Mitsubishi Outlander PHEV Forum

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 Post subject: Maintaining SOC while towing a caravan / under heavy load
PostPosted: Tue Jan 20, 2015 1:11 pm 
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Location: Netherlands, Utrecht area
Normally, the PHEV will maintain a minimal SOC for the drive battery (>26% at low speeds, >30% at higher speeds). If the SOC drops below these values, the car will engage in serial hybrid mode in an attempt to generate more power and recharge the battery to an acceptable level. The result is a racing engine, which is not comfortable. And worse, if this happens when your are pushing it (climbing, towing, …) the car may not be able to recharge sufficiently, not even in serial hybrid mode, eventually resulting in the so called Turtle Mode.

Normally, when driving solo, this will not happen very quickly. The racing condition may occur, but the Turtle Mode will be seen very seldom. Now, when towing a caravan it is a different situation. As soon as you start climbing, you need more power than the car can produce in parallel mode. As a result, the battery will be drained to its lowest acceptable value, serial mode will be engaged and the engine will rev up uncomfortably. But chances are you need even more power than the car can produce in serial mode. This means that the battery will continue to be drained, eventually resulting in Turtle Mode.

This is why Mitsubishi suggests to activate Save mode when you anticipate heavy towing or climbing in order to save as much SOC as possible. As a matter of fact, Save mode doesn’t do the trick, because even in Save mode the SOC may drop below the set level when extra power is needed and the lost SOC won’t be restored when you get back at normal power demand. Effectively, the set level is lowered. So, Charge mode is in order. But even in Charge mode you will not be able to maintain SOC, let alone enhance it.

Is it because our little engine is too small? I don’t think so. Of course, 89 kW is not a lot for hauling 3700 kg shaped like an IKEA warehouse. But measured over a lengthy trip, in average you need far less power than 89 kW. I fact, in average you need less than what the engine can produce in parallel mode at trailer towing speeds (approx. 42 kW). A useless conclusion for a normal car, as average power need is meaningless. Not so for a hybrid car where you can convert a decent amount of surplus power into energy to be stored in the battery, and reuse that energy later to aid the engine when there is a power shortage. And yet, we are not able to maintain a decent SOC.

This is my assessment of why we are not able. It is sophisticated guessing, based upon what I see on my OBDII scanner and more, while driving at approx. 90 km/h steady (60 mph), with the engine engaged in parallel mode (either because you are in Charge mode or because the engine is depleted). Please feel free to shoot at it.

===============================

I believe that the engine is programmed with a strong preference to output a specific amount of power, only depending on speed (or revs, if you like). Let’s call that preference the target output. This target output for a specific speed may have been determined by the engineers by adding the amount of power you normally (flat road, no wind) need for maintaining that speed plus the amount of power they believe is acceptable to throw at the battery (max charge current). This seems to be confirmed by the fact that power output of the engine goes down a notch as soon as the SOC increases to above 50% (max charge current is reduced) and some more notches when SOC increases even higher.

When power need is temporarily reduced (down hill, tail wind, lifting the throttle, …), the actual power output has to be less than the target output. Otherwise, the max charge current would be exceeded. The actual power output can also be higher than the target output when you temporarily need more power (head wind, up hill, pushing the throttle deeper, …). But, and this is crucial, before the power production actually increases, the engine will try to stay at the targeted output by reducing the charge current. Until the charge current is finally reduced to 0, only then the actual output is increased. The power normally used to recharge the battery is used to even out fluctuations in power need. Within limits, of course. As a matter of fact, I even like to believe that the Cruise Control actually controls speed by manipulating the generator resistance, not the throttle. Maybe this is why our Cruise Control is so accurate. Also, fuel consumption under CC is extremely steady.

