variably resisting a dc motor

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Old 08-17-15, 10:14 PM
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variably resisting a dc motor

First off im new to the site. If anyone knows of a more appropriate forum for my questions please feel free to direct me.

I am currently in the middle of a vintage moped build. 1978 Puch Newport with a list of mods that would take an hour for me to type. I stumbled upon a cheap electronic turbocharger on ebay. Realizing that putting a turbo on 70cc 2 stroke is taboo, i was quickly entertained by the idea. It is basically a dc fan motor that packs air into the cylinder. More air allows more fuel which creates more horsepower. I need to be able to control the speed of the fan in order to sustain preferred stoichiometric curve. My goal is to fabricate a quarter twist throttle not only pulling the throttle cable to the carburetor, but also controlling the speed of the fan as well. I am picturing something similar to a rheostat or variable resistor.. electrical just isnt my strong suit. Again, if im in the wrong spot i apologize. Once i start on a problem i cant stop.

Thank you!
 
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Old 08-17-15, 11:19 PM
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Welcome to the forums.

Electronic turbocharger...... what will they think of next.

Unfortunately the turbo needs to track the demand of the engine as well as throttle position.
Without a comparator circuit that won't be easy.

When you want to vary the speed of a DC motor you need to use a PWM (pulse width modulation) circuit. It's like an electronic speed control. Instead of varying the voltage to the motor it actually turns it on and off at a fast rate.

Fortunately these controllers are sold everywhere and are easy to wire. 12volt in, motor out and a variable speed control.

RioRand Upgraded 6V-90V 15A DC Motor Pump Speed Controller
 
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Old 08-18-15, 02:18 PM
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Thank you for the information.

Due to the lack of moving parts and the simplicity of these small engines, adapting a forced induction to work wont be much of a challenge after i get the electric motor producing a predictable amount of air and some not so simple calculations.

A question i have regarding the PWM controller is if the control knob is interchangeable. Adapting this to the throttle that i have will be much simpler if the control knob is a 90* or quarter turn knob. Is there an option for a quarter turn knob? Either by changing the knob out or modifying the knob itself?

Again thank you.
 
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Old 08-18-15, 05:22 PM
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If you want to talk crazy why not look at other fuels either as a standalone switch or in conjunction with your electric turbo. Gasoline has a relatively narrow stoichiometric range and when you burn more fuel you'll also have to concern yourself with cooling. Methanol has a broader range so it's much easier to reliably get a bang and you won't have cooling issues like with gas.
 
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Old 08-18-15, 06:31 PM
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The pot that is used is probably a 3/4" turn type. You want something that works in a narrower range or less turning action.

You should try it with what it comes with first. You may find that you don't need all the way down or all the way up. You may also be able to change the value of the control affecting its action.
 
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Old 08-18-15, 06:36 PM
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Oh, heck, if you are going to go crazy use nitro-methane and nitrous oxide.
 
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Old 08-18-15, 07:01 PM
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This moped is being designed as a daily driver. Methanol in two strokes is recommended for short duration racing only. You would basically be reducing fuel mileage by half. Methanol is hygroscopic meaning it absorbs water. Its corrosive to metals seals and gaskets. Because fuel tanks filters and carbs are ventilated to the atmosphere methanol must be drained from the fuel system everyday especially in humid coastal climates. Also methanol fuels are much less tolerant of lean air/fuel mixture and can detonate violently.
 
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Old 08-18-15, 07:06 PM
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@Pjmax
Very good insight. I will give this unit a shot. Now onto machining the throttle housing too accept a cable and the PWM knob in a gear like fashion.
 
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Old 08-19-15, 03:49 AM
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I really doubt it will make any difference in performance but guess you will find out if you try it. but they really probably wont produce any boost just don't spin nearly fast enough.
there is other ways to get power out of a 2 stroke though like a tuned pipe or pulling the cylinder and porting it or even dropping the cylinder to raise compression if there is clearance to do so.
 
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Old 08-19-15, 05:49 AM
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So, you're DIY hacking a electric turbo to a '78 moped and still want to call it a daily driver?

Have you looked at supercharges used on RC controlled models. A lot of work has been done in that area developing things that work.
 
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Old 08-19-15, 03:35 PM
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@alan73
Do you have experience with these electric turbos? It seems that if they can produce enough air to feed a four cylinder car engine they can surely do so with an engine less than 1/4 the displacement. Although I am seeing nearly 13,000 rpms I feel the electric turbo can keep up. If I can tune it to be useful is a different story. But not many people have done it. These are uncharted waters.

As far as two stroke science goes I am fairly confident in my abilities. I have read A. Graham Bells book, Gordon Jennings book, and MacDizzys guide. All multiple times. A 70mph 70cc single speed ped isn't hard to obtain. I'm literally doing this because, who the **** does that!
 
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Old 08-19-15, 05:36 PM
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Getting a 70cc bike to go 70mph can be done but pushing it that hard I think will make it "fussy". Trying to call it a daily driver is what I have trouble with. It sounds like a fun project though. Keep us updated.
 
 

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