How much max power really goes to the rear wheels?
 

Hi all.

I just read something on Edmunds.com that got me wondering about the AWD system of the 08 Highlander. I was hoping that you could comment on your experience driving in the snow/ice, specifically driving up/down hills. I'm especially interested in this statement: "There's no center differential and the V6 engine never provides power to the rear wheels".

If possible, it would really put it in perspective for me if you could compare it to other AWD systems you may have tried before, such Audi's Quattro, Volvo's Aldex (SP?) or Subaru's.

This is part of the articlle I found on Edmunds:
"Given the presence of the rear-mounted electric motor, the Toyota Highlander Hybrid is technically a four-wheel-drive vehicle. Most of the time, though, it functions in front-drive mode; the electric motor engages the rear wheels only when extra traction or torque is needed. Bear in mind that this setup differs significantly from the 4WD/all-wheel-drive system on the regular Highlander: There's no center differential and the V6 engine never provides power to the rear wheels. The upshot is that buyers shopping for a serious snow vehicle may not find the hybrid Highlander robust enough to meet their needs.".

This is the link to the article: http://www.edmunds.com/toyota/highla...08/review.html

Thanks!

Re: How much max power really goes to the rear wheels?
 

The rear electric motor develops 67 hp and 96 foot-lbs of torque. That 96 ft-lbs is available all the way down to 0 rpms because electric motors develop full torque at stall unlike a gas engine. This is enough torque to wildly spin the rear tires in a deep snow condition if not moderated by the traction control.

You don't need a lot of torque in snow conditions. You need traction and ground clearance for deep snow. Get some good chains and store them in the vehicle just in case and you will be fine unless you slide off-road into a ditch or unless the drifts are so high your front bumper becomes a snow plow.

Some argue that the traction control system on the Highlander Hybrid may be a bit overzealous in preventing wheel spin in low traction situations. This applies equally to the rear drive and the front drive (which has an over abundance of torque and power from both the engine and electric motor on call.

The Highlander will not let you madly spin the tires when stuck in a snow drift or on glare ice for that mater. You can press the gas pedal all the way to the floor under these conditions and the engine will ignore your input and remain idling while the wheels may very slowly rotate trying to gently capture traction. This is very disconcerting to some who like to power themselves out of being stuck by smoking all the tires while rocks snow and mud go flying everywhere.

Re: How much max power really goes to the rear wheels?
 

The rear electric motor develops 67 hp and 96 foot-lbs of torque. That 96 ft-lbs is available all the way down to 0 rpms because electric motors develop full torque at stall unlike a gas engine. This is enough torque to wildly spin the rear tires in a deep snow condition if not moderated by the traction control.

You don't need a lot of torque in snow conditions. You need traction and ground clearance for deep snow. Get some good chains and store them in the vehicle just in case and you will be fine unless you slide off-road into a ditch or unless the drifts are so high your front bumper becomes a snow plow.

Some argue that the traction control system on the Highlander Hybrid may be a bit overzealous in preventing wheel spin in low traction situations. This applies equally to the rear drive and the front drive (which has an over abundance of torque and power from both the engine and electric motor on call.

The Highlander will not let you madly spin the tires when stuck in a snow drift or on glare ice for that mater. You can press the gas pedal all the way to the floor under these conditions and the engine will ignore your input and remain idling while the wheels may very slowly rotate trying to gently capture traction. This is very disconcerting to some who like to power themselves out of being stuck by smoking all the tires while rocks snow and mud go flying everywhere.

Re: How much max power really goes to the rear wheels?
 

So about 25 to 30%. Probably sufficient for most situations.

Now a stupid question from a beginner in hybrid technology. Is max power from the electric motor only reached when the battery at 100% of charge? In other words, if the battery is at 50% of charge, am I only getting 50% hp/torque from it?

Thanks!

Re: How much max power really goes to the rear wheels?
 

Voltage, mostly, stays at a constant level as a battery discharges. It is that voltage which determines how much power is available, so the system should be capable of delivering close to %100 at all times. The battery is never allowed to fully discharge anyway. The system imposes artificial limits to maximize battery life.

Also, don't forget that the power doesn't have to come from the batteries, but can come from the generator as welll (in fact that is more efficient if the motor is already running and producing power).

Re: How much max power really goes to the rear wheels?
 

Thanks guys, that's good information.

By the way, I tried the GMC Acadia yesterday. I heard they are going have a hybrid version based on what they are doing with the Yukon. The Acadia is quite a bit bigger than the HH and unfortunately the driving dynamics feel like it. Interior finish isn't as nice as the HH but let me tell you that GM came a long way in the last few years. I like the 60/40 split 3rd row seats and bigger cargo area with the 3rd row up. Of course, power is nowhere near what the HH feels like. Hopefully the hybrid powertrain will really improve that.

Re: How much max power really goes to the rear wheels?
 

[quote=Don R;156435]The rear electric motor develops 67 hp and 96 foot-lbs of torque. That 96 ft-lbs is available all the way down to 0 rpms because electric motors develop full torque at stall unlike a gas engine. This is enough torque to wildly spin the rear tires in a deep snow condition if not moderated by the traction control.



