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    1. Global Moderator EPilot's Avatar
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      05-18-2017 11:48 PM #1
      Why hasn't any electric car manufacturer ever put a RAT (Ram Air Turbine) in or on their cars? Supposedly they can produce anywhere from 5-70 kW of energy so why hasn't a car company tried such a device to offset the loss in stored energy from driving?
      Seems like you add a smaller RAT somewhere on the car and use aerodynamics to speed up the air going to the RAT to generate even more power.

      From Wikipedia
      A typical large RAT on a commercial aircraft can be capable of producing, depending on the generator, from 5 to 70 kW. Smaller, low airspeed models may generate as little as 400 watts.
      A RAT for those who might know what it is.



    2. Member Robstr's Avatar
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      05-18-2017 11:50 PM #2
      Quote Originally Posted by EPilot View Post
      Seems like you add a smaller RAT somewhere on the car and use aerodynamics to speed up the air going to the RAT to generate even more power.
      Doesn't work that way. Adding one of these would only decrease efficiency, except it it only popped out when decelerating.
      All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others.

    3. Global Moderator EPilot's Avatar
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      05-18-2017 11:51 PM #3
      Quote Originally Posted by Robstr View Post
      Doesn't work that way. Adding one of these would only decrease efficiency, except it it only popped out when decelerating.
      Want to explain why it wouldn't work since you seem to have some understanding of the way RATs work that I don't?

    4. Member mx5er's Avatar
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      05-18-2017 11:59 PM #4
      I first heard about RAT during a TV show about the Gimli glider. The Gimli glider was a Air Canada flight originating from Montreal to Edmonton where the aircraft ran out of fuel.

      No fuel means no electrical power. So the pilot deployed the RAT so the aircraft had electrical power.

    5. Member GTI_2.0T's Avatar
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      05-19-2017 12:44 AM #5
      Quote Originally Posted by EPilot View Post
      Want to explain why it wouldn't work since you seem to have some understanding of the way RATs work that I don't?

      I'm guessing conservation of energy.

      I'd think the energy required to overcome the drag of a turbine will offset any energy gained.
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    6. Global Moderator EPilot's Avatar
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      05-19-2017 12:52 AM #6
      Quote Originally Posted by GTI_2.0T View Post
      I'm guessing conservation of energy.

      I'd think the energy required to overcome the drag of a turbine will offset any energy gained.
      I think that point is probably why.
      There is a patent for it.

      https://www.google.com/patents/US8098040

      Ram air driven turbine generator battery charging system using control of turbine generator torque to extend the range of an electric vehicle
      US 8098040 B1

      ABSTRACT

      A system and method to convert the ram air energy resulting from the movement of an electric vehicle through the air mass into electric energy to recharge the energy storage devices of the vehicle while minimizing the apparatus caused drag effect on the vehicle, thereby extending the driving range of the vehicle between external charging. At least one ram air driven turbine is positioned within the vehicle, the turbine driving a mechanically coupled generator to charge the battery. Ram air is ducted in the front of the vehicle to cause the turbine generator to rotate and output electrical energy to charge the battery. The effect of variation in vehicle speed on both turbine generator output and turbine caused drag is optimized by adjusting the pitch angle of the turbine blades. At least one included ultra capacitor will implement a pre-programmed charge/discharge profile to reduce charge resistance electrical loading on the turbine generator and enable continued battery charging with minimal increase of turbine caused drag.


    7. 05-19-2017 06:38 AM #7
      Same reason why they don't have giant fans in front of wind turbines.

      Electric vehicles already have electricity. Aircrafts need a way to generate electricity and therefor they sacrifice some efficiency for this.

    8. Member MrRoboto's Avatar
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      05-19-2017 08:22 AM #8
      I imagine that because electric cars generally spend most of their time at low speeds where a RAT won't work, carrying that extra weight (and cost) to implement such a solution probably won't be worth it. The additional aero drag at high speeds would probably negate the advantages, even if you put it behind a NACA duct I imagine.

      The usage is different for aircraft though. Efficiencies don't matter much when you need hydraulic pressure to work flight controls if all your hydraulic pumps have stopped, either due to pump or engine failure.

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      05-19-2017 08:40 AM #9

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      05-19-2017 08:46 AM #10
      I suppose if you could get past the aerodynamic drag, sizing would be an issue. To have a generator to create sufficient power, the size might be larger than the real estate available on the car. Sort of like solar panels; just not enough room to make enough power.

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      05-19-2017 09:16 AM #11
      It's about as feasible as direct exhaust injection.

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      05-19-2017 09:25 AM #12
      Quote Originally Posted by Unilateral Phase Detractor View Post
      It's about as feasible as direct exhaust injection.

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      05-19-2017 09:28 AM #13
      Quote Originally Posted by Unilateral Phase Detractor View Post
      It's about as feasible as direct exhaust injection.
      Maybe we could invite Feliks in to build it and have VigorousZX design it into the car.

      This thread is going places.

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      05-19-2017 09:41 AM #14
      Quote Originally Posted by EPilot View Post
      From Wikipedia
      A typical large RAT on a commercial aircraft can be capable of producing, depending on the generator, from 5 to 70 kW. Smaller, low airspeed models may generate as little as 400 watts.
      Low airspeeds in planes like that are very, very high speeds in cars. So even less power than the 400 watt figure. And additional drag of such a device means the car needs to use more power to move it. EVs already have electric regeneration in braking. Doing that regeneration via the air isn't really necessary and the added weight of the equipment would be counterproductive.

