Which acceleration from 0 mph has the highest MPG?

Discussion in 'Gen 3 Prius Fuel Economy' started by lexel, Jan 21, 2015.

  1. slimothy

    slimothy Junior Member

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    Thanks! That was what I was looking for.

    I do find that there is some speed-dependency in the HSI, where higher vehicle speeds shift the scale on the HSI, e.g., higher revs or kW would be at the edge of the ECO zone at 50 mph but well into the PWR zone at 25 mph.

    I also find when climbing a 7%-ish grade from 25mph to highway speeds that staying below 2500 rpm seems to get slightly higher fuel economy; but isn't too far off until 3000 rpm.

     
  2. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Thanks, here is the BSFC chart I was remembering:
    [​IMG]
    Thanks Toyota!

    There is a broad range of rpm, ~1,400 - ~3,200, that stays in the area of peak engine efficiency. Closer to the center avoids excursions outside of the peak efficiency area.

    Bob Wilson
     
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  3. walter Lee

    walter Lee Hypermiling Padawan

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    I drive in very hilly terrain in high traffic density ( traffic light density is about one traffic light signal every 0.5 kilometers or 1/3 of a mile) where traffic jams are frequent. I found that ECO mode tended to frequently cause the battery level to drop below three bars in my daily commute (15.8 miles, +55 traffic lights) - this caused the Prius ICE to automatically recharge the HV battery which cause the Prius MPG to drop. Switching to PWR or normal mode helps reduce the drain on the HV battery - but switching to the ICE at a lower speed during acceleration - has almost eliminated the drain on the HV battery levels and boosted the Prius' overall MPG.

    The optimization of MPG on the Prius as well as other cars is to make the most efficient use of the momentum generated by the acceleration. The heavier the car the more costly braking is when the car must comes to a complete stop and loses all of its momentum. The 3rd generation Prius is a fairly heavy car so braking is expensive wrt to fuel efficiency. However, if a Prius doesn't have to brake then its MPG jumps up. For example, during the summer, I time-shifted my 15.8 mile commute to when the traffic lights are all turned off which allowed me do greatly reduced the need to apply the Prius' brakes -- my Prius' MPG (as measured by a ScangaugeII AVG Xgauge) jumped from 65 mpg to +80 mpg per trip ( I've did this several times to meet the PriusChat 800 mile on one gas tank challenge). Every time the Prius has to brakes it loses momentum generated by the acceleration and this causes its overall MPG to drop. The Prius' regenerative braking recaptures some of the energy during the braking process but not all - the Prius' regenerative braking only recaptures about 10% to 50% of the braking energy. The most important hypermiling technique one learns is to maximize MPG is to minimize the amount of braking required. No matter how efficient the acceleration - the amount of braking either by the driver or the on board computer will cause the Prius' MPG to drop. The Prius is more difficult to hypermile than a regular car because the Prius has extra modes where it will automatically brake when the driver is not applying the brakes and when the car is not in cruise control - a driver seeking to optimize a Prius' overall fuel efficiency has to be aware of this.

    In addition, as Bob Wilson has previously charted for us on PriusChat, as the Prius' speed goes over 25 mph its overall MPG will start to drop -- regardless of how efficient its acceleration was.


    There is a Wayne Gerdes-Hobbit method of accelerating with a ScangaugeII - accelerations events are limited by LOAD (LOD) Xgauge so that 70 <= LOD <= 80. The Gasoline used per hour (GPH) can vary greatly based on whether the Prius is going uphill (+1.4) , going on an approximately flat part of road (1-.8) , or going downhill (<.6) when the LOD is fixed at 75. Simply put - it is more efficient to accelerate when going downhill and less efficient to accelerate when going uphill. This phenomena is called gravity assist [1].

    [1] Gravity assist - Wikipedia, the free encyclopedia
     
    #43 walter Lee, Feb 6, 2015
    Last edited: Feb 6, 2015
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  4. tpenny67

    tpenny67 Active Member

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    I think this is because it's taking into account the efficiency of the total system, not just the engine. The engine can only deliver a fixed amount of torque directly to the drive wheels, so as it revs higher it's spinning MG1 faster to generate electricity to provide more torque via MG2.

