GM gets to grips with AWD traction
BY RON HAMMERTON | 23rd Nov 2012
‘Active On-Demand’ is likely to make its Australian debut on the Opel Mokka, the Barina-based small SUV that has been undergoing engine and transmission calibration at Holden’s Lang Lang proving ground this year.
Opel Australia is yet to announce its plans for Mokka, which is already on sale in Europe and Britain under Opel and Vauxhall badges and which will appear in the US as the Buick Encore.
Mokka is known to be on the wish list of the local Opel branch which launched its operations here with Corsa, Astra and Insignia in September.
The closely related Chevrolet Trax, which is also getting Holden calibration treatment for both local and international markets, has been confirmed for the Holden line-up from about mid year, but Holden says that version is unlikely to get the magnetic AWD clutch system on cost grounds.
The AWD system might provide a point of difference between the Mokka and Trax, just as it will in North America where the up-market Buick Encore version will get the system over the Chevrolet version.
Active On-Demand AWD was developed in collaboration with Borg Warner, and is fundamentally a scaled down version of a similar electro-magnetic system employed in the bigger, luxury Buick Enclave in the US.
In a media release about the Buick Encore, GM says the system uses electro-magnets to engage a clutch at the rear axle “preemptively, when the vehicle is at rest” because wheel slippage is most likely to occur just as the vehicle begins to move.
“If no slip is detected, a computer control automatically disengages the preemptive torque to the clutch once the Encore is underway, returning the system to a primarily front-drive operation,” GM says.
This provides the best of both worlds – AWD traction when it is needed most, on take off, and fuel-saving two-wheel drive for most other driving.
Systems that rely on wheel slippage to trigger AWD engagement are less than ideal in many circumstances, such as soft sand, steep slopes or icy road surfaces.
In these conditions, breaking traction can cause the vehicle to bog down or slide before the rear wheels can share the load.
More complex and expensive systems, such as Haldex, automatically send a portion of the drive torque – say 10 per cent – to the rear wheels, to help avoid this issue.
GM says that once the vehicle is underway, sensors constantly monitor yaw rate, lateral and longitudinal acceleration, steering angle, wheel speed, accelerator pedal position and engine revs and torque, providing data to the control unit which decides when to engage the multi-plate clutch for AWD.
Depending on road conditions, up to 50 per cent of the torque can be sent to the rear axle.
On Mokka, the entire AWD system weighs just 65kg.
GM says Active On-Demand not only provides a light-weight solution for compact SUVs but provides rapid activation and deactivation, more control over the transfer of torque, more refined performance in tight turns and parking situations and better integration with safety systems such ABS and stability control.
In Europe, Mokka comes with a choice of three common GM engines – a base 1.6-litre normally aspirated petrol four cylinder producing 85kW and 155Nm, a 1.4-litre turbo petrol four producing 103kW and 200Nm, and 1.7-litre CDTi diesel that generates 96kW and 300Nm.
While the front-drive diesel Mokka with a six-speed manual transmission uses 4.5 litres of fuel per 100km on the combined test cycle, the AWD model with the same engine and transmission combination uses 4.9L/100km/h.
Opel Australia is tight-lipped about which Mokka configurations it is considering consider for this market.
