Help me understand the real MPG during EV driving

Discussion in 'Gen 4 Prius Fuel Economy' started by Awap, Apr 8, 2019.

  1. Awap

    Awap Member

    Joined:
    Mar 22, 2019
    44
    27
    0
    Location:
    Boulder, CO
    Vehicle:
    2019 Prius
    Model:
    LE AWD-e
    Okay, so I've been doing "pulse & glide" etc. and was happy to get a brief round-trip of "63 mpg".

    But I feel like there must be a more precise way to think about that "100 mpg" instantaneous value I'm seeing during EV driving. After all, that battery juice came from gasoline (since I don't have a plug-in). True, that gas was already accounted for in the mpg readings during the time the ICE (internal combustion engine) was active, so it's kind of like "free energy" when some of that gets used again during EV driving. But, at some point the battery runs low and the ICE has to kick in to recharge the battery. So, it isn't really "free" energy -- there must be some actual "conversion factor", and I'm 99.9% certain the real mpg during EV is not exactly 100! Maybe it's 87? 204? I'm just wondering if there's a more accurate way to "model" how EV-based driving factors into the overall mpg.

    Also, does the car's calculated mpg end up close to the real-world mpg?
     
  2. bisco

    bisco cookie crumbler

    Joined:
    May 11, 2005
    94,254
    42,718
    0
    Location:
    boston
    Vehicle:
    2012 Prius Plug-in
    Model:
    Plug-in Base
    the battery shouldn't run low during pulse and glide

    however, given mpg on your screen includes total distance travelled, so that is accurate.

    mpg is a misnomer for electric driving, just as miles/kwh would be for a gasser.
     
    #2 bisco, Apr 8, 2019
    Last edited: Apr 8, 2019
  3. The Professor

    The Professor Senior Member

    Joined:
    Apr 5, 2018
    529
    734
    0
    Location:
    UK
    Vehicle:
    2017 Prius
    Model:
    Excel
    It works like this... In both a traditional car, and hybrid, you'll use some fuel to increase your speed (increased kinetic energy), or get to the top of a hill (increased potential energy). You then start decelerating (that can include regenerative braking in the hybrid), or going down the other side of the hill. This is where the two cars differ. The traditional car burns off all that energy as heat, either through the friction brakes, or by adiabatic heating (engine braking). That energy is lost forever. Your hybrid, however, converts roughly 80% (that's your 20% conversion loss) of that kinetic or potential energy into electricity, and stores it in the battery.

    Now you start accelerating or going up a hill again. The traditional car will have to use fuel again to do that. But the hybrid can use its stored electrical energy to reduce the amount of fuel needed.

    The 100MPG instantaneous reading is just the limit of the gauge. You can also set it to 150MPG maximum in the settings. In reality the MPG is infinite when you're using the electric motor only, as zero fuel is being used.

    In reality it's a bit more complicated than that. The Prius uses an Atkinson Cycle engine, which is super-economical over a very specific rev range. The computers use the electric motor to try and keep the ICE running at that super-economical speed. When that speed is too fast for your current energy requirements, it uses the spare energy to charge the battery. When you need more energy (e.g. going up a hill or accelerating briskly), then it uses the electric motor to supplement the ICE in an attempt to keep the ICE at that super-economical speed. It can also opt to turn the ICE off entirely if it would be more economical to do so (very low energy requirements, or decelerating, or going downhill). That's when you see the EV light come on.

    If you're wondering where that super-economical speed for the ICE is, then look at the screen with the accelerator gauge on it. Press the accelerator until the bar rises to the very top of the light blue area, but not into the red. That's your sweet spot. If you can accelerate from a standstill and hold the accelerator at that position, you'll start getting scores of 95 or above for acceleration. If you're pulsing and gliding, again use that accelerator position for the pulse. During the glide, press the accelerator really gently, so that there is no blue bar visible at all (you're trying to get rid of the tiny amount of regenerative braking the car applies by default). If you don't do this, you're regenerating, and regen wastes 20% of your energy.
     
  4. Awap

    Awap Member

    Joined:
    Mar 22, 2019
    44
    27
    0
    Location:
    Boulder, CO
    Vehicle:
    2019 Prius
    Model:
    LE AWD-e
    Thanks! That really gives me a much clearer way of visualizing what's going on.

    In your 5th paragraph I assume you're talking about the Hybrid System Indicator? That's p. 127 in my 2019 owner's manual:
    Prius2019p127HSI.png
    Sorry, no color in this view... Is the "sweet spot" that you're describing near the top of zone 2 (right below the PWR zone)?

    This sounds different from what I've heard elsewhere, i.e. "TheTunneys" (
    ) - focuses on Lexus but has similar displays to older Prius - which I assume was just a flattened out version of the current HSI shown above. At around 12 minutes he seems to be saying (crediting "The Hobbit") that if you are in that upper region (the top half of zone 2 in the image above), you want to avoid the upper 1/4 or lower 1/4 of that section. That would make sense to me based on your 4th paragraph, since the ICE wants to be in its "optimal" zone.

