Prius HSI display hypermiling guide

Discussion in 'Fuel Economy' started by yota4me, Mar 3, 2014.

  1. yota4me

    yota4me New Member

    Is there an expert version of an illustrated Prius HSI display hypermiling guide?

    Included here is a sample of what I’m referring to.

    Along with instant mpg on Scan Gauge, the HSI is my standard economic driving guide. I find the instant mpg very helpful to fine tune optimum accelerator pedal pressure at any given time.

    I’d like to apply all the hypermiling techniques to the Prius hybrid, keeping things as simple as possible. The HSI seems pretty simple.

    My average mpg, per the onboard mpg tracker has been roughly 62-65 mpg in my PriusC. That’s with 40 psi in the original tires, without pulsing, using EV to accelerate to max (30-35 mph).

    Recently, I changed the ATF fluid & noticed increased drag & lower mpg, which I assume is due to higher viscosity of new fluid vs 26,000 mile old. In spite of the reduced mpg from new ATF fluid, I’ve been able to increase average mpg to roughly 68-72 by increasing tire psi to 46, adding pulsing, limiting EV only acceleration to 15 mph then jumping to ICE acceleration as noted in sample HSI guide.

    Now, if I could perfect these hypermiling techniques, run 50 psi in LRR Michelin Energy Saver tires, use summer blend, real gas….

    Last edited: Mar 3, 2014
  2. jdk2

    jdk2 Member

  3. yota4me

    yota4me New Member

  4. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Jdk2:

    Hobbit screwed it up again with his insistence on higher RPM running in a higher load range. I tried to get him to back off with the gen II and here he is doing it again with the gen III. There is a big difference and one of the reasons why there are 70 + mpg drivers and 60 mpg drivers. He is the pushing readers to the latter. :(

    HSI is also variable depending on SoC.

    And you need an SG-II or similar to watch the tach as the gen III Prius liftback HSI does not provide real ICE-Off indication. You can deduce warp stealth out of it however.

  5. jdk2

    jdk2 Member

    Hi Wayne,

    The reason I linked that article was for illustration purposes only to the OP. It was written quite a while ago, probably right after the Gen III was introduced. I'm certain there have been others written since but I was made aware of that link when I first bought my 2010 Gen III late in 2009 to try and understand how the HSI worked.

  6. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Jim:

    Thanks for the clarification.

  7. Jerry Liebler

    Jerry Liebler Member

    Wayne, are you simply saying that it is more economical to use less power?
    I have observed that 1/4"right of center on the HSI corresponds to 13hp or 10KW of ICE power output and when the bar is above the space between the W and R of PWR that is about 40 HP or 30KW. This is the range of best ICE efficiency according to a widely published map of the 1.8L Prius engine. Hobbit suggests that operating in this range is OK but does not mention losses in the "transmission". Greater power flow will result in greater losses so operating with lower power flow will get better fuel economy. Or what do you mean by the first paragraph?
  8. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Jerry:

    Yes. The maps do not translate to the road and have to be rethought for the best fuel economy. It is an analog process beyond the maps.


  9. waltermlee

    waltermlee Well-Known Member

    The Hatchback Prius Atkinson cycle motor most energy efficient mode is when the engine is running between 60% to 80% Load while running from 900 to 1300 RPMs while burning from .46 to 1.00 gallons per hour. IMHO a 3rd gen Prius needs to be coasting with the engine off about half the time to get even close to +60 mpg. The Prius loses most of its energy when it makes a full stop (if the Prius curb weight wasn't so high stopping wouldn't be so costly energy wise). As the Prius driving on a flat smooth road at about 70 F degrees in a non stop circuit goes from 25 mph to 75 mph its MPG drops from +90 mpg to 44 mpg due to aerodynamic drag. As the driving temperature drops from 75 F degrees to 32 F degrees, the Prius fuel efficiency can drop by as much as 30% due to thermal loss.
  10. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Waltermlee:
    Now that is what I am talking about. Great post!

  11. RedylC94

    RedylC94 Well-Known Member

    In that case, at three times the speed, the car is about twice as efficient in terms of distance traveled per unit fuel consumed. However, the energy required per mile to overcome aerodynamic drag is 9 times as high at 75 mph, so the engine is more efficient then.
    We tend to confuse car efficiency (distance traveled per unit fuel) with engine efficiency (energy output per unit fuel input). The former peaks at a much lower speed than the latter, on any car, because of aero drag.
    Last edited: Mar 29, 2014
  12. some_other_dave

    some_other_dave Well-Known Member

    There's an argument about that very thing over at ecomodder lately. A relatively-new (but very knowledgeable, all-in-all!) guy has been confusing those two points...

  13. waltermlee

    waltermlee Well-Known Member

    Yes. I did not make that clear. My bad. The on-board computer tries to run the ICE efficiently when ever it is needed and that every time you stop and then restart an ICE there is some loss in energy efficiency. The reason a Prius get good MPG at 25 mph is that aerodynamic drag is minimal at this speed and because at that speed the Prius will turn off the ICE frequently so it uses very little fuel at this speed so this makes for a very energy efficient car but not necessarily the most efficient use of the ICE due to the energy loss every time the ICE is started and then stopped. The Atkinson cycle engine minimizes the loss but there always is a tiny bit of loss. When a Prius goes at 75 mph - the ICE is on all the time so there is no loss from stop or starting - this more energy efficient for the ICE but at those speeds the Prius is spending over 50% of its energy over coming aerodynamic drag and so its MPG drops.

    A car's energy efficiency is also dependent on the driving environment: light traffic, no stop signs , warm weather, dry smooth roads surfaces, no cross winds (or a tail wind), and a down hill grades- can all helps boost a car's energy efficiency. It's when the driving environment isn't favorable: like cold weather, heavy head winds, rough road surfaces , stop and go traffic jams, and uphill road grades - that a hypermile's skill is really challenged. ..
    Last edited: Apr 21, 2014
  14. Jerry Liebler

    Jerry Liebler Member

    Interesting, energy loss starting and stopping the ICE may explain why my high speed pulse & gradual deceleration (DWL with ICE still running) seems to get better fuel economy than pulse and glide. I've averaged 70 MPG(73displayed) at 50 mph average on a 30 mile round trip. The technique was pulse to 62 keeping the HSI in ECO then back off the pedal to where the HSI is just right of center till speed drops to 50 then pulse again.
    Note, I have a plug in Prius so I can force a glide below62MPH by pulsing the EV button.
    Last edited: Oct 31, 2014

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