EV, HV, MPGe calculations...

Discussion in 'Prime Fuel Economy & EV Range' started by HypersonicPrime, Oct 1, 2017.

  1. HypersonicPrime

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    Hey everyone. I looked through the threads and found no direct answer for this question.

    Background:
    On conventional gasoline vehicles I would document the gallons I filled and the odometer readings and would keep a log of MPG for each trip between fill-ups. On an old Mercedes I converted to run on either diesel or straight vegetable oil, I recorded gallons of diesel and diesel miles and gallons of SVO and SVO miles to get miles per gallon on each fuel and then an overall (method similar to PHEV MPGe calculations). I find this information interesting and want to do something similar for the Prius Prime.

    What I'd like:
    I'd really like to know my performance in HV mode (miles/gal), EV mode (miles/kWh), and overall (MPGe) on a time/distance basis of my choosing (not Eco Diary, which doesn't seem to have all the info I want anyways). I'm trying to find the easiest way to record/watch this sort of information. Here are my thoughts:
    1. The MPG listed on all of the screens seems utterly useless to me because it's pegged at 199.9 miles/gal if you're driving around on EV all the time. It seems to be total miles (HV+EV) per gallon of gasoline (HV). I will therefore call it tMPG. It really gives no indication of how you're doing in HV or EV mode or even overall. It'd be a good indicator of MPG if you never plugged-in, but this is a plug-in hybrid...
    2. The Drive Monitor 2 shows miles/kWh since last resetting energy consumption. This looks like the best way to look at EV performance on a user-specified interval. If I want to track it, I supposed I'd have to write this number down periodically and reset energy consumption each time I want to start over.
    3. The Drive Monitor can show information for each trip meter. It also shows EV driving ratio. I suppose if I reset Trip A each time I reset the energy consumption, I can track the miles and EV driving ratio. This way if I have 1000 miles on the trip and a EV driving ratio of 90%, 900 miles are EV and 100 miles are HV.
    Potential Solution:
    1. Reset energy consumption and Trip A.
    2. Drive.
    3. At the end of a trip (or on a periodic basis), write down
      1. Miles from Trip A meter (total_miles)
      2. tMPG from Trip A meter (tMPG)
      3. EV driving ratio from Drive Monitor for Trip A (ratio_EV)
      4. EV performance from Drive Monitor 2 (perf_EV)
    4. Calculate HV driving ratio (ratio_HV): 1-ratio_EV.
    5. Calculate gasoline miles (gas_miles): total_miles*(ratio_HV).
    6. Calculate gallons of gasoline used (gal_used): total_miles/tMPG. This will only work when the tMPG is not pegged at 199.9.
    7. Calculate EV miles (EV_miles): total_miles*ratio_EV.
    8. Calculate energy used (E_used): EV_miles/perf_EV.
    9. My EV performance (EV miles per kWh used) is simply perf_EV.
    10. My HV performance (HV miles per gal used) is tMPG*(ratio_HV). This will only work when the tMPG is not pegged at 199.9.
    11. My overall performance (total miles per total energy consumed) is total_miles/((gal_used*33.7kWh/gal+E_used)/(33.7kWh/gal))
    Conclusion:
    This seems a little cumbersome, although wouldn't take too much time with a spreadsheet. Also, it won't work when you do enough EV driving to push tMPG over 200. Any suggestions/advice? What do I do when tMPG >200? Is there a way I can get most of what I'm looking for more easily?
     
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  2. Sam Spade

    Sam Spade Senior Member

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    It seems that you have "contracted" the Obsessive Compulsive Disorder that befalls many new hybrid owners. ;)

    If all of that REALLY makes you happy......forge ahead.
    But my advice is: Worry less, drive more. (y)
     
  3. HypersonicPrime

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    Some people like to spend many hours per week playing fantasy football. Some of us like to spend one hour every week or two looking at our energy efficiency. Everyone's got their own thing. I have been interested in engineering and energy efficiency long before owning a hybrid. From what I can tell, there is no good screen to look at HV, EV, or overall energy economy in the Prius Prime, but that's why people buy this vehicle. Many people who play fantasy football are interested in the scores--not just the game. Many people who buy a Prius are interested in its efficiency--not just driving it. Also, a drop in energy economy unrelated to driving habits can help predict maintenance issues. Anyways, I'm hoping some people out there might have looked into this and have a technical answer.
     
