn.
Computer controls, electric solenoids and data communication between computers are all used to provide smooth traction regardless of the driving conditions and optimize fuel economy too. That’s modern all-wheel drive.