PHEV propulsion tech vs. Twin motor 4WD lock ==> Peak power?

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mrqz

Well-known member
Joined
Nov 6, 2017
Messages
50
Hi guys,

There've been quite a lot of mediation regarding this topic web-wide.
However there seems to be an underlying gearing related issue, never specifically addressed and that's been bugging me for the last few months of owning my PHEV.

Here it goes: front axle and rear axle are said to be independent and driven by the front motor and rear motor/differential respectively. As there's said to be no central differential joining up the forces.

It has also been said that up to 100% of drive is on the front wheels in normal circumstances, power distribution to rear taking place only upon traction issues.

Based on the above then, how can the REAR MOTOR on the rear axle deliver it's full power without the 4WD lock mode switched on?

Isn't it that the rear motor only delivers its 82 BHP peak power when 4WD lock mode is on? In fact, I haven't experienced any differences in the acceleration when 4wd was on. But then, how can full power be applied through the given mechanism? There's must be something we (me) miss...
 
mrqz said:
It has also been said that up to 100% of drive is on the front wheels in normal circumstances, power distribution to rear taking place only upon traction issues.
This only applies to "parallel hybrid mode", when the ICE is running and propelling the front wheels directly. But even then, when power provided by the ICE is not meeting power demand, the rear E-motor will join in. When power provided by ICE in parallel mode + full power of rear motor is still not enough, the car will switch to series mode and both motors together will do the job*).

*) Up to approx. 120 km/h. Above that, the ICE in parallel mode can provide more power than the front motor can and series mode makes no longer sense.
 
Nor can you experience any difference in acceleration, as 4WD lock only functions up to 40 kph. It is meant as a traction-increasing device at low speed, not a power enhancer.

If you floor the pedal, all available power will be delivered to all wheels anyway, so the distribution by the 4WD lock would not add anything.
 
jaapv said:
Nor can you experience any difference in acceleration, as 4WD lock only functions up to 40 kph. It is meant as a traction-increasing device at low speed, not a power enhancer.
Most of this may be true, but the "as 4WD lock only functions up to 40 kph" bit is not. When you select 4WD mode in parallel hybrid mode, the ICE will deliver 50% less power to the front wheels and the rear motor will step in to compensate for this. One effect of doing this is that your batter will drain faster than you would think was possible, when towing a caravan :x
 
I stand corrected for the cut-out speed, but it means that total power, and with that throttle-down acceleration, actually decreases at speed if you activate 4WD lock.
 
anko said:
mrqz said:
It has also been said that up to 100% of drive is on the front wheels in normal circumstances, power distribution to rear taking place only upon traction issues.
This only applies to "parallel hybrid mode", when the ICE is running and propelling the front wheels directly. But even then, when power provided by the ICE is not meeting power demand, the rear E-motor will join in. When power provided by ICE in parallel mode + full power of rear motor is still not enough, the car will switch to series mode and both motors together will do the job*).

*) Up to approx. 120 km/h. Above that, the ICE in parallel mode can provide more power than the front motor can and series mode makes no longer sense.

That makes sense... this is what i was screwing up then. The statement only refers to the ICE motor - got it!

And thanks to all for your valuable input so far.
It seems that in terms of available power... we can comfortably say that the rear motor will deliver all its power upon there's a need (for power; not for traction, that is!)
Put it in practice: if you push on the gas at 20kph, all power that the 2 (plus ICE - if pushed hard) motors are capable of, gets capitalized every time - in 2WD mode.*

*Debate is ongoing if 4wd lock mode _decreases_ power at throttle-down acceleration as it allegedly halves power delivery of the ICE on the front wheels. (And as ICE can't propel the rear ones, it stays at 50% power delivery :?: )
 
jaapv said:
I stand corrected for the cut-out speed, but it means that total power, and with that throttle-down acceleration, actually decreases at speed if you activate 4WD lock.
don't think so. Because in a throttle-down situation you would not be in parallel hybrid mode ;-)
 
mrqz said:
anko said:
mrqz said:
It has also been said that up to 100% of drive is on the front wheels in normal circumstances, power distribution to rear taking place only upon traction issues.
This only applies to "parallel hybrid mode", when the ICE is running and propelling the front wheels directly. But even then, when power provided by the ICE is not meeting power demand, the rear E-motor will join in. When power provided by ICE in parallel mode + full power of rear motor is still not enough, the car will switch to series mode and both motors together will do the job*).

