# Bad Analogy



## Ziggythewiz (May 16, 2010)

Also Transmission ==> Transmission

Double transmisions means we need half (or less) the gears.


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## skooler (Mar 26, 2011)

major said:


> I found this post in a thread today and want to comment without hijacking that thread. This electric motor => transmission analogy was started by Bill Dube (of Killacycle fame), I believe. While I consider Bill a friend and acknowledge his expertise and accomplishments, I strongly disagree with the subject analogy.
> 
> The electric motor is an energy conversion device. It converts electrical energy to mechanical energy. It uses magnetic energy in that process.
> 
> ...


Just to throw in another deviation...

Battery ==>> Fuel tank

Motor controller ==>> Fuel Injection system (carburetor / ecu, injectors etc.)

Electric motor ==>> ICE

Transmission ==>> Transmission


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## Woodsmith (Jun 5, 2008)

Fair enough, it is only what I have learnt here.


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## GerhardRP (Nov 17, 2009)

skooler said:


> Just to throw in another deviation...
> 
> Battery ==>> Fuel tank
> 
> ...


The reason to draw analogies is to understand the operational limiting factors in a system. I think we are mislead by insisting that the motor and engine fit the same purpose. Reading any of the threads on EV performance, it is clear that the battery is the limiting factor: 
when considering range, it is the fuel tank: but when considering power, it is the engine, limiting the maximum horsepower. 
Drag cars should be rated by mass and battery horsepower at the maximum power point, as ICE vehicles are rated by the maximum engine horsepower.


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## major (Apr 4, 2008)

GerhardRP said:


> Reading any of the threads on EV performance, it is clear that the battery is the limiting factor...


I disagree. The limiting factor of power can be any of the components in the system. It is rarely (if ever) the fuel tank in the ICE system. But it is also not necessarily the battery nowadays with modern Lithium products.


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## ken will (Dec 19, 2009)

How about:
The c-rate of a battery system is a limit,... like the maximum flow rate of a fuel pump.


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## major (Apr 4, 2008)

ken will said:


> How about:
> The c-rate of a battery system is a limit,... like the maximum flow rate of a fuel pump.


You can always get a bigger fuel pump, right? You have some batteries with 50, 90 or higher C-rates. And you can always get a bigger battery to increase power. Point is: The battery is not necessarily the power limit to the system.

BTW, what is the C-rate of gasoline?

And we are losing context a bit here. Woodsmith used the analogy for a street conversion application, not a drag racer.


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## Ziggythewiz (May 16, 2010)

Even for a dragster there's no limit. Everything is a limit. You upgrade the weakest point to improve. Battery doesn't matter if the controller can't channel it. The controller doesn't matter if it smokes the motor. The motor doesn't matter if the tires don't hold. 

Nothing matters by itself, it's all a system. For a street car you define the requirements and get stuff that meets them, so there is no weak link (per the requirements). For a dragster you upgrade the weak link, then the next, etc until you run out of money. Then you go beg for more money. Rinse, repeat.


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## tomofreno (Mar 3, 2009)

How'bout controller ==> carburetor or injector system? Both control the flow of energy in time (power) for their respective powerplants, e motor or ice. And transmission ==> transmission.


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## steelneck (Apr 19, 2013)

Q. How much power can 9" motor X produce?

A. That question is like asking how many HP a 2L ICE can produce. The answer can be anything, 20 HP just as well as 1000, or even more. The ICE is a system of many parts, just as an electric traction system is.

Fuel tank = range limiter = batteries.

Intake and exhaust = ICE breathing limiter = controller _and_ batteries

Engine = physical limitations = motor

The transmission act the same, and have the same purpose in both cases.


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## PStechPaul (May 1, 2012)

I don't see why such an analogy is needed or helpful. Sometimes, for those who are weak in electricity and magnetism, but good with mechanical systems that can be more easily visualized, the "water analogy" can help. A reservoir can be a battery or a capacitor, as either stores energy. Lifting the tank higher creates more pressure, which is analogous to voltage. A pipe has a certain resistance to water flow, and the resistance increases with length and decreases with diameter, just like a wire. Water running through a pipe also has inertia, or momentum, that increases with volume and speed. A valve is a switch, and if you close a valve quickly there will be a spike in water pressure (called water hammer), which is similar to an inductive spike. You can add air-filled pipes near the valve to absorb the pressure, functioning as a snubber.

You can convert mechanical energy to water pressure using a pump, and you can vary the bore and stroke for higher pressure or volume, just as an electrical generator can be designed for high voltage or high current. There are pressure regulators on plumbing that are like voltage regulators. You can use water pressure feedback so a small amount of force can control a large amount of current, like a MOSFET. And you can use a pump in reverse to make a hydraulic motor, or a water wheel as an alternate design. There are flow meters and pressure meters corresponding to voltmeters and ammeters.

I have heard that it is theoretically possible to design a hydraulic microprocessor. At least, it is possible to make fluidic logic gates, which are the building blocks:
http://en.wikipedia.org/wiki/Fluidics


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## steelneck (Apr 19, 2013)

@PStechPaul: Yes you are right. If EV's where the the norm, it would probably be more difficult to explain a complete "ICE-system" for someone used to think in terms of EV's, but today people like that hardly exists, we are all sort of indoctrinated/brought up to think in the ICE-way. The old "water analogy" is a good one that probably even predates the ICE.


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## rochesterricer (Jan 5, 2011)

I think the analogy is mostly used when discussing the size of the motor to be used. Assuming the rest of your components remain the same(controller, battery pack, etc.), two different sized motors will put out the same power, as long as they can handle what you throw at them. The RPM that you make the power will change, but the overall power output should be the same. This is somewhat like changing the final drive ratio, which may be another reason the transmission or differential analogy is popular. 

I asked a question that revolved around this analogy when I first joined this community. The analogy isn't based so much on what the components do, but rather how they relate to one another. It is used because many newcomers assume that simply using a larger motor will result in more power.


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