Choke

"PartyPooper2012" (PartyPooper2012) 05/19/2017 at 09:15 • Filed to: None

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No. Not that choke. The other one - you try to start your leaf blower... or carburetor car - you need to close the choke.

Why? What does it do to combustion?

I pour gasoline on a pile of hay and throw a match, it will fire. Ask me how I know. No choke required.

What does the choke do inside the engine?

BvdV - The Dutch Engineer > PartyPooper2012 05/19/2017 at 09:19

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The choke is a mechanical method to reduce the air/fuel ratio in the carburetor. By closing a butterfly valve, thus only allowing a small volume of air to pass, the mixture will be richer, leading to it being ‘easier’ to combust in a cold engine. In a modern fuel injected engine this is achieved by injecting more fuel.

PartyPooper2012 > BvdV - The Dutch Engineer 05/19/2017 at 09:24

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That’s the allegation. It makes it ‘easier’ to start because fuel/air is richer.

In theory it should light just fine without it. Plus if I flood the carb, then no one is starting anything. It will be too rich. So in that case, wouldn’t it make sense to open the choke and try starting more fuel plus more air should in theory ignite... unless fuel is up to the spark plug.

BvdV - The Dutch Engineer > PartyPooper2012 05/19/2017 at 09:35

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I started to doubt my answer, and it seems the changed a/f ratio (a/f does change with air temperature) is only partially the reason:

Choke valves are important for normally aspirated gasoline engines because small droplets of gasoline do not evaporate well within a cold engine. By restricting the flow of air into the throat of the carburetor, the choke valve reduces the pressure inside the throat, which causes a proportionally greater amount of fuel to be pushed from the main jet into the combustion chamber during cold-running operation. Once the engine is warm (from combustion), opening the choke valve restores the carburetor to normal operation, supplying fuel and air in the correct stoichiometric ratio for clean, efficient combustion.

Reference https://www.physicsforums.com/threads/why-engine-choke-works.632720/

So it seems the choke mainly helps changing the conditions under which the fuel mixes with air.

Thomas Donohue > PartyPooper2012 05/19/2017 at 09:40

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Allegation? No....chemistry and the principles of combustion.

But hey, you go out there on a cool summer morning and keep yanking the pull string on your lawnmower until your arm falls off.

Tekamul > BvdV - The Dutch Engineer 05/19/2017 at 09:53

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Your 2nd point is the correct answer, but it doesn’t fully explain what’s happening. They describe it as a change in pressure, but it’s also a change in intake velocity.

RamblinRover Luxury-Yacht > PartyPooper2012 05/19/2017 at 09:53

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When you pour gasoline on a pile of hay, you have enough liquid gasoline on the hay to make sure there’s enough evaporated into the air to go “boom”. Since a carb is normally providing *exactly* the right amount of gas to *be* that vapor and nothing more, dropping pressure to suck more fuel out of the carb and reduce the air is exactly the right choice. Too much air, too little fuel *as vapor*, and it won’t burn. The gas you needed to be vapor and actually *in* the air will be trickling off somewhere and only burning slowly if it ignites because too much is still liquid. More fuel, more liquid, less air, lower pressure ->more fuel as vapor, actual function.

Flooding is when there’s so much fuel *as vapor* that there isn’t enough air for it all, and it’s burning/half-burning/condensing in such a way that it’s fouling the plugs.

PartyPooper2012 > BvdV - The Dutch Engineer 05/19/2017 at 10:07

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Thanks for giving it a second look.

Your second answer makes more sense. You close off choke, so when engine cranks, more pressure is triggering more fuel into the chamber and with more fuel droplets lingering in the air, combustion is more likely.

Is it always the case that you need to start the (cold) motor with choke closed?

I have a chainsaw and instructions call to close choke, yank cord till motor makes noise, open choke and try starting it. It’s a terrible translation from chinese and I can’t figure out if the instruction want me to start the saw and get it running or if they want me to crank motor over and then open choke and start it. The whole noise bit is a bit confusing, but I am veering off topic. Now that I know what choke does, it seems i should get the saw running with choke closed.

PartyPooper2012 > Thomas Donohue 05/19/2017 at 10:08

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until one understand how a gizmo works, it’s all magic and mystery and allegations.

Smallbear wants a modern Syclone, local Maple Leafs spammer > PartyPooper2012 05/19/2017 at 10:15

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Cuts off a large portion of airflow, allowing for a richer mixture, making cold starts easier.

Smallbear wants a modern Syclone, local Maple Leafs spammer > RamblinRover Luxury-Yacht 05/19/2017 at 10:17

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In other news, you can *technically* put out that fire in the hay by pouring more gas on it. You just have to deliver lots of it, really really fast...

Thomas Donohue > PartyPooper2012 05/19/2017 at 10:22

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That’s fair. I’ve been known to call ‘shenanigans’ from time to time.

