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Question: Prius transaxle, tire diameter, RPM

Discussion in 'Gen 2 Prius Technical Discussion' started by F8L, Oct 13, 2011.

  1. F8L

    F8L Protecting Habitat & AG Lands

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    I'm having a bit of trouble understanding how the HSD system reacts to tire diameter changes.

    In a conventional autox or manual transmission equipped vehicle you can change tire diameter and this will affect your final drive ratio and thus the RPM at a specfic speed. E.g. you change tire size to something larger in diameter and your RPM "at a specfic speed will drop, acceleration will decrease and your mpg will usually go up. Change to a smaller diameter tire and RPM at a specific speed will increase, acceleration will increase and mpg will decrease.

    Does the HSD system respond the same way or does it continually adjust the final drive ratio depending on conditions? Thus tire size will not change the general RPM at a specfic speed.
     
  2. GasperG

    GasperG Senior Member

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    How is the speed measured in Prius, with RPM off the wheels? Changing wheels to bigger diameter (without the car knowing it) would result in speedo showing less than actual speed, odo would show less miles than you really traveled, your MPG would drop (miles driven would be wrong). You would gain a higher top speed, higher speed with electric only, acceleration would be slower (HSD can't produce max HP at slower speeds).
    Of course you would have to measure the actual speed with GPS or multiply indicated speed and miles with diameter change.
    Here is one calculator:
    Tire size calculator
     
  3. David Beale

    David Beale Senior Member

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    The best way to describe the HSD engine RPM is as a load based system. The engine RPM depends on the load, not the speed or wheel diameter. It's set that way to select RPM for best fuel economy while delivering as much power as needed. Until you get MG1 up close to its' max. RPM, then the engine must rev. higher to protect it.
    But good luck measuring it. There are a lot of variables in the load, and I doubt you could ever compare engine RPM and get the same reading twice. ;)
     
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  4. Ct. Ken V

    Ct. Ken V Active Member

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    Hi,guys,

    I didn't look at the "Tire size calculator" yet, but MPG in the Prius is calculated by the on board computer from the "on" duration time of the #1 cylinder's fuel injector & NOT by data from the speedometer, odometer (for distance covered), or wheel speed sensors (basically only used to compare each wheel speed to the others to activate traction control if any of them get to far out of the synchronization range of the others). Your car doesn't know you changed tire size, so therefore can't report to you any new differences.

    Speedometers are typically driven by a gear at the output shaft end of the transmission (whose ratio is engineered at the factory for the standard size tires the car is being shipped with). Because of fender clearances (when turning the steering wheels), any change in wheel size (diameter) usually results in a lower profile tire being recommended (aftermarket) or installed (at the factory) to also keep the speedometer & odometer accurate [by not varying the rolling radius or the circumference of the tire (usually referred to in tire size comparison charts as "revolutions per mile")] since in most cases there isn't another speedometer gear to change to.

    Since the car "doesn't know" you changed tire size it can't report any of the new deviation errors to you. The only way you could see the differences would be if you had a "fifth wheel" hanging off the rear bumper (the way Consumer Reports or Car & Driver tests their cars) or by checking your GPS's indicated speed & distances while driving (as GasperG already mentioned). Toyota's navigation system data (just like a stand-alone GPS) is fed from satellite positioning info only & gets no data from the car's systems.

    Both GasperG & F8L have good handles on the effects of the tire size change. I tried to explain the same things to people either here or over at PriusOnLine some time back & they said I was full of it. The only things I wonder about the 2 previous poster's thoughts are MPG & acceleration. The car's "reported" MPG wouldn't drop (since it's based on the "on" time of the #1 cylinder fuel injector), but the "actual" MPG would be greater than the "reported" MPG because you're covering more ground with each rotation of the larger tire.

    By the same token, the car's acceleration shouldn't be affected because I'm thinking the car's performance should be based on the performance of the engine & drive-train gearing (that aren't changing---only the tires), so a 1/4 mile distance (standard test?) should be reached sooner since you're covering more ground with each tire rotation. If you're going to go by the other standard test (0 to 60 MPH), then the car's "reported" speed of 60 MPH shouldn't change because of no change in the engine/drive-train set up, but in actuality the "true" speed of 60 MPH (if checked by radar) should be reached sooner (again, because of more distance covered per tire rotation).

    F8L, since the Prius isn't equipped with a tach, the only way you can see your RPM's "at a specific speed" is if you have a Scangage installed in your Prius (& that will only be the "reported speed" of the car's speedometer & NOT the "actual speed" as would be clocked by radar). So, yes, the tire size change will affect the general RPM's for the ACTUAL "specific speed".

