How does pulling a string start a lawnmower or a weed eater?

[sevensages]

TL;DR Summary

How does pulling a string start a gas-powered lawnmower or a gas-powered weed eater?

I don't know for sure how pulling a string would start a gas-powered lawnmower or a gas-powered weed eater.

My speculation is there are generators in gas-powered lawnmowers and gas-powered weed eaters with pull strings, and I speculate that pulling the string on a gas-powered lawnmower or a gas-powered weed eater would turn a piece (possibly called a rotor) in a generator, which would generate electricity. Then my supposition is that this electricity would go to a spark plug and make a spark, starting the gasoline engines in lawnmowers and weed eaters.

Is my speculation correct? If not, how does pulling a string cause a gas-powered lawnmower or a gas-powered weed eater to start?

 

[jack action]

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https://www.sourcefy.com/articles/how-does-a-lawn-mower-ignition-system-work/

 

[sevensages]

​

https://www.sourcefy.com/articles/how-does-a-lawn-mower-ignition-system-work/

From watching the video, I suppose that my speculation in the OP is correct. Thank you for that video.

 

[jack action]

Yes.

 

[berkeman]

From watching the video, I suppose that my speculation in the OP is correct.

Except that the "generator" is an integral part of the engine, and generates the spark anytime the motor is turning. It's just like your Internal Combustion Engine (ICE) car -- the starter motor spins the engine to circulate the fuel/air mixture and rotate the generator for the electrical spark generation. There is no separate electrical generator that is spun by the starter rope in your examples.

 

[sevensages]

Except that the "generator" is an integral part of the engine, and generates the spark anytime the motor is turning. It's just like your Internal Combustion Engine (ICE) car -- the starter motor spins the engine to circulate the fuel/air mixture and rotate the generator for the electrical spark generation. There is no separate electrical generator that is spun by the starter rope in your examples.

Pulling the drawstring turns a flywheel with a magnet on it, which moves the magnet over the wire going to a spark plug. This movement of a magnet over a wire generates electricity. It is a type of generator. This electricity causes a spark in a spark plug. I was basically correct.

 

[berkeman]

I was basically correct.

Yes, but it's better to just say, "Pulling the starter rope spins the engine, which allows the motor to start up in its normal running mode".

 

[sevensages]

Yes, but it's better to just say, "Pulling the starter rope spins the engine, which allows the motor to start up in its normal running mode".

I disagree. Just saying "Pulling the starter rope spins the engine, which allows the motor to start up in its normal running mode" does not say anything about pulling the starter rope generates electricity to make a spark in the spark plug.

 

[berkeman]

What if you just spin the engine without using the starter rope? Do you think the engine would start?

https://www.superstock.com/asset/male-pilot-trying-turn-plane-propeller-hand/4186-18482405

 

[hutchphd]

I disagree.

But no one is asking for your opinion. This is good brcause your proposed answer (C-) is misleading and @berkeman gives a much more succinct description (A+). The pull- string just makes the gas engine go 'round: when that occurs the engine is designed to keep running. Their is no special "starting" generator. connected to the string

 

[sevensages]

What if you just spin the engine without using the starter rope? Do you think the engine would start?

View attachment 360332
https://www.superstock.com/asset/male-pilot-trying-turn-plane-propeller-hand/4186-18482405

If I spin the entire engine, no. If I spin the flywheel, yes.

 

[sevensages]

But no one is asking for your opinion. This is good brcause your proposed answer (C-) is misleading and @berkeman gives a much more succinct description (A+). The pull- string just makes the gas engine go 'round: when that occurs the engine is designed to keep running. Their is no special "starting" generator. connected to the string

No. I don't think you or berkeman really added anything

 

[Baluncore]

No. I don't think you or berkeman really added anything

It takes several different answers from different members to satisfy a questioner. Then later, others will find the thread through Google, and try the answers to see if one fits from their perspective.

Meanwhile, someone who was uninformed about small engine starting techniques yesterday, should not be designing the curriculum today. If you don't like some of the answers, and express that feeling, you will get fewer answers to choose from next time.

