Thanks for the kind words, left and right. I feel encouraged (sorry for that) to try to explain why the car will eat away SOC when using 4WD mode while towing. Although not many will tow or use 4WD or do both at the same time, it might still provide people with some insights that could be helpful elsewhere.
The ICE has, when engaged, a strong preference for working at a relative load of 75% load (so 75% of available torque at given RPMs). I think it is safe to assume this is the sweetspot: the load at which it works most efficiently. Why would a higher load be more efficient? For one thing: higher load at same RPMs => more kW at same RPMs => more kWh per unit of time at same RPMS => more kWh per number of revolutions of the engine => less internal resistance to overcome per kWh => …. In parallel mode, under most circumstances, you will need far less than 75% of the power available with the RPMs associated with your speed. Normal cars deal with this by shifting into a higher gear: same speed, less RPMs, higher relative load.
We cannot shift into a higher gear was we have only one if them. So, instead we artificially increase power demand by bringing into play the generator. Surplus ICE torque (or power, if you like) is fed into the generator which transforms this into electrical energy which is used to recharge the battery. Of course, instantaneous fuel consumption goes up, but the extra ICE output is not wasted. It flows into the battery, to be used later. Of course, some conversion losses are involved, but I am convinced that these losses are outweighed by the increased efficiency of the ICE at higher load. Otherwise, why would they bother? Tada, IMHO this is the advantage of a hybrid car in a nutshell.
Small side step: The amount of power the battery is willing to accept depends on actual SOC: the higher the SOC, the less power the battery will accept. At some point it means we cannot reach the optimal 75% load because at 75% load the generator would be producing more power than the battery is willing to accept. Opinions differ on whether or not driving around with high SOC results in a measurable reduction of overall fuel economy. But that is a different story ;-)
So, the ICE has a preference for 75% load. It will take the load associated with driving and add a load associated to charging to reach this 75% load. When load associated with driving goes down (driving on a down ramp, tail wind, etc), charging load is increased. When load associated with driving goes up (driving on an up ramp, head wind, …) charging load is decreased. We can see this result in a very, very constant instantaneous fuel consumption. Also, our Cruise Control does not play with the throttle to maintain constant speed, it plays with the charge current to achieve this. IMHO, this explains why CC is so very effective, when it comes to maintaining a constant speed.
When the load associated with driving reaches 75% of the power, available at the current RPMs, charging simply stops. Only when the load associated with driving exceeds 75% of available power, the ICE will work harder, still at the same RPMs associated with your speed.
Another small side step: When the load associated with driving exceeds 100% of available power, the battery will send power to the E-motors, so they can support the ICE. When the load associated with driving exceeds 100% of available ICE power + 60 kW from the battery, the car will switch to serial mode, allowing the ICE to produce even more power.
Now, imagine load associated with driving is 50% of available power at current RPMs. In non 4WD mode, the ICE will produce 75% of available power, 50% out of 75% used for driving the front wheels, 25% out of 75% used for recharging. Now you engage 4WD mode. The ICE wil still produce 75% of available power. 25% out of 75% will be sent to the from wheels. Another 25% out of 75% will be sent (via the generator) to the rear E-motor and the last 25% will be sent to the battery. Nothing really changes, apart for some additional conversion losses that have been introduced.
Now, imagine load associated with driving is 75% of available power at current RPMs. In non 4WD mode, the ICE will produce 75% of available power, 75% out of 75% used for driving the front wheels, nothing left for recharging. Now you engage 4WD mode. The ICE wil still produce 75% of available power. 37.5% out of 75% will be sent to the from wheels. Another 37.5% out of 75% will be sent (via the generator) to the rear E-motor and nothing is left for recharging. Again, nothing really changes, apart for some additional conversion losses that have been introduced.
Now, imagine load associated with driving is 90% of available power at current RPMs. In non 4WD mode, the ICE will produce 90% of available power (it will step up), 90% out of 90% used for driving the front wheels, nothing left for recharging. Now you engage 4WD mode. 45% of available power will be sent directly to the front wheels. As, from a pure ICE perspective, the load associated with driving is less than 75%, the ICE will work at 75% relative load, the sweetspot. Now only 30% remain. These 30% are sent to the rear motor, but the rear motor needs 45%. The missing 15% are taken from the battery. The draining has started.
Now, imagine load associated with driving is 100% of available power at current RPMs. In non 4WD mode, the ICE will produce 100% of available power (it will step up), 100% out of 100% used for driving the front wheels, nothing left for recharging. Now you engage 4WD mode. 50% of available power will be sent directly to the front wheels. As, from a pure ICE perspective, the load associated with driving is less than 75%, the ICE will work at 75% relative load, the sweetspot. Now only 25% remain. These 25% are sent to the rear motor, but the rear motor needs 50%. The missing 25% are taken from the battery. The draining goes even faster.
