MPG in Hybrid Mode

Discussion in 'Prime Technical Discussion' started by CEJ, May 13, 2019.

  1. CEJ

    CEJ Member

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    As a newbie to Prime (but veteran to Prius), I very well could be doing something wrong. Just can't figure out what it could be. If I fully charge, then carefully put it into HV each time I start out and avoid turning anything on, seems pretty straightforward.
     
  2. jerrymildred

    jerrymildred Senior Member

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    I finally have time to peruse this interesting thread now that I'm home and unpacked. It's gotten so long, that I didn't have time to read every word of it, but it looks like @jb in NE started down the right path in post #98. I learned a lot in this 6,000 mile road trip, but it's hard to explain. But I'll try.

    Here's what's happening.

    First, don't think of EV/HV portions. It's a battery, plain and simple. Yes, the display SOC percent it the portion reserved for EV, but it's all one battery. Not the reason for what you're observing, but important to remember just the same.

    Second (and this is the reason), there are threads about using charge mode and then using up the charge and recharging. And that's the key. Have you noticed that at speeds under about 65-70 mph, the car does that automatically? It will go to EV all by itself and stay that way for a mile or two running down the battery charge. It much more aggressive than the regular Gen 4 Prius. Then it will charge the battery back while you only get maybe 35-40 mpg. Then it repeats.

    If you stop before it puts all that borrowed charge back (and you probably will because it will immediately put itself back in EV at that point) then that's the new benchmark for the battery management. So, it you drive for example 10 miles the battery might have dropped to 95-98% and you shut it off, now that's what it wants to maintain. So you start up and go a few miles. The car uses & replaced charge till you stop again. It will almost for sure be a little more depleted unless you stop just as it gets charged back up. And so it goes, ratcheting down the SOC.

    So. If you drive 20 miles or you drive 200 miles at speeds low enough (<65 mph or maybe 70), the SOC will only go down to a certain level and that level is a matter of where you are in a given charge/discharge cycle. You may, in fact, lose less EV range from driving 200 miles than 20 miles in HV if you happen to stop just as the car finishes putting back the electrons it borrowed while it had itself in EV. I was able to drive several hundred miles a day on the trip I just finished and not lose any EV range because the charge just kept going up & down and I wasn't stopping.

    The only way to keep the SOC at 100% (or any other fixed point) is to stop after it goes through a discharge/charge cycle and shut it off as it's ready to put itself back in EV.

    I suggest you try it yourself if you can get out on a 60-65 mph highway and just drive 200-300 miles without shutting it off. I think you'll see what I mean. It's pretty hard to explain concisely and clearly.
     
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  3. CEJ

    CEJ Member

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    Thanks for your input, Jerry. It will take me a bit to fully digest what you have said and I will probably have some questions. Yes, I do understand there is just one battery, with a certain percentage of the charge on it (roughly 75%) reserved for EV and the other 25% for HV. I will be going on a 200mi trip this week so I will be able to try out your suggestions.
     
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  4. CEJ

    CEJ Member

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    Are you saying that the charge will get depleted and then restored to its original state if you keep driving, but if you stop when it is down to a certain level, it will only rise to that level when you drive again? What is this charge that is being taken used for? Even though you are driving in HV mode, does it shift temporarily shift into EV for some reason? How does your theory jibe with SK's results give just above. He never seems to lose more than a few percent on SOC. I've been down into the 50's.
     
  5. jerrymildred

    jerrymildred Senior Member

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    That's correct. Whatever the SOC is when you shut down/start up, that's what it will try to maintain in HV. On top of what I previously said, if the engine is cold, it will borrow from the battery quite a bit to let the engine warm up gently and that needs to get put back. So short trips where the engine sits long enough to cool make it even more pronounced.

    Yes, exactly. You'll see the EV light come on and go off as it does this. The regular Gen 4 will also pop into EV by itself at similar speeds. I was riding with a friend in his 2017 Prius four and it acted like the Prime in HV mode. Back and forth between EV and HV, but not for as long as the Prime since the battery is smaller. It seems that the car can charge itself efficiently enough and use the EV charge efficiently enough to make up for the losses incurred when charging up the battery. Previous generations do something similar but only at low speeds like 35 and under or something like that.

