AWD via rear axle motor

Honest John Back Room » Motoring discussion
What is life like with your car? Let us know and win £500 in John Lewis vouchers | No thanks
View Threaded
Thu 2 Jun 2022 11:19
AWD via rear axle motor - movilogo

For ICE cars, AWD is typically achieved via shaft connecting front and rear axles.

But for many AWD hybrid cars, there is no driveshaft and a separate motor spins rear wheels to make it AWD when more traction is necessary.

My question is, what happens when traction battery has no charge to spin the rear motor? Does it mean AWD car becomes FWD only?

Also, when rear wheels are driven by driveshaft, it is guaranteed that they will rotate at same speed.

But if front and rear axles are unconnected, then somehow the computer needs to ensure that both axles are spinning at same speed. So this must be achieved via software then?

Seems unnecessary complication over conventional driveshaft mechanism.

Reply | Report as offensive | Link
Thu 2 Jun 2022 11:55
AWD via rear axle motor - mcb100
A hybrid doesn’t completely flatten its traction battery - it’s constantly recharging when in motion, either by regenerative deceleration or drawing power from the ICE.
Even if the dashboard display is showing ‘empty’ it’ll still function as a hybrid.
Reply | Report as offensive | Link
Thu 2 Jun 2022 13:32
AWD via rear axle motor - craig-pd130

My old BMW 225xe used exactly this system. It worked seamlessly. The car's control software never lets the main traction battery get completely exhausted: if running battery-only, the ICE started automatically when the main battery reached 6% charge, and when the ICE is running, there's a constant low level of charge going to the battery. There's also a 'charge' mode in which the ICE runs all the time, to preserve battery charge if the journey will involve entering a ULEZ at some point.

Reply | Report as offensive | Link
Thu 2 Jun 2022 14:17
AWD via rear axle motor - bathtub tom
Also, when rear wheels are driven by driveshaft, it is guaranteed that they will rotate at same speed.

But if front and rear axles are unconnected, then somehow the computer needs to ensure that both axles are spinning at same speed. So this must be achieved via software then?

The wheels will tend to rotate at the same speed, because they're on the same road. The axle driven by the electric motor doesn't lock the wheels when it's not being powered, so neither axle will tend to spin the wheels (unless one pair of wheels is on ice or similar).

Reply | Report as offensive | Link
Thu 2 Jun 2022 16:20
AWD via rear axle motor - Terry W

For many folk AWD is a very occasional need when traction is limited at slow speed - snow, slightly muddy field at the gymkhana or day out. Most vehicles with decent tyres will never need AWD.

In over 50 years and 500k of motoring I have never needed AWD, although it has been close a few times. I would tend not to go where there may be a problem anyway.

This is separate from the essential requirement where the vehicle is frequently off road, towing heavy loads, more extreme weather etc. The mechanical approach to AWD adds weight, fuel consumption, complexity, cost.

For the occasional need it make more sense to design cars with in-hub, low power electric motors, deployed at low speed only (up to 20mph?). Power assistance can be controlled with sensors, just as electric power steering retains weight in the system.

Powering the system would place insignificant load on batteries in a hybrid or wholly electric vehicle. Lighter, less complex and probably cheaper. With some ingenuity existing hubs may even be capable of redesign to allow retrofit to existing FWD vehicles.

Reply | Report as offensive | Link