    In a traditional car with an automatic transmission (in either "D" or "O/D"), engine RPM's in relation to wheel RPM's is pretty consistent over an acceleration range [increase engine speed = directly related increase in wheel speed (fixed gearing except for a small loss due to slippage of the torque converter)]. With a standard transmission car in high gear (4th, 5th, or 6th---depending on the car), the same holds true except for no RPM loss due to slippage because there is no torque converter. So with a traditional car when starting to climb a gradual hill & pressing down slightly to maintain a "specific speed", your engine RPM's should remain about the same (unless you have to downshift the manual gearbox or the automatic "automatically" kicks down a gear).

    With the Prius' HSD continuously variable transmission (CVT), if you approach the same gradual up-hill & press the accelerator lightly to maintain a "specific speed", the engine RPM's will increase (as will the fuel consumption rate) because the "continuously variable" transmission is "kicking down" in a sense (although not like the "kick-down" of a traditional "step" automatic transmission) trying to maintain the same speed range.

    By the same token with a traditional car going DOWN the same slope, the engine RPM relationship to wheel speed RPM's would again remain fairly constant, while in the Prius the engine RPM's go to zero as the engine shuts off, leaving the wheel RPM's (however high they may be) to turn the motor generator to re-charge the HV battery. And remember too that in reverse even up to the top speed (which I think is 25 MPH), the gas engine doesn't run at all (unless the HV battery level is already low & needs more juice). In reverse only the electric motor drives the Prius (because you can't reverse the directional rotation of the gas engine & the HSD CVT contains no reverse gear cluster).

    I only hope now that I haven't confused everybody.

    Edit : I was in "Reply" mode just before David Beale made his post (Yes, I'm a very slow "hunt & peck" typer--- never had a typing course).

    Ken (in Bolton,Ct)
     
  5. xs650

    xs650 Senior Member

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    You can't calculate MPG without knowing how far you have gone. The same signal that is used for the odometer is normally used to determine that.

    Since ABS has become nearly universal, speedometers/odometers are often (almost always?) driven by a computers interpretation of the wheel speed, not a gear on the transmission output.
     
  6. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    This is what I think I know:
    MG2 is a fixed ratio to road speed, if you change tire circumference, you change MG2's speed.
    MG2 has high torque at low speeds and low torque at high speeds, so if you change tire circumference you will alter the torque of MG2.
    The engine runs to provide enough additional torque to meet the driver's request. There is never a fixed RPM to ground speed ratio, so you can't mess it up. The engine will provide the extra torque needed. (to it's ability)
    In some edge cases, the engine RPM is non-optimal for MPG, so MG1 does not over rev, these are rare below 42, above 63 and below 100 MPH. (I can only find a Gen1 simulator, can anyone link a Gen 3 simulator?)

    MG1 provides resistance to the engine so torque flows to the wheels, so long as it is within it's rev limits, we can ignore it. (except as an electrical power source for MG2)
     
  7. Ct. Ken V

    Ct. Ken V Active Member

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    xs650,

    Thanks for pointing out my goof about including the odometer in my statement about the MPG calculations. If I had gone back & proofread the statement [especially the part I myself had included about "(for distance covered)" after the word odometer], I hopefully would have noticed how absurd that sounded : without knowing how many miles were traveled (odometer, of course) for the amount of fuel consumed ("on" time of the #1 fuel injector).

    However, I was not aware that (for the most part?) there are no longer gears being used in the transmission output shaft area to drive a mechanical speedometer cable (my "speedometer cable/output shaft gear" knowledge is of cars up through the mid-90's). I'm learning something new about the evolution of the automobile every day, especially here. Thanks.

    Ken (in Bolton,Ct)
     
  8. seilerts

    seilerts Battery Curmudgeon

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    Ken, at least through mid 2000s, there are output shaft speed sensors in most transmissions, especially truck and SUV. But they are magnetic reluctor type, rather than a physically connected gear. The Prius definitely calculates speed from ABS however, probably due to the extremely tight integration of ABS with the HV ECU due to regen, and/or the fact that HSD is complex enough already.

    There are aftermarket products that will reshape ABS wheel speed sensor signals in order to correct speedometer error if you put on a significantly larger or smaller tire.
     
  9. Hal W

    Hal W New Member

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    There is also the electric motor that helps when you add size to the tires and or more weight to the car.Adding big tires can effect more than just handling and RPM? Hal