Although the rear motor is theoretically capable of developing 67 HP and 96 ft-lbs torque, the HiHy's traction battery is a 45 KW/60 HP max sustained output unit, and this is the maximum electric assist possible between the two motors. Thus the total HP rating of 268, with 208 delivered by the engine.

I suspect the reason the front motor's capacity is so large (123 KW/165 HP) is for reliability as it is used so frequently. And maybe this makes its ability to start and spin the ICE up to speed quicker when demanded, with a transient battery draw of more than 45 KW.

Pete

Re: How much max power really goes to the rear wheels?
 

Re: How much max power really goes to the rear wheels?
 

[quote=PineywoodsPete;156788]

Re: How much max power really goes to the rear wheels?
 

[quote=wwest;156839]
Although the rear motor is theoretically capable of developing 67 HP and 96 ft-lbs torque, the HiHy's traction battery is a 45 KW/60 HP max sustained output unit, and this is the maximum electric assist possible between the two motors. Thus the total HP rating of 268, with 208 delivered by the engine.

You're forgetting that the front generating capacity can be "diverted" (battery charge mode) to help power up the rear drive motor.

I suspect the reason the front motor's capacity is so large (123 KW/165 HP) is for reliability as it is used so frequently.

No, it's so that when that "motor" is acting as a generator it can more rapidly recharge the hybrid battery while simultaneously providing FULL power to the rear 67HP motor and whatever HP is needed for the other "front" motor.


Your point about oversized front motor being more related to regen capacity is well taken.

But the total sustained combined power of the two motors cannot exceed 45KW/60HP, as that is the maximum sustained electrical energy flow the battery will supply - analogous to the maximum total air/fuel mixture flow possible in an ICE, which is the limiting factor in power output regardless of engine displacement.

And that's why the 4WDi version has no more total combined HP at 268 than the FWD, in spite of the additional 67 HP motor.

Pete

Re: How much max power really goes to the rear wheels?
 

[quote=PineywoodsPete;156853]There are 3 electric motors, not two in the Hi Hy 4WD. MG1 ( in front) is used for starting the engine and for converting engine power into electric power to be shared by the other two motors: MG2 (also in front) and MGR (in the rear). MG2 in front and MGR in the rear can use the battery's 45kW PLUS the amount of electric power generated by MG1 directly from the gas engine.

Thus, both electric drive motors (MG2 and MGR) can concurrently consume much more than the 45kW battery maximum power output. The rear motor can indeed receive the full 45kw if it needs it from the battery and MG1 in combination.

The battery provides 288 volts DC at full charge. The three electric motors all operate at 650 volts AC. An inverter using high power insulated gate bipolar transistors up-converts the 288 volt DC battery voltage to an alternating 650 volts to make the motors more efficient (less resistance losses allowing smaller diameter copper wire in the motor windings).

Note that the 4wd Hi Hy accelerates faster than a 2wd Hi Hy even though the maximum HP is the same on both. The 4WD distributes the torque better to all 4 wheels so that the throttle is not as inhibited by traction control to prevent wheel spin.

Re: How much max power really goes to the rear wheels?
 

[quote=Don R;156932]
Unless I am very mistaken, MG1's AC output is connected electrically only to the traction battery through the inverter, not directly to MG2&R also. If you have different info, I'd appreciate a reference!

I don't believe you will get - in a steady state - more than 45KW electrical power however it is split between the three (or two) motors, though brief spikes could exceed that. If more sustained power was actually available on maximum demand, why would the total 268 HP spec be the same for the two versions?

ICE generated electrical energy is very inefficient, on the order of only 20% of input energy (ICE thermodynamic efficiency of 30% or less, times mechanical to electrical generator efficiency of about 70%) as I recall. So using MG1 generated power under max power situations going forward would be grossly inneficient, actually subtracting from performance due to the mechanical>electrical> mechanical energy conversion losses.

Pete

Re: How much max power really goes to the rear wheels?
 

For anyone interested, I found this article: http://autos.aol.com/cars-Toyota-Hig.../expert-review

It includes this particular paragraph about rear wheel power:

"The all-wheel-drive system in the gas models provides a full-time 50/50 front/rear torque split. In Hybrid models, the AWD system is front-drive biased, but when it detects slippage, the rear-mounted electric motor can kick in to deliver up to 25 percent of the available power to the rear wheels. Both systems will help you get the kids to school on snowy days."

Re: How much max power really goes to the rear wheels?
 

[quote=PineywoodsPete;156940]Please recheck your source. See the diagrams below from Toyota. MG1 and MGR can receive power directly from MG1 through the inverter while completely bypassing the battery.

The thermal to mechanical efficiency of an engine is typically less than 30% but all gasoline engine powered vehicles suffer from this loss so this isn't considered in the hybrid/non-hybrid efficiency comparisons.

Hybrid motor/generator mechanical/electrical conversion efficiency is much better than 70%.

Study the Toyota power split device. The engine, MG1 and MG2 and the front wheels are all mechanically connected by gears in a 4-way planetary gear set.

