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Author Topic: Frontal Area / Cd numbers  (Read 23899 times)

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Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #240 on: May 28, 2019, 08:02:28 PM »
Given the proposed hybrid-of-sorts propulsion system discussed in this thread, maybe one should look at what the big electric streamliners are doing for traction (and traction control), driveline motors (or wheel hubs), speeds obtained, vehicle weight and aerodynamics for some historical reference. 

Also excuse the off-topic subject query:  With rotary engines spinning axial permanent magnet generators -- are the rotary engines to be throttle actuated or set at a fixed RPM for maximum output?  Will the electric wheel hubs be driven by the driver actuated throttle input and pulling "juice" as needed via a software (traction) control system -- or controlled by how?  No black box (power management system) necessary between said both to make it work?

Hi SuperL...Traction control for this design should be easy to achieve.  As I envision it, wheel speed sensors will allow the motor controllers to see which wheels are spinning relative to each other and automatically adjust power to each wheel as needed to maximize acceleration given available traction.  We won't use a foot throttle because that too susceptible to driver nervousness.  A hand throttle lever (like a boat) will allow the driver to feed in IC power to the generators at a fixed rate which will in turn supply high voltage current to the motor controllers.  The controllers will direct current to the wheel motors based on programming fed by real time wheel sensor data.  The controllers will attempt to maximize motor acceleration to whatever limit electrical current is being received from the generators.  If for example the driver had the IC engines at full throttle wheel speed would ramp up progressively based on available traction to each wheel.  There would be no need to try and throttle the IC engines up and down during a run as controller programming will compensate for wheel spin effectively acting as the acceleration throttle.  Wouldn't want the IC engines to run at max output constantly, the driver's job is to push the IC throttle forward in tune with forward thrust and increase in speed.  The driver would most likely use a bump and run approach to advancing IC power letting the car build speed progressively as traction allows.  Pulling the hand throttle back would automatically kill power and slow the car when needed for whatever reason.  The whole scenario could be programmed to maximize acceleration but that's something that will need to be evaluated with testing.  Thanks...Terry

Offline 7800ebs

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Re: Frontal Area / Cd numbers
« Reply #241 on: May 29, 2019, 09:52:43 AM »
I've done some work for  https://www.wrightspeed.com/  .  Interesting comment by an engineer there, when they cut power, the truck rear tires would "lock up". Sliding to a stop...

So plan on a programmable "ramp down" .

FYI.. I don't believe anything is "easy" when you are dealing with electronics and a salt environment.

bob

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #242 on: May 29, 2019, 12:48:35 PM »
I've done some work for  https://www.wrightspeed.com/  .  Interesting comment by an engineer there, when they cut power, the truck rear tires would "lock up". Sliding to a stop...

So plan on a programmable "ramp down" .

FYI.. I don't believe anything is "easy" when you are dealing with electronics and a salt environment.

bob

Great point Bob...negative torque is created when one backs off the throttle and could lead to  the wheel motors locking up unless there are adequate programming controls to configure current routing between the generators and motors.  Woldesemayat & Nam (2017) identified causes and solutions that can be applied to controller programming to either eliminate the problem allowing coasting, or more desirably in this application induce degrees of electromotive braking to begin slowing the car as soon as the IC power is cut.  Braking rate can be programmed based on speed allowing the chutes to do their job initially and applying greater amounts of negative torque at lower speeds to bring the car to a stop.  The great thing about EV propulsion is that almost everything is programmable.  Thanks... Terry

References:

Lee, H., Woldesemayat, M. L., & Nam, K. (2017). Zero Torque Control for EV Coasting Considering Cross-Coupling Inductance. IEEE Transactions on Industrial Electronics, 64(8), 6096–6104. doi:10.1109/tie.2017.2681973

Offline jacksoni

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Re: Frontal Area / Cd numbers
« Reply #243 on: May 29, 2019, 02:12:45 PM »
I would think a Vernier throttle more like small plane ( all I've flown were single engine) which can be "slammed" open or closed or twisted to gradually increase or decrease power would be better than just a "boat" like lever.
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Offline manta22

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Re: Frontal Area / Cd numbers
« Reply #244 on: May 29, 2019, 02:22:13 PM »
If you are using  PM DC torque motor, the braking torque problem is fairly simple. Since the motor is now acting like a generator, controlling the electrical load on the "generator" will control its braking torque. Polyphase AC motor braking torque control is a much more difficult problem.

Regards, Neil  Tucson, AZ
Regards, Neil  Tucson, AZ

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #245 on: May 29, 2019, 04:20:17 PM »
I would think a Vernier throttle more like small plane ( all I've flown were single engine) which can be "slammed" open or closed or twisted to gradually increase or decrease power would be better than just a "boat" like lever.

That may be just the ticket jacksoni; I've never flown an airplane so that throttle control is new to me.  I'll study up on it...thanks.  Terry

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #246 on: May 29, 2019, 04:34:08 PM »
If you are using  PM DC torque motor, the braking torque problem is fairly simple. Since the motor is now acting like a generator, controlling the electrical load on the "generator" will control its braking torque. Polyphase AC motor braking torque control is a much more difficult problem.

Regards, Neil  Tucson, AZ

Hi Neil...yes the Yasa P400 RC gen/motors I'm designing around are 700v DC permanent magnet axial flux motors.  The standard 80mm cartridge model falls a little short of my peak power goals but Yasa offers custom windings to get there.   I'm confident stacked Yasa gen/motors and controllers can deliver what my power train design calls for.  Thanks for the info... Terry

Offline TD

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Re: Frontal Area / Cd numbers
« Reply #247 on: May 30, 2019, 11:10:56 AM »
Very interesting discussion IMO, thanks for sharing it.   :cheers:

If you are using  PM DC torque motor, the braking torque problem is fairly simple. Since the motor is now acting like a generator, controlling the electrical load on the "generator" will control its braking torque. Polyphase AC motor braking torque control is a much more difficult problem.

