How Does Electric Steering Work?

Discussion in 'Gen 3 Prius Technical Discussion' started by Maine Pilot, Aug 29, 2011.

  1. Maine Pilot

    Maine Pilot Senior Member

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    None of my searches was successful on this subject, so here goes: How exactly does the power steering work in a Prius? Is it similar to the power brakes or airconditioning; i.e., an electric motor takes the place of a fan/serpentine belt and powers up a hydraulic pump; or does it use another method?
     
  2. xpcman

    xpcman Senior Member

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    Re: HowDoes Electric Steering Work?

    No hydraulic pump used in the Prius system.
    Electric power steering (EPS or EPAS) is designed to use an [ame="http://en.wikipedia.org/wiki/Electric_motor"]electric motor[/ame] to reduce effort by providing steering assist to the [ame="http://en.wikipedia.org/wiki/Driving"]driver[/ame] of a [ame="http://en.wikipedia.org/wiki/Vehicle"]vehicle[/ame]. Sensors detect the motion and [ame="http://en.wikipedia.org/wiki/Torque"]torque[/ame] of the steering column, and a computer module applies assistive torque via an electric motor coupled directly to either the steering gear or steering column. This allows varying amounts of assistance to be applied depending on driving conditions. The system allows engineers to tailor steering-gear response to variable-rate and variable-damping suspension systems achieving an ideal blend of ride, handling, and steering for each vehicle
     
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  3. Tideland Prius

    Tideland Prius Moderator of the North
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    There's no pump (and hence no fluid). Like you said, an electric motor takes the place of a hydraulic pump (and associated parts such as a belts and fluids) to assist the driver. When driving in a straight line, no assist is given. This is where the electric power steering (EPS) system has its advantage. It doesn't require any power when you're driving in a straight line. When you turn, the electric motor draws on power to assist you. Switching from hydraulic to EPS can boost fuel economy by 2%.
     
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  4. Maine Pilot

    Maine Pilot Senior Member

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    Thanks for the replies.

    Since the use of the electric power steer, brakes, etc. contribute to improved MPG, why is it then, when the ICE is activated, MPG drops so low, at least according to the "instant fuel consumption gauge?" Since the Prius has a relatively small (Corolla) engine, one would think MPG would be phenomenal with all these efficient systems.
     
  5. Mike Dimmick

    Mike Dimmick Active Member

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    MPG is related to a) speed and b) rate of fuel consumption. It is miles-per-hour divided by gallons-per-hour. The rate of fuel consumption is related to the power demand. Toyota published this graph, for the Gen 2 Prius, in an SAE white paper:

    [​IMG]

    Each of the lines marked with a number is a contour, like the terrain heights on a map. It indicates an efficiency number measured in grams of fuel input per kilowatt-hour of energy transferred, at each combination of engine speed and torque produced. Or, in grams-per-hour of fuel input per kilowatt of power produced. The red line around the left and top sides of the graph is the engine speed and load that the car tries to stick to - it is more efficient to load up the engine than to increase its speed, up to about 90% of full load.

    That's not terribly helpful without knowing where the power outputs are, but for some Gen 3 dealer material they provided these graphs:

    [​IMG]

    (You can see that for the Gen 3, Toyota decided that flooring it should go to wide-open throttle for maximum power, above 40 kW output, rather than trying to preserve maximum efficiency as on Gen 2.)

    The rate of fuel consumption in gallons per hour is then g/kWh multiplied by the power output at that point in kW, multiplied by the conversion factor from grams to gallons (density of the fuel).

    The corner where it starts to increase engine speed rather than load is roughly where the electric motors hit their speed limits and the car is forced to run the ICE when cruising. This is deliberate of course! The increase in fuel consumption as the throttle is closed at low power demand is substantial.

    Cruising on city roads, the speed is low and so is the efficiency, so the car switches the engine off and drives on electrics, until the battery is somewhat depleted. The car then loads up the engine more heavily than current power demand requires - still on the minimum revs - in order to recharge the battery. Because it's running harder, it's instantaneously consuming more fuel (so instantaneous MPG is lower), but this is amortized over the whole discharge/recharge cycle, so more efficient overall.

    There are a number of recommendations for rate of acceleration for best economy; the one I usually go with is trying to maintain instantaneous MPG at half of current speed in MPH. (This advice came from the US, I actually use 0.6 x MPH due to 20% larger gallons.) That's a rate of about two gallons per hour. Other suggestions are trying to maintain the engine speed in the 230 or 220g/kWh zones indicated (tricky to do by ear!), or finding the point where no energy is taken from or sent to the battery (indicated by no arrow to or from the battery on the energy display). On the Gen 3, the Hybrid System Indicator is another way of deciding how hard to push: see HSI for some ideas.


    Low-rolling-resistance tyres, great aerodynamics, and electric ancillaries, all contribute to reducing the power required to cruise at a particular speed. However, a Prius in body and ancillaries, but with a Corolla engine and slushbox, would get nowhere near.
     
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  6. Paul58

    Paul58 Mileage Miser

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    While Mike provided the PhD Level explanation that made my head hurt halfway through his post, I'll offer a much more simplistic answer... Weight! The Prius is considerbly heavier than a Corolla. The added weight of the Hybrid System (batteries, inverter, two large electric motors, extensive cabling, etc) puts a lot more load on the Prius engine than is placed on the Corolla engine. Also, the Atkins Cycle engine in the Prius has less HP/Torqe than the Corolla engine even though they are both 1.8L engines.
     