*) Up to approx. 120 km/h. Above that, the ICE in parallel mode can provide more power than the front motor can and series mode makes no longer sense.

That makes sense... this is what i was screwing up then. The statement only refers to the ICE motor - got it!

And thanks to all for your valuable input so far.
It seems that in terms of available power... we can comfortably say that the rear motor will deliver all its power upon there's a need (for power; not for traction, that is!)
Put it in practice: if you push on the gas at 20kph, all power that the 2 (plus ICE - if pushed hard) motors are capable of, gets capitalized every time - in 2WD mode.*

*Debate is ongoing if 4wd lock mode _decreases_ power at throttle-down acceleration as it allegedly halves power delivery of the ICE on the front wheels. (And as ICE can't propel the rear ones, it stays at 50% power delivery :?: )
At 20 KPH the ICE will not cut in in parellel mode. The revs would be far too low.
It would, however,start up in series mode to generate the electricity the battery cannot provide.
 
jaapv said:
mrqz said:
if you push on the gas at 20kph, all power that the 2 (plus ICE - if pushed hard) motors are capable of, gets capitalized every time - in 2WD mode.*

*Debate is ongoing if 4wd lock mode _decreases_ power at throttle-down acceleration as it allegedly halves power delivery of the ICE on the front wheels. (And as ICE can't propel the rear ones, it stays at 50% power delivery :?: )

At 20 KPH the ICE will not cut in in parellel mode. The revs would be far too low.
It would, however,start up in series mode to generate the electricity the battery cannot provide.

That's correct and that's how i meant it - series mode for the ICE and - if acceleration continues - from a certain speed it switches to parallel. (Oh, I love the PHEV way, it just plain works :) )
 
jaapv said:
At 20 KPH the ICE will not cut in in parellel mode. The revs would be far too low.
It would, however, start up in series mode to generate the electricity the battery cannot provide.
True, but odd. At 20 km/h, the battery alone can provide more power than the motors can handle without exceeding max torque :?
 
mrqz said:
That's correct and that's how i meant it - series mode for the ICE and - if acceleration continues - from a certain speed it switches to parallel. (Oh, I love the PHEV way, it just plain works :) )
As soon as battery + ICE in parallel mode can meet demand (which is at least above 60 km/h) parallel mode will kick in. As long as battery + ICE in parallel mode cannot meet demand, it will stay in serial mode (again, up to 120-ish km/h).

Interesting phenomena: when accelerating harder in the 60 - 120 km/h window, the battery will drain less quickly :lol: I have used this when towing, when maintaining SOC is more a priority than economy is. Whenever it 'threatens' to switch to parallel mode, kick the throttle a little bit harder. Consumption goes through the roof, but SOC will not be devastated.
 
anko said:
mrqz said:
That's correct and that's how i meant it - series mode for the ICE and - if acceleration continues - from a certain speed it switches to parallel. (Oh, I love the PHEV way, it just plain works :) )
As soon as battery + ICE in parallel mode can meet demand (which is at least above 60 km/h) parallel mode will kick in. As long as battery + ICE in parallel mode cannot meet demand, it will stay in serial mode (again, up to 120-ish km/h).

Interesting phenomena: when accelerating harder in the 60 - 120 km/h window, the battery will drain less quickly :lol: I have used this when towing, when maintaining SOC is more a priority than economy is. Whenever it 'threatens' to switch to parallel mode, kick the throttle a little bit harder. Consumption goes through the roof, but SOC will not be devastated.

Sry, what does SOC stand for in this case? BTW, i always thought that ICE in parallel mode is of much more help (power) than in serial... But then it seems not the case - probably due to the lower rpms? Or maybe its about the switchover from serial to parallel (short interval with no ICE drive) causing the problem when towing?
 
jaapv said:
State Of Charge.

Ahhha, so then it's all clear now... If you'd lose the charge, then what would it do with the odd ICE alone :)
thanks for the interesting point. A great example of what lies under a not-so-simple propulsion technique - even a bit more of complexity. :lol:
(I'd have a hard time explaining the mechanic though, if any of these quirks worked not exactly as expected - an incidence that should never happen)
 
mrqz said:
Sry, what does SOC stand for in this case?
Sorry about that ;-)


mrqz said:
BTW, i always thought that ICE in parallel mode is of much more help (power) than in serial... But then it seems not the case - probably due to the lower rpms?
Indeed. Below 60 km/h, parallel mode is not even available as revs would be way too low. Don't forget, you effectively only have a 5th (or 6th?) gear. At 120 and a bit km/h, an ICE in parallel mode can produce as much power as an ICE in serial mode. This is because the output of the generator is limited to 60 kW and the ICE in parallel mode at 120 and a bit km/h produces 60 kW at best as well, given the associated revs. Above that speed, the ICE in parallel mode has to upper hand.

