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FUEL-CUT while in EV-BOOST

Discussion in 'Gen 1 Prius Plug-in 2012-2015' started by john1701a, Aug 10, 2013.

  1. john1701a

    john1701a Prius Guru

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    Since introduction of the plug-in model Prius, those wanting to undermine its acceptance have been exploiting the misconception related to the 100 km/h (62 mph) threshold.

    Unfortunately, we haven't had a good way to prove the system doesn't actually work as they implied. When traveling at speeds faster than the threshold, the engine RPM isn't 0. That gives the impression the engine is running. We can point out how little horsepower is required to maintain travel at higher speeds, but that message is difficult to convey when there isn't anything observable to support the claim.

    Fortunately, I made an unexpected observation. My morning commute started with an errand that needed to be run, rather than just jumping on the highway like I'd usually do. That meant approaching from a different direction at a slower speed. Further altering the routine, I got stuck behind a string of traffic slowed by a big truck. The end result was cruising along with an engine not fully warmed up yet.

    Since my speed was 65 mph, I was in EV-BOOST mode instead of EV. That means the engine was spinning, but not necessarily consuming any fuel... the aspect of operation we've never really had a good method of proving. Even though the RPM readout was 992, the temperature of the coolant wasn't rising. In fact, it was dropping! That's basically impossible for a combustion engine... since they are so terribly inefficient, losing most of their energy from fuel as heat through the exhaust & cooling system.

    But there I was, watching the coolant temperature drop. It was a new bit of evidence confirming the engine was in FUEL-CUT mode... only spinning, without use of fuel.

    It takes very little energy for the engine to spin without any compression. There's just unimpeded air passing through. The power from the electric-motor and battery-pack were more than enough sustain travel. Seeing the value go lower was great. I had enough electricity available to make the observation twice too, once when the temperature started at 175°F and the other 181°F. Both times I watched 7°F disappear one degree at a time before intentionally speeding up to watch it switch from dropping to rising.

    It was pretty exciting to stumble across new data like that.
     
    #1 john1701a, Aug 10, 2013
    Tracksyde, 3PriusMike and chesleyn like this.
  2. pwp1943

    pwp1943 PHEV Afficionado

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    Great assessment!

    One of the things I’m interested in understanding is how the EV Boost works. My objective is to find the optimal point of operation of the ICE (rpm?, power?, gph?) from a Scangauge II. If I can find that point, then I will let the battery do its job. It should provide just enough energy to maintain the 65 mph speed limit with the ICE operating at an optimal level. My assumption is this will be the best I can do for mpg until the battery is depleted.

    Do you thinks Toyota’s computer system already does this?

    Paul
     
    #2 pwp1943, Aug 10, 2013
  3. Ian Mayo

    Ian Mayo Junior Member

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    Would a small coil set close to a spark-plug HT cable generate enough power to light a LED on the dashboard to indicate the engine is running?

    No spark MUST mean no ICE power.

    I can't believe that the Toyota engineers would allow the ignition to spark when there is no fuel.
     
    #3 Ian Mayo, Aug 10, 2013
  4. drysider

    drysider Active Member

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    ScanGauge shows throttle position (TPS). When the throttle is closed, the engine is not running. Mine is closed at a TPS 0f 14. There is also a fuel cutoff reading which watches the fuel pump. It needs to be calibrated...the website has info on both of these.
     
    #4 drysider, Aug 10, 2013
  5. hill

    hill High Fiber Member

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    John, doesn't this necessarily beg the question then - of how fast can the mg/gear sets spin before it's necessary to burn fuel?
     
    #5 hill, Aug 10, 2013
  6. john1701a

    john1701a Prius Guru

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    It's another way of looking at what we've been talking about all along. Simple terms, like a specific RPM of either MG, accomplish that too.

    In others words, it's an effort to portray operation in a way an ordinary consumer or newbie would readily understand & embrace. What few parameters would owners want to watch on a regular basis?

    We've seen that coolant temperature is an effective means of explaining when the engine will shut off. You just watch that value and know exactly when it will happen.
     
    #6 john1701a, Aug 10, 2013
  7. bfd

    bfd Plug-In Perpetuator

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    I have also observed this - and I can confirm 992RPM as being the engine speed at which things start happening, like temp dropping and MPG going to 9999. This has almost always occurred for me on a downhill at speeds greater than 60MPH while in EV mode. As the glide slows the vehicle, the EV kicks back in, it's usually noticeable when it does. But I've watched the coolant temp drop steadily during my 1 mile downhill, particularly on colder mornings. ScanGauge is indeed handy for these observations. Just never put 2+2 together before reading this.
     
    #7 bfd, Aug 10, 2013
  8. CharlesH

    CharlesH CA HOV Decal #5 on former PiP

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    The main reason the ICE spins when above 62mph is the mechanical design of the Power Split Device. With the ICE off, one of the gears (the one attached to the ICE) is not spinning, and there is a limit to how fast the gears can be spinning relative to each other. One mode of operation to deal with this is to have one of the motor/generators turn the ICE without fuel or compression. Another is to have the ICE running but with minimal power output. In the first case, you will see 999mpg; in the latter, you will see some large but not "infinite" mpg, like 85mpg. All depends on the decisions made by the control unit.
     
    #8 CharlesH, Aug 15, 2013
  9. ItsNotAboutTheMoney

    ItsNotAboutTheMoney EditProfOptInfoCustomUser Title

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    In the Gen 3 liftback, a high-speed glide has an RPM of 960. 992 RPM is the normal minimum RPM. 992 RPM rather than 960 RPM makes sense for a higher-speed glide, but despite that the dropping coolant temperature, I'm not sure that's a guarantee. Check your RPM the next time you have an easy warm-up. Also, you might want to watch GPH. It's 0.02 in fuel cut and I'm not sure how low it goes when pumping.
     
    #9 ItsNotAboutTheMoney, Aug 15, 2013
  10. 3PriusMike

    3PriusMike Prius owner since 2000, Tesla M3 2018

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    I don't have a scan gauge...but I've seen the same things 2 or 3 times. In EV, accidentally going 65 mph (rather than staying below 62), slight downhill, ICE goes on, but then mpg goes to max.

    Mike
     
    #10 3PriusMike, Aug 15, 2013
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