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Figuring out optimal driving strategies

Discussion in 'Gen 1 Prius Plug-in 2012-2015' started by Electric Charge, Oct 4, 2012.

  1. Electric Charge

    Electric Charge Active Member

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    So today, I changed my driving strategy from using EV mode when doing less than 40mph and dealing with start/stops to using EV mode as long as I can when I leave for work. This means that now about 70% of the battery is spent doing 55mph in hilly areas, while also going through a village with a 30mph speed limit and a major intersection.

    I thought for sure that my original method would give me the best results (average 75mpg for my 2-way commute, around ~20 miles each way, using half of the traction battery each leg), but after trying this today, I am surprised to hit 95mpg.

    While some of it makes sense, I am now wondering what else I am missing. Is anyone aware of a guide/thread which explains how to get the most bang for the gallon?

    Maybe we can start collecting this info here, as I am sure that I and other PiP newbies still have a lot to learn.
     
  2. 9G-man

    9G-man Senior Member

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    Good job! I gather you can't charge at work....
    The key to excellent gas mileage in the PiP, if your drive distance is beyond the EV range, is to switch back and forth when its most efficient.
    Typically its better to accellerate and climb hills on gas (HV), then downhill, slow-speed, maintaining speed on EV.
    If there is a predominant elevation change one way vs the other, use more HV for the "climb" leg.
    That "55MPH hilly" stretch could actually be efficient in EV mode if you can maintain momentum and speed, on the declines, with very little battery draw (just a peak of bar on the HSI indicator). Obviously highspeed interstate travel is not possible in strict EV, but in can be blended by selecting EV mode. I've found its not the best use for battery capacity, unless its a very very short interstate distance and/or charging is an option when you get there.

    Bottom line, using EV to accelerate and climb hills uses more EV battery "range" than actual distance traveled. So, it can be wasteful if the EV range could be used in lower load circumstances. But only if the drive distance exceeds your EV range, or you can't charge on both ends.

    I make a 14 mile trip to my boat regularly and with smart HV/EV use I can get 225MPG for the drive. My Wife and daughter get about 150MPG for the same drive. It all depends on when you use the HV/EV.

    I don 't know if you are a previously experienced regular Prius driver, but,
    If you are looking for something really basic on how to use the HSI in HV mode, check this out.
    Knowing how to drive a PiP in HV mode is just as important as knowing when to use EV mode.
    Secret....HV mode offers some significant EV range too, below 46mph......
     
    Ayerscreek likes this.
  3. If you monitor your instantaneous mpg meter you will find that good consumption can be obtained on flats and downhill portions in HV ! Between 50- 100 + mpg. Very often, 80 mpg +. Save the EV for going up small hills (guaranteed 100 mpg PLUS )
     
  4. Electric Charge

    Electric Charge Active Member

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    Switching to EV when doing ~30mph is what I was doing, which resulted in 75mpg. Doing the first 2 of the 3 55mph legs (these 2 legs being hilly as well) on EV is what gave me 95mpg, hence the confusion.

    I will check the video out. I do try to stay on battery in HV mode as much as possible, which does help a lot once I no longer have access to EV.

    That somewhat confirms why my mileage was better today, since it was the first time I used pure EV to climb these hills. Really wish I could afford the Advanced model, so I have a HUD, paying too much attention to the console now as it is :/
     
  5. ukr2

    ukr2 Senior Member

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    I drive 14.6 miles daily one-way with 3 miles of 65 mph highway and charging at Home and Work.

    I've tested the trip in EV all the way and run out of battery 1-1/2 miles early and got 196 mpg to Work and 171 mpg going Home.

    Then I tested switching to HV manually just before getting on the Hwy and switching back to EV at the end of the Hwy. When I got to Work, I had 2.6 miles of battery left with 189 mpg. Going Home, I had 2.4 miles of battery left with 133 mpg. I don't like having any battery left.

    Today, I bumped the ICE to be warm before the Hwy and left it in EV on the Hwy. When I got to Work, I had 1.4 miles of battery left with 223 mpg. When I got Home, I had 1.2 miles of battery left with 172 mpg. Again, I don't like having any battery left.

    So which was is Best?

    Since EV all the way uses all the Battery and still gives high mpg, it might be the Best way.

    I wish the PIP had a Data Recording system. Taking photos of the displays While Driving is dangerous and probably illegal !!!
     
  6. Whichever procedure leads to the highest mpg, that is the goal.
     
  7. miscrms

    miscrms Plug Envious Member

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    I think that's important to remember, and sometimes lost in the trees of EV range etc. In general you always want to use up the battery as much as possible on every trip, but ultimately whatever uses the least fuel is where you want to go. Balancing efficiency between ICE and EV really only comes into play when you know you have to drive significantly beyond the batteries range.
     
    Andyprius # 1 likes this.
  8. Agree.
     
