How does it work exactly?

Discussion in 'Gen 3 Prius Main Forum' started by LDB, Oct 19, 2014.

  1. LDB

    LDB Member

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    Does the Prius run on battery power only under certain circumstances? Here's my scenario. I live fairly centrally in a small town. I help out a friend now and then at her business which is 1.7 miles away. The first 1.4 miles are in the neighborhood with a maximum speed limit of 30mph. The last 1/3 mile is a main road with a 45mph limit. The same 1.4 miles applies if I'm headed to the grocery store or to get breakfast.

    I'm curious if some or all of that 1.4 miles could/would be driven on battery only, saving even more gasoline, or if the gas engine runs all the time. Along with that, what is the effect of needing a/c or heater?
     
  2. css28

    css28 Senior Member

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    The first mile or so is the least efficient for the Prius.
    It's fundamentally a gasoline powered car and the engine needs to warm up before it will run efficiently.
    The Prius can run electrically for a mile or so under some conditions but generally not until it's warmed up.
    There's no free lunch here--electric motivation is best when driving by engine would be at its worst (stop and go, parking lot cruising). Whatever battery you use has to be made up with engine use anyhow (and with losses in the charging/discharging cycles).
    Using the AC for cooling draws the battery down faster. Heating requires the engine to be warm and will force the engine to run more initially.
     
  3. Mendel Leisk

    Mendel Leisk Underfoot

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    Electric mode will only be engaged by the car when it's decently warmed up. With a 1.7 miles commute that's not going to happen. I wouldn't try forcing it into EV mode either. I'd leave the heater and A/C off for such a short run, as much as possible. In particular the A/C, which won't be much effective in such a short distance.
     
  4. kbeck

    kbeck Active Member

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    You're thinking about it the wrong way. It's like this:
    1. Prius is a full hybrid: It can be powered from the gas engine, the electric motors/battery, or both.
    2. Fundamentally, one puts in gasoline in the tank, exhaust comes out the tailpipe, and when that's going on, the car moves. So, from that perspective, the Prius is not an electric car: It's a gas car, albeit with good gas mileage.
    3. The Prius engine complex is computer controlled, in spades. The people who wrote the program wanted good gas mileage, decent pickup, and a reasonable driving experience. So, the Prius will, willy-nilly, switch from gas engine to electric motors to both or neither as the programmers desired, based upon the demands (how far down the gas pedal is pushed) of the driver.
    4. If the driver has a passing familiarity on how the guts work, the driver can make the car have better gas mileage. However, this point is also true for a driver pushing the pedal on a conventional car; being light on the gas pedal helps both conventional cars as well as Priuses on gas mileage. Being a lead foot driver will make the gas mileage go down on both types of cars.
    So, in terms of how it works: The gas engine is an Atkinson-cycle motor. Main point: It's more efficient than the standard, Otto-cycle motor used in most gas-powered cars. Trick with Atkinson motors: Great efficiency, lousy torque. So, thwap a pair of electric motors and an "interesting" planetary gearset into the car in place of a transmission, rigged so the gas engine can make the car move forward, the electric motors can make the car move forward (with the gas engine stopped), and torque to the driving wheels is a combination, computer controlled, of the gas engine torque and the electric motors torques. So: Electric motors have great torque; combined with the Atkinson motor, the pair (motors/gas engine) is comparable in driveability to a gas-only motor, alone. At much better efficiency.

    Now that the electric motors are in place, one gets several interesting benefits:
    1. When the gas engine is running, the electric motors and planetary gearset form a continuously variable transmission towards the driven wheels. What this means: For a given power level from the gas engine to the driven wheels, the effective gear ratio is adjusted to keep the gas engine running in its high-efficiency area. (Plus or minus electric losses going from one motor generator to another; this is presumably adjusted for by the people who did the programming.)
    2. There's a battery pack present into which energy can be dumped or extracted. It's not meant to hold much; I believe somebody calculated the total amount of available energy as about two tablespoons of gas. But this is where the torque comes from when needed. It's also where energy can go when braking (regenerative braking). It might be noted that pulling energy out of the wheels, through the motor generators, into the battery, and back out again to the motor generators for acceleration is a pretty inefficient process; but, whatever it is, it's better than dumping all that motive energy into heat with standard brakes, which is what normal cars do.
    3. On a Gen III, when conditions permit, and typically at low speeds somewhere below 60 mph, the gas engine will stop and the car will run on electrics. This partly gets around the inefficiency of gas engines, in general, at low power output. So, if one is in stop-and-go driving, the gas engine will kick in when one accelerates to speed with a boost from the motors as well, then go off and run on battery while at constant speed (below 30 or 4o mph, typically), then regenerative braking when one comes to a halt, repeat, lather, rinse. This is why city driving MPG numbers are a hair better on a Prius than the distance driving. Note that it is possible to run the traction battery down whilst running around like this in traffic. At that point the gas engine will kick in and charge the battery, even if the car is at a dead halt. Which doesn't do gas mileage any good.
    Note, however, that despite all the stop-and-go improvements on a Prius: If one drives in a straight line at 30 mph, one will get much better gas mileage on a Prius than if one does stop and go.