I have created a little graph that tries to visualise the above. On the horizontal axis you see how much power is needed for driving. On the vertical axis you see how much power is produced (or drained from the battery). Normally you would expect these two to be the same. To a large extend they are. Any surplus power is used for recharging the batteries (forget about the dark green surface for now):


Image

Sorry for Dutch labels. In English they are:
- E assistance
- Extra charging
- Standard charging
- ICE in serial mode
- ICE in parallel mode

I have used the below parameters for the diagram. Some guessing was involved, but more important than their accuracy was the fact that they allowed me to draw the above diagram:

- Max power output in parallel mode at 90 km/h (60 mph) : 42 kW
- Power needed for maintaining speed: 22 kW
- Max charge current: 8 kW
- Target output: 30 kW (22 kW for driving + 8 kW for charing)

As the amount of power needed to maintain speed increases (as we travel from left to right along the horizontal axis) we see 5 'zones':

I: 0 - 22 kW: Under very good circumstances (tail wind, down hill, etc.) 8 kW goes to the battery (light green area) and less than 22 kW goes to the wheels (orange area). The target output is not achieved as it can not be consumed. The blue arrow from engine to battery is lit. Charging is maximal.
II: 22 - 30 kW: Under less optimal to normal circumstances (light head wind, slight incline, …) more and more power goes to the wheels and less and less power goes to the battery. The target output is achieved. The blue arrow from engine to battery is still lit, but the charging current has been reduced from 8 kW to 0 kW by the time we reach the 30 kW mark.
III: 30 - 42 kW: Under unfavorable conditions (strong head wind, moderate incline, …) more than 30 kW goes to the wheels, nothing goes to the batteries. Engine reserves are activated and the target output is exceeded. No charging is going on. All blue arrows are off.
IV: 42 - 102 kW: Under bad circumstances (strong incline, fast acceleration, …) more power is needed than the engine can produce in parallel mode. The E-motors will drain power from the battery and assist the engine. The blue arrow from the battery to the wheels is lit.
V: 102 - 120 kW: When it gets even worse, the engine will drop out of parallel mode and the car will be able to achieve the max output of 120 kW (max output for this speed).

As said, with just the car, you will need about 22 kW for driving, sometimes a little bit less, sometimes a little bit more. That means, in general 8 kW is being fed to the batteries, sometimes a little bit less. On the upside, you seldom need more than 42 kW, so with a somewhat adjusted driving style, you will very seldom drain power from your battery.

While towing a trailer, you will need more like 30 kW, sometimes more, sometimes less. The average power need is almost equal to the target output of the engine. This means during normal driving very seldom a little bit of charging may take place, but most of the time there will be no charging at all. To make things worse, rather often you will need more than 42 kW to maintain your speed and you will frequently drain power from the battery. An onramp of a bridge is enough to require e-assistance. Down goes the SOC!

The engine has enough power reserve to keep charging, even when 30 kW is needed for driving (42 - 30 = 12 kW reserve). But the engines preference to stick to the target output gets in the way. Had the target output been 35 kW instead of 30 kW, then you would get the same diagram, but including the dark green area. Even while towing a caravan, more / more often surplus power would be sent to the batteries. Perhaps enough to maintain SOC.

I believe just a small increase of target output is necessary to achieve this. As with the EV heating discussion, I think it should not be to difficult to add another function to the ECO button:
- ECO ON = 30 kW target output
- ECO OFF = 35 kW target output

(An extra function would be required for 2.5 meter wide caravans, as those probably require an even higher target output :mrgreen: )

When you are concerned with overloading the engine: Max output is not affected. Overal average output is not affected either. There will less periods of serial mode (so less engine racing). As a plus, the engine runs at a higher load and thus a higher efficiency. I think fuel consumption may actually go down, as you might actually end up in EV mode at some point.

Comments are welcome :-)


Last edited by anko on Wed Jan 21, 2015 5:34 am, edited 4 times in total.

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 Post subject: Re: Maintaining SOC while towing a caravan / under heavy loa
PostPosted: Tue Jan 20, 2015 2:18 pm 
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Location: Essex, England
After all that, I'm glad I only use mine on the flat, and never tow anything! ;) . Having said that, it would appear to be logical (within the set parameters - "some guessing involved"). But I think we're getting into the realms a bank of buttons labelled "Eco", "Eco/Save", "Eco/heat", "Climb", "Hyper-drive", "Warp Factor 9", etc, etc. Unless they can be incorporated into the MMCS, or software implemented.