  9. 9G-man

    9G-man Senior Member

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    My technique gets 250-300 MPG (short) trip mileage and 95-100 MPG tank mileage.
    Thats with 98 mile Interstate commutes several times a week.
     
    Andyprius # 1 likes this.
  10. miscrms

    miscrms Plug Envious Member

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    I agree with the conclusion, but would like to propose a slightly different explanation.

    To start with, route optimization and driving style is certainly key. Once you've figured that out, you've basically determined the amount of mechanical work that has to be done. The key to using the minimum amount of energy to accomplish a fixed amount of work is to optimize efficiency. The efficiency of the electric motor should be pretty consistent across speeds and loads, so really the key is using the ICE (if you have to use it) as efficiently as possible.

    To understand ICE efficiency its helpful to look at the efficiency map for the engine. Here are two versions, as each is a bit lacking in detail.

    [​IMG]

    [​IMG]

    The first is useful as it has units on the Torque axis, and the second is handy as it includes the power curve overlay. What these curves basically show is the efficiency of the engine in grams of fuel, or energy input, divided by the mechanical power output in kW. This is traced out as a 3d contour map vs. the load on the engine (torque) and the engine's rpms. Neither graph labels the contours, but it seems safe to assume 10g/kw per contour based on what labels are there. Like most engines, the Prius engine is most efficient at high loads and fairly low rpms and least efficient at light loads and high rpms. This is the basic idea behind pulse and glide. By running the ICE either in the high efficiency zone or not at all, you can increase overall efficiency pretty significantly. The Prius engine is notable in that overall efficiency is high, and that peak efficiency band is pretty wide.

    When you step on the gas in the Prius, you are basically telling the computer I want this much power. This can either be to accelerate the vehicle, or in response to a change in terrain such as climbing an slope, or just to supply the required power to maintain a given speed. When you ask for a given amount of power, the computer will decide on a "gear ratio" to try and produce that power at the most efficient rpm and torque possible. There's also another layer of complexity in that the computer can also decide to use electric drive or regen drag to increase or decrease the load on the ICE to try and improve efficiency.

    To get a sense of the power in kW required in different circumstances, the ev calculator linked below can be useful to generate some data points for a Gen 2 with a net 0 weight change. Gen 3 should be similar, as its weight and air resistance that dominate this calculation.
    EV Calculator

    Column 1
    0 [TH]Eqv Acc mph/s[TH][TH][TH][TH][TH][TH] 1 [TH] 2
    1 [TR][TH]Speed / %Grade[TH] -3% [TH] -1% [TH] 0% [TH] +1% [TH] +3% [TH] +10% [TH]+20%
    2 [TR][TH]20 mph[TD]-1.24[TD]1.29[TD]2.55[TD]3.82[TD]6.35[TD]15.2[TD]27.3
    3 [TR][TH]40 mph[TD]-0.8[TD]4.26[TD]6.78[TD]9.31[TD]14.4[TD]32.0[TD]56.2
    4 [TR][TH]60 mph[TD]2.98[TD]10.6[TD]14.4[TD]18.2[TD]25.7[TD]52.3[TD]88.6
    5 [TR][TH]80 mph[TD]11.8[TD]21.9[TD]27.0[TD]32.0[TD]42.1[TD]77.5[TD]125.9


    The 10% & 20% grades are included not so much because you're likely to regularly climb that steep, but because the force required to climb a 10% and 20% grade should be roughly equal to the force required to accelerate a Prius at 1mph/s and 2mph/s respectively.

    Looking at the contours, the "sweet spot" for ICE efficiency seems to be roughly 10-30kW output. So then looking at the table, any conditions that fall within (or maybe near) the 10-30kW range are probably good candidates for using the ICE. I think its important to reiterate that this doesn't mean its more efficient to drive fast, accelerate all the time, and go up hills ;) It means the ICE is converting fuel to mechanical power most efficiently in those regions. Driving faster and going up hills requires more work, so even if done more efficiently you'll likely use more fuel to get where you're going. Accelerating quickly as long as you are in the efficiency band is ok, the only real penalty comes in if you drive faster. The assumption here is you have to drive a certain speed to be safe or expedient, you have to go over certain hills to get where you need to go, and you have to stop and start occasionally. That's all part of your route optimization, which is related but different.

    Anyway, that's all pretty theoretical, but might at least give you all a place to start when trying to decide when to use HEV vs. EV or some additional insights into what might be going on behind the scenes. I'm sure the reality is more complicated than this, but maybe it helps a bit :)

    Rob
     
  11. miscrms

    miscrms Plug Envious Member

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    The other piece of the puzzle is the efficiency of the electric motor/inverter which is shown on the bottom of page 65 of this ORNL report:
    http://ecee.colorado.edu/~ecen5017/notes/OakRidge_2010Prius.pdf

    As expected its very good over a very wide range. Its basically within 10% of peak efficiency everywhere except <1000rpm and <10Nm. At low speeds I would assume the boost converter will be off, so looking to the 225V contour at the bottom of page 66 shows even better efficiency at low rpms.