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  4. bisco

    bisco cookie crumbler

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    the problem with ben 4 is, toyota decided in their ultimate wisdom to track hv miles only when the engine is running, and the rest, ev miles. so, you really have no hv mode miles, just gasser mode.
     
  5. AnneLW

    AnneLW Member

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    This is how I am doing it (numbers rounded off). Somebody check my math and correct any wrong assumptions:

    At 500 miles, my car "thinks" it has gotten 100mpg. I presume this means it has used 5 gallons of gas.:

    According to the "EV%" I have been in EV 65% of the time. I presume this is base don miles driven, not hours driven.

    Thus, actual miles in HV = (1 - .65)(500) = 175 miles.
    175 miles per 5 gallons = 35 miles per gallon.

    I would certainly expect my car to be doing better than that, but as luck would have it, my HV miles have been almost entirely uphill. Or did I calculate wrong?

    If it's right, OP could do a "reset" at a pre-planned time and do the math going forward.

    __________________________________

    NEXT QUESTION
    at $2.80/gallon and 1 gallon per 35 miles:
    $2.80/35 = $.08/mile (8 cents per mile)

    at $0.13 per kwh and 4.0 miles/kwh
    .13/4.0 = .0325 (3.25 cents per mile)

    As long as my electricity rate is lower than 32 cents per kwh, then electric is cheaper?
     
  6. Lee Jay

    Lee Jay Senior Member

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    You calculated it wrong. The EV percentage includes Ev time while in Hv mode, which is the normal hybrid way of operating. Since all (or at least most) of that Ev energy came from gasoline, you have to include those miles as gas miles, not Ev miles.

    Bottom line, you can't use Ev percentage this way for anything.
     
  7. MelonPrius

    MelonPrius Active Member

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    I'm glad you are interested in this. I'm hoping for a Prime to be my next vehicle and have been looking for the same info. Please post your findings.
     
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  8. HypersonicPrime

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    Hey @AnneLW! I agree with your calculation of 5 gallons of gas and EV% being miles driven (not time--confirmed with owner's manual). I also agree with your calculation of 35 mpg. However, this is your mpg only when the engine is on. This is why it seems low because when operating as a hybrid (engine cycling on and off) your overall mpg would be closer to the ~55 EPA value.

    After thinking about it more, I believe my methodology in the original post will work although it's actually a comparison of engine-on mpg and engine-off miles/kWh, not HV Mode mpg and EV Mode miles/kWh. I think this is what @bisco and @Lee Jay are saying.

    As for your question at the end, I think the following is a more accurate calculation. From the car's perspective, it's really running as a HV or EV. If you never charge it, the engine is still not on the entire time and it performs better than 35 mpg, so HV/EV should be the comparison. Since I don't think we can calculate Mode-based efficiencies easily, I'd use the EPA-reported mpg/mpge values. With values of 54 mpg for HV and 133 mpge for EV and your cost values, that's (2.8 $/gal)/(54 miles/gal)=5.19 cents/mile on HV and (0.13 $/kWh)/((133 miles/gallon)/(33.7 kWh/gal))=3.29 cents/mile on EV, making the cutoff 5.19/3.29*13 cents/kWh=21 cents/kWh, from a purely financial viewpoint. I've seen some people try to compare a direct conversion, neglecting the difference in efficiencies of utilizing each "fuel" (($2.8/gal)/(33.7kWh/gal))=8 cents/kWh). At 13 cents/kWh, this would mean you'd want to use HV all the time! Not true! That method needs the 8 to be multiplied by 133/54=2.46 to again get the proper value of 21 cents/kWh.
     
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  9. Lee Jay

    Lee Jay Senior Member

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    Here's how I calculate mode-based efficiency.

    Most of the time, I'm in Ev mode all day, every day. The Eco-Diary provides miles/kWh, mpg and total miles for each day. What I do is assume that mpg=199.9 means all Ev mode, so I calculate that day in miles/kWh. If it's less than 199.9, that means I was in partial Ev partial Hv that day and I don't now how much, so I assume all Hv for that day and just use the provided mpg. Most of the time this works because I primarily use Hv on long trips.
     
