Behaviour of non-full-time FWD-based AWD systems. When does the AWD kick in?

[Eug]

Driving this FWD-based AWD RAV4 Hybrid around, I was surprised to learn that EVERY TIME the car accelerates from a dead stop, it turns on AWD. It doesn't matter if it is slush or dry pavement or not, AWD is used. Then once it gets up to moderate speed, it shuts off AWD and runs in FWD mode.

I wasn't expecting this behaviour. I expected it to stay in FWD mode until the car's computer noticed the front wheels slipping, and then would turn on AWD to help. This is the impression of how these systems worked at least on some older traditional mechanical AWD systems on cars that were predominantly FWD.

It should be noted that the Hybrid's AWD design is different, because the rear wheels are powered by a separate rear electric motor. There is no mechanical connection between the front engines and the rear wheels.

Is this a fundamental design difference of the Hybrid AWD, or are newer gas-only non-full-time AWD vehicles behaving the same way these days?

 

[SearchMaster]

It should be noted that the Hybrid's AWD design is different, because the rear wheels are powered by a separate rear electric motor.

I know nothing of the RAV-4 Hybrid but I think that's your answer. The primary reason for a hybrid is to offset the MPG losses when accelerating from a dead stop/near dead stop. That's when a gas engine is least efficient, so the electric motor takes over.

So if the rear wheels have a separate motor designed to improve the vehicle's efficiency from a dead stop, why wouldn't it use it every time?

 

[Eug]

It seems I'm behind the times. I'm told this has been a feature of some other AWD cars as well in recent years.

http://www.wheels.ca/car-reviews/2013-chevrolet-trax-a-compact-power-pack/

The Trax AWD system is interesting. Engineers thought that if the vehicle started from a stop in front wheel drive and then encountered slippery conditions, when the AWD kicked in, it might be noticeable and bothersome to the driver.

So, at every stop, the Trax starts out in AWD and if there’s no slip detected on any of the wheels, it unobtrusively shifts itself into FWD at 3 to 5 km/h for less drag and optimum fuel economy. In the AWD model I drove, this was totally undetectable. Nice.

- See more at: http://www.wheels.ca/car-reviews/20...act-power-pack/#sthash.jhu9XhtP.LqDMZxNx.dpuf

I didn't think of the fuel economy advantage though. Good point there too.

So there are several potential advantages to this approach. With other cars there would be a fuel economy hit, but with the RAV4 Hybrid, there could actually be a fuel economy benefit with this approach. However, the prevention of slippage (which commonly occurs when accelerating from a complete stop) is probably the most important one. I guess the other advantage is the performance boost from utilizing both electric engines in a situation where torque from a gas engine is low.

 

[LTC8K6]

This is what is irritating about the Jeep Cherokee and it's variants. When the rears engage, it's very noticeable.

My friend's 2014 V6 Cherokee will chirp the fronts if you hammer it from a stop, then the rears come in with a noticeable bump.

It's taking off in FWD and then trying to engage the rears, which doesn't work very well, imo.

http://www.gizmag.com/jeep-cherokee-ecotrac-disconnecting-all-wheel-drive-system/30211/

 

[LTC8K6]

Typically, there are also other sensors which will engage AWD, such as an inclinometer and a rain sensor. Both are present on the Cherokee.

 

[ChronoReverse]

Does it give a jerk in the RAV4 hybrid? Because if not then that's the way I would have designed it since the rear motors are electric-only.

For hybrids, it makes sense to maximize electric use at low speeds and acceleration then transition to the engine as speed levels off.

 

[Eug]

No, I can't feel the rear wheels engage. The only reason I know it's working is because there is a screen telling me where the power is going.

In this picture I stole off the internet, you'll see the yellow lines indicating battery power is going to the front and rear wheels. The gas engine is completely off here (although in many cases the gas engine will be on too - depends on the acceleration).

Well, it's not the only way I know it's working. The other hint is that it doesn't slip in wet conditions when accelerating from a stop, whereas my FWD car with the same tires occasionally does if I'm not delicate with the gas pedal.