If any input/output to a planetary gear set is left open (freewheeling), no power can be transmitted between the other input/outputs.

In order for the engine to be able to turn the wheels, MG1 must be able to apply an engine-opposing force to the gear set which in turn generates electricity which in turn is sent to MG2 which in turn contributes positive motive force to the gear set. If MG2 could only supply 45KW of power, the engine could easily overpower MG2 and spin it backward in a partial freewheel mode preventing much power from being sent to the front wheels from the engine.

The total power sent to the front wheels equals sum total of
1) the engine power
2) less the power used by MG1 to generate electricity
3) plus the power from MG2 supplied electrically by MG1
4) plus the power from MG2 supplied by the battery
5) less the power from MG1 sent to charge the battery
6) less conversion efficiency and friction losses.



Attachment 3511
Attachment 3512

Re: How much max power really goes to the rear wheels?
 

[quote=Don R;157235]

Re: How much max power really goes to the rear wheels?
 

[quote=Don R;157235][quote=PineywoodsPete;156940]

Please recheck your source. See the diagrams below from Toyota. MG1 and MGR can receive power directly from MG1 through the inverter while completely bypassing the battery.

The thermal to mechanical efficiency of an engine is typically less than 30% but all gasoline engine powered vehicles suffer from this loss so this isn't considered in the hybrid/non-hybrid efficiency comparisons.

Hybrid motor/generator mechanical/electrical conversion efficiency is much better than 70%.

Study the Toyota power split device. The engine, MG1 and MG2 and the front wheels are all mechanically connected by gears in a 4-way planetary gear set.

If any input/output to a planetary gear set is left open (freewheeling), no power can be transmitted between the other input/outputs.

In order for the engine to be able to turn the wheels, MG1 must be able to apply an engine-opposing force to the gear set which in turn generates electricity which in turn is sent to MG2 which in turn contributes positive motive force to the gear set. If MG2 could only supply 45KW of power, the engine could easily overpower MG2 and spin it backward in a partial freewheel mode preventing much power from being sent to the front wheels from the engine.
(end quote)


Thanks for the links and more explicit diagrams than Toyota offers, Don, clarifying the dual power flow from MG1. But I can't imagine that in any situation the ICE and MG2 would be engaged in a 45 KW or more power struggle - at that level, a cumulative 90 KW/120 HP before any net motive effect. Possibly you could explain this in more detail. I thought the idea was Synergy, not Mortal Combat!

Diagram #2 suggests that MG2 is active whenever the ICE is driving the wheels, through MG1 generation, rather than only when we see the instrument icon showing MG2 activity. Is this because the MG1 input prevents freewheeling when MG2 is not in power mode? Surely this wouldn't require much energy input for a steady state speed of, say 45 MPH where MG2 is not active, and which may require only 10 or so ICE HP to sustain, with instateneous FE readout of 35-40 MPG. Where you do see a huge drop in FE is when the traction battery is depleted and MG1 is re-charging actively, where FE will fall to half that under the same conditions.

Seems to me the biggest practical result of the dual flow from MG1 is to allow more than double the power in reverse, where the ICE can't directly contribute: 45 KW from the traction battery, and an available direct 50 KW from ICE-driven MG1 to MG2/MGR. Another reason for MG2 to be so large, especially in FWD models. I recall reading that in the early Prius, limited backup power was a problem, and was battery output only, I believe.

Thanks again for your input, Don, in clarifying this complex hybrid system.

Pete

Re: How much max power really goes to the rear wheels?
 

MG1 and MG2 can be considered an electric torque converter. MG2 has copious torque at low RPMs where the engine is weak on torque. Thus under heavy acceleration, the engine is allowed to spin up in RPMs quickly, generating power in MG1 which immediately converts into high levels of low RPM torque from MG2 to get the vehicle moving quickly and more efficiently than a conventional hydraulic torque converter. In this mode, the engine is trading its horsepower for MG2's torque.

MG1 and MG2 also play the role of continuously variable transmission as they directly control the engine RPM for optimal efficiency at all speeds and power demands without the need for shifting gears and clutches found in a conventional transmission.



Keep in mind that whenever the vehicle is being powered forward under engine power, both MG1 and MG2 are always energized and double-converting power. Neither motor can take a break while the engine is providing power, as the gearset will then freewheel and power will cease flowing to the wheels.

When the vehicle is cruising at speed under light engine power, a different more efficient operating mode engages that is not described in the diagrams I provided. Rather than MG1 generating power and MG2 immediately supplying it back to the planetary gear set, the reverse occurs. MG2 generates power and MG1 immediately supplies it back as mechanical torque. Since MG2 has an extra gear reduction of 2.478 to 1 in its connection with the engine than MG1, it needs to generate less power to oppose the engine torque during steady state cruising. This reduces the amount of double power conversion taking place (mechanical -> electrical -> mechanical) for higher efficiency operation at a steady cruise.

Re: How much max power really goes to the rear wheels?
 

Thanks again for your detailed explanations; I think I finally have the big picture down now. One final question: how efficient are the new motor-generator designs in the various mechanical<--->electrical conversions taking place? My 70% figure must have come from much older AC genset designs.

Pete