Am I doing it right?

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

  1. Gageraid

    Gageraid Well-Known Member

    I believe what I'm attempting is the P&G technique with the engine still on.
    So am I doing it right...
    Let's say I'm on a back road with the speed limit of 35, would I be saving gas if I give it has to speed up to 42mph and let it coast for a few seconds back to 35 (in drive), or would this waste gas versus just cruising at a constant speed?
    I know this method at least tricks the scangauge to rear higher average numbers. I'm about to mess around and shut off the fuel cutoff setting because I'm about 99 percent sure my engine doesn't do it, or very rarely.
     
  2. RoadWarrior222

    RoadWarrior222 rockit serjun

    I am not too familiar with scangauge, but I thought it could read injector duty cycle, if that says you're getting zero fuel, you're getting zero fuel.

    Hiiiiighly dependent on ECU and transmission programming though, my Note seems to do fuel cut down to 1000 rpm and holds lockup down to that, the Escort only seems to do it down to 1800ish, and the lockup doesn't hold as low either... My old '88 Voyager even has SOME fuel cut but just about the only way to stay in it long is to be engine braking downhill at about 50mph+
     
  3. Gageraid

    Gageraid Well-Known Member

    It can read gallons per hour which is one of the parameters I have on right now.
    TPS, LOD, GHP, and average is my setup right now.
     
  4. ksstathead

    ksstathead Moderator

    P&G would have you shift to neutral for a glide.

    I would not expect great fuel savings from leaving it in D as described because you are not gliding. You need to remove the power train connection.
     
  5. Gageraid

    Gageraid Well-Known Member

    Got ya. I don't plan on putting it in neutral. I'd still like to find out if I'm tricking my Scangauge, or if this is actually working.
     
  6. 08EscapeHybrid

    08EscapeHybrid Moderator

    Chances are if your vehicle is compatible with a Scangauge, it has fuel cutoff. Some vehicles more eagerly enter fuel cut mode than others. Pretty much you don't want to use fuel cut anyway unless you're going down a steep hill or mountain. Fuel cut on moderate descents takes too much momentum away which has to be replaced by burning more fuel when the injectors come back on. What type of vehicle do you have? (year/make/model/engine/transmission?)
     
  7. ksstathead

    ksstathead Moderator

    I don't know what you gain by negating DFCO. It saves fuel every time you unexpectedly need to throw away momentum, like a light changes or you get cut off.

    Otherwise, you are idling for nothing.

    I can think of no time when DFCO costs you fuel. Needlessly using DFCO instead of maintaining momentum costs you fuel.

    What is wrong with neutral?
     
  8. timw4mail

    timw4mail Well-Known Member

    What do you do with your right foot when driving in neutral?
     
  9. Gageraid

    Gageraid Well-Known Member

    A little confused by this post. I always coast in gear to lights... There is no information on whether my vehicle has DFCO or not.
    Also, there are no reports of continued use of using neutral like this in my vehicle. I don't wanna be the first to screw it up.
     
  10. Gageraid

    Gageraid Well-Known Member

    Makes sense.
    2008 Jeep Liberty 3.7L 4 speed automatic.
     
  11. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I usually stretch it out on top of the dash. Helps to prevent cramping.

    Wait, that's not right. That's what I used to do when I still employed the cruise control.
     
  12. ksstathead

    ksstathead Moderator

    Coasting in gear is not gliding. You are throwing away momentum.

    PRND&L each have their uses. You don't have to use N, but it would extend your glides.

    Other things being equal, best thing is not ever over accelerating vis-à-vis the next light, and just using a constant low throttle. Since that is not always achieved, and since a big ICE is not efficient at low throttle, P&G was born.

    Think of it this way, in gear, your ICE is always turning, whether or not fuel is flowing. The friction/heat loss of that is not insignificant.

    Some trannies don't reengage as quickly/smoothly as others, and AT can never match a FAS'd MT, but N is available to you. Try it when you will have to stop anyway, and maybe work up to using it when you will reengage D at low speed.
     