    But I'm surely confusing things... most of the information online about this is based on older Priuses so probably the engine has changed since then.
     
    pmabraham likes this.
  5. Awap

    Awap Member

    Joined:
    Mar 22, 2019
    44
    27
    0
    Location:
    Boulder, CO
    Vehicle:
    2019 Prius
    Model:
    LE AWD-e
    Just caught a mistake in my post above - "the ICE wants to be in its optimal zone" - that's not right because the Prius keeps the ICE in its optimal zone, by transferring energy to or from the battery.

    So, I think near the lower end of that upper half of zone 2, the ICE is charging up the battery, which wastes energy in the conversion. So it makes sense you'd want to avoid that end. But I'm still not clear exactly where you want to be in the upper zone, or why.
     
  6. The Professor

    The Professor Senior Member

    Joined:
    Apr 5, 2018
    529
    734
    0
    Location:
    UK
    Vehicle:
    2017 Prius
    Model:
    Excel
    I think you've got it. You want to be at the top of zone 2 like you thought. Anything below that incurs a greater amount of conversion losses in the long term. Anything higher (red zone) burns through your battery and fuel more quickly because the ICE is not running at its most economical speed and you're using the maximum amount of electrical energy, which means that statistically it's less likely to be available later to optimise ICE speed. If you drop into the lower half of zone 2 then you're likely to be using electrical only and again that means you're just incurring greater conversion losses later as that battery needs to be recharged if it is to be available to optimise ICE speed.

    In short, the top of zone 2 means the ICE is producing the most energy for a given amount of fuel, and it's delivering as much of that energy to the wheels as it can (i.e. not incurring unnecessary conversion losses).

    All that being said, if you know you're heading mostly downhill for the next couple of miles, you may as well use the battery more. A fully charged battery can't recover any more energy. So you want to avoid situations where your battery is full (0 or 1 bars left empty at the top of the display) as the Prius will start limiting the flow of energy to the battery at that point. There are other situations where you might know better than the computers too.

    In the short term, you will get better economy keeping the accelerator anywhere lower than where I'm suggestion, but all you're really doing is using the battery more, which makes you believe you're getting better MPG (and indeed you are for that short time), but in the long term you're just increasing conversion losses and reducing the car's ability to keep the ICE at it's optimal speed. Remember that, as you said earlier, all the energy in the battery came from the ICE via one route or another, and hence it all suffered a 20% conversion loss. So it's generally true that your best plan is to allow the ICE to run at its most economical speed whenever you're accelerating, and avoid regen when gliding, keeping regen only for when you want to actually decelerate the vehicle or control downhill speed, and a avoid the friction brakes (they kick in once the brake pedal pressure moves the accelerator graph to what would be past the bottom of zone 1 on the diagram).
     
    alanclarkeau likes this.
  7. alanclarkeau

    alanclarkeau Senior Member

    Joined:
    Feb 24, 2016
    6,762
    7,344
    0
    Location:
    near Brisbane, Australia
    Vehicle:
    2016 Prius
    Model:
    N/A
    One problem with the MPG method of fuel measuring - is that it can cause that problem - in that you could be achieving INFINITY MPG (ie ZERO FUEL use), but a PRIUS hasn't got a way of showing infinity MPG. With litres/100km like most of the world uses, ZERO means NONE, and the bigger the number, the more fuel you use. TOYOTA must have decided that 100 MPG is as good as they show (tho The Professor indicated that can be changed to 150. (I believe Japan uses km/litre, which is similar to MPG concept.)

    I've heard your question quite a few times since being on PriusChat. I suspect that there are 3 types (at least) of people who buy PRIUS.
    • #1 One just drives it - and gets very good economy;
    • #2 Another thinks - "this is an EV" - and tries to sort out in their mind how the ICE fits in with it. And sometimes overthinks it - but probably improves economy a bit;
    • #3 The other thinks "this is a very efficient ICE" car - assisted by a battery. And, yes sometimes overthinks it too - and also probably ends up improving economy just the same as #2.
    I think it's good that we think about how our cars work. I can still remember my thinking the first time I ever took a driving lesson. I'd devoured information about cars, about how engines, gearboxes, clutches, worm&roller steering mechanisms etc work. And, as I took off in the shopping centre carpark the first time, I could visualise the engine spinning the flywheel, the clutch engaging and the car ... actually moved. I asked my Dad about something after a while - and came to the conclusion that he knew that you put your foot on this pedal and it has this outcome - had no idea what was happening. It's possible to do the same thing with PRIUS (I do) - but, then some people aren't interested, they just drive - and that's fine too - my Dad did it till he was 85 and never thought about what was causing it to work.
     