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 Post subject: Re: Maintaining SOC while towing a caravan / under heavy loa
PostPosted: Tue Jan 20, 2015 2:24 pm 
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Joined: Tue Jan 20, 2015 2:16 pm
Posts: 191
I had hoped that there would be a definitive guide to towing when I read the title, as I pick up my PHEV next week and will be towing with it (great emphasis placed by dealer on its ability to tow 1500kg when I was looking). Pending its arrival, all I seem to read is lots of theory about how it may be possible to do this or that re SOC and wouldn't it be nice if.... but nothing concrete about how. My dealer pretty much left it at "suck it and see", which I think is all I'm left with.

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 Post subject: Re: Maintaining SOC while towing a caravan / under heavy loa
PostPosted: Tue Jan 20, 2015 2:50 pm 
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Joined: Tue Aug 05, 2014 1:32 pm
Posts: 1796
Titan wrote:
I had hoped that there would be a definitive guide to towing when I read the title, as I pick up my PHEV next week and will be towing with it (great emphasis placed by dealer on its ability to tow 1500kg when I was looking). Pending its arrival, all I seem to read is lots of theory about how it may be possible to do this or that re SOC and wouldn't it be nice if.... but nothing concrete about how. My dealer pretty much left it at "suck it and see", which I think is all I'm left with.


There were some posts about 6 months ago - might have been jaapv - boasting about how good it was at towing but I can't remember which thread - so try the search option at the top of the screen.


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 Post subject: Re: Maintaining SOC while towing a caravan / under heavy loa
PostPosted: Tue Jan 20, 2015 3:10 pm 
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Joined: Thu Dec 18, 2014 11:14 am
Posts: 46
In my short recent experience, the engine screams quite a lot when towing something over 1,000kg if it involves any kind of inclines. A 2ltr turbo diesel engine is probably a better bet for anything heavier than a trailer.


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 Post subject: Re: Maintaining SOC while towing a caravan / under heavy loa
PostPosted: Tue Jan 20, 2015 3:14 pm 
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Joined: Mon Dec 16, 2013 1:18 pm
Posts: 4111
Location: Netherlands
greendwarf wrote:
Titan wrote:
I had hoped that there would be a definitive guide to towing when I read the title, as I pick up my PHEV next week and will be towing with it (great emphasis placed by dealer on its ability to tow 1500kg when I was looking). Pending its arrival, all I seem to read is lots of theory about how it may be possible to do this or that re SOC and wouldn't it be nice if.... but nothing concrete about how. My dealer pretty much left it at "suck it and see", which I think is all I'm left with.


There were some posts about 6 months ago - might have been jaapv - boasting about how good it was at towing but I can't remember which thread - so try the search option at the top of the screen.

I have never towed a caravan in my life...I sometimes rent a little trailer when I’m clearing out the garden - a rare event.

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 Post subject: Re: Maintaining SOC while towing a caravan / under heavy loa
PostPosted: Tue Jan 20, 2015 9:40 pm 
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Joined: Wed Oct 29, 2014 8:23 pm
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Location: Dee Why
Hi,

Nice summary on how it all seems to come together. So, the generator has a maximum output of around 40kw?
I have not bought one yet and still considering all things as towing a light caravan (around 1200kg) will be on the cards.
One thing you have not mentioned is engine braking, which is obviously only done by regeneration. How will it work in the mountains on say a long steep downhill run, with the van on the back. I would assume if you approach that situation with a nearly full battery, there is trouble with smoking brakes ahead, as there can be little or no regeneration taking place.
Conversely, at what point when towing up a steep hill will the whole show stop? Do the electric motors alone (as there can be no parallel mode) have enough torque to keep things going, let alone re-start on a steep incline when required?