    It seems to me that the HSI is an indication of how much torque is being supplied to the drive wheels. In the ECO area, the engine is supplying most of the torque and not much electricity is being generated. In the PWR region and above, the MG1 -> MG2 path becomes a significant source of torque, which is less efficient due to electrical losses. At low speed, the engine doesn't need to rev fast to spin up MG1, so the HSI fills very quickly.

    Have you ever noticed that if you hold the HSI in a steady position, the speedometer changes at a very steady rate across a wide speed range?
     
  5. Mendel Leisk

    Mendel Leisk Sand Pounder

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    There's a hill behind our place, the main drag climbs it, with a lot of jogs and switchbacks. There's also a power line that goes straight up the hill, with a steep path, frequented by folks looking for cardiac workout.

    That was our goal, but to reach the start of that path you need to walk up the first stretch of the hill-climbing road. Walking along you see the denizens from up the hill taking on that road in their cars, with straining engines. It really drives it home: hill climbing with a ton or two of steel is hard work.
     
    #45 Mendel Leisk, Feb 6, 2015
    Last edited: Feb 6, 2015
  6. CR94

    CR94 Senior Member

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    Obviously it's easier to accelerate going downhill, but efficiency isn't the same as ease. Efficiency is useful energy delivered, divided by energy consumed. Accelerating downhill sometimes requires zero energy, in which case efficiency is zero, or undefined.
     
  7. CR94

    CR94 Senior Member

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    Right. That's why, I believe, best overall efficiency occurs at a lower engine speed when the car speed is slow than when the car speed is higher. The HSI takes that into account.
     
  8. slimothy

    slimothy Junior Member

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    That does make sense, I should look at the MG1/MG2 torques at different speeds and throttle inputs.
     
  9. lexel

    lexel New Member

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    New test:
    Car warm, EV mode possible and performed a few accelerations from 0 to 30 km/h with EV only (no ICE). Measured the time past at reaching 30 km/h compared with two methods. No ScanGauge measurement required for that test.
    1) Acceleration just below 1/2 the ECO-line (from 0 - 30 km/h)
    2) Acceleration at 1/4 the ECO-line (from 0 - 30 km/h).
    Result:
    1) 10.4 sec, 10.3 sec, 10.2 sec, 11.5 sec
    2) 18.4 sec, 18.2 sec, 18.7 sec, 19.2 sec
    So acceleration methode 2 is slower than methode 1 but lower current is drawn. Unfortunatly i have forgotten to record the AMPs drawn for both methodes:(. Time x current = capacity drawn, used for both methodes is maybe equal.
     
    #49 lexel, Feb 8, 2015
    Last edited: Feb 9, 2015
  10. lexel

    lexel New Member

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    Now measured the current drawn for both methodes acceleration from 0 - 30 km/h with the EV Motor. All off, only light on (about 1.7A subtracted from total measured current)
    1) 10,6 sec at 27.3 A (both avg)
    2) 18,6 sec at 15.3 A (both avg)
    If we look at the capacity drawn = Amps multiplied with current we have for methode
    1) 289,4 As (Ampere seconds)
    2) 285 As

    Conclusion: Methode 1) and Methode 2) draws about the same capacity using the same energy.
    Methode 1) Generates about 1,75 times more power (18,6 sec/10,6 sec)
     
