'Coasting' - What's Happening?

[Claymore]

There has been some discussion about 'coasting' in this thread;

http://www.myoutlanderphev.com/forum/viewtopic.php?f=9&t=1976 and elsewhere (can't find the posts again) but searches haven't produced any further info for me, maybe I should just wait a few more days and read the manual.

I seem to remember someone (Anko is prime suspect :idea: ) saying that even when 'coasting' in B0 some energy will still flow from the batteries to negate the drag from the electric motors, and whilst lying in bed thinking about my long coasts in neutral with my Skoda Superb ( :roll: ) with no drag from the engine (which was still idling nevertheless) I couldn't rationalise how you could actually be 'coasting' if the E motors were still connected and being turned by the wheels so the question is;

Are the E motors always mechanically connected to the wheels either directly or via the CVT (another question)? or can they be physically disconnected by (say) moving the CVT selector to 'Neutral' instead of leaving it in 'gear' at setting B0?

I hope this question does not set a new low standard of my ignorance of the car.

Jim B

 

[jaapv]

There is no CVT, the electric motors are always coupled to the wheels. The ICE couples and decouples through a clutch. Again no CVT or such.
There is no CVT selector either. The thing that looks like one is just an electric switch.

 

[Neverfuel]

There is no CVT, the electric motors are always coupled to the wheels. The ICE couples and decouples through a clutch. Again no CVT or such.
There is no CVT selector either. The thing that looks like one is just an electric switch.

Just to add a bit to that, the electric motors are also used to provide REGEN. When a motor is spinning with no power to it, it becomes a generator, a spinning motor, even with power applied to it tries to create a current as well. It is known as "induced or back EMF". As the motor spins faster, the induced current becomes larger, so there is a point where the power from the motors drops off quite rapidly. A small current is applied to offset this when coasting, or when the engine is being used to drive the front wheels in parallel hybrid mode. From memory I think it is 2.5kw.

 

[anko]

Indeed, like Jaap says, no CVT whatsoever. The amount of energy used to make the E-motors drag-free depends on your speed. The faster they spin, the more power is needed. Whether this is linear … don’t know.

This image shows how during coasting energy is taken from the battery for this purpose:

Scenario: While driving aprrox. 100 km/h in B2 take your foot of the pedal and then switch to B0.

Taken from the top down on the left most side of the graph you see:

- Orange line is speed reducing
- Blue line is RPMs reducing
- Red line is engine output, dropping to 0 almost instantly when I take my foot off the pedal
- Green line is SOC (not relevant here)
- Thick blue line is fuel consumption, dropping to approx 2.2 l/100 km when I take my foot of the pedal
- Teal line is battery input kW, dropping from about +24 kW to -3.1 when I take my foot of the pedal, and slowly going ‘back up’ to -2.1 as speed reduces and les power is needed to eliminate E-drag.

This tells us that when coasting fuel is used to overcome the internal resistance of the ICE, but no output is delivered by the ICE. And battery power is used to overcome internal (E) resistance of the E-motors. But again, no output is delivered.

Based on the collected data I could have added more traces showing that the generator took 0 kW from the engine, so is not generating any power. And that both E-motors are not outputting any mechanical power. But are taking little electric power.

Where the amount of power taken from the battery is less at lower speed, fuel consumption kind of stays the same. This is (IMHO) because fuel consumption is already expressed in terms of l/100km. Had it been l/h, than it would have gone down.

BTW: had I lifted my foot while in B0 and not hit the brakes either,it would have been a totally different picture. As the engine would have continued to produce output, used to recharge the battery.

 

[Claymore]

Thanks for all of your replies.

Maybe I should have titled this 'Coasting for Dummies' - and being one of the 'dummies' (prob non PC now) is the reason I didn't post in the 'tech' section.

Maybe I should say that I've had a fair bit of experience with lower powered electric power systems including various iterations of Lithium cells if only through fairly extensive use of electric powered r/c model aircraft and e - bikes but do understand many of the principles of battery, motor and switch technology albeit at a fairly basic level but with this car every time I think I'm about 80% there (big fan of the Pareto Principle) I take the proverbial two steps back...

......so in Anko's example shown above, along with the other answers provided, I'm fine now with the knowledge that the motors are permanently connected (and possibly at least one reason why the car is speed limited) and also understand the principle of the reason for the increasing drag of the motors with increasing speed and the consequent desire to counter that (even if I'm not convinced that that is an exercise in futility unless the 'cost' of the drag is more than the 'cost' of defeating it).

If the example had been one of pure EV state at 100 kph (which I believe the car is capable of given the right external circumstances and sufficient SOC), being driven in B0 and taking the foot off the pedal then electromechanical interactions described above are easy to follow.

There are a couple of points in Anko's example that niggle away at me (only in the sense that I don't understand the affect they have on the outcome;

1. In the example you start the coast in B2 then select B0. Would it make any difference of you had been in B0 when you started the coast other than the fact that in the space of time in between the change there would momentarily be some regenerative braking?

