Multiple Power Sources on vehicle chassis- Same ground

[newman180]

I am working on a Senior Design project for Mechanical Engineering Degree, so electrical understanding is not my best skill.

I have a vehicle chassis whose two stroke engine is started with a starter motor which only has a positive lead (no negative lead). I assume it pulls its ground from the engine block as the spark plug transformer is grounding to the block from the battery source.

I am attempting to run the ignition system on a separate lower voltage source than the starter motor but I am unsure how to pursue the ground. If I were to ground the ignition/engine block to the lower voltage source with the positive lead going to the ignition transformer and also ground the higher voltage source to the engine block with the positive lead going to the starter motor, will this work?

I can't completely wrap my head around my logic, but I want to say no because I cant understand how the powered source will follow the correct ground to complete the loop.

Thanks in advance!

 

[Skogsgurra]

You should not ask such a question. It only confuses.

If you were outside the car and had several power sources*, you would probably connect all your power sources with negative to ground and positive going from each source to its "consumer". We forget about vintage (British mostly) cars. They sometimes had a reverse order and that thinking seems to be alive in some quarters. We do not mention them any more. Just forget about them.

The car is exactly the same thing. Only that the car body now is your "Ground". Or Common. Or Chassis.

So, even if thinking is sometimes good and necessary, don't think. Just do it**. ***

------ read text below only if you need a few more points of view -----
*Yes, there are also bipolar power supplies. Like a +/-12 V supply (with 24 V between + and -). We don't mention them here. They may confuse. But, actually, the same principle applies: Zero to Ground. Only that Zero is the voltage between + and -. See? I shouldn't have mentioned it.

**And always remember to put a fuse or other protective device in the positive lead. Never put it in the return lead (the Ground wire).

***Thinking is for hobbyists. A professional doesn't have to think about basic things. He knows how it is done - and just does it.

I do not want to sound condescending here. But it is such a basic thing that it is almost impossible not to.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[newman180]

I don't know if I should be laughing but that was hilarious haha.

I understood the example about outside the car but then you further confused me. Especially the part that you said "I shouldn't have mentioned it"...

Anyways, I think I understand that I can have 1 battery (12v) and another batter (24v) grounded to the same chassis (or piece of conducting material) as long as the positive lead's don't interact with each other.

I just don't completely get why... I always view DC as "pushing electrons in a single file line" and the + and - have to be in a loop for the "line" to go through. What I don't get is what stops said "electron" from the 12v battery to not jump over and into the 24v line, or vice versa?

They don't teach this in intro to EE haha.

 

[Skogsgurra]

Like the Salmon (Salar Salar) they always want to return home to mate

Think of the Ground as a pool of water. Your batteries suck water from that pool and puts water to different "consumers". The consumers dump the used water into the pool from where it is sucked up again. The pumps do not mind if they get water that have been elsewhere before.

Electrons are like water molecules. They are all equal.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[newman180]

haha your a funny guy.

The second is a great analogy. I completely understand now! Makes me rethink a lot of things...

 

[Skogsgurra]

Good. One last thing: You say "What I don't get is what stops said "electron" from the 12v battery to not jump over and into the 24v line, or vice versa?"

I'll let you in on a secret of the trade - it is called INSULATION. It is like having a tube with high-pressure oil and another one with lower pressure oil and a common oil sump. The walls of the tubing keeps the different systems apart. Tube wall == Insulation. Easy as that.

I admit that this addition may not have been necessary. But I wanted you to be fully educated. You can now put EE and add "seasoned" to it on your business card.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[newman180]

Gotcha. I didn't mean jumping through space but merely changing paths on the ground...Anyways, I do understand insulation, but I appreciate you taking your time to help clarify everything for me!

 

[waross]

I am attempting to run the ignition system on a separate lower voltage source than the starter motor

It is very uncommon for a small engine to have different voltages on the starter and on the ignition.
Also, it is very common for two cycle engines to have a magneto to fire the spark. An attempt to feed voltage into the magneto may not be a good idea.
A third possible issue, some small engines have a small alternator but many two cycle magnetos have a provision to charge the battery.
Check your specs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[newman180]

Thanks for the post, originally there was only one power source, you are correct.
It is a two stroke, but unfortunately no magneto, just a constant drain on the battery.