anko
Well-known member
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.
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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):
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
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):
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