Sorry if I missed it, but I don't think I've seen mention of where the energy recovered under braking goes.  In a modern diesel-electric locomotive the generated power is dissipated as heat in the braking grid resistors mounted atop the car body.   In a contemporary EV the generated power is used to recharge the battery.

Tim

 

Offline Doc B.

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Re: Frontal Area / Cd numbers
« Reply #248 on: May 30, 2019, 01:25:14 PM »
I am way out of my realm here, but don't most racing organizations require a throttle with a return spring that forces the throttle to always return to idle position if control pressure is removed? I wonder if a throttle that can be set in position like a boat or aircraft throttle (I think that is what is being described here) would be allowed.

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #249 on: May 30, 2019, 03:53:12 PM »
Very interesting discussion IMO, thanks for sharing it.   :cheers:

If you are using  PM DC torque motor, the braking torque problem is fairly simple. Since the motor is now acting like a generator, controlling the electrical load on the "generator" will control its braking torque. Polyphase AC motor braking torque control is a much more difficult problem.

Sorry if I missed it, but I don't think I've seen mention of where the energy recovered under braking goes.  In a modern diesel-electric locomotive the generated power is dissipated as heat in the braking grid resistors mounted atop the car body.   In a contemporary EV the generated power is used to recharge the battery.

Tim  

Hi Tim...no we haven't discussed where the energy goes when power to the wheel motors is cut and they effectively become generators feeding electromotive force back into the circuit that drives them under IC power. Up front I'm not an engineer...mechanical, electrical, or otherwise.  What I'm commenting on here is my understanding of how this circuit would work under the conditions described.  I may be half wrong or entirely wrong but I don't think so unless or until shown otherwise.  

If the electrical circuit between the generators and motors is broken or disconnected there will be voltage but no current flow so the gen/motor mechanism will free wheel to some theoretical maximum voltage the circuitry can withstand. All well and good for idling or coasting but contributes nothing to braking.  When the circuit is closed however the gen/motors switch roles and electromotive force or EMF generated by the wheel motors is effectively driving the IC gen/motor connection.  The degree of force the IC gen/motors sees is determined by voltage the controllers are programmed to power the now reversed circuitry.

In my vision of the design, when power to the IC engines is cut (throttle off switches off ignition spark and fuel injectors) the 8 rotors and spinning generators become an active flywheel.  Reversed polarity EMF from the wheel motors attempts at low voltage to slow rpm of the flywheel and reverse rotation of both the generators and IR rotors. EMF Voltage is progressively increased as IC rotor rpm comes down to the point where the IC gen/rotors actually begin to rotate in the opposite direction and begin ramping up flywheel rpm absorbing braking energy generated by the wheel gen/motor EMF.  During the spin down to IC flywheel reversal the aluminum rotor housings and separator plates become heat sinks acting as resistors to dissipate heat build up in the gen units. The oil and coolant pumps all being electric motor driven would continue to run in standard rotation to keep oil and coolants flowing until IC flywheel eventually stops.  

In a rotatory engine there are no valves or cams to contend with; reverse rotation is possible where the engine acts as an air pump under compression load dumping compressed air back into the atmosphere through intake ports and turbo housing.  When the car comes to a stop the IC flywheel is still spinning and can spool down under is own internal friction load.  I'm open to correction or clarification on any of this if someone with better knowledge knows reasons why this wouldn't work.  Thanks...Terry
« Last Edit: May 30, 2019, 10:37:04 PM by Simspeed »

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #250 on: May 30, 2019, 04:01:45 PM »
I am way out of my realm here, but don't most racing organizations require a throttle with a return spring that forces the throttle to always return to idle position if control pressure is removed? I wonder if a throttle that can be set in position like a boat or aircraft throttle (I think that is what is being described here) would be allowed.

You may be right Doc... I'm not familiar with the rules on this.  it shouldn't be a problem if this approach isn't allowed.  We could work with a forced idle position spring by setting an EMF coast mode between between throttle up and full throttle off positions, where EMF braking would only be triggered in the throttle off/power off position.  Thanks... Terry

Offline robfrey

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Re: Frontal Area / Cd numbers
« Reply #251 on: May 30, 2019, 04:02:07 PM »
I don’t see it just locking up the wheels when you lift your foot off the throttle.
There are many ways to skin this cat.


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Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #252 on: May 30, 2019, 10:38:13 PM »
I don’t see it just locking up the wheels when you lift your foot off the throttle.
There are many ways to skin this cat.

Rob Freyvogel
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AA/BFS

I agree Rob.

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #253 on: May 31, 2019, 06:34:13 PM »
I cleaned up the rear section of the car and shortened the chassis a couple of feet by removing the inboard support bracing for the outboard wheel motors.  I also added the chute tube and support structure and designed the chute cone doors to open and close with an actuator when triggered.  This design only needed 10° of movement to clear the chute path.  Don't know if that's enough to keep the tether lines from whipping them apart at speed.  We only have room for one 6" chute tube in this design, can we pack 2 chutes in that?  Thanks...

Offline Simspeed

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Re: Frontal Area / Cd numbers
« Reply #254 on: May 31, 2019, 06:35:22 PM »
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