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      05-19-2017 10:03 AM #15
      Quote Originally Posted by GTI_2.0T View Post
      I'm guessing conservation of energy.
      I'd think the energy required to overcome the drag of a turbine will offset any energy gained.
      Exactly. If you're cruising down the highway any power you gained from the RAT is LESS than the power lost from the RAT's extra drag regardless of the configuration. This is an absolute thermodynamic certainty. It has nothing to do with the car going too slow for it to work (it's a bad idea at any speed). That's because the power a RAT generates comes directly from its drag: the force of airflow on the blades is how it generates power (actual power generated is somewhat less than its drag).

      You could use one like you use regenerative braking. But regenerative braking systems are probably better in every measurable way.
      Last edited by Robstr; 05-19-2017 at 10:10 AM.
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      05-19-2017 10:15 AM #16
      Quote Originally Posted by Robstr View Post
      Exactly. If you're cruising down the highway any power you gained from the RAT is LESS than the power lost from the RAT's extra drag regardless of the configuration. This is an absolute thermodynamic certainty. It has nothing to do with the car going too slow for it to work (it's a bad idea at any speed). That's because the power a RAT generates comes directly from its drag: the force of airflow on the blades is how it generates power (actual power generated is somewhat less than its drag).

      You could use one like you use regenerative braking. But regenerative braking systems are probably better in every measurable way.
      You'd need it to only deploy while braking or decelerating, meaning it would need a hatch and motor to pop the door open and flip this thing out, then close it when deceleration is complete. That's a lot of added weight and complexity not to mention the space it would take up. Regen braking can all be done in the electric motor, it's all contained, and any extra weight is minimal.
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    17. Senior Member AZGolf's Avatar
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      05-19-2017 10:23 AM #17
      Quote Originally Posted by GTI_2.0T View Post
      I'm guessing conservation of energy.

      I'd think the energy required to overcome the drag of a turbine will offset any energy gained.
      This is correct. As for the notion of only popping the RAT out when slowing down, that's unnecessary because all electric cars even back to the 1st gen Prius and Insight have done regen at the wheels when slowing down, which is far more efficient than collecting it through wind.

      So ultimately it can be summed up as such: The OP wants to know why EVs don't do regen, which is starting from a false standpoint because they all already do regen in the most efficient way possible.

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      05-19-2017 10:31 AM #18
      Quote Originally Posted by EPilot View Post
      Want to explain why it wouldn't work since you seem to have some understanding of the way RATs work that I don't?
      If something could create more energy than it takes to move it then we'd have a perpetual energy source which bends physics.
      I'm just a regular Joe, with a regular job. I'm your average white, suburbanite slob.

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      05-19-2017 11:44 AM #19
      This is why.

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    20. Senior Member bzcat's Avatar
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      05-19-2017 01:23 PM #20
      Quote Originally Posted by AZGolf View Post
      This is correct. As for the notion of only popping the RAT out when slowing down, that's unnecessary because all electric cars even back to the 1st gen Prius and Insight have done regen at the wheels when slowing down, which is far more efficient than collecting it through wind.

      So ultimately it can be summed up as such: The OP wants to know why EVs don't do regen, which is starting from a false standpoint because they all already do regen in the most efficient way possible.
      Came to post this.


      Also, RAT is a device that converts kinetic energy from motion (change in velocity) back into battery storage. Exactly the same physical conversion of energy from a change in velocity as brake regen. Except brake regen is much more efficient at lower speeds.

      RAT trades a slight decrease in very high air speed to generate electricity (e.g going from Mach 0.85 to Mach 0.849). Brake regen trades a large decrease (e.g. in % terms going from 60mph to 0) in relatively slow ground speed to generate electricity. RAT won't be effective in cars because the change in velocity in absolute terms (i.e. going from 60mph to 0) is not a meaningful change in air speed.

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      05-19-2017 01:54 PM #21
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      05-19-2017 02:01 PM #22
      this thread

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      05-19-2017 02:32 PM #23
      You would be better off lining the underside of an electric car with piezoelectric power generators. They would generate electricity from the movement/vibration of the car. Would not work when not moving though.

    24. Global Moderator EPilot's Avatar
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      05-19-2017 02:39 PM #24
      Quote Originally Posted by bzcat View Post
      Came to post this.


      Also, RAT is a device that converts kinetic energy from motion (change in velocity) back into battery storage. Exactly the same physical conversion of energy from a change in velocity as brake regen. Except brake regen is much more efficient at lower speeds.

      RAT trades a slight decrease in very high air speed to generate electricity (e.g going from Mach 0.85 to Mach 0.849). Brake regen trades a large decrease (e.g. in % terms going from 60mph to 0) in relatively slow ground speed to generate electricity. RAT won't be effective in cars because the change in velocity in absolute terms (i.e. going from 60mph to 0) is not a meaningful change in air speed.

      Thank you for the post and information. Now off to my newest fascination.


      Gassy seats lol

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      05-19-2017 02:53 PM #25
      Vehicle engineers don't exist in a vacuum, and they constantly borrow from one another. Want to know how automobiles ended up with disc brakes and anti lock braking systems?

      If the RAT was a feasible idea and had good performance to the expense of R&D and manufacturing costs, it would be on the car. This idea is similar to those pot metal turbines that drop into your intake a swirl the air for more efficiency. In a world where engineers fight for fractions of a MPG across the drivetrain, if there was a $14.99 plus shipping and handling fix to get 10+ mpg, it would already be on the car.

      Also, the amount of kinetic and potential energy an aircraft has to swap to generate electricity far exceeds what my old LEAF would have, unless you took my LEAF up to FL350, accelerated it to .76 mach and dropped it back to earth.
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