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

[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.

 

[sophiecentaur]

Most regular rotary IC engines need to be turning already before they can actually run. A starter motor, a handle or a coiled cord can get it going. The fuel needs to be mixed with the air and then you need the mixture to be compressed before it 'burns'. It's a big ask for some engines (elderly or infirm) because it starts off cold and moving very slowly. You need a clutch arrangement to gradually apply the load to the engine as it starts because it needs to be moving so you can't just engage the stationary wheels.

An 'external combustion engine' (a steam engine uses high pressure steam from an external boiler) already has its supply of high pressure steam so you can let the already high pressure steam into the cylinder and it will turn the engine over even from rest. Unfortunately, a steam engine is not as efficient as a petrol engine so, after a few years of steam engines, nearly everyone moved to internal combustion.

This is also true for many electric motors; they are high torque when the start to turn.

 

[jack action]

This is also true for many electric motors; they are high torque when the start to turn.

I have a bench grinder to which I removed a faulty start capacitor years ago. Like in the following video, I have to start it manually every time.

​

 

[sophiecentaur]

I have to start it manually every time.

Many induction motors do not have torque at low / zero speed without a bit of a fudge. But AC/DC motors are what we learn about first and the magnetic fields are just right for good starting.

All conversations about Engineering have exceptions and tend to be divergent.

 

[.Scott]

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.

As demonstrated in the "Flight of the Phoenix", 1965.
I have used several of the methods described here - the string pull, the propeller throw, the roll the car and engage the clutch. Also, planes that use magnetos actually have two separate sets that can be engaged one at a time - so that each can be tested while still on the ground.

 

[gmax137]

When I was a kid we had a lawnmower that was started with a pull rope - a short length of clothesline with a knot at one end. The knot fit into a little groove or slot in the pulley, you wrapped the rope around and pulled. My dad's advice to 8 year old me: "be careful."

 

[Klystron]

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.

View attachment 360371
https://photos.com/featured/chuck-yeager-standing-by-f-86-sabre-bettmann.html?product=art-print

Been a while since I read General Yeager's books, but your recollection sounds correct. While @Flyboy likely has later knowledge, I heard several USAF expressions for starting a jet aircraft engine with a running aircraft including blowdown.

Observed two successful blowdown starts while on active duty. The first at Da Nang airport during Vietnam War involved a Boeing transport aircraft after a successful infrastructure attack that destroyed other aircraft, portable APUs and other aircraft support equipment. Once the left engine caught from exhaust from a similar running jet aircraft in front, I think the right engine was started by cross-bleed. See this article for diagrams and explanations.

The second blowdown start occurred at a very remote electronic warfare (EW) range. I was tooling along in a Dodge pickup trying to find a relocated radar. An experimental jet aircraft was stalled on a flat apparently with APU troubles. Mechanics and pilots push-backed a running trainer jet* so that the exhaust entered the front scoop of the stopped engine. The 'X-plane' fired right up but with deafening loud noise and smoke.

Concerning 'shotgun shell' starters the same article mentions:

Early models of the KC-135 Stratotanker, a military derivative of the Boeing 707, even featured cartridge start systems. These used explosive charges to spin up engines in seconds—an extreme method designed for Cold War-era urgency.

*T-38 Talon IIRC starting an A-12.

 

[Baluncore]

Also, planes that use magnetos actually have two separate sets that can be engaged one at a time - so that each can be tested while still on the ground.

Aircraft required reliable spark-ignition piston-engines. Magnetos and spark plugs were critical components, so they were duplicated, and always operated in parallel. The duplicate circuits were not engaged one at the time, so much as disengaged one at the time.

Both magneto circuits would be enabled, except during the power test before takeoff. Then each magneto on the aircraft would be disabled sequentially, and the drop in RPM of that engine noted. If that "mag-drop" in RPM, fell by more than the specification, the flight would be aborted, because the other magneto circuit was faulty.

 

[Flyboy]

Whomst pong’d?