    People get good gas mileage on a Prius by driving it, well, like a normal car. You can get somewhat better gas mileage numbers by avoiding jackrabbit starts and abrupt halts, just like with a "normal" car. There's a minor trick, mentioned in the car user manual: If you're traveling in a straight line at lower speeds (below 40 mph), the car might be running the gas engine; by coming off the accelerator a bit, it'll switch to electric, at which point one can go back to where the accelerator was in the first place, and one will be running along at the same speed with the gas engine stopped. It helps a bit, it's not that big a deal, and there's a gauge selection on the dash that helps with doing it, if one is so inclined.

    Then there are the hypermilers who float around here. These are the zanies who can get 40 mpg on a Caddy Seville. They do stuff. They like the Prius. They can take a stock Prius, with or without extra instrumentation, and get north of 70 mpg where you or I would be lucky to get 50 mpg. The technique they use that I hear the most of is something called, "Pulse and Glide", where they speed more-or-less abruptly up to a certain velocity, come off the gas to a particular set-point, and slowly glide and slow down to a second speed, then repeat. As might be imagined, doing this in traffic doesn't win any favors. But to those who have hobbies, why not?

    In conclusion: By and large, one can get 50 mpg or thereabouts on a Prius with little or no effort, and by just driving it like a normal car. (This is not true for most commercially available hybrids: Toyota did a good job here.) With a bit of effort, one can do better; and the car will get worse gas mileage (like other cars) in cold weather. It handles OK, not great, but I've had worse-handling cars in the past.

    Hope this helps.

    KBeck

    (changes added for English errors.. And saying that the car's a RWD. I know it's not, sorry 'bout that.)
     
    #4 kbeck, Oct 19, 2014
    Last edited: Oct 20, 2014
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  5. LDB

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    Thanks for all the input so far. It is just what I wanted to know. My hero is the tortoise not the hare. I drive with a very light foot. It will work well on the get go but won't do much good for regenerative braking as I very often only have to lift off while most others are on the brakes. I thought there might be a possibility that a fair amount of my local 2-3 mile drives might use no gasoline but I guess not. I think with my usual light foot I'd do well on the gasoline side though.
     
  6. DoubleDAZ

    DoubleDAZ Senior Member

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    With your that kind of driving, you might want to consider the PiP. I didn't check to see how much more they cost and I could be wrong, but I don't think it goes through the engine warm-up cycle like the regular Prius does. Therefore, your 3.4 mile round-trips should be all electric. The drawback is you'd need to plug it in, though I don't know how often, I'd guess every couple of days. Someone will correct me if I'm wrong.

    With the regular Prius, it will have to warm the engine virtually every time you start it, assuming it cools off when you're helping your friend. Being in Houston helps though since it doesn't get very cold there. Mind you, I think you'd still get good mileage, but I have no idea what it would be under those conditions and I don't know what mileage you'd consider worth buying a Prius for.
     
  7. CaliforniaBear

    CaliforniaBear Clearwater Blue Metallic

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    Hi kbeck,

    That is a fantastically good explanation. I'm going to copy it to give to friends that ask about the Prius. However!!... The Prius is front wheel drive. I know its difficult for us old timers to forget that differential full of 90-weight and the screech of rear tires at the start of a drag race. Sometimes I forget why I walked down the hall into another room to do something.