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 Post subject: Re: Maintaining SOC while towing a caravan / under heavy loa
PostPosted: Tue Jan 20, 2015 10:25 pm 
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Joined: Mon Dec 01, 2014 11:30 am
Posts: 3404
Location: Netherlands, Utrecht area
HHL wrote:
Hi,

Nice summary on how it all seems to come together. So, the generator has a maximum output of around 40kw?
I have not bought one yet and still considering all things as towing a light caravan (around 1200kg) will be on the cards.
One thing you have not mentioned is engine braking, which is obviously only done by regeneration. How will it work in the mountains on say a long steep downhill run, with the van on the back. I would assume if you approach that situation with a nearly full battery, there is trouble with smoking brakes ahead, as there can be little or no regeneration taking place.
Conversely, at what point when towing up a steep hill will the whole show stop? Do the electric motors alone (as there can be no parallel mode) have enough torque to keep things going, let alone re-start on a steep incline when required?

Max generator output is 60 kW (dark blue surface in the diagram).

My caravan is 1500 kg and on a trip from the Netherlands to the foot of the French alps, I encounters the turtle mode just once. But it didn't slow we down. Must say, I've been driving no faster than 95 km/h.

Correct on the breaking. But before you get to a downhill run, you must first deal with an uphill run. The only way of starting a downhill run with full batteries, is by charging at the top. That seems to be not a good idea, both from a safety and a financial perspective.

The E-motors have plenty of torque, especially at low speeds. I guess it is more / better usable than you will get from most normal cars, as it is available from 0 km/h, only to start fading away at 58 km/h. To have that amount of torque at 0 km/h in normal cars, you depend on clutches that wear off or torque inverters that run hot. At some point the e-motors may run hot and reduce their their output. Haven't seen that happening yet.

UK based Practical Caravanning have said it tows well enough, but they said it had troubles getting away on a steep incline. This goes against all theoretical models, as in theory this would be one of the very best cars to pull away under load. The EV system effectively acts as an infinitive torque inverter or CVT if you like. Also it goes against user experiences shared on the Dutch forum. I am tempted to believe they have confused the sound of a revving engine with trouble. But, then again, they are not laymen. Who will tell? I am not worried about it anyway. You may want to check out the next piece of footage. The owner said, we wasn't even on snow tires, but that is a different topic :mrgreen:
https://drive.google.com/file/d/0B1KVb1 ... MtSFk/view


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 Post subject: Re: Maintaining SOC while towing a caravan / under heavy loa
PostPosted: Tue Jan 20, 2015 11:42 pm 
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Posts: 3404
Location: Netherlands, Utrecht area
SMB wrote:
A 2ltr turbo diesel engine is probably a better bet for anything heavier than a trailer.

I do not agree with you. Compared to my previous car (which happened to be a 2.2 liter Diesel, an Outlander even) this cars saves me 200 - 400 euro before taxes + 450 euro after taxes. Every single month. It pays easily for my holidays. And more. And in the mean time, my fossil fuel consumption is down by more than 50%. If I was not willing to accept some downsides or compromises for this, indeed I should have bought another car. And so should 95% of us.

So far, the car has suited me well. Even with my 1500 kg caravan. Maybe it is not optimal, but to me it is more than good enough. I am not disappointed at all. Does that mean I shouldn't dream / think about how it could be better? I think not. Because that's what inspires progress. ;)


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 Post subject: Re: Maintaining SOC while towing a caravan / under heavy loa
PostPosted: Wed Jan 21, 2015 12:10 am 
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anko wrote:
My caravan is 1500 kg and on a trip from the Netherlands to the foot of the French alps, I encounters the turtle mode just once. But it didn't slow we down. Must say, I've been driving no faster than 95 km/h.

And how did you use the various modes during this trip? Did you drive in normal mode or utilize Save/Charge at all?

(BTW I can't seem to see your image in your first post. I've tried different browsers but all seem to just show an image tag but no link or image itself. Not sure what I'm doing wrong but obviously some setting on my computer somewhere)

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