Yes, magento-type ignition is still used in aviation due to the simplicity and self-contained nature of it. You can suffer a complete electrical bus failure and still run your engine in a piston-engine aircraft, something that you notably cannot do in your personal vehicle. They’re not inherently more reliable than a modern electronic ignition, though, usually requiring major inspection and maintenance every 500 hours or so. That’s anywhere from about once or twice a year on a flight school bird to every five years or so on someone’s “I fly just enough to stay current” bird.

There’s a growing movement to electronic ignition with an integrated generator driven off the magneto drive gear, but it’s still restricted to Experimental/Amateur Build aircraft at this time.

Dual redundant magnetos do provide added safety, but also provide more uniform combustion and improved efficiency. Airplane piston engines use relatively large cylinders and low RPMs, so getting a proper burn is hard with one spark plug. Two plugs solves that, and provides redundancy. Win/win.

As for APUs… they actually predate the practical jet engine by several years. Famous example of that is the B-29, which had a small piston engined APU aboard to help provide enough electricity to start the mighty R-3350 engines.

Jet engines have usually used some form of external power source for starting basically from their inception. The famous Me-262, for example, had a direct-drive APU in the intake nose cone: a little 2-stroke diesel engine with a pull starter which spun the engine up to the self-accelerating point. Electric starters were used for smaller engines basically from the outset, but they typically used an external power supply like a Ground Power Unit (GPU) to drive the starter. Batteries just couldn’t provide enough current for the long spool times required to start a turbine engine, and the only reason they can do it on smaller jets today is improved efficiency on the starter motors and the two-spool design of modern engines reducing the rotating mass the starter needs to turn. Even then, most midsize business jets and up will feature a small gas turbine APU, typically a Honeywell unit, which provides extra electricity to assist the battery/batteries during the start.

Larger engines, or smaller ones where weight is a constraint, will use a high volume, low pressure (~40 psi) air turbine starter to spool the engine. This is the kind that would have been used in 2nd and 3rd generation combat jets, due to the lighter weight and simplicity, although those were typically externally powered by a GPU, often referred to as a “huffer cart”. A variety of these existed, and still exist today. You’ll occasionally see them in use at airports when they have an INOP APU.

On that note, the APU on larger aircraft primarily provides the large volumes of compressed air needed to spool up an airliner engine, but also provide electrical power, and occasionally hydraulic power via a direct pump, although that is rare.

Starting an engine with the jet blast from another engine is definitely in the category of “well, we’re screwed if we don’t get an engine started!”, and is not something taught in training. The folks involved probably used a variation of the in-air start approach, but you probably have to watch your turbine temps really closely… I can see that turning into a hot start really quickly.

Cartridge starters are basically a small solid rocket motor firing into a really durable starter turbine to accelerate the engine to self-accelerate speed in single-digit seconds instead of the more typical 15-20 seconds. I can’t recall any civilian designs that used it. There’s a piston engine equivalent, the shotgun starter, which may have seen some limited applications in civilian designs but again was mostly military.

 

[Baluncore]

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

Thinking about the question at a deeper level, I realise I have never had that problem.
The life-long problem I have, is working out why, when I pull the string, it does not start.

 

[DaveE]

OK, since this thread has drifted way far away from lawn mowers, here's my personal favorite.
On dive trips the boats need compressed air to refill tanks. There are always full scuba tanks lying around. So many dive boats have diesel engines that use a pneumatic starter powered by a scuba tank. Compressed air starts the engine. Then the engine runs a compressor to refill all of the scuba tanks. All this system needs is diesel fuel and one full scuba tank. Very reliable in the marine environment.

 

[sophiecentaur]

All this system needs is diesel fuel and one full scuba tank.

Brilliant!

 

[.Scott]

Both magneto circuits would be enabled, except during the power test before takeoff. Then each magneto on the aircraft would be disabled sequentially, and the drop in RPM of that engine noted. If that "mag-drop" in RPM, fell by more than the specification, the flight would be aborted, because the other magneto circuit was faulty.

