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 Post subject: 4wd lock
PostPosted: Mon Oct 01, 2018 7:54 am 
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Joined: Wed Apr 18, 2018 5:17 pm
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I've noticed that using 4wd to climb very steep hills makes a huge difference to progress. Much less struggling, especially when starting off on a steep hill. I suspect it's because the load is being shared between 2 motors.
Is there any disadvantages to driving generally in 4wd?


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 Post subject: Re: 4wd lock
PostPosted: Mon Oct 01, 2018 7:58 am 
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Location: Netherlands
Not that I've noticed. I believe it switches off above a certain speed anyway.

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 Post subject: Re: 4wd lock
PostPosted: Mon Oct 01, 2018 10:22 am 
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Joined: Mon Dec 01, 2014 11:30 am
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Location: Netherlands, Utrecht area
seaneb wrote:
I've noticed that using 4wd to climb very steep hills makes a huge difference to progress. Much less struggling, especially when starting off on a steep hill. I suspect it's because the load is being shared between 2 motors.
Is there any disadvantages to driving generally in 4wd?
I assume you are talking about speeds < 65 km/h? Then the load is shared between 2 motors also when you do not use 4WD Lock.


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 Post subject: Re: 4wd lock
PostPosted: Mon Oct 01, 2018 10:24 am 
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Location: Netherlands, Utrecht area
jaapv wrote:
Not that I've noticed. I believe it switches off above a certain speed anyway.
When driving > 65 km/h in parallel hybrid mode, 4WD Lock causes the load to be distributed evenly over the ICE and the rear motor. This goes on at least up to 100 km/h.


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 Post subject: Re: 4wd lock
PostPosted: Mon Oct 01, 2018 5:53 pm 
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My original question was aimed at the wear and tear of using 4wd most of the time; is the rear motor designed so that it will not be damaged with prolonged use or is it just meant to be used occasionally for short periods? Is there any effect on fuel and charge consumption over 2wd also?


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 Post subject: Re: 4wd lock
PostPosted: Mon Oct 01, 2018 9:11 pm 
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Location: Netherlands, Utrecht area
I posted this on FB a few days ago. The relevant bit is where it says "caveat 2", but I think you need the whole piece to have some context.

Quote:
First of all, you do not need all that much power to tow a caravan. In essence, even our not-all-that-powerful 2.0 l ICE is powerful enough to tow a decent size caravan.

At speeds below 40 MPH, there should not be an issue at all. Serial hybrid mode allows the ICE to rev freely and produce up to 60 kW of electric power. In most conditions, more than enough to propel car + caravan AND recharge the battery while at it. During rather brief moments, it may not be enough, but you will likely have plenty of SoC to use the battery as a source for extra power. A couple of years ago, I have done several hours of testing in very hilly terrain (yes, we have some of that in the Netherlands when you know where to look). And, not knowing then what I believe to know now, I was surprised to see my SoC kept going up instead go down.

Driving at speeds above 40 MPH is where the issues begin. Because the ICE is linked to the front wheels, revs are limited and therefor so is power output. At 55 MPH the ICE revs at approx. 53% of max revs. Looking at the torque curve of the ICE, this means that you have (at best) 50% of maximum mechanical power (89 kW) available, which means approx. 45 kW. Looking at the AVERAGE power demand over longer distances, 45 kW would still be enough to keep you out of trouble. When demand is more than 45 kW, the battery can be used a s a source for extra power. When demand is lower, the battery can be recharge a bit.

Unfortunately, this is primarily theory. In practice, the ICE has a strong pre-programmed preference to work at a relative load of 75% in order to maximise efficiency (this is where the sweet spot is). At 55 MPH, this would result in about 34 kW (75% * 45 kW). So, when power demand is less than 34 kW, the battery can and will be recharged. But only the difference between actual demand and 34 kW is used (which typically is not a whole lot), as that 34 kW brings the load back to the most efficient 75%. When power demand is higher than 34 kW, the ICE will produce more than 34 kW (in order to keep things going) but it will not produce even more kWs to recharge the battery. Not even when demand is between 34 and 45 kW and there are still some reserves. (When load is above 45 kW, the battery + E-motors are needed to support the ICE)

So, like it or not, while towing a decent size caravan at decent speeds over decent distances, SoC will drop, no matter what you do. Now, IMHO, this only becomes an issue when you hit 20% SoC. As soon as SoC hits 20%, a new hysteresis cycle comes in play: the engine will switch to series hybrid mode allowing it to rev freely and produce more power. This additional power is used to increase SoC back to 22%. When 22% is reached, parallel hybride mode is engaged again. Smart thinking, I would say.

But of course, from an economy perspective, serial hybrid mode is worse than parallel hybride mode, as it requires two energy conversion steps. However, if you ask me, the impact on your overall lifetime economy is not huge if you ask me. Especially when taking into account that most likely most lies are driven without caravan anyway. More annoying is the reduced comfort. The constant switching between an engine revving at give or take 2300 RPM and an engine revving at 4100 RPM is a bit exhausting. But still more an annoyance than a real problem. Only when you hit 20% SoC at a time of high power demand (going uphill on the motorway), the ICE may not be able to maintain / increase SoC, not even in serial mode. This is when the Turtle may appear. To prevent all this (for now and the rest of the trip), we would like to maintain an as high as possible SoC.