Also, when the AWD shuts off, say once I hit about 30 km/hr, I can't feel that either. Completely seamless transition.

 

[ChronoReverse]

That's really nice. If only Toyota did a sedan sized AWD hybrid in Canada. I'd be all over that.

 

[Yuriman]

I wouldn't say it's strictly true that gasoline engines are "less efficient" when accelerating from a stop. Gas engines are able to convert gasoline into mechanical energy most efficiently when running at high load at a relatively low RPM (let's say 70-90% load at 1750-2500RPM, depending on your displacement). Electric assist lets you accelerate quickly without revving the engine up (keeps you in the most efficient RPM range regardless of your power demands), or allows you to have a smaller engine while maintaining the same power (smaller engine = less pumping losses, less friction, runs at a higher percent load at a given RPM). This is an oversimplification, but essentially, Toyota embraced the "keeping the engine at optimal RPM" approach, with their hybrid synergy drive, with the 1.8L engine basically always running at 2000RPM and varying power by either assisting or dragging with the electric motor, while Honda downsized their engines. Because of this, you find Toyota's hybrids pulling ahead in stop-and-go situations where you'd normally be revving your engine up a lot, while Honda's hybrids tend to perform better on the highway, where the smaller engine shows the most benefits.

It's likely that AWD can't engage in your RAV4 above a certain speed because the electric motor in the rear is geared to provide a lot of torque at low speeds, and is disengaged at higher speeds due to RPM limits (because of Toyota not wanting the added complexity of a second gearbox in the rear).

 

[skyking]

The honda AWD system in our old Pilot was on from the start, and phased out completely at 30 MPH. I can confirm this because many times over the years I could safely do 35~40 on snow covered mountain passes, but I could never get there
You could feel the rear drive fade away. It was mildly annoying at times.

 

[malbojah]

On know on my Element the AWD kicks in when a speed difference is detected between the front and rear wheels. I practice in the snow as much as possible, sliding the front then having the rear come on the flip the back end around.

 

[brainhulk]

 

[Eug]

The honda AWD system in our old Pilot was on from the start, and phased out completely at 30 MPH. I can confirm this because many times over the years I could safely do 35~40 on snow covered mountain passes, but I could never get there
You could feel the rear drive fade away. It was mildly annoying at times.

What model year?

 

[LTC8K6]

Most of these systems are still active at higher speeds and will still engage the rears or fronts if slippage is detected. Not sure why the Pilot was not doing so, I think it should have been.

My Jeep is always driving all 4 wheels with a slight rear bias of 48/52. It adjusts the bias according to wheel slip or speed differences. It can go from 0/100 to 100/0 bias or anything in between. Normally with no traction problems, it is running at 48/52.

 

[gorcorps]

That's pretty common with most AWD systems these days. My Rogue did that, and all of the documentation about the AWD system was very up front about how it worked and why. My highlander not only uses the rear wheels from a dead stop, but I also believe any time it accelerates from what I've read.

 

[Eug]

Interesting. When accelerating but not from a dead stop, the RAV4 does not use the rear wheels most of the time, but sometimes does.

That would be different behaviour from a Highlander AWD then.

 

[gorcorps]

Interesting. When accelerating but not from a dead stop, the RAV4 does not use the rear wheels most of the time, but sometimes does.

That would be different behaviour from a Highlander AWD then.

I don't know if it ALWAYS does it, but from the way the manual describes the function it'll try to use the rear wheels during acceleration. It doesn't say if it's only fast acceleration or anything, so I'm not positive what the cutoff may be (or even how much of the rear wheels it'll try to use).

 

[skyking]

What model year?

05

 

[Eug]

I don't know if it ALWAYS does it, but from the way the manual describes the function it'll try to use the rear wheels during acceleration. It doesn't say if it's only fast acceleration or anything, so I'm not positive what the cutoff may be (or even how much of the rear wheels it'll try to use).

