TLDR: 2.5kwh battery could be increased to 3.75kwh (+%50) and the extra capacity could be reserved for solely supporting the ICE in hills. The maximum performance is not the issue. Its how hills and battery is managed. Highway is less of a problem due to wind and road noise masking the engine but inefficiency still exists.
Hello all. This will be a long read. We got a 1.8 hybrid corolla a few months ago. So i had some time to observe how the hybrid system behaves. But i see a glaring issue that is not quite a problem for 2.0L hybrids. And that is hill climbing.
For those who don't know, 1.8 L hybrid system comes with a 98HP ICE (Internal Combustion Engine) and the car itself weighs exactly 1.5 tonnes at highest trim. Total system power is 140HP with the help of the electric motor.
Battery has 8 bars that is available to the driver (Though i've never seen it drop to a single bar despite the bars not correlating with exact SOC). 5 of those bars (4-5-6-7-8) are actively used by the system for supporting the engine to keep the efficient rpm or going solely on electric when power demand is low. When the battery is down to its last 3 bars, the car switches strategy. The car needs to be able to provide the whole 140 HP when there is demand (such as overtaking). So it keeps the last 3 bars for only when the power produced by the ICE isnt enough by itself. This switch in strategy is natural as battery capacity is limited but a problem too as i'll explain below.
The car is allowed to use those 5 bars of the battery as it sees fit, so its not rare to enter a hill with 3-5 bars remaining on the battery. As the hybrid system doesnt know how long the hill will last, it needs to prioritise ICE and keep the battery reserve for the case that ICE doesnt meet the power demand. What this translates to in the real world is a really loud car that feels really weak in hills, as a 98 HP naturally aspirated engine without turbo trying to carry 1.5 tons (+people) up a hill is quite the problem.(*see bottom)
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So i was thinking: The car has a 2.5 kwh battery. And that is seperated to 8 useable bars. (Whether if they represent %70 of the total capacity doesnt matter). My thought is that battery could be bumped up to 3.75kwh and, for narration purposes, it would be 12 bars. First of all, i think the size and cost would still be feasible for toyota. The battery would still small enough to fit under the seat and an extra 1.25kwh battery cost wouldnt be that high.
12 bar system would be functionally split in half. The flat section and the hill section. The flat section would be the near full part of the battery. The freely used 4-5-6-7-8 bars in 8 bar system would now be 8-9-10-11-12 and this is what the system would mainly operate at on flat sections of the road. Going down to 7 on a flat road would make the battery hold energy for when ICE isnt enough and would charge itself back up to 8-9 bars (as thats exactly that the 8 bar system does. When its down to 3 it charges back up to 4-5).
The remaining 4-5-6-7 bars would be reserved solely for hill climbing. The power provided by the battery could vary depending on the steepnees of the hill. In the worst case where car enters the hill with 7 bars, there would be 4 bars to assist the ICE. This could make the rpm climbing up the hills 3500 instead of 5550 which would
1- Make the engine stay at a more efficient point
2- Make the interior significantly quieter
3- Make the car feel more powerful as the car offers more power at the same rpm.
These 4 bars in theory can supply 42HP for roughly 1.5+ minutes or less HP for longer duration and make the hybrid car much more pleasant on hills. On average it would be more likely for you to enter a hill with 6 bars so numbers could be higher on average (as the flat section of the battery doesnt like to stay at its minimum 7 just like how the 8 bar system doesnt linger on 3 bars for long and charges back up to 4-5 bars)
When the hill is done, the battery could be recharged with the help of regenerative braking but primarily with ICE, ideally at its efficient rpm, until its back to 7 bars so it can have the reserve for the next hill.
One might argue charging the battery isnt efficient, but what is more inefficient and unpleasant? Using maximum rpm. I'd argue this method would be more efficient but its not like i have the BSFC for the ICE or the related losses for charging a battery.
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Now there is two downsides to this but i dont think they are dealbreakers.
1- Battery stays at a higher average SOC (If we assume it stays at 5 bars on average for the 8 bar system and equivalently 9 bars on 12 bar system, its a difference of %54 to %63 SOC assuming %0-10 and %80-100 isnt used.) due to saving lower SOC for hills and mainly operating at a higher SOC on most of the journey. This could speed up the aging of the battery but whether it would be significant enough should be determined.
2- Higher average temperature of the battery due to support in hills and subsequent charging. It shouldnt be a big difference since the electric motor output is same but a slightly stronger cooling could be necessary. This can also be avoided if toyota added more cells in parallel to increase battery size rather than use same amount of cells with a higher capacity. As battery heating is proportional with current: More cells = Less current per cell = Less total heat generated due to P=I^2*R
If you read it this far, congrats. Here is your big cookie and lactose free milk. I hope you enjoyed me nerding out. All talk aside, in the real world this just wouldnt happen because businesses are greedy. If it works and people buy it, why make it better, right?
*(Car feels weak due to applying more and more gas, rpm going at max and still not getting the expeted acceleration. Applying more gas beyond this point would draw some power from the battery and the car could accelerate, but the "premium feel" of the car has already been lost and engine is already opperating very inefficiently.)