  11. walter Lee

    walter Lee Hypermiling Padawan

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    A roller coaster and a soapbox derby racer are two examples of vehicles that rely only on gravitational forces to create acceleration, speed, momentum, and work (distance x mass). Any vehicle (with or without an engine) will have less work to do as long as the vehicle is working in the direction of gravity. A powered vehicle's energy efficiency is basically the same after 45 minutes of warming up - but the cost of warming up from a cold start climbs as the driving temperature drops below 60 degrees Fahrenheit- the bigger the engine the higher the cost of warming up. If one uses a engine block heater the cost of warming up mitigated the ICE cold start cost but the Prius does not normally come with an EBH. An ICE running efficiently does not translated to high MPG if the vehicle remains parked. A Vehicle's ICE energy efficiency is necessary for high MPG but it is not the only contributing factor to High MPG. High MPG also depends on the transmission, wheels/tires, and the driving environment. The driving environoment is a bit tricky part of this equation. Aerodynamic drag can be lessen by a tail wind ( wind moving from the rear to the front of a vehicle) when the vehicle is moving forward. With no wind, as the Prius goes over 25 mph aerodynamic drag eats away at its efficiency as Bob has charted out. In addition, if the roads are not smooth or dry - then as rolling resistance increases the Prius' energy efficiency drops. Driving with snow tires or with chains on the tires will increase road traction in the snow but it lower a Prius' MPG.

    With respect to experimenting with the Prius fuel efficiency - the DOE tested the Prius on the ICE efficiency already -- they had data logging OBDII-laptop monitor to get real time rpm/fuel usage data. Initially I tried to use RPM on my Scangauge II to get the best MPG but then realize that low RPM doesn't equate to the best use of power. A low GPH doesn't always equate to the best MPG either. The answer I found was this - - conserving the vehicles momentum so it doesn't have to stop and getting the most distance for every ounce of gas burns is what increases MPG. Smaller more aerodynamically efficient, lighter weight vehicles tend to get the best MPG.
     
    #51 walter Lee, Feb 16, 2015
    Last edited: Feb 16, 2015
  12. lexel

    lexel New Member

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    To take the electric energy into acount to evaluate which acceleration methode is best, i have measured the fuel burned during charging the battery:
    Car is standing somewhere and battery SOC (state of charge) is dropping to 39,5%. -> ICE starts an i measured the fuel burned until charging to 50% SOC. Result: 0.08 liter for araound 2457 As (=39,5% to 50Soc%). A full battery has a capacity of 6,5 Ah=23400 As, 10.5% of this is 2457 As. So 1l gas is equivalent to 30712 As.
    Based on my info provided for EV acceleration from 0-30km/h, e.g. 0.0093 l is burned for that acceleration.

    With that i found that every acceleration speed area has his different fuel efficiency. So i realized that its all about fuel efficiency at a certain speed.
    Have done a lot of data measurements now, also measuring the time travelled during different acceleration methodes.
    Here are my results of total fuel burned during different acceleration methodes, and sorted by speed areas:
    Acceleration 30-50 km/h little bit over 1/2 ECO Line = 0.0022l
    Acceleration 30-50 km/h at 75% ECO Line = 0.0019 l
    Acceleration 30-50 km/h at 75%-100% ECO Line = 0.0016 l
    Acceleration 30-50 km/h at low PWR Line = 0.0012 l

    Acceleration 20-30 km/h with EV = 0.0031 l
    Acceleration 20-30 km/h little bit over 1/2 ECO Line = 0.0033 l
    Acceleration 20-30 km/h at 75% ECO Line = 0.0034 l
    Acceleration 20-30 km/h at 75%-100% ECO Line = 0.0032 l

    Acceleration 0-20 km/h with EV = 0.0062 l
    Acceleration 0-20 km/h little bit over 1/2 ECO Line = 0.0098 l
    Acceleration 0-20 km/h at 75% ECO Line = 0.0101 l

    Summary: Based on above data there is for each speed area a different optimum acceleration methode (most efficient methode)!
    That means from 0-20km/h EV acceleration rules. Between 20-30km/h methodes are very similar.
    Very interesting is the area between 30-50 km/h: Here is an acceleration at the low PWR line best!

