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What is the better tactic for maximizing overall fuel economy

Discussion in 'Gen 2 Prius Fuel Economy' started by Aegison, Feb 18, 2010.

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  1. Aegison

    Aegison Member

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    Assume that a light has just turned red some distance in front of you. The road is flat; you've been travelling at 40 mph. No one right behind to be troubled by how you approach the stopped traffic. What is the best tactic for maximizing your efficienty?

    (1) Glide up close to the stopped traffic and then brake as hard as needed to stop.

    (2) Take foot off "gas" pedal and let the pseudo "engine braking" (by the electric motor/generator) slow you down and send electricity to the battery until you reach the stopped traffic and brake to come to a complete stop.

    (3) Take foot off gas pedal and use the brakes lightly and send electricity to the battery until you reach the stopped traffic
     
    #1 Aegison, Feb 18, 2010
  2. ksstathead

    ksstathead Active Member

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    Best fuel economy should come from preserving the highest possible speed, meaning regen early and go a little slower for a longer period, gliding without regen and arriving at the light as it turns green with the greatest speed that is safe and legal. If you brake later you will have to brake more which involves at best conversion losses and at worst heat waste via friction braking.

    Whether slowing down by the brake pedal or the softer 'let of the gas' level is of little importance in my estimation, so long as you stay out of friction braking.

    That's not the question you asked, but I don't get points for maximizing regen. I do get fuel savings points for maximizing fuel economy. Conserve momentum.

    Great thread topic. I've been pondering it and look forward to responses from you and others.
     
    #2 ksstathead, Feb 18, 2010
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  3. Aegison

    Aegison Member

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    Your're right, and I changed the thread's title to fuel economy. Thanks !


    Postscript -- I just found out that I can't (or don't know how to) change the title which shows on the index page -- only the smaller print title right over the post once you've gone to the thread.
     
    #3 Aegison, Feb 18, 2010
  4. don_chuwish

    don_chuwish Well Seasoned Member

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    Someone asked this a while back and got a huge runaround about "just don't stop". I'm curious too if it makes any difference whether you coast or lightly use the brake pedal (say for a speed limited downhill even, not necessarily for a stop).

    - D
     
    #4 don_chuwish, Feb 18, 2010
  5. Jeff_M

    Jeff_M Junior Member

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    "Assume that a light has just turned red some distance in front of you. The road is flat; you've been travelling at 40 mph. No one right behind to be troubled by how you approach the stopped traffic. What is the best tactic for maximizing your efficienty?"

    Assuming I'm familiar with the light and can reasonably predict when it will turn green, my logic is to gradually reduce speed by gliding or coasting with the objective of reaching 20' behind the nearest vehicle when my velocity is 20 mph, at the same instant the traffic is accelerating from the same velocity through the green light. My target battery SOC is usually 2-3 bars.

    That being said, I've only driven our Prius 1000 miles in 6 months (wife is the usual user). So I'm sure you're going to get much better advice from the great user community on this site. Thanks for the good question!
     
    #5 Jeff_M, Feb 18, 2010
  6. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
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    Right or wrong, here's what I do (within the parameters provided in the original post. Individual situations require individual actions):
    I let off the pedal as soon as the light turns yellow. This will recapture as much high-speed regen as I can get, as ksstathead said. This will also slow me down. While I'm coasting and regenning, I count the number of cars at the light in front of me. Multiply that number by two and that's how many seconds it will be before the last car starts to move. Ideally, I want to come no closer than 20 feet to the back of the last car in the event that something unforeseen happens such as a right-turner, a sleeping driver or a false start (where someone jumps forward then realizes they have nowhere to go and then hit the brakes (this makes everyone in line start and stop again before starting again)). In order to maximize fuel economy, I want to maintain a speed greater than 15mph. I have found this to be a good minimum speed to re-accelerate from (below, I discuss higher-speed approaches). So after doing the math and calculating the timing, I fluctuate between regen and glide in order to hit my target.

    There are possibilities of variation. For example, if there's no one behind me, I will regen for a while and then, depending on my state of charge, pulse and glide between 35 and 40 mph stealthing occasionally just so long as I time it correct to never encroach on the 20-foot buffer between me and the last car in line.
     
    #6 TonyPSchaefer, Feb 18, 2010
  7. BAllanJ

    BAllanJ Active Member

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    You would reduce losses from air resistance by having the lowest speed, so if you're going to have to stop...ie really long red... My guess is brake moderately down to whatever speed where you can then just glide up to the light without braking. My bet is that this thinking is too simplistic because there may be efficiencies in regenerating at certain current levels to the battery, but this would make the maximum kinetic energy available for regeneration.
     
    #7 BAllanJ, Feb 19, 2010
  8. don_chuwish

    don_chuwish Well Seasoned Member

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    Methinks this requires ScanGuage measurement to get a real answer. Several runs at the same starting speed, braking/slowing distance to a full stop and note the readings. Since I don't have one, couldn't say what the specifics would be. But I'm inclined to believe that the difference isn't significant enough to sweat unless you've already mastered every other hypermiling technique and want to squeeze out another fraction of a mpg.

    - D
     
    #8 don_chuwish, Feb 19, 2010
  9. F8L

    F8L Protecting Habitat & AG Lands

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    I keep things simple...

    In the above scenario I would let off the gas and glide (no arrows) as long as I could and as I approach the stopped cars I let off the gas completely to regen and recapture some energy while assising in slowing down. The extra energy could assist in an EV only take off from the stop light or stop sign.