 

[berkeman]

If I spin the entire engine, no. If I spin the flywheel, yes.

You do understand that the flywheel is rigidly connected to the rest of the rotating mass in the
engine, right? What learning resources have you been using so far to understand how ICEs work?

 

[jrmichler]

TL;DR Summary: How does pulling a string start a gas-powered lawnmower or a gas-powered weed eater?

there are generators in gas-powered lawnmowers and gas-powered weed eaters

When it's only function is to power spark plugs, the system is known as a magneto ignition (search the term) system. Piston airplane engines still use magneto ignition systems, although some are now using an electronic ignition system.

 

[hutchphd]

No. I don't think you or berkeman really added anything

You know, I believe that you think that. Be assured that there will be no further wasted effort from me to respond to any question from you.

 

[Baluncore]

How does pulling a string start a gas-powered lawnmower or a gas-powered weed eater?

It is not only small garden toys that are started by pulling a string.
I have a 1950s Caterpillar D6 bulldozer here that weighs over 17,000 kg. Its small gasoline donkey engine, with a magneto, is started with a piece of string. That donkey shares lubrication oil and cooling water with the big diesel engine. When it has all warmed up, the engines are coupled together, so the spark ignition (SI) engine, turns and starts the compression ignition (CI) diesel engine, then the SI donkey is no longer needed.

 

[Rive]

If I spin the entire engine, no.

Yes, instead.
Old times it was about manually cranking the engine (crankshaft) with a handle. Or to push start it. Both is about getting the engine in motion till it can start operating normally.

The only case I can recall which was about manually cranking up a flywheel is for some WWII tank engines.

Quite different business, I would say.

 

[Lnewqban]

Then my supposition is that this electricity would go to a spark plug and make a spark, starting the gasoline engines in lawnmowers and weed eaters.

One or many successive sparks alone are unable to start a gasoline engine.
Many things must simultaneously harmonize for self-sustained internal combustion to happen.
Have you observed people pulling those strings many times but not making an engine start or continuos running beyond a couple of cycles?

 

[Averagesupernova]

Mopeds have used a mechanism which winds up a spring with multiple pumps of a pedal. There is a clutch that is engaged to turn the engine. No cord-pull start.
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Motorcycles with a kick starter do something similar but the motion on the pedal is directly coupled to the crankshaft instead of winding up a spring.
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Golf carts have a starting mechanism that can turn the engine either direction for forward or reverse travel. I believe the system for keeping the battery up doubles as a starter. Coils around the flywheel I suspect.
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Pony/donkey motors as described by @Baluncore .
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The picture of the plane with someone turning the propeller posted by @berkeman .
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The video linked by @Rive I have not watched but undoubtedly it gets the engine turning.
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Pushing a vehicle down a hill to get it rolling and then engaging the clutch to get the engine to turn. May not work on the newest vehicles. I know it still works on older fuel injected vehicles with an ecm.
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Tractors like these are started by grabbing the flywheel and turning it.

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International Harvester put the engine inline so an actual crank couples up to the front end of the engine directly to the crankshaft.

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They all have one thing in common. They turn the engine by some means to get the fuel/air mixture drawn in as it would in ordinary operation and to fire the spark plug in the same manner as it would in ordinary operation. No special generator used only during starting for firing spark plugs. The same mechanism is used during starting that is used during normal running.
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Go back even farther in time and there are large stationary non-diesel engines that used glow plugs. They are still started the same way. Get them turning.
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@sevensages it seems you just want to 'be the most correct'. You are not.

 

[berkeman]

And for a variation on the engine starting theme...

I remember reading in one of Chuck Yeager's books about when he flew P-80 and F-86 jets in the Korean War, how they sometimes had to improvise to get some of the jets started. (My memory of the details is a bit fuzzy, but I think this is correct.) The jets normally have an Auxiliary Power Unit (APU) that supplies electricity to spin up and start the main jet engine, but when the APU of a plane was not working, they would turn a running fighter jet directly in front of the other plane and use the jet exhaust directed into the 2nd plane's intake in order to get their turbine spinning and start the jet. They had a name for this technique, but it's NSFW so I won't include it in this post.

https://photos.com/featured/chuck-yeager-standing-by-f-86-sabre-bettmann.html?product=art-print

 

[Averagesupernova]

Shotgun shell for starting. Second vid is more descriptive.