    So here is my acceleration methode (from 0 - 50 km/h) which beats Waynes:
    lexel methode: From 0 -20 Km/h use EV, from 20-30 km/h accelerate more and more from above 1/2 ECO line to full ECO line. At 30-50 km/h use the low PWR area.
    This methode generates the least gas consumption for an acceleration from 0 - 50 km/h!

    lexel
     
    #52 lexel, Feb 16, 2015
    Last edited: Feb 16, 2015
  13. SageBrush

    SageBrush Senior Member

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    Either of these two rules of thumb work fine:
    1. rpm between 1500 - 3000
    2. instant MPG = MPH, or within about 20% of equality
     
  14. Blizzard_Persona

    Blizzard_Persona Senior Member

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    Thats some serious testing Lexel, good stuff and thanks for taking the time to get those results and posting.

    Can someone translate Lexels Method from KPH to MPH…

    Thanks
     
  15. bwilson4web

    bwilson4web BMW i3 and Model 3

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    MPH ~= KPH / 1.6

    Bob Wilson
     
  16. macman408

    macman408 Electron Guidance Counselor

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    Nice! Some other bits of data that you could glean from your measurements (though additional data could improve the accuracy):

    Charging the battery is about 27% efficient. (A gallon of gas has ~120 MJ of energy, which is about 31.7 MJ/liter. Taking the nominal battery voltage of 273.6V times 30712 Amp-seconds yields ~8.4 MJ. 8.4/31.7 ~= 27%)

    According to some articles that discuss the next-generation Prius, the current generation ICE has 38.5% thermal efficiency. If the battery charging were happening at that peak point of efficiency, then the efficiency of the planetary gears, motor, and charging the battery would only be about 70%. On the other hand, the best thermal efficiency might be at a slightly higher-power point, which could reduce the lifespan of the battery, so they may sacrifice some engine efficiency there for battery reliability. I'm not sure I've seen what the transmission efficiency is, but 90-something percent is probably not a bad guess. I'd bet that the efficiency of the MG and charging is probably at least 80%, maybe even close to 90%.
     
  17. bwilson4web

    bwilson4web BMW i3 and Model 3

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    Tables make the data a little clearer
    Km/h mph power
    1 0-20 0-12.5 EV
    2 20-30 12.5-19 1/2 to full ECO
    3 30-50 19-31.25 low PWR
    • ~29 Km/h (18 mph) - peak mileage speed (my benchmarks from 2003 Prius)
    • low PWR - ~3,200 rpm, the peak, cooled, EGR value measured via miniVCI
    This profile shows the change in speed is roughly proportional to the change in power. The lowest speed range is in the battery region and the highest speed range at the peak engine efficiency. Without going into the math, the vehicle kinetic energy looks to be proportional to the power setting. Begging the question, what about higher speeds? . . . "low PWR" is the best range. FYI, the car is fairly brisk at this speed, certainly adequate for Huntsville AL urban driving.

    Bob Wilson

    ps. The following table tags make it easy:​
    "table=head" enclosed in "[]" - starts a table with the next line the column headers
    "," or "|" separate columns with each line being a row
    "/table" ends the table enclosed in "[]"​
     
    #57 bwilson4web, Feb 17, 2015
    Last edited: Feb 17, 2015
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  18. lexel

    lexel New Member

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    Thank you for your feedback.

    Nominal voltage is 202 V for Prius III.

    Thanks for the nice table. Which higher speeds are you interested?

    Open is the little question if there is a optimum from 30-40km/h. I have tested from 30-50km/h. It could be that a little bit lower than PWR line is better here in this speed area.
    Range 0-30km/h is completely tested.

    lexel
     
  19. The Electric Me

    The Electric Me Go Speed Go!

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    Good god.
    I love this place, and I love you guys...

    But sometimes you all are a little heavy on the charts, graphs and scientific analysis.

    IMO just accelerate safely, but aggressively, get up to speed and then let off.
     
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  20. lexel

    lexel New Member

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    Bob there are some conversion errors in your table.

    Correct is

    Km/h mph power
    1 0 - 20 0 - 12.5 EV
    2 20 - 30 12.5 - 19 (pass through) 1/2 to full ECO
    3 30 - 50 19 - 31 low PWR

    lexel​
     
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