I was traveling on a 60 km/hr road at about 50 km/hr and decided to floor it just to see what happens with the rear motor.

With moderate acceleration, the rear motor did not kick in. But when I put the pedal to the metal, the rear motor activated too to provide that extra boost. I didn't feel a transition. I just felt faster acceleration.

I have yet to try the highway though.

I will also have to try a hill. I live on a hill but it's a residential neighbourhood so I don't want to accelerate too much there. However, I've now seen a Toyota video that says that on an incline if necessary, it can activate the rear motor to provide additional torque, proportional to the degree of incline.

These are all scenarios where AWD can be activated, with slippage never having to occur. Cool.

P.S. I didn't even know inclinometers were a thing for cars until yesterday.

 

[Kelvrick]

So after reading this thread and doing no other research, is the basis of Toyota calling the Rav 4 AWD the fact that the rear tires can be engaged by the electric motor? There is no transfer of power from the gasoline engine to the rear wheels, and the most power the rears can/will ever get is the maximum available from the rear electric motor.

This would be completely different than traditional gas only based AWD systems that transition power through the use of differentials. Frankly, the power bias of traditional engine-only based AWD systems all depends on what the manufacturer's goals are. Usually, "performance" based models will have power going to all four wheels, but will have more of it going to the rear.

I wonder how strong the front and rear motors in the Rav4 system are and if one is stronger than the other.

 

[Eug]

Yes. The rear electric motor is completely independent and the rear wheels can only derive power from that rear motor. It is physically impossible for the rear wheels to get power from either the front electric motor or the gas engine, since there is no mechanical connection from these to the rear wheels. These strictly are for the front wheels.

The rear motor is 50 kW or 67 HP. Dunno torque numbers but low speed torque should be good because it's electric. Front electric motor is 105 kW or 141 HP and front gas engine is 2.5 L 150 HP. Combined HP for entire car is 194 HP (or 197 HP according to some literature).

So no, don't go off-roading over big boulders with this thing, because the rear motor just isn't strong enough. It was not designed with this purpose in mind. However, IMO it is a good design for urban & suburban AWD in slippery conditions.

 

[Brian Stirling]

In a decade or so when there's more hybrids and in particular, second generation hybrids with no mechanical drive train and an electric motor at each wheel you'll have, for the very first time, TRUE 4WD. None of the AWD or 4WD vehicles on the road are TRUE 4WD as the best they can manage is some degree of limited slip. But, when you have an electric motor at each wheel and the sensors that go with it then you have TRUE 4WD.

I'd put the best current 4WD system at more like 3WD in actual performance so when all four wheels can control torque with very fine resolution and fast response then you get actual 4WD. You also save about 10% that is due to the losses in transfer from the engine to the wheels due to various reductions and gear losses as well as pumping losses in autos. Yes, there's loss in the electrical delivery system, but no where near the losses with a mechanical drive train.


Brian

 

[MongGrel]

Glad I don't need it really these days, I guess.

 

[thomsbrain]

Toyota's use of rear torque in their "AWD" hybrids from a stop has nothing to do with traction and is purely a function of maximizing the electric power to the wheels while the sleeping engine wakes up. On a hybrid with "AWD," there's going to be all kinds of weird behavior on the electric-only axle that has nothing to do with traction and is just about energy management.

The Honda CR-V sends power the rear wheels during all starts as a way of minimizing suspension pitch. Again, nothing to do with traction.

Other than that, most FWD-based systems send power rearward in reaction to loss of traction at the front. Depending on mechanical design and software the reaction times, speed ranges, torque splits and the ability to appropriately move torque when one or more wheels is completely unloaded varies.

Symmetrical AWD systems send torque 50/50 at all times until a wheels slips, then they can shuffle it around. Subaru's systems cannot appropriately move torque when a wheel is completely unloaded (all power goes to the spinning wheel).

 

[nk215]

Lexus RX450h (2015) does the same thing. From a stop, it will engage the rear motor regardless of the road condition. Hard acceleration - not from a stop - does not engage the rear motor on dry roads.