 

[hutchphd]

Shotgun shell for starting. Second vid is more descriptive.

Many WWII bombers used shotgun shells to start the motors turning (I watched the movies). If I remember correctly the Titan II missile had a characteristic starting "whoop" that was produced by a small rocket motor which was fired internally to initiate the turbopumps for the first stage propellants.

 

[DaveC426913]

Many WWII bombers used shotgun shells to start the motors turning (I watched the movies).

Used to great dramatic effect on a Fairchild C-82 Packet by Jimmy Stewart in "Flight of the Phoenix (1965)".

 

[DaveC426913]

I disagree. Just saying "Pulling the starter rope spins the engine, which allows the motor to start up in its normal running mode" does not say anything about pulling the starter rope generates electricity to make a spark in the spark plug.

It does if you understand how an ICE works.

It is not sufficient to simply spark the plugs - that's frankly a minor component - the engine must be turning over, which operates the pistons, compressing the fuel and also operating the valves. These are all integrated components and must all work in lockstep for the engine to fire up and continue running. That's really what you're doing.

Note that, in the case of diesel engines, there are no spark plugs, so the electrical component is irrelevant, but the turning-over of all the mechacoal components remains the primary goal.


Further, if all one had to do was fire the spark plugs, the pull on the cable would be very easy. The reason it is so hard to pull, and why it needs repeat pulls, is because one is physically turning over the mechanical components. Again, that's the primary purpose.

No. I don't think you or berkeman really added anything

Well, they did add wisdom; I think what you meant to say was "I don't think they added anything in a way that I understood." which is a very different sentiment.

 

[hutchphd]

Used to great dramatic effect on a Fairchild C-82 Packet by Jimmy Stewart in "Flight of the Phoenix (1965)".

Bingo......that was the movie in my head !

 

[Baluncore]

Note that the "shotgun shell", used as a gas generator to start engines, was actually a cartridge not a shell, and it contained no shot, just loose-packed black powder, and the percussion cap to ignite it. The engine was positioned to a set mark, after TDC, then the cartridge was fired.

 

[DaveC426913]

Synchronicity!

Watching a mystery set in 1936. Character is talking about the magneto in the car.

First time I heard the term, and now twice in the same day!

 

[sevensages]

It does if you understand how an ICE works.

It is not sufficient to simply spark the plugs - that's frankly a minor component - the engine must be turning over, which operates the pistons, compressing the fuel and also operating the valves. These are all integrated components and must all work in lockstep for the engine to fire up and continue running. That's really what you're doing.

Note that, in the case of diesel engines, there are no spark plugs, so the electrical component is irrelevant, but the turning-over of all the mechacoal components remains the primary goal.


Further, if all one had to do was fire the spark plugs, the pull on the cable would be very easy. The reason it is so hard to pull, and why it needs repeat pulls, is because one is physically turning over the mechanical components. Again, that's the primary purpose.

I stand corrected.

 

[hutchphd]

Very good. Ignorance is much more serious when mixed with arrogance.
This is a lesson we are all learning recently, I hope.

 

[Ian Telcher]

TL;DR Summary: How does pulling a string start a gas-powered lawnmower or a gas-powered weed eater?

I don't know for sure how pulling a string would start a gas-powered lawnmower or a gas-powered weed eater.

My speculation is there are generators in gas-powered lawnmowers and gas-powered weed eaters with pull strings, and I speculate that pulling the string on a gas-powered lawnmower or a gas-powered weed eater would turn a piece (possibly called a rotor) in a generator, which would generate electricity. Then my supposition is that this electricity would go to a spark plug and make a spark, starting the gasoline engines in lawnmowers and weed eaters.

Is my speculation correct? If not, how does pulling a string cause a gas-powered lawnmower or a gas-powered weed eater to start?

Pulling the cord rotates the crankshaft, setting the motor in motion, allowing the electric spark and combustion to do their job.