  13. RedylC94

    RedylC94 Well-Known Member

    Buried inside Edwin's joke about that, he made a good point. While you're in neutral is a good opportunity to rest that right foot, stretch your tired ankle, whatever. (Going so far as to prop it up on the dash might be a small problem in case you unexpectedly have to brake, though.)

    Besides moving the car, it takes energy just to turn the engine over each revolution, so you want to minimize the number of unnecessary engine revolutions in your journey. Neutral coasting is only one of several ways of doing so.
     
  14. RoadWarrior222

    RoadWarrior222 rockit serjun

    Well having an inkling of the way Chrysler do things, I'm gonna take a guess that it's only in lockup from 40mph upwards, so you might not get much fuel cut until you're on 50mph roads and can do 50-40
     
  15. RoadWarrior222

    RoadWarrior222 rockit serjun

    Yah but the thing is, do you do better spinning the motor at it's lowest efficiency (idle) or using fuel at higher efficiency (pulse at 50-80% throttle) to get momentum to drive it through friction at probably 20% drivetrain loss there. If your BSFC is more than 20% better in the pulse regime than the idle regime, then you probably do better in fuel cut than taking longer glides in neutral.

    Theoretically, that motor being a lopped V8 should/oughta have similar bsfc map to the 318 which maybe has data.
     
  16. 08EscapeHybrid

    08EscapeHybrid Moderator

    I will pretty much guarantee you have fuel cut. I'm not sure how eagerly it will enter fuel cut mode though. There's a set of parameters that must be met first, and its different from vehicle to vehicle.

    For example, my 1998 GMC Sierra (5.7 L V-8, 4 speed automatic with 4wd) enters fuel cut quite easily. Here's what it requires:

    Engine RPM > 1500 and zero throttle input for > 3 seconds.

    Cutoff continues until RPM < 1000 or throttle input is detected.

    Set one of your gauges on your scangauge to report open/closed loop operation. On the highway accelerate up to speed and at the top of a decent hill take your foot off the accelerator and count the seconds until your scangauge reports open loop. That's how long it takes to enter DFCO in your vehicle. You can then experiment to find your minimum activation RPM and at which RPM it cancels. When it cuts off you may notice your engine braking to slightly increase and/or your RPM's drop slightly. When it reactivates your injectors you may feel a small surge. I'm so in tune with my K1500 I can feel when it cuts in and out, and don't need a scangauge to tell me.
     
  17. RedylC94

    RedylC94 Well-Known Member

    BSFC is infinite (or a divide-by-zero issue, actually) at idle, so that's easy. The second half that sentence is the interesting half. Fuel cut might theoretically be better in some cases (i.e., in a car that maintains fuel cut down to a relatively low engine speed, and a transmission that chooses to spin the engine at a speed only a little above that), but likely not in other cases (transmission doesn't have high enough gear, or downshifts to increase engine braking).
     
  18. RoadWarrior222

    RoadWarrior222 rockit serjun

    Yeah if you sit crosslegged on your hood overnight wearing nothing but yak butter, reaching out to feel oneness with the inner mysteries of your vehicle....


    ... you'll probably catch a cold and the neighbours will gossip.... but it really only takes a bit of listening and paying attention, slight tone change, slight nudge. I was oblivious to it for years, now I can tell when other people vehicles I'm riding in are doing it if I'm alert.
     
  19. 08EscapeHybrid

    08EscapeHybrid Moderator

    Great. What am I going to do with all this yak butter then?

    [​IMG]
     
  20. ksstathead

    ksstathead Moderator

    Pulses are identical either way: Fuel to climb from say 25 mph to 40 mph at a high load and low rpm will take x amount of fuel and cover y amount of distance regardless of what is done between pulses.

    It will take more pulses to cover a given distance if the glides are shortened due to DFCO. That should outweigh the idle fuel consumption during NICE-ON.

    But the math would involve the idle fuel consumption per glide, times the number of glides, less the pulse fuel consumption times the EXTRA number of pulses over the given distance.
     

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