I could find a good photo, but this one (from a Cessna 172) will do:

The labelling says "Off", "Magnetos R, L, Both", and "Start". That red switch to the right is to keep the electronics offline when you're playing with those magnetos.
@Baluncore accurately describes the procedure as I understood it until a magneto actually failed on me. I was a student pilot practicing at Warrenton, VA but my home base was Hartwood, about 10 miles away (and no longer in service). I ran through the regular start-up checklist with my instructor watching. When I switched to "L", there wasn't just an RPM drop, the engine began sputtering to a stop - and so I shut it down. My instructor was alarmed and did not immediately believe that a magneto was bad. I assured him that the instruments were off (thus safe from electrical damage) and that the magneto was had, in fact failed. After another demonstration, he was convinced.
But then there was an issue of economics. I was renting the plane from Hartwood where they had their own aviation mechanic. So the instructor wanted me to fly with the dead magneto. This time I was a bit incredulous - but after confirming what he wanted I continued. At 3000ft AGL, he asked why I had so much altitude. I explained that I was making sure I had a glide path down to either the Warrenton or Hartwood runways at all time.

 

[DaveC426913]

All this system needs is diesel fuel and one full scuba tank.

Call me paranoid but the idea of using a diesel engine to fill scuba tanks (or even have the two near each other) fills me with fear.

I've heard too many incidents ^{[citation needed]} of divers getting sick or dying because of tanks contaminated with bad air. (Usually it's oil, but still). The effects of contaminated air on the blood stream are multiplied by the pressurization.

"...one must still be very careful about what is drawn into the intake, especially when it comes to carbon monoxide (CO). CO, a product of engine exhaust, is deadly and can be drawn into a compressor’s intake if the intake is placed too near a potential source of CO."
https://dan.org/safety-prevention/d...ination-a-case-for-education-and-maintenance/

"There was recently a thread about CO poisoning on a live aboard and a Russian diver died, while several others were injured (some fairly seriously). CO can be a real concern in poorly maintained or gasoline/diesel powered compressors."
https://scubaboard.com/community/threads/bad-air.236144/post-3595659

(Clearly these are not official incident reports. Finding those might be a little more involved, if anyone wishes to challenge my allegation.)

 

[Averagesupernova]

If the OP is still around, here is a few pix of a small engine I was working on yesterday. You can see the stator part of the magneto. It is defective. No breaker points, all timed off of the magnet in the flywheel. The primary and secondary coils as well as the electronics that provide the function of the breaker points are all potted in this unit. Also is a pic of the magnet in the flywheel. I had cleaned the surface up with some sand paper as sometimes the layer of rust can build up to the point that the flywheel and stator rub. This will prevent the magneto from working.

 

[Averagesupernova]

Compressed air starts the engine. Then the engine runs a compressor to refill all of the scuba tanks. All this system needs is diesel fuel and one full scuba tank.

Used to be the norm to use compressed air in semi tractors for starting. With air brakes there was always a need for air anyway. There was a great need to take care to have a reliable tank that could be isolated since it's not practical to expect the whole system to stay aired up long term. Watch the movie Mad Max The Road Warrior. The semi used in that movie had a pneumatic starter. I suspect many large stationary engines still do use air for starting.
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Edit: I wonder if any truckers carried a scuba tank? Lol

 

[DaveE]

Call me paranoid but the idea of using a diesel engine to fill scuba tanks (or even have the two near each other) fills me with fear.

Yes. You're not paranoid IMO. Anyway, I want the guy filling my tanks to be worried. You must take great care with exhaust and intakes. The truth is dive operations fill tanks on land, unless it's a long excursion.

 

[jedishrfu]

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

As an aside, I always liked the German aircraft engineer who took charge of reengineering the plane and then later causally mentions he works with model planes. I think he also used a sliderule to do it.

 

[Janus]

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.

The advantage to the magneto system with aircraft is that you can lose your entire electric system and the engine will keep going. Aircraft also use 2 magnetos, so if one fails, you can still keep the plane aloft in order to limp to an available airport.

