When car is cold, glide in neutral?

Discussion in 'Gen 2 Prius Technical Discussion' started by tom1l21, Jul 5, 2009.

  1. tom1l21

    tom1l21 Member

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    When the Prius is initially warming up, it can be frustrating when you are trying to glide downhill or even on a flat surface and the engine will run. Isn't it better to pulse, then put it in neutral then put it back into drive and pulse again, so that the engine can warm up exclusively when you pulse? I use this method too when I see a big hill up ahead that I know will warm the engine up and I dont' want to warm the engine up gliding to it. What do you think?
     
  2. JimboK

    JimboK One owner, low mileage

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    I think you're at least partly correct. It's not an issue for me because I can shut the ICE down during warmup with the EV button, but I can think of one circumstance where that doesn't work: in extremely cold weather. What I found myself doing in those conditions is seeking somewhat different routes (generally higher speeds) early in the the commute where the ICE would have to run anyway for propulsion, shortening the warmup time as much as possible and letting that energy actually move me.

    As for whether ICE-on gliding is more efficient in neutral than by feathering the pedal, I can't say for sure (I know of no one who has studied it). But considering the short duration of the warmup cycle, any savings one way or the other likely is negligible. One consideration: If the car happens to hit 157F (the S3/S4 temperature transition) at >34MPH and the car is in neutral, the ICE won't shut down; it will if you're feathering the pedal. No auto-shutdown is fine if you're monitoring temperature with ScanGauge or other device and shift out of neutral at that point, but that takes active driver input and steady attention to temperature. I think I'd prefer the passive approach and keep it in D.
     
  3. tom1l21

    tom1l21 Member

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    Does the ICE still run when the car is in neutral though? When it's in drive with ICE gliding, it says that its running anywhere from 67-88mpg, but when I put it in to neutral, it always says 99mpg. Thats why I figured I would let it warm up when I actually had to use the ICE.
     
  4. JimboK

    JimboK One owner, low mileage

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    Depends on whether it's running when you first shift. While in neutral, the ICE remains in whatever condition it is in when the shift to neutral is made. If it's on when shifted, it stays on (regardless of temperature); if it's off, it stays off.

    99 MPG doesn't mean infinite MPG. I've seen instantaneous MPG readings above 99 MPG with the ICE on under very low demand.
     
  5. Bob64

    Bob64 Sapphire of the Blue Sky

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    If the engine is cold, then chances are it'll be running. My experiences that the engine basically shifts itself into neutral, using approx .18 gph when your feathering it. Same fuel usage when its idling. However, the amps drawn is near 5amps, depending on the speed....

    My suggestion: either way is good, but using neutral saves a tiny bit more electric power (which may impact glide distance).
     
  6. Norm611

    Norm611 Junior Member

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    As Bob said, the ICE will remain running if it was running when you put it in neutral. One way you can check is by putting the car in neutral, then letting the car slow down (or slow down with the brakes). As the speed drops below 15-20 MPH you will see the MPG drop. If the engine is burning ~0.18 gph, your mileage will be greater than 100 MPG if you are traveling faster than 18 MPH, and less than 100 MPG if your speed is less than 18 MPH.

    Norm
     
  7. nerfer

    nerfer A young senior member

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    Watching the engine load with the ScanGauge, you see this is actually pretty common. In these conditions, if you slow down to under 15 mph, quite often you'll see the instantaneous reading start to drop to upper 90's, then 80's, but there's still no arrows shown from the engine. It's actually still running, but basically in idle.

    About putting it in neutral, there's an obvious difference for at least the first mile in winter conditions (below freezing, more obvious at below 20'F). Then when you put it in neutral, your instantaneous mpg jumps from 15 or 20 mpg to 80 or better. The engine load drops from about 70% down to about 30%. So there's an obvious improvement, but with the engine not running as hard, I wonder if it warms up slower and you end up just extending the warmup time and it all comes out the same?

    The better answer is using a engine block heater (EBH). Hopefully I'll put one in before this winter starts. I meant to do it last year, but never did, and between that and the coolant thermos not pumping right (fixed in Feb. under the service bulletin) and my short commute, I got terrible mileage.
     
  8. daniel

    daniel Cat Lovers Against the Bomb

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    Please note that it is dangerous to put the car in neutral while moving, as in an emergency you have no power. Note also that in neutral the engine will not charge the battery.

    In many states it is illegal to drive in neutral, because as noted above, it is unsafe. The amount of gas you will save, if any, by using neutral is negligible.
     
  9. JimboK

    JimboK One owner, low mileage

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    Driving downhill is illegal in many states, but I know of no state that outlaws it on level terrain or uphill. If you know of any, please provide a citation.

    The laws of course were written with safety in mind. But that was decades ago when manual transmissions (and deceleration by downshifting) were more common and brakes less effective. It is my opinion they have outlived their usefulness, at least for passenger vehicles. I don't agree that the lack of instant power is an issue. I know of no emergency situation that requires acceleration in an immediate and reflexive way like other avoidance maneuvers (e.g., braking or swerving).
     
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  10. hill

    hill High Fiber Member

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    I can't find the page right now ... but theres a WAY cool mod that a couple PC'ers have done (both are good, but slightly different) by making up a little bread board with the appropriate resistors to fool the system into thinking the ICE is warmed up to S2 or S4 ... I can't remember. I'll see if I can find it.

    Edit:
    see the following:
    http://hiwaay.net/~bzwilson/prius/pri_temp.html

    http://priuschat.com/forums/gen-ii-prius-modifications/36342-another-thermistor-hack-2.html


    enjoy!

    .

    .
     
  11. alanh

    alanh Active Member

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    Note however that the thermistor hacks will probably make your emissions worse.
     
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