    Regards CalBear
     
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  8. GrumpyCabbie

    GrumpyCabbie Senior Member

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    The Prius is a petrol car with electric assistance. Whilst it can run on electric alone, the set of parameters to allow this are quite limited. In absolutely ideal circumstances (outside temperature, battery temperature, no a/c etc) you might be able to eke out a mile on electric but the return journey would be much less efficient than it should as the car tried to charge up the battery as well as propel the vehicle.
     
  9. cycledrum

    cycledrum PSOCSOASP

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    Electric drive will be used for low to very low load (power) demands. You may be only going 8 mph, but accelerating up to 20 or more MPH and acceleration demands power. Accelerate at a normal rate with gas engine.

    You won't get big MPG numbers on 1 or 2 mile trips, maybe 40 tops on gage.

    Nothing wrong with buying a Prius for 9k miles a year driving. I'm about to hit only 27k miles total and 3 years comes up Nov 16th.

    One more thing, there are many circumstances where no gas will be flowing to the engine, even over 60 MPH: Lift up on accelerator, boom, no gas to engine. You see that on instant MPG gage.
    This car is crazy good on gas, especially in mild SF Bay Area weather.
     
  10. SageBrush

    SageBrush Senior Member

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    Superb post, Kbeck.
    My only nit to pick is implying the car is a RWD.
     
  11. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    All Prius are front wheel drive.
     
  12. cycledrum

    cycledrum PSOCSOASP

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    Just a couple things to add to kbeck's post. Front wheels are drive wheels on Prius.

    Pretty sure 42 mph is the cutoff for pure electric propulsion (under light load like super smooth road no headwind and for 1/2 mile if lucky) because of battery power and motor speed limitations.
     
  13. SageBrush

    SageBrush Senior Member

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    OP,
    Read Kbeck's post for a very good overview.

    Here is the 'Prius for Dummies' version: The regular Prius is a gas car. Fancy stuff goes on under the hood with electricity to make the petrol engine more efficient (higher MPG.) If you force the car to run on only electricity today, you WILL pay for it tomorrow with petrol, and typically you will end up paying more overall than if you had just let the car do it's thing.

    That is about right for Gen2; Gen3 is 45ish. These numbers presume the threshold is reached from a lower speed.

    Knock off ~ 3mph if your starting point is a higher speed. Hysteresis is a bitch.
     
    #13 SageBrush, Oct 19, 2014
    Last edited by a moderator: Mar 1, 2015
  14. kbeck

    kbeck Active Member

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    Think I read somewhere that when the second set of planetary gears were added to the 2nd motor generator on the introduction of the Gen III, the top speed at which the gas engine could be stopped was around 60 mph. Gen II cars had a lower limit.

    As you well-read guys know, above some speed the gas engine has to be turning over in order to prevent one motor generator or the other from overspeeding and potentially causing damage. However, if one is going downhill and the power demand for the gas engine happens to be nil, I believe the gas injection will turn off, the spark plugs will turn off, and the gas engine will be spun by the motor generators as required to keep the overspeed from occurring.

    KBeck
     
  15. SageBrush

    SageBrush Senior Member

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    ^^ About 60 mph for the PiP.
    Right you are about going downhill; I let cycledrum slide on the inaccuracy because he said 'electric propulsion,' which I took to mean not potential energy.
     
  16. Rebound

    Rebound Senior Member

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    I have a Plug-in Prius, so I would travel the entire 3 miles on battery, as long as I don't accelerate too hard and if I keep the heater off.

    The AC is electric, but the heater is conventional, meaning that it relies upon the engine's heat. If I turn the heater on, the engine starts immediately when I turn the car on. So on moderate days, I make sure the heater is set to the coolest temperature, and I use the seat heaters for heat and the AC to defrost the windshield. But if it's really cold, I've just got to bite the bullet.
     
  17. hybridbear

    hybridbear Member

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    A plug-in hybrid would be best for you in that scenario. The Plug-In Prius doesn't count as you cannot get decent acceleration on battery power only. A Ford Energi model or a Chevy Volt might be a better fit for you rather than buying a Prius.
     
  18. budrow56

    budrow56 Junior Member

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    I have a 2012 prius v and and getting 52.5 mpg city and 56 highway.
     
  19. css28

    css28 Senior Member

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    Truthfully though you wouldn't be getting that mileage with 1.7 mile trips.
     
  20. budrow56

    budrow56 Junior Member

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    True on 1.7 mile trips you would barely use gas at all it would be on ev power unless you were climbing hills.
     
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