 

[sophiecentaur]

the German aircraft engineer who took charge of reengineering the plane and then later causally mentions he works with model planes

A great (but infuriating) character. The book was really fab. The film was good for its generation, but needed a plot twist to avoid the fact that their plane crashed after takeoff during the filming and there were deaths but no money to make another plane.

 

[sbrothy]

I simply love how you people can get a 50 message thread out of how starting a lawnmover works! I really feel i found my tribe!

 

[jedishrfu]

A great (but infuriating) character. The book was really fab. The film was good for its generation, but needed a plot twist to avoid the fact that their plane crashed after takeoff during the filming and there were deaths but no money to make another plane.

Wow, I didn't know about the plane crash and deaths. I'll have to read more on it.

While not common, it did happen in many movies like Charlton Heston's Ben Hur and the chariot race. Perhaps that's why actors get such high pay for hazardous duty. Over the years, I've heard so many of these stories.

 

[Janus]

Aircraft required reliable spark-ignition piston-engines. Magnetos and spark plugs were critical components, so they were duplicated, and always operated in parallel. The duplicate circuits were not engaged one at the time, so much as disengaged one at the time.

Both magneto circuits would be enabled, except during the power test before takeoff. Then each magneto on the aircraft would be disabled sequentially, and the drop in RPM of that engine noted. If that "mag-drop" in RPM, fell by more than the specification, the flight would be aborted, because the other magneto circuit was faulty.

Once, while on a training flight with my instructor, we suspected that we had lost a magneto, but after landing, we checked everything out, and couldn't find an issue. The instructor reported it to the airport mechanic so they could give it the once over to make sure.

 

[sophiecentaur]

I'll have to read more on it.

The wiki article mentions it.

 

[JT Smith]

I don't think we can have all of these diversions without at least mentioning how retractable pull cords in lawn mowers, vacuum cleaners, etc. sometimes stop retracting easily/reliably.

When I was about ten years old I decided I would fix the pull cord on our Briggs & Stratton lawn mower. I had tools. I took it apart. At some point I got to the coiled spring and when I pulled it out it inflated in way that Alan Guth would have been proud to have witnessed. Needless to say...

 

[jedishrfu]

That's got to be a dangerous activity pulling out a loaded spring. Mechanics risk their lives when replacing springs, shocks, and other loaded devices. One wrong move and the thing will slip with tremendous force.

 

[Averagesupernova]

That's got to be a dangerous activity pulling out a loaded spring. Mechanics risk their lives when replacing springs, shocks, and other loaded devices. One wrong move and the thing will slip with tremendous force.

There's typically not a lot of torque on a recoil starter assembly for a lawnmower. Not to say the spring can't get away and maybe give someone a good startle. Doing real physical damage will likely not occur unless it gets to an eye. I've had many of them apart. If it's just a broken cord there is usually no reason to disassemble to the point of dealing with a spring.
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The real danger in springs working on vehicles and such is suspension. Semi trucks use air brakes and the canisters that actuate the brakes can have a spring. They will be a two chamber canister. One chamber activates the brakes when air is applied. The other releases the brake when air is applied. That one functions as a parking brake and is the one with a spring. People have been killed messing with those not knowing the dangers.

 

[Baluncore]

People have been killed messing with those not knowing the dangers.

Above all, recognise and identify stored energy, lest it will destroy you.

 

[sophiecentaur]

There's typically not a lot of torque on a recoil starter assembly

Above all, recognise and identify stored energy, lest it will destroy you.

Precisely. It's Energy wot does all the damage. The torque on the starter assembly has to be low enough for your (fit) granny to start the mower or you'll need to be doing it yourself.

The most comical pull cord starting process is with an old outboard motor on a small boat. The skipper can get very cross.

 

[JT Smith]

That's got to be a dangerous activity pulling out a loaded spring. Mechanics risk their lives when replacing springs, shocks, and other loaded devices. One wrong move and the thing will slip with tremendous force.

The problem was that there wasn't enough stored energy to pull the flimsy cord back in. My plan (I was ten years old) was to wind it tighter.

Our old vacuum cleaner has the same problem. I'm not going to try and repair it but I'm not afraid of it.