    I suppose one thing you could try is using the ICE to accelerate from a stop to 10 or 15 mph to avoid that lower efficiency area in the motor/inverter contour and get the most out of the battery capacity. The ICE should be decently loaded from acceleration and at a reasonable rpm due to "gearing" so it should be decently efficient. Again that probably only applies if you know you are going to deplete the battery and have to use the ICE somewhere.

    Rob
     
  12. ukr2

    ukr2 Senior Member

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    Highest mpg AND run out of battery just before you get to your charging station.
     
  13. lensovet

    lensovet former BP Brigade 207

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    seems to me that basically trying to "outsmart" the car is a silly exercise, if anything. the only time it makes sense to manually toggle between EV/HV modes is when you have some strange reason to conserve battery, such as when you know that you won't be able to fully recharge. but basically, your goal should be to fully deplete the battery as much as possible. toggling out of EV mode into HV mode achieves the exact opposite goal.
     
  14. jim335

    jim335 Member

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    Thanks for the data above. I will examine more closely when I have time.
    I have tried lots of strategies for more ev miles, but finally gave up and just use them from the start of my trips. That was getting me about 12.4 miles of EV. But I have added a battery from Plug in supply to my plug in, last week, and now I just drive (any way I like) and am getting 27 to 28 miles of ev.
     
    Andyprius # 1 likes this.
  15. Details......?
     
  16. jim335

    jim335 Member

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    miscrms likes this.
  17. miscrms

    miscrms Plug Envious Member

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    In general you are correct. The Prius is usually smarter than the driver at this point ;) Where this is not true is when you know things that the car doesn't. For example, if you know you are going to have to drive significantly over the range the battery, say 20 miles. In that case, you know you will have to use the ICE at some point. By choosing to use the ICE when its most efficient to do so, and using the battery when the ICE would be particularly inefficient it should be very possible to achieve lower overall fuel usage than just letting the car run in EV until the battery is gone and then switching to the ICE. Even so its still easy to end up worse off as the OP discovered. The conditions in which it might be better to use HV are very specific, and maybe not really well understood yet. If the Optimized his HV usage he might be able to beat just straight EV, but its not always easy.

    But if you will be driving close to or less than the range of the battery then it probably makes more sense to just leave it in EV. Some folks try to use HV to get the best possible EV range but this can actually result in more fuel usage which is counter productive. This was true of the ukr2's first example in the post above. Just driving EV ended up using less fuel for the commute than trying to be smarter than the car and using HV for certain types of driving. On the other hand his second example showed better total mpgs even with using less battery capacity overall. I guess that just shows that there are other factors that some into play. In this case it may be that the ICE was required during the highway stretch even in EV mode, so it may be that letting the ICE warm up before the highway stretch increased its efficiency on the highway enough to overcome the efficiency loss associated with using a bit less of the battery.

    Rob
     
  18. Electric Charge

    Electric Charge Active Member

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    On my way to work this morning, I decided to turn off EV mode when I had around 0.3 mile left on battery. I did this just to see if I would be allowed to switching it back to EV when being this low (thought I read somewhere that I had to have a certain amount of charge left before I can switch to EV), and it would let me. So this made me think.

    Now I am able to recharge the traction battery by the usual braking/down hill coasting, and as this charge goes back into the battery, bumping it up to 0.5 really quick (didn't try any longer as I was close to the office) I can keep driving in EV mode.

    Perfect example: there is a stretch of road which has a 30mph limit, but because of the small inclines, I can't stay on battery when in HV mode (even when in ECO). However, with having access to EV again, I can 'conquer' these small hills, without engaging ICE. Doing this, my average mpg went up by 10, to 100mpg for my 42 mile commute.

    So is this a valid strategy, or was it just a coincidence that my mpg bumped up (maybe I'm becoming a better hypermiler, I don't know)? Would love your thoughts. Hopefully my rambling made sense ;)
     
  19. bisco

    bisco cookie crumbler

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    i have found you also get a small ev bump from the ice when it comes on.
     
  20. 9G-man

    9G-man Senior Member

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    Yes, that's a valid strategy. And if you want to get really snazzy, notice the fact that regen in EV can be "stacked" for more EV range.
    In contrast to regen in HV, whick tends to be used up in HV mode for better HV efficiency. Some here feel that selecting EV mode for big regen opportunities is a better way to use (later) the recaptured energy.


    example: driving around in HV mode, with minimal EV range left, you can select EV for your big braking (regen) opportunites. Then switch back to HV for continued driving. You'll be "stacking" or increasing EV range for later use. I see this only useful on really long trips where there might be an opportunity to gain something of significance

    For typical driving, I like to try and use all my EV range, and my HV stealth (EV) range by the time I get home to charge.