  10. Oniki

    Oniki Active Member

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    The Prime keeps a log of monthly miles/kwh available to the driver.
    If the owner tracks charges and they are empty to full then the EV miles are known since a full charge is 5.6 kWh
     
  11. Lee Jay

    Lee Jay Senior Member

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    I don't know about you, but for me, more than 99% of my charges are NOT "empty to full".
     
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  12. Salamander_King

    Salamander_King Senior Member

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    I've been keeping a daily log of actual kWh charge used and EV miles driven. Those two value are not something displayed nor can be calculated from the value displayed. It is very cumbersome to record the end of EV miles driven while driving a car (since there is no other way to record actual EV miles driven) and for that record to hold, I have to keep the car on EV mode only and never switch to HV or EV Auto while still have EV range left. This record gives me actual miles driven on EV mode/kWh. However, in actuality, some amount of kWh are used for powering things other than the motor, so it still is not TRUE EV efficiency, IMO. Also, it will not give me actual HV mode mpg until the next fill up for calculation of gas used.
     

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  13. PriusPrime4849

    PriusPrime4849 Junior Member

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    The best and most fundamental method of tracking electric and gasoline usage is to treat the car as a black box which consumes electricity and gasoline, then measure the amount of each that enters the car. Then we can compute MPGe for electricity and MPG for gasoline, and the cost of operation on each. As previously discussed, gasoline use can be computed accurately from data supplied by the car. But it turns out that the amount of electricity consumed in charging can only be determined accurately by measuring the power input to the charging adapter.

    If you are using the included Level 1 charging adapter to charge the car, plugging it into an inexpensive watt hour meter such as the P3 P4400 Kill A Watt Electricity Usage Monitor (available from Amazon) is a convenient way to measure the power used for charging the battery (including any losses in the internal charger). Some Level 2 charging adapters have watt meters built in; that's worth some extra money since 230 volt watt hour meters tend to be expensive.

    In pure EV driving, the amount of power used to charge the car is typically about 10% to 15% or even 20% greater than the usage computed from miles/kWh. If the engine runs during the trip, the power needed to recharge can be significantly less than the miles/kWh usage.

    Example: On a 32.3 mile trip, miles/kWh was 4.6, which is 155 MPGe (= 4.6 * 33.7). Based on this MPGe, electricity consumed was 7.00 kWh (= 32.3 / 155). Actual kWh to charge was 6.59, which is 0.41 kWh less. Since the trip was uphill on the average, there was no net regeneration and the engine must have contributed at least 0.41 kWh to the electric drive system (the reduction in charging power is larger, since charging is only about 85% to 90% efficient).

    Rather than waiting for an infrequent fill-up, we can closely approximate gasoline usage from in-car data. As has already been mentioned, daily miles traveled divided by miles per gallon is the number of gallons used for each trip. Unfortunately, the limit of 199.9 MPG displayed in the car means you could use up to 0.15 gallons of gasoline on a 30 mile trip without being able to measure it. If Entune shows shows daily mileage greater than 199.9, use that value instead. In the example above, the car showed the daily MPG as 199.9, but Entune showed 346.1 MPG; thus the trip consumed 0.09 gallons of fuel.
     
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  14. prairiedog53

    prairiedog53 New Member

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    Is it not true that the ICE is ALWAYS running in HV mode?
     
  15. Lee Jay

    Lee Jay Senior Member

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    Yes, it is NOT true.
     
  16. walterm

    walterm Active Member

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    Right - in HV mode the Prime acts like a 'regular' Prius, where the ICE runs as needed but stops when the car is stopped, when decelerating the battery is charged and then can drive the car solely electrically for a short while, etc.
     
  17. Oniki

    Oniki Active Member

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    In routine driving in HV mode the ICE is off ~ 30% of miles. They are all counted as EV miles so far as I know
     
  18. Lee Jay

    Lee Jay Senior Member

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    Not for miles/kWh. I don't know about the Ev percentage stat.
     
  19. Oniki

    Oniki Active Member

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    I was only talking about the EV %
     
  20. prairiedog53

    prairiedog53 New Member

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    Thanks everyone for this information! I've had some difficulty finding good descriptions of HV mode. More seems available on other modes, but I've been trying to learn what goes on in HV mode. Incidentally, don't have a Prius yet, nor have I ever ridden in one. Nor have we actually seen a Prime in real life... only recently has Toyota begun to roll them out in BC. So we wait to experience firsthand.... (y)
     
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