Between 60 and 120 km/h, the ICE in serial mode has the upper hand. But as serial mode is less efficient than parallel mode (two additional energy conversions in the process) the car has a strong preference for parallel mode. Only when you drive it so hard that parallel mode + battery is insufficient, the car will switch to the less economic serial mode.
 
anko said:
At 120 and a bit km/h, an ICE in parallel mode can produce as much power as an ICE in serial mode. This is because the output of the generator is limited to 60 kW and the ICE in parallel mode at 120 and a bit km/h produces 60 kW at best as well, given the associated revs.
Between 60 and 120 km/h, the ICE in serial mode has the upper hand

Gaining momentum just to lose it again :) I'm not sure i get that - let's bring it down to Nm's.
Two E-motors have a total 332 Nm of maximum torque as per specs. Acceleration from 60 to 120: ICE in serial mode driving the generator/charging the batteries - whatever it does, it can't get out more of the twin motors than 332 Nm of torque to put on the wheels and accelerate the car. Whereas the ICE in parallel mode (even in 5th gear) can only add to this - with its own 190Nm's (even if this figure is particularly lower on lower rpms)

Where's the logic (wrong) :?:
 
A few observations that may help us understand what / how / why:

It is very popular to say "power does not matter, torque does" these days. Although this may be true when talking about "at the wheels" it is (IMHO) not true when talking about "at the engine". Because, in order to produce torque at the wheels you need power. So, we must look at how much power the ICE can send to the wheels, taking into account ICE power curve and gearing (as it may dictate ICE revs).

Although we have a single gear transmission when in parallel mode, the car effectively transforms into one with a CVT when in serial mode. There is no longer a fixed relation between ICE speed and wheel speed. In serial mode, the ICE can run whatever amount of revs it sees fit, where in parallel mode the amount of revs are directly related to vehicle speed.

At 120 km/h in parallel mode, the ICE will do approx 3250 RPM (1000 RPM per 37 km/h), sending max 60 kW to the wheels. At lower speeds, the amount of power that can be sent to the wheels comes down with speed (almost linearly, as we have a relatively flat torque curve). At higher speeds, the ICE can send more power to the wheels (up to 89 kW at 170 km/h).

At 120 km/h in serial mode, the ICE can do 4100 RPM and produce 70 kW of mech power, which results in 60 kW of electrical power (after losses in the generator). So, at 120 km/h the same amount of power can be sent to the wheels as in either mode. But at lower speeds, the ICE can still send the full 60 kW of power to the wheels. On the other hand, at higher speeds, the ICE can still send only 60 kW to the wheels.

So, below 120 km/h, the ICE can send most power to the wheels when in serial mode. Above 120 km/h, the ICE can send most power to the wheels when in parallel mode. Therefor, below 120 km/h the car will switch between modes depending on power demand, where above 120 km/h the car will always select parallel mode (as serial mode only adds losses without offering extra power).

Now let's add the battery to the equation. Indeed the E-motors together can deliver the 332 Nm of torque but they are also limited to a total output of 120 kW.

Up to 60 km/h, the motors are restricted by their torque limits. Power will go up linearly from 0 to 120 kW (ignoring the fact that E-motors have low efficiency at low revs). Above 60 km/h, the motors are restricted by their power limits. Power will stay constant at 120 kW, but torque will linearly go down from 332 Nm to .... half of that at 120 km/h.

Max available power is:
- between 0 and 60 km/h: 0 - 120 kW (serial mode)
- between 60 and 120 km/h: 120 kW (serial mode)
- between 120 and 170 km/h: 120 - 149 kW (parallel mode)

Or:
- between 0 and 60 km/h: 0 - 120 kW (serial mode)
- between 60 and 120 km/h: 90 (*) - 120 kW (parallel mode)
- between 120 and 170 km/h: 120 - 149 kW (parallel mode)

(*) Estimating about 30 kW output max from the ICE at 1625 RPM.
 
anko said:
.....

Although we have a single gear transmission when in parallel mode, the car effectively transforms into one with a CVT when in serial mode. ...

....

At last!!! :D
 
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