    This is easy for every day driving and not having to think about much. Tony's advice is good. I think that in some situations just gliding to a complete stop (or gliding and timing lights and traffic) may be even better but that is only tolerable in certain circumstances. :)
     
    #9 F8L, Feb 19, 2010
  10. sfvb

    sfvb New Member

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    You forgot one. The idiot that has to pass you to hurry up and get to the stop light.
     
    #10 sfvb, Mar 1, 2010
  11. TonyPSchaefer

    TonyPSchaefer Your Friendly Moderator
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    I get asked that all the time. The only answer is : recalculate and adjust accordingly.
    Those people tick me off. :tsk:

    Though this morning there was an example of just the opposite. A guy in a truck who figured it out at the very first light. There are four stoplights within the first mile of my morning drive. He and I were headed in the same direction. He picked up on my coasting to the first light (there were already waiters) and hung back. When the light turned green we moved on but the next one was red with an oncoming vehicle waiting to turn left; we coasted and he hung back. We both turned right at the next light and there's a few miles of multi-lane so he passed me civilly and seemingly without malice. It helps to note that for that first leg, the street is three lanes wide and he probably could have gunned it around me if he wanted.
     
    #11 TonyPSchaefer, Mar 1, 2010
  12. richard schumacher

    richard schumacher shortbus driver

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    Assuming that the "native" foot-off-the-gas regeneration rate is optimal for the battery, do that. Without knowing or assuming when the light will turn green/traffic ahead will resume moving, you cannot do better. No, I can't prove this :_>
     
    #12 richard schumacher, Mar 2, 2010
  13. Aegison

    Aegison Member

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    I appreciate the good advice given so far.

    However, indulge me as I re-phrase the question with which I started this thread, as I feel like the guy who asked what time it was but was instead told how to build a watch.

    I know it's best to conserve momentum and not have to come to a complete stop as traffic starts moving on the green light. Etc.

    So, let's change the scenario -- You've just come around a bend in the road and see for the first time that you're approaching traffic which is stopped at a train crossing, and the wait has just begun. The road is level. No chance you'll coast up when the cars begin moving no matter what you do. No side streets to duck down. No earlier warning. The way into the parallel universe is blocked. etc. You're going to reach a line of stopped vehicles in a given amount of distance, and have to come to a complete stop yourself. No way out.

    And so, my question is still, essentially, which is more fuel efficient:

    1. using "pseudo engine braking" and then using the brakes "at the last minute" to stop yourself when you reach the stopped cars? or
    2. using the brakes continuously during the slow-down and stopping completely when you reach the stopped cars? or
    3. I guess I should also include gliding as far as possible and then braking to a stop, just to cover all the bases.
    I guess another way to say it is, simply: which is more fuel efficient -- pseudo "engine braking" or use of actual regenerative braking?

    Thanks,
    Rick
     
    #13 Aegison, Mar 3, 2010
  14. 2009Prius

    2009Prius A Wimpy DIYer

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    I suppose the other way to formulate the question is: which braking method is the most efficient in regeneration? Assuming your engine is fully warmed up such that once you let go the gas pedal the engine shuts off, I would apply even braking until I stop behind the stopped cars. Because if I don't brake until the last moment and then apply maximal brake, I may exceed the max current the HV battery is programed to receive and thus lose some regenerated energy. Just my 2 cents. :)
     
    #14 2009Prius, Mar 3, 2010
  15. ksstathead

    ksstathead Active Member

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    There is no energy lost to friction braking if you do not use the brake pedal, so if that gets you down to 7 mph or less in time, go that route and only brake for final stop.

    There is minimal loss to friction braking as long as you keep the hsi within the regen region, so brake early enough to stay within that. I suspect that longer, lighter regen is more efficient than shorter, stronger regen, but cannot back that up.
     
    #15 ksstathead, Mar 3, 2010
  16. uart

    uart Senior Member

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    And even if it's not more efficient it's still going to be easier on the battery. :)
     
    #16 uart, Mar 4, 2010
  17. Tamyu

    Tamyu New Member

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    I don`t know the real answer to this and have a III not II, but just watched the DVD (Japanese) that came along with our Prius... And there is a situation almost exactly the same as this with "engineer advice".
    They say it is best to brake lightly to provide a long period of light regen if you know you will need to come to a stop. It says you will gain more charge than a short period of strong regen. Whether that also applies to the II or is true in real life situations, I have no idea. :) But it was in official Toyota Japan stuff.
     
    #17 Tamyu, Mar 4, 2010
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  18. qbee42

    qbee42 My other car is a boat

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    1. B mode braking wastes energy. Don't use it for normal braking.

    2. Continuous braking spreads out the regeneration and helps avoid friction braking. This is generally the most efficient.

    3. Fast braking at the end can exceed regeneration limits and force friction braking. Not as efficient.

    Use #2 for normal braking. An additional detail: light regenerative braking (foot off the gas coasting) is less efficient than using a bit of the brake pedal. There are some fixed losses. With light braking, much of the energy is consumed by the fixed losses, so not as much gets into the battery.

    Tom
     
    #18 qbee42, Mar 4, 2010
  19. 2009Prius

    2009Prius A Wimpy DIYer

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    Glad to know my hunch

    http://priuschat.com/forums/gen-ii-...izing-overall-fuel-economy-2.html#post1075916

    got the official Toyota approval! :D
     
    #19 2009Prius, Mar 4, 2010
  20. usbseawolf2000

    usbseawolf2000 HSD PhD

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    It depends on how long the red light would last. If it is going to take a while, I would start to brake. If I expect it to turn green, I would glide. I I am not sure, I would regen-glide (foot off accelerator).
     
    #20 usbseawolf2000, Mar 4, 2010
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