Temperature and ICE vs Battery

In other threads, SmellyTofu and alanclarkeau mentioned their experience that temperature can have a big effect on the use of ICE vs traction battery (motor) to drive the wheels. In other words, hybrid torque strategy (eg Liu 2008, Najem 2021) changes with temperature. This is not unreasonable; because ICE, traction battery, inverters and EMs all need temperature management. Thermal management involves control of power drawn from the device; and/or removal of heat, which consumes energy. An effect on torque strategy may be either direct (a temperature parameter in the hybrid system control algorithm) or indirect (eg increased temperature reduces traction battery charging more than discharging, so SOC decreases, causing an increase in the ratio of ICE:EM drive torque). It may also be predictive (eg Yang 2021).

To assist in this analysis, I logged various temperature and torque PIDs in a rav4 hybrid AWD, using OBDLink. I could not find enough power PIDs to make that comparison (and too many assumptions were required to calculate power from torque). There are lots of other temperature PIDs, some of which are initial or maximum temperatures at specified locations. I am uncertain whether torque is measured or estimated (mapped), and whether it is specified at the same place for the ICE and motors. For example, if engine torque is specified at the crankshaft, it would need (at least) a correction for gear ratio for equivalence to an axle torque. It is difficult to determine instantaneous gear ratios in an e-CVT. Some torque PIDs may be compound. For example, the Toyota planetary gearbox dynamically splits ICE torque between generator and wheel torque.

Some PID outputs were null (rear motor brake regeneration), some were not what I expected (rear motor torque ratio), and sign was unpredictable (torque from the ICE and drive motors was usually positive, torque from the generator and brake regeneration was usually negative, but there were exceptions in all cases). Here are some graphs from the logged data:


The most obvious (and understandable) effect was that uphill grades (+ve pitch) generally called for ICE and motor torque, whereas downhill grades (-ve pitch) were often associated with regenerative and generator torque. I can not discern a broad effect of temperature on hybrid torque strategy under these conditions. Many of the readings are very spiky, and the uncertainties mentioned above may interfere with interpretation. Various ratios that I examined did not help. It is possible to imagine various effects of various temperatures over short time periods, but these do not hold up over the journey. An effect might be clearer under controlled conditions with only one factor varied, but this would miss all the important interactions while driving. An effect might be revealed by more sophisticated mathematical (component) analysis, but this is beyond my ability.

That is not to say that SmellyTofu and alanclarkeau are mistaken. If they are right, there may be too many other parameters in the HS control algorithm for me to discern the temperature effect from logged PIDs under these (relaxed urban) driving conditions. With the exception of some fairly direct effects on cooling fan operation, I am not sure that anyone has been able to reveal whether and how temperature PIDs are used in control. Unfortunately, Toyota does not reveal to car owners the parameters, let alone the algorithms, used to control operation of the cars that we own. Sigh.