    Not sure which results you mean. One possible theories is that his drives are longer than yours. The real test should be your upcoming 200 mile trip. On the main display (the one with the speedometer), turn on the sub screen so you can see the current fuel consumption bar graph. It's on page 200 of my 2017 manual. Then you can see that when it comes out of EV and starts recharging the battery, it's getting well below 50 mpg. It will stay down there till it restores the previous SOC and then go back to EV. That's assuming that the car is not going uphill, or accelerating or driving so as to put more load on the drive train than the computer thinks will work for that sort of operation. Once you get up to about 70 mph, it won't put itself in EV. When it's going back and forth, though, mine seems to get almost charged up and then go along at about 50 mpg, apparently adding energy to the battery more slowly before it reverts to EV again.

    When I first observed this behavior, I wondered if it was a good idea or not. But it does seem to return good results. And it makes sense to me to let it borrow from the battery around town like that since you're making short trips and should be able to charge soon. So, if it does bleed down like what you're experiencing, it's saving gas since you should be able to replace the charge more cheaply with the EVSE cable than with the ICE. And on a long road trip, it should preserve that SOC within a few percentage points for when you get to the next city.
     
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  6. Salamander_King

    Salamander_King Senior Member

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    I think Jerry has the correct answer. It is the short repeated trips that cause a gradual drain on the traction battery. As I stated, I have never tested holding the traction battery SOC for longer than a single day mostly on a single long trip. For that situation yes, PRIME is capable of holding close to 100% of SOC.

    As I am experimenting holding SOC without any charge for multiple days of my commuting (~18 miles one way), short repeated trips show gradual loss of traction battery SOC. Depending on the condition, I am assuming that can be temperature, the way the car is driven, and/or other driving conditions, this loss can be as little as 1% as seen on my Day 1 morning commute, but can also be as much as 4% loss as seen on my Day 3 morning commute. Once the SOC is lost, with short repeated trips, there is not enough regen to take place to replenish the SOC to the original level, and as Jerry pointed out each time the car is shut down, the now lower SOC is set as the full capacity.

    I have done this experiment only for 3 days now and with only 2 days commuting (4 trips), but my SOC has now decreased to 89%. I am not sure if this will continues to decrease past 80% or not. I will report after tomorrow's commute.

    Screenshot 2019-05-26 at 11.11.29 PM.png
     
    #106 Salamander_King, May 26, 2019
    Last edited: May 26, 2019
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  7. CEJ

    CEJ Member

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  8. CEJ

    CEJ Member

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    My head is swimming a bit trying to assimilate all this great data and info you two pros are throwing out. Naive questions: What determines how much EV mode and charge are introduced into the HV drive mode? In pure HV mode, with EV charge at 0%, I get very good mpg, so are you saying that in the former case (HV with available EV charge), it is more economical to steal some of that mode and charge than not (which, of course, you couldn't do if the charge were 0%)? And I don't quite understand if you start/stop/start why you can't charge up any further than where you were when you stopped. It looks, SK, like if you keep going, you could wind down like I did to 50% or even lower.
     
  9. Salamander_King

    Salamander_King Senior Member

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    I think a certain amount of traction battery is always used at the start-up of ICE. This is true regardless of whether you have 100% SOC for EV or 0% SOC left for EV. If there are any SOC the car uses that and if the SOC is 0% it uses the reserve of battery below 0% EV level during the engine warm-up cycle. The amount needed for this initial warm-up can be as low as 1% but can be a higher amount depending on the condition like temperature, AC use, and your driving conditions like how fast you go and how quickly you accelerate, and how long and how steep of hills you must climb. With my driving condition, it varies from 1% to 4%. If your drive is long enough, regen braking will replenish used SOC all the way to where original, but with very short trips there is usually not enough regen. Of course, if you happen to live on top of a long hill, and your short trip is just a few miles downhill. You are most likely to be able to regenerate all the lost SOC when you reach the destination at the bottom of the hill, but when you are coming back uphill, you will likely to lose quite a bit of SOC and have no opportunity to regenerate that lost SOC. So, repeating short trips with the mixed train but zero-sum up and down will still likely to lose some SOC after a few repeats.