2. In the example, the ICE motor is running. Was that because the car was using ICE power for the drivetrain or in 'Charge' or 'Save'? If the ICE was powering the drivetrain and CVT is left in B0 is there also engine braking or is the CVT programmed to physically disconnect the ICE from the drive creating a real 'neutral' condition?

I apologise for all of these stupid questions and in doing so wonder how any of you guys actually get any work done - I have one image of a dentist drilling with one hand and tapping out messages on his iPad with the other....but I don't want to start more thread drift!

Thanks to you all,

Jim B

 

[anko]

1. In the example you start the coast in B2 then select B0. Would it make any difference of you had been in B0 when you started the coast other than the fact that in the space of time in between the change there would momentarily be some regenerative braking?

It turns out there are two modes of coasting in parallel mode:

A - Engine idles along and battery is used for eliminating E-drag (see graphic)
B - Engine keeps making an effort in charging the battery (and does so at a pretty high rate as no engine power is needed for propelling the car)

As long as you are in B0, you can switch between these two modes by briefly hitting the gas pedal (switches from A to B) or the brake pedal (switches from B to A).
When you are in B1+ when you start coasting, you wil always be in mode A. In order to switch to mode B, you must first select B0 and then briefly hit the gas pedal. This is for example what happens when you cancel out of CC, as CC implies B2 or higher.

2. In the example, the ICE motor is running. Was that because the car was using ICE power for the drivetrain or in 'Charge' or 'Save'? If the ICE was powering the drivetrain and CVT is left in B0 is there also engine braking or is the CVT programmed to physically disconnect the ICE from the drive creating a real 'neutral' condition?

It was in Charge mode. During the same trips I did some other tests, trying to 'reveal' the behaviour in the car when coasting in parallel mode with various B-levels. Unfortunately, traffic was way to dense to get clear pictures, but what I got out of it does answer your question, I think:

In this section, I lifted the gas pedal at 100 km/h in B0, B1, ... B5. Funny enough, I cannot remember which order I did these and the picture is not clear enough to tell. But it does show that engine RPM (blue line) and speed (Orange line) stay synced all the time, so the engine / generator assembly is not decoupled from the front axle.

BTW: I did replace the green SOC trace with a gold coloured Generator trace, which shows the amount of mechanical kWs absorbed by the generator, which is a rough measure for the amount of electrical kWs generated.
BTW2: fuel consumption stays relatively high during this test (5.1 l/100km is the minimal value). Which doesn't line up with my previous post where I explained / showed that the engine would idle when coasting in B1+. Need to think this over ..... in the Technical Section.
BTW3: please stop saying CVT

 

[Claymore]

:lol: Ha! I promise to write out 100 times "Single Gear Automatic". But will wait a while in case that's wrong too :?

Whenever my kids try to blame someone else for something I always tell them, when they are 'pointing their finger', to look where the other 3 are pointing (the 5th one is a thumb) so I'll do the same whilst blaming all the posts in the 'Tech Section' quoting 'CVT' which I have absorbed whilst skimming through thousands of posts.

More thinking time required.

Thanks again.

JimB

 

[anko]

I know, I know. I have added to the confusion by writing something like "in essence we have the optimal electronic CVT", referring to the fact that in serial mode there is no fixed relationship between engine RPM and vehicle speed. Sorry for that

Back in 2013, a Dutch magazine wrote about a "CVT like experience". Many others copied there "CVT" but left out the "like". We (PHEV-drivers) ended up being told by non-PHEV-drivers that we did not know what the setup of the car was, because all the magazines told them there was a CVT .....

In case you want to know, this is what we have were many others have there CVTs:

You see a belt somewhere? Or a rubber? :lol:

 

[jaapv]

(and possibly at least one reason why the car is speed limited)

The speed limitation probably helps with the inherent back-EMF power loss of the electric motors and avoidance of mechanical stress on the components at high revolutions, but the real reason is that at top speed the ICE is directly connected to the drive train and is rev-limited.

 

[Neverfuel]

(and possibly at least one reason why the car is speed limited)

The speed limitation probably helps with the inherent power loss of electric motors and avoidance of mechanical stress, but the real reason is that at top speed the ICE is directly connected to the drive train and is rev-limited.

And the motors are limited to 14000rpm on the front, 10000 on the rear - both permanently connected to the wheels through reduction gears. Please see a full explanation here:

As Anko says, the motors and engine are the same as are the tyre sizes.

But it might be possible if the primary and secondary reduction gears have been altered in the new model. This would give a faster acceleration and / or a higher top end speed.

In the old model we know certain facts:

1. The maximum RPM of the engine is 4,840.
2. The maximum RPM of the front motor is 14,000
3. The maximum RPM of the rear motor is 10,000
4. The maximum RPM of the generator is 14,000
5. The final reduction gearing for the front motor is 9.663 (2.55 X 3.789)
6. The final reduction gearing for the rear motor is 7.065 (1.956 X 3.611)
7. The final reduction gearing between engine and front transaxle is 3.425 (0.903 X 3.789)
8. The final reduction gearing between the engine and generator is 2.736
9. The speed is limited to 106mph
10. The tyre size is 225/55/R18

If we work out the diameter of the tyre we can work out its circumference. This will tell us the RPM of the transaxles:

(Diameter of the rim 225mm X Aspect ratio 0.55) X 2) = 247.5mm (Convert to inches / 25.4) = 9.74 inches add the radius of the tyre +18 = 27.74 inches diameter.