The obvious thing is: Depart with a full battery and use Charge mode from the moment you start your trip (or at least as soon as the car will allow it). But there are a few caveats to remember:

Caveat 1: Lifting the throttle may cause the ICE to idle instead of recharge the battery, even when in Charge mode.

When you lift the throttle while driving in B1 or higher, the ICE will go idle (and still use a bit of fuel in order to prevent it from dragging) and not recharge the battery. Quickly switching to B0 AFTER lifting the throttle does not change that. You really have to lift the throttle WHILE already in B0, to get the ICE to seriously recharge the battery. And when you do, it will go rather quick.

Keep in mind, driving in CC implies B2 or higher. So, just cancelling CC means the ICE will idle. After cancelling CC, you will have to switch to B0 and briefly tap on the gas (which is counter intuitive after cancelling CC) in order to get the ICE working for you. As soon as you tap the brake (or engage B1), the ICE starts idling again. Switching back to B0 and taping the gas briefly restarts the charging again. And so on.

Caveat 2: Using 4WD Lock is killing for SoC

Imagine at some moment in time, power demand is 90% of available power at current speed (so roughly 40 kW). In normal (non 4WD lock mode) the ICE will run at 90% load and all power needed will be delivered by the ICE via the front wheels. In 4WD Lock mode, half of the power needed (20 kW) will be delivered by the ICE via the front wheels. The ICE will run at 75% load (and not 90% as before) and produce 14 kW surplus power. This is used by the rear motor, but the rear motor needs 20 kW to produce the other half of the needed power. So, the missing 6 kW must be supplied from the battery. So you see, an hour and a half will be more than enough to completely deplete a fully charged battery, even with Charge mode enabled.

Okay, in the above story there are some simplifications and some corners have been cut, but in general I believe it to be correct. At least for my MY14. It may help you to understand how your car is behaving (not just while towing, BTW) and it might help you to maximise SoC.


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 Post subject: Re: 4wd lock
PostPosted: Tue Oct 02, 2018 1:42 am 
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Joined: Mon Mar 12, 2018 2:18 am
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Location: Yorkshire end of M1, UK
seaneb wrote:
My original question was aimed at the wear and tear of using 4wd most of the time; is the rear motor designed so that it will not be damaged with prolonged use or is it just meant to be used occasionally for short periods? Is there any effect on fuel and charge consumption over 2wd also?

There is no 2wd. Power is always supplied to the 2 motors at lower speeds. I'm not sure what happens at higher speeds when the ICE goes into parallel mode.

Pressing the Lock button causes the S-AWC software to mimic a locked centre differential, though there is none.

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 Post subject: Re: 4wd lock
PostPosted: Tue Oct 02, 2018 1:54 am 
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Joined: Mon Dec 01, 2014 11:30 am
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Location: Netherlands, Utrecht area
ThudnBlundr wrote:
I'm not sure what happens at higher speeds when the ICE goes into parallel mode.
Please see my post on caravan towing above ;-)

ThudnBlundr wrote:
Pressing the Lock button causes the S-AWC software to mimic a locked centre differential, though there is none.
To a certain extend, it does. But keep in mind, where a real center diff would be able to transfer power from the front motor to the rear axle (or from the rear motor to the front axle) S-AWC cannot. So, when you loose traction on one axle, you loose roughly half of the available torque / power.


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 Post subject: Re: 4wd lock
PostPosted: Tue Oct 02, 2018 2:15 am 
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Joined: Mon Mar 12, 2018 2:18 am
Posts: 357
Location: Yorkshire end of M1, UK
Thanks for that. I did read your post, but I wasn't clear how much power goes to the electric motors in parallel mode when Lock is not pressed. The car must deliver some power to the electric motors to overcome drag. They also supply more when extra power is required, for instance to climb hills, but do they supply more than "tickover" in regular driving?

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 Post subject: Re: 4wd lock
PostPosted: Tue Oct 02, 2018 3:58 am 
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Joined: Mon Dec 01, 2014 11:30 am
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Location: Netherlands, Utrecht area
ThudnBlundr wrote:
Thanks for that. I did read your post, but I wasn't clear how much power goes to the electric motors in parallel mode when Lock is not pressed. The car must deliver some power to the electric motors to overcome drag. They also supply more when extra power is required, for instance to climb hills, but do they supply more than "tickover" in regular driving?
Power is provided to both motors AND EVEN the generator to kill drag. The rear motor receives a tiny bit of extra power so that a small positive torque is created and (according to Mitsubishi) a "4WD characteristic is maintained". The rear axle is pushing (a bit) rather than dragging.

When looking at PHEVWatchDog, you see rear motor torque between 0 and very small positive (1 or 2 Nm) where front motor is always 0. Thought it would be more.


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