  7. xs650

    xs650 Senior Member

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    Surprisingly (or maybe not), the Prius is only 242 lbs heavier than a Corolla.

    With both Prius batteries in the rear, I wouldn't make any big bets about the Prius having more weight on the front wheels either.
     
  8. qbee42

    qbee42 My other car is a boat

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    It is phenomenal. The instantaneous mileage on regular cars is even worse, in general.

    Recall also that all energy used by a Prius comes from burning gasoline. Since the engine is not always running, when it does run it is burning gas for immediate needs and repaying an energy debt from using the battery. The total amount of gas burned is less, but the instantaneous amount can be higher, since it is repaying that borrowed from the battery.

    Tom
     
  9. Maine Pilot

    Maine Pilot Senior Member

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    Mike's explaination still leaves me with questions. I couldn't understand the second graph, so that might explain some of my confusion.

    He said that Toyota's engineers decided to make the Prius' ICE more efficient by squewing its power curve to deliver more torque--but why? OK, better acceleration, but isn't that counterproductive to fuel economy?

    Operating close to W.O.T. seems counterintuitive for obtaining best MPG. My interpretation of the first (top graph) is ~1700 RPM looks like the best fuel efficient area.

    I'm not an engineer, so please be gentle as any of you explain this to me.

    I guess "I want it all;" meaning the highest fuel efficiency in operating in EV, hybrid and ICE modes.
     
  10. Tideland Prius

    Tideland Prius Moderator of the North
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    An ICE works most efficiently under load (which is why you may have seen threads here that mention that too slow an acceleration may actually use more fuel than accelerating more briskly). You want to rev the engine to that sweet spot (by the looks of the graph, anywhere between 2,200rpm to 3,300rpm which is roughly between the ECO area and the PWR area on the HSI).

    More torque allows the car to accelerate faster given the vehicle weight. If the engine doesn't have enough torque to move the vehicle at a reasonable pace, chances are that the driver will dip the throttle further to get up faster (and thus use more fuel) or spend longer accelerating (and thus using more fuel).

    He's not saying that you should be accelerating at WOT. He's just mentioning that the difference between WOT in the Gen 2 and the Gen 3 is that in the Gen 2, the car will try to retain maximum efficiency and thus limit output. With the Gen 3, Toyota is trying to appeal to a broader market and thus at WOT, the program has been changed to allow the engine to produce maximum power instead. Based on the graph, it only affects the upper half of the curve so really under normal operations, you're still benefiting from the torquier 1.8 litre engine and maximum engine efficiency.

    This is the way I see. Hopefully it's correct.
     
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  11. macman408

    macman408 Electron Guidance Counselor

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    I'd make just one correction here...

    In general, it seems like keeping the HSI at the top of the ECO area, just before the PWR area starts filling, corresponds to about 2,000 RPM. This will vary with battery SoC, however; at a high SoC, RPM will be lower, using more battery power to make up for less engine power, in order to get back to the magic 60% value that the computer wants to maintain. The opposite applies at low SoC, in which case you very well might hit 2200-3300 RPM where you mention in the HSI.
     
  12. Tideland Prius

    Tideland Prius Moderator of the North
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    Good point. I wasn't even thinking low SoC, just normal 6 bars but definitely something to clarify to those that don't realise that the HSI bar is a sliding scale.
     
  13. fuzzy1

    fuzzy1 Senior Member

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    Worst instantaneous MPG under changing conditions is not all that important. Whole-trip MPG is what counts.

    That said, my Prius handily beats my Subaru on both counts. The Suby is now easily capable of low-30s MPGs for decent trips. Under the conditions where the Prius' MPG drops into the teens, the Suby's ScanGauge displays single digit MPGs.
     
  14. fuzzy1

    fuzzy1 Senior Member

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    I believe it also varies with speed. From some testing a couple years ago: "Max RPM while still in this ECO area varies with speed -- 1660 rpm at slow street speed of about 20-25 mph, about 2400 rpm at 60 mph, and a solid 3200 rpm at 79 mph."

    Hobbit wanted to know the maximum flat road speed while still keeping HSI in ECO. I ran out of test space before finding that limit.
     
  15. qbee42

    qbee42 My other car is a boat

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    With a normal car, the ICE must be able to move the car over a wide range of loading. Generally there are only three or four gears, so the ICE finds itself operating at a lot of sub-optimal levels of speed and load. This forces design compromises that reduce mileage.

    The Prius, on the other hand, has in infinite range of gearing and an electrical propulsion system to help. Because of this, the ICE in the Prius can be designed to operate at a fairly narrow range of efficient speeds and loads. The battery is used to even out the loading.

    When the load is very light, the ICE can turn on and off as needed. This allows a brief run at higher efficiency to be spread over a longer interval of driving. Think of it as being like a kid on a skateboard: kick, glide, kick, glide. The ICE comes on (kick) and adds energy to the system, then turns off (the glide).

    Tom
     
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  16. Maine Pilot

    Maine Pilot Senior Member

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    Great analogy, Tom
     
  17. GBC_Texas_Prius

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    This thread has me wondering how the electric steering works on a Prius. Reading wikipedia that it applies variable torque either to the steering shaft or the steering gears isn't much insight.
     
  18. xs650

    xs650 Senior Member

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    This should be a little clearer than that particular Wiki article.
    Electric Power Steering

    Short version.
    There are torque sensors on the steering shaft. The more torque applied to the shaft, the more the electric motor assists.
     
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