    Carlos Sá likes this.
  8. Awap

    Awap Member

    Joined:
    Mar 22, 2019
    44
    27
    0
    Location:
    Boulder, CO
    Vehicle:
    2019 Prius
    Model:
    LE AWD-e
    Professor, thanks again and I think i am really getting it now... just had 53 mpg on an uphill commute, aiming for upper half of top half of zone 2. It's hard to "aim" for the sweet spot during fast-and-slow traffic though!

    One question though, that I don't think you addressed. TheTunneys (video above, at 12 min) cites The Hobbit as source for this coloring: Highway speeds.png
    ... saying that it's best to stay in the green zone on that image (middle of upper half of Zone 2). From what you're saying, I believe that upper red sliver he shows just below PWR, which he calls inefficient, you would call the efficient zone. So I'm just trying to get a clear idea of what part of the display I'm aiming for. Again, most of the info online is based on older Priuses so I'm wondering if the sweet spot has changed over the years.
     
  9. The Professor

    The Professor Senior Member

    Joined:
    Apr 5, 2018
    529
    734
    0
    Location:
    UK
    Vehicle:
    2017 Prius
    Model:
    Excel
    That's an older Prius. I'd imagine it's a different drive train, different engine with a different sweet spot, different electric motors, different battery, different aerodynamics, different software... And so on. At the very least it's a different representation of engine speed on a different guage so there's no direct comparison.

    Also, everything I said is a generalisation. Different journeys and different routes may prefer slightly different throttle positions. Even different weather conditions can make big differences to fuel use. Understanding those intricacies, and how to adapt to them, is the art of hypermiling.

    Try different accelerator positions when starting from a stop and see which gives you the best score for acceleration/starting on the hybrid system indicator screen. The score seems to be a function of how much fuel you actually used (directly, or indirectly with battery use) to travel the distance you traveled while accelerating. Just do it safely. :D My first real test of automatic emergency braking may have been while watching that screen too much. ;)

    SM-T813 ?
     
    alanclarkeau likes this.
  10. Awap

    Awap Member

    Joined:
    Mar 22, 2019
    44
    27
    0
    Location:
    Boulder, CO
    Vehicle:
    2019 Prius
    Model:
    LE AWD-e
    I was going to ask, "What's that funny blue zone in the background that moves up and down behind the acceleration bar". I found it on p. 128 - "Eco accelerator guidance"... quoting the manual below:

    --------
    which can be used as a reference operation range for using the accelerator pedal according to driving conditions such as starting off and cruising.
    The “ECO Accelerator Guidance” display changes according to the driving status, such as when starting off or cruising.
    It is easier to drive in an Eco-friendly manner by driving according to the display showing the accelerator pedal operations and staying within the “ECO Accelerator Guidance” range.

    p. 225:

    ●When starting off:
    While staying within the “ECO Accelerator Guidance” range, gradually depress the accelerator pedal and accelerate to the desired speed. If excessive acceleration is avoided, the “Eco-Start” score will increase.
    ●When driving:
    After accelerating to the desired speed, release the accelerator pedal and drive at a stable speed within the “ECO Accelerator Guidance” range. By keeping the vehicle within the “ECO Accelerator Guidance” range, the “Eco- Cruise” score will increase.
    ●When stopping:
    When stopping the vehicle, early releasing the accelerator pedal will cause the “Eco-Stop” score to increase.
    ----------

    I'll keep an eye on that and see if it agrees with what we discussed. In the meantime I just had a nice round trip (just a few miles) practicing your techniques, clumsily - 46 mpg uphill and 95.8 mpg downhill (average 71, I guess).

    It does seem a dangerous distraction... I hope to learn the "feel" of the right accelerator position, eventually without looking.
     
  11. The Professor

    The Professor Senior Member

    Joined:
    Apr 5, 2018
    529
    734
    0
    Location:
    UK
    Vehicle:
    2017 Prius
    Model:
    Excel
    That's the Eco Accelerator Guidance bar, as you discovered.

    It takes what I said previously to the next level, taking into account battery level and the energy requirements hinted at by the driver.

    When accelerating, put the accelerator level at the very top of that guide (you'll find it's usually where I said, at the top of area 2). It may like to hang around much lower then that if the ICE is off or you've been traveling at a constant speed for a bit and it thinks you're trying to maintain speed, so always remember you're in control and it's advising you based on what it thinks you're trying to do (accelerate or cruise). Give it a nudge of acceleration to make it realise you're wanting to accelerate if needed. When maintaining a speed, keep your accelerator anywhere in the range the guidance bar gives to do that. You'll find the guidance bar slowly drops bit.

    Based on how well you do the above, will determine your scores for acceleration and cruising.
     
    Tideland Prius likes this.
Loading...