ZF’s decoupleable all-wheel drive system for cars delivers up to 5% reduction in fuel consumption compared to conventional AWD
22 April 2013
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| Simulation of CO2savings using ECOnnect decoupleable AWD system. Source: ZF. Click to enlarge. |
ZF has developed an all-wheel drive system for front-transverse installation in passenger cars that, through an automatic decoupling function, can reduce fuel consumption by up to 5% in comparison with conventional all-wheel drives, according to the company. Additionally, the front-transverse architecture allows for a combination with the new ZF 9-speed automatic transmission. ZF showed the ECOnnect system, first presented at a technical workshop in October last year, at the 2013 SAE World Congress.
Currently, vehicles with classic, controllable all-wheel drive (AWD) do not have better fuel economy than vehicles with permanent all-wheel drive; even when the second axle is trailing, the driveline is active, which causes spilling and friction losses.
ECOnnect relies on the fact that all-wheel drive is unnecessary in the majority of day-to-day driving situations. For constant, straight-ahead driving, powering all four wheels presents no advantages for vehicle dynamics nor for handling. But as soon as the subsurface or the driving mode changes, there are clear advantages to having two driven axles.
All-wheel performance with simultaneously lower fuel consumption thus requires activation and deactivation of the all-wheel drive that is imperceptible to the driver and with as little fuel loss as possible, within a few hundred milliseconds.
In the front-operating 2WD ECOnnect system, the entire all-wheel driveline, from the angle drive to the propshaft and the rear axle drive, comes to a standstill. The automatic decoupling takes place in the front, directly after the main transmission, via a ring gear disconnect via dog clutch. The rear drive unit (RDU) features a twin clutch system without differential—one clutch for the left wheel and another for the right. Thus, there are no drag losses that increase fuel consumption in the all-wheel components.
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| ECOnnect system. Click to enlarge. |
To engage the rear drive, first the propshaft is synchronized, then the dog ring is engaged. The entire process, including build-up of rear axle torque takes between 190-270 ms, ZF said.
The weight-optimized construction of the all-wheel drive system also has a fuel-saving effect.
In addition to the fuel savings through complete decoupling, the all-wheel drive system also has advantages for driving dynamics. The two clutches at the rear axle enable a free transverse distribution of the drive torque between the left and right rear wheels. Consequently, through the classic hang-on idea (demand-oriented distribution of torque between front and rear axles), it is also possible to use the effect of torque vectoring. This causes the yaw moment of the vehicle to be actively influenced by individual wheel control of the rear wheels. On changing subsurface or going into curves, traction and driving stability are thus improved simultaneously.
This is all well and good, but I know from experience driving AWD systems in Alaska that waiting 190-270 ms for AWD to kick can be the difference between controlling your car when it hits black ice and calling 911 from the roadside ditch. If you really need AWD it needs to be on when your wheels start to slip, fuel economy benefits not withstanding.
Posted by: Nick Lyons | 22 April 2013 at 11:52 AM
...of course, sometimes black ice is such that you just can't drive on it safely, AWD or no...
Posted by: Nick Lyons | 22 April 2013 at 12:12 PM