    Jerry thinks when the car is shut-off with some SOC used, this lower SOC is now full capacity the car tries to regenerate. I do not know the mechanism of this, but since with short repeated trips you will not have enough regen opportunity, even without this new cap, the SOC will continue to lose every time you stop the engine and cool down, so you have to repeat the warm-up cycle. I don't know if my car will continue to lose SOC below 80% as you saw on your car each time I make a short trip, or the decline stops at 80% and car will try to maintain the SOC at that level as some others suggested. I will be able to answer that question in the next few days as I continue my daily commute experiment.
     
    #109 Salamander_King, May 27, 2019
    Last edited: May 27, 2019
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  10. jerrymildred

    jerrymildred Senior Member

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    @Salamander_King explains it well. Probably the best explanation is just experience. You'll learn a lot just by observation.

    As for the SOC benchmark the computer wants to maintain when you start up, that's just the way it's programmed. They could have programmed it to remember the max SOC from the previous trip but it would take more fuel to get that built back up. It would be contrary to the design objectives of minimal pollution and maximum fuel economy. And with repeated short trips, it might be impossible anyway.

    Also, remember that the manual advises against keeping the battery at a high SOC for long periods. So it doesn't make sense to keep driving day after day in HV with a full or nearly full battery if you want that battery to provide the best performance 10-15 years from now.

    On the 6,000 mile trip my wife and I just completed, I tried to keep about a 10-30% SOC for driving through small cities, or just getting out of town in the morning. I had very few chances to plug in. I think I got to plug in before hitting the road five times and always used at least 30% of the charge getting out of town. I then bled it down some more any time I got into a slower zone.
     
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  11. CEJ

    CEJ Member

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    Thanks to you both. Some light is beginning to dawn! Can you find holes in the following: As has been emphasized, there is one battery and about 25% of its capacity is reserved for HV driving and the remainder (as high as 100% SOC or 75% of total battery capacity) can be used for EV mode if desired. If SOC is at 0%, HV mode will operate and the HV charge is used for warmup, the electric motor, etc.,and it is used in conjunction with ICE to propel the car. You can see, in this case, the EV charge fluctuate on the bar meter as it loses charge and then is replenished by the generator and reg braking; at times the ICE will even go off for a bit. Is there any real difference in the operation of the car when you operate in HV mode with non-zero percentage SOC-- except that the charge used and replenished is shown in the change of SOC percentage and HV fraction of the charge on the bar meter is shown as staying constant? And, as Jerry says, the system for reasons of its own (battery life preservation or whatever), won't let the SOC percentage get too high.
    By the way, Jerry, is the benchmark you are referring to 80%?
     
    #111 CEJ, May 27, 2019
    Last edited: May 27, 2019
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  12. jerrymildred

    jerrymildred Senior Member

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    I think you've got it! (y)

    It really is hard to express verbally without some sort of video illustrations. There is one possible difference between the car's behavior in HV when it has EV range available and when EV range is depleted, but this is just my theory. I suspect that, if there is EV range available and you've selected HV mode, the car may be dipping more deeply into the battery when it puts itself in EV than if the EV range has been used up. At least it seems to me that the cars stays in EV longer in that situation. But I don't have enough experience with those conditions to call it as a conclusion base on observation. It's just a suspicion that I'm looking at to see if it might be valid. In the grand scheme of things, it's not really important. Bottom line is that this is an incredibly efficient automobile.
     
  13. farmecologist

    farmecologist Senior Member

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    Too bad they didn't think to throw in a 'road trip' mode. Seems like it would have been rather simple to implement. However, I suppose that would be yet another knob to toggle ( and invite complaints )! (y)
     
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  14. CEJ

    CEJ Member

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    Another thought occurred. When in HV mode with EV range available, all the energy stolen will probably not be all replenished as we have seen (unlike HV mode with EV range unavailable) so the meter-shown mpg will be artificially high by a bit. This was borne out in my experience when I was getting 70-71 in HV with no EV range available and around 75-76 with it available. Maybe your theory plays into this too.
     
  15. Salamander_King

    Salamander_King Senior Member

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    So here are my 3rd commute trips results (4th day since the start but I did not drive on Sat). Well, surprise, surprise, even during short 18 miles trips, regen is capable of replenishing more SOC than car uses for the warm-up and other things if you drive the car to maximize the regen braking. This was not what I was expecting from what Jerry has suggested.
    There is no hard set benchmark or cap the car has to maintain as long as you drive the car to maximize regen braking, which is in contrary to the car's mission of being the most fuel efficient.

    After my two days of commuting trips (~18 miles each way), the original 100%SOC and 32.3miles GOM was down to 89%/29.4miles. Overnight parking somehow had 1% SOC loss this morning indicated by 88%/28.2miles. On my way to work starting with 88% SOC and on forced HV drive, I did something I don't usually do. Instead of driving to maximize EV range, I intentionally drove to maximize regen braking. With more HV acceleration out of EV range followed by coasting to charge the battery, I was able to replenish my SOC to above where I started. On my way to work, I was able to increase the SOC to 91%. Then on my way home, I was able to regenerate 9% of SOC back to 100% full SOC with initial 32.3miles GOM indicated.

    Screenshot 2019-05-27 at 12.44.42 PM.png
    IMG_20190527_114610 (1).jpg
    IMG_20190527_121824.jpg

    I now know, there is no hard cap on SOC level the car imposes. It can be regenerated all the way to 100% even if it is down to 88%, albeit using more gas doing so. If you drive very conservatively and have fewer regen opportunities, then SOC does lose some amount gradually each time you drive.

    Regardless, as Jerry said it, the car is amazingly efficient.
     
    #115 Salamander_King, May 27, 2019
    Last edited: May 27, 2019
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  16. jerrymildred

    jerrymildred Senior Member

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    Exactly. Since you're going to need to plug in to replace what you lose, it will give a falsely high mpg calculation. That's why I don't pay as much attention to mpg as to cost per mile. That's kind of a given with a PHEV like the Prime. There's really know way to be perfectly accurate with mpg. But it's still pretty close.
     
  17. CEJ

    CEJ Member

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    These results are fascinating and answer a lot of questions. When I take my 200mi trip this week, it will be interesting to see if I can come anywhere close to duplicating this. Thanks for giving us such detailed quantitative information!
     
  18. CEJ

    CEJ Member

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    I notice that you are not in Eco driving mode. Do you prefer normal mode?
     
  19. Salamander_King

    Salamander_King Senior Member

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    Oh! That's a news to me. LOL :eek: I was not paying any attention to that logo. I must have hit that button inadvertently and did not notice the change. I looked back my photo record, and seems this change happened on May 16th. lol I was always in ECO mode and have never switched that setting from Week 2. I have tried both NORMAL and POWER mode briefly on my Week 1 after purchasing my car, but settled to ECO mode. I used ECO mode almost exclusively on my Gen3 also. As far as I know Eco and Normal have very little effect in EV range, but throttle response and AC use are different. There was a thread up that is discussing Eco vs Normal under HV drive having significant difference in ICE and EV use. That may explain the fact I was able to regenerate all the EV SOC lost from the warm-up while driving HV. Worth the consideration. I will try if I can do the same regeneration of EV SOC using ECO mode.

    Here is the thread: Eco vs. Normal: HV vs. EV | PriusChat
     
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  20. CEJ

    CEJ Member

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    Wow, please report back! It will be very interesting to know if Normal mode is better for replenishing SOC than Eco mode.
     
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