Convert this to a circumference:

27.74 inches diameter X PI (3.142) = 87.17 inches / 12 (convert to feet) = 7.26 feet.

So the car travels 7.26 feet for every revolution of the wheel.

As the car is limited to 106mph we can now work out the maximum RPM of the wheels and therefore of both transaxles:

106 (MPH) X 5280 (Feet per mile) / 60 (seconds in a minute) = 9328 feet travelled.

If we divide the feet per minute by the distance travelled in 1 revolution of the wheel we get the maximum RPM of the wheels (transaxles):

9328 / 7.26 = 1285 RPM (at the wheels).

Therefore:

Front motor RPM at max speed = 1285 X 7.065 (rear motor reduction gear) = 9071 RPM, 928 RPM below its limit of 10,000 RPM.

Rear motor RPM at max speed = 1285 X 9.663 (front motor reduction gear) = 12,417 RPM, 1583 RPM below its limit of 14,000 RPM.

Engine RPM at max speed = 1285 X 3.425 (Engine to differential reduction gear) = 4,401 RPM, 439 RPM below its limit of 4,840 RPM.

Generator RPM at engine RPM of 4,401 = 4401 X 2.736 (engine to generator reduction gear) = 12,041, 1,958 RPM below its limit of 14,000.

If we now used the upper limits of the engine, front & rear motors, and the generator, what would be possible?

Front motor RPM limit 14000 / 9.663 (front motor reduction gear) = 1448.83 RPM at the wheels. 1448.83 X 7.26 (distance travelled in one revolution) = 10,518 feet travelled. 10,518 feet X 60 (seconds in a minute) / 5280 (feet in a mile) = 119.52 MPH. So, to operate the front motor at its limit of 14,000 RPM the car would have to travel at 119.52 MPH.

Rear motor RPM limit 10000 / 7.065 (rear motor reduction gear) = 1415.42 RPM at the wheels. 1415.42 X 7.26 (distance travelled in one revolution) = 10,276 feet travelled. 10,276 feet X 60 (seconds in a minute) / 5280 (feet in a mile) = 116.77 MPH. So, to operate the rear motor at its limit of 10,000 RPM the car would have to travel at 116.77 MPH.

Engine RPM limit 4840 / 3.425 (engine to differential reduction gear) = 1413.14 RPM at the wheels. 1413.14 X 7.26 (distance travelled in one revolution) = 10,259 feet travelled. 10,259 feet X 60 (seconds in a minute) / 5280 (feet in a mile) = 116.57 MPH. So, to operate the engine at its limit of 4,840 RPM the car would have to travel at 116.57 MPH.

As the car is always driven by its engine at top speed, we can conclude that the front and rear motors will always be operating within their limits even if the car were to operate at the maximum engine RPM of 4,840. The generator would be disconnected anyway, but what if it wasn’t?

At 4,840 RPM of the engine the car would be running 2.736 times faster due to the engine to generator reduction gear. 4840 X 2.736 = 13,242 RPM, 758 RPM below its limit of 14,000.

Okay, what did that prove?

Well, if the new car has got different primary or secondary reduction gears, from the ones we have, the balance of acceleration, top end speed or both could be changed quite significantly - without having to make changes in the specs of the engine, front or rear motors or the generator. As all we have to go on at the moment are the published specs of the new model, but have got more detailed information on our model - we can only wait and see.

Maths lesson over, I’m off for a cuppa.

 

[Claymore]

It's a nice picture Anko but unfortunately it doesn't illuminate me.

I suspect my grey matter is now similar to the PHEV drive battery at close to 100% SOC where any further attempts to charge will be inefficient at best and futile at worst (and most likely).

Jim B

BTW, here's my favourite vehicle with a very nice Continuously Variable Transmission (Suzuki Burgman 650);

 

[jaapv]

Indeed, and come to think of it, 4850 Rpm is rather low for a petrol engine to be red-lined. The unit should be long-lived.
The engineers seem to have allowed for a 10% exceeding of the maximum speed when running down an incline.

 

[Neverfuel]

It's a nice picture Anko but unfortunately it doesn't illuminate me.

I suspect my grey matter is now similar to the PHEV drive battery at close to 100% SOC where any further attempts to charge will be inefficient at best and futile at worst (and most likely).

Jim B

Nice post Claymore.

Great minds think alike, and apparently at the same time!

See my last post:

http://www.myoutlanderphev.com/forum/viewtopic.php?f=10&t=2010

 

[Neverfuel]

Indeed, and come to think of it, 4850 RpM is rather low for a petrol engine to be red-lined. The unit should be long-lived.

I don't suppose there is any other way to do it if you want to bring in parallel at 40 mph but are limited through fixed gears by the motor maximum rpm's. But as you can see from my calcs there is a LITTLE room for manoeuvre. Now just to find someone who could chip itM :twisted: