Am I doing it right?

Discussion in 'Fuel Economy' started by Gageraid, Aug 12, 2014.

  1. 08EscapeHybrid

    08EscapeHybrid Moderator

    As a general rule, I only allow fuel cut when I wish to throw away excess momentum anyway. (slowing down, stopping, preventing too much speed gained going down a steep hill or mountain)
  2. RoadWarrior222

    RoadWarrior222 rockit serjun

    Nope, need about 5HP worth to overcome friction in motor and drive accessories to keep it at 800rpm or so, some will hold high idle until VSS reads zero. Anyway, 5HP at possibly as high as 400g/hp an hour vs 5HP worth of extra drag, financed at 120% of ~180g/hp an hour. (Or 6HP drag accounting drivetrain loss)
  3. RedylC94

    RedylC94 Well-Known Member

    Nope, you forgot the meaning of the "B" in BSFC. "Brake" means net output, not including that internal friction, or pumping losses. Net output is zero by definition at idle. You're evidently thinking of ISFC ("I" for "indicated," referring to power delivered to the piston by gas pressure in the cylinder.)

    Such details aside, I agree with your main point (at least as I understand it) that it will take more fuel to turn the engine over each revolution at idle than to turn it over with kinetic energy stored from power delivered with a reasonable load on the engine. However, in fuel cut, the engine will likely turn over more times during the "glide" phase, which could consume more energy than idling, depending on the details.
    Last edited: Aug 13, 2014
  4. RedylC94

    RedylC94 Well-Known Member

    Yeah, something like that. That comparison would be interesting to compute if we had a complete set of realistic numbers to work with. Bottom line probably is that fuel cut will work out better (when you don't need to be slowing down anyway) only if the gearing and engine friction are such that "engine braking" is relatively light.
  5. Gageraid

    Gageraid Well-Known Member

    So to conclude... it seems like I should drive a steady speed since I'm not willing to throw it in neutral.
  6. ksstathead

    ksstathead Moderator

    That is my feeling.
  7. 08EscapeHybrid

    08EscapeHybrid Moderator

    Yes, and steady speeds are an excellent tactic to employ. My FEH does extremely well just setting the cruise control at speeds 65 MPH or lower.
  8. Gageraid

    Gageraid Well-Known Member

    Thanks for the input, everyone. I'm very good at maintaining speeds for mpg, was just curious is this other tactic was worth it or not.
    I'm sure I'll have another question in the near future. Off to continue hypermiling in a big ol' SUV. :flag:
  9. some_other_dave

    some_other_dave Well-Known Member

    I can think of a way that even leaving it in gear doing P&G could produce better average FE numbers than steady-state. If your transmission (auto, I assume?) allows lots of slip between the engine and transmission, you could pretty much get some of the benefit of P&G without going to neutral.

    In general, you'd get more benefit from going to N. Just as you'd get even more from turning the engine off--with the possibility of "minor" little problems like trashing the transmission or having to put it in Park to re-start the engine. (Depends on the car; some are fine with engine-off gliding, others are not.)

    You could always try it out for a whole tank if you want to see if the SG is reporting the situation accurately.

  10. 08EscapeHybrid

    08EscapeHybrid Moderator

    The full size GM trucks respond very well to just lifting off the accelerator and letting the transmission coast at about 40 - 50 MPH.
  11. WriConsult

    WriConsult Super Moderator

    Unless your vehicle completely freewheels while coasting in "D" (and very few do), you're throwing away fuel by pulsing and coasting in gear. In effect, coasting in gear is still using engine braking, which throws away valuable momentum to keep the engine spinning for no reason at all. Regardless of what the SG says, you will likely get worse mpg than simply driving steady-state.

    You can still get big gains by doing your "glides" by coasting in "N", without having to kill the engine. But if you're not willing to at least throw it in N (and I'm not sure why), then there's no point. Just stick to steady-state or DWL.
  12. RoadWarrior222

    RoadWarrior222 rockit serjun

    Yah the bigger the vehicle the more it seems to like that approach. My Voyager, the TC stall speed is 2200ish so it feels like you're just warming the transmission fluid to keep steady state at city speed. Then when it coasts in gear, it's really not coupled to the engine much at low speed. So best city driving regime with that is just to give it "blurps" of 2200 about once or twice every block and let it coast in gear the rest of the time.. that gets you about 23mpg city vs EPA 16.

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