PartyPooper2012 > Thomas Donohue 05/19/2017 at 10:27

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sort of hard to call carburetor activity shenanigans... unless it’s out drunk all night coming home with lipstick on the air intake

KusabiSensei - Captain of the Toronto Maple Leafs > RamblinRover Luxury-Yacht 05/19/2017 at 10:32

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Something something Dalton’s Law something something.

It’s effectively a partial pressure problem. If you are limiting the pressure of *air* in the plenum, then if you have a vaporizing liquid, you will get more liquid (which starts the enrichening), and due to Dalton’s Law, it will vaporize more readily, yielding a more easily combusted mixture in the cylinder.

RamblinRover Luxury-Yacht > PartyPooper2012 05/19/2017 at 10:33

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“Close choke” in this case meaning that they want you to operate the choke and crank until you get fire. Not that crazy a thing when a motor *resists* excess fuel flow to the point you have to crank a lot to get any excess fuel in the system. Once it’s down the throat and the engine has fired, that tiny bit of excess heat can be enough to no longer need the choke, and the suction it provided will have pulled fuel through the carb and primed it up.

That secondary function seems to be common on ~20hp Kohler v-twins - they seem to need a good bit of cranking with the choke on even when it’s warm outside.

It’s also possible for a two-stroke to have the carb tuned so that it dumps *so much* fuel with the choke on that it probably can’t even run at full choke past the initial firing. Helpful way to make sure there’s enough two-stroke oil in the crank case and to seal the rings... maybe.

Thomas Donohue > PartyPooper2012 05/19/2017 at 10:37

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It would be more like calling shenanigans on an explanation, or the properties of physics, because I just don’t want to think about it at the time. Because whisky. Ironically I need to go outside to mow the lawn now.

RamblinRover Luxury-Yacht > KusabiSensei - Captain of the Toronto Maple Leafs 05/19/2017 at 10:38

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Yep. Or, if you’re demented and English messing with a French design, you design a Solex that has a side passage and uses the main butterfly as a gate, forcing air to travel *very fast* through a side passage which has a gating valve to govern the relative volumes of air and fuel, and which is using that side passage to venturi extra fuel from the air speed - then and *only then*. Said side-tube-boobery also featuring a fuel heater, because of reasons. Start procedure: pump gas no more than twice, take foot *off* the throttle, choose one of two choke positions, either of which will be turning on the heater because of reasons (lack of a separate switch).

It’s like having a whole separate three-position-throttle heated carburetor of nutjob attached to the main one.

PartyPooper2012 > Thomas Donohue 05/19/2017 at 10:40

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close that choke before you start the mower... Remind your carburetor to quit with the shenanigans and start

themanwithsauce - has as many vehicles as job titles > PartyPooper2012 05/19/2017 at 10:53

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ANd here I was all prepared to have a conversation about the first picture but......let’s move on!

As others have said, the choke adjusts the air/fuel mixture to make it easier to combust at cold temperatures. It’s a pretty dirty reaction though, as most engines are designed to be efficient when warm and use the heat they generate along with the compression from the piston to combust cleanly.

What I’ll add on to the discussion is that most engines with a choke are single piston motors. Every revolution that piston makes is responsible for moving the whole of the rotating assembly of the engine. Hell, even two cylinders are under a lot of work upon a cold start. On my KLR, I have a 650CC cylinder. THat’s a big, heavy sucker to get moving. So when the engine is cold, that sucker is big, heavy, and not wanting to move very well. - Without manually forcing the engine to throw more gas and less air in there to generate some heat and keep the explosions going, the engine won’t run. The cold block will cause the combustion to be less efficient. The cold oil increases the losses of energy from each rotation of the piston. The single cylinder won’t get enough energy to spin the crankshaft and will stall out. BUT! You use choke and the extra fuel means you get enough energy to keep the engine spinning while the block heats up. On mine, the choke forces the enigne to stay at 3000 rpm too. It idles just fine at 1000 when warm though.

Hope it helps.

DipodomysDeserti > PartyPooper2012 05/19/2017 at 11:03

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As has already been mentioned, a choke both helps in richening the air/fuel mixture. Depending on the engine and your A/F set up, you don’t always need the choke closed in order to start cold engines. I had a ‘63 Corvair which came with automatic chokes. They started failing so I removed the chokes completely. It took a little longer to start in cold weather, but would start and run well after five minutes or so. My ‘66 GMC has a four barrel Edelbrock carb with no choke mated to a SBC350, and it starts fairly easily even in cold weather. It will run a bit sluggish for the first five minutes. I also ride a ‘73 CL450 as my part time daily. I’ve done some custom work to the engine and it is kick start only. In cold weather it will need the choke closed in order to start. However, I have to slowly open it within the first minute or the engine will die again. If it is warm out the engine will start with the choke open within the first three kicks.

PartyPooper2012 > themanwithsauce - has as many vehicles as job titles 05/19/2017 at 11:03

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You know... it’s a free country. If you want to tell me/us about that first image, I and the rest of us here will gladly read about your escapades.

But yes, your explanation makes sense.

I understand the whole cold thing warm thing. Makes sense. What others pointed out is that closing off the choke creates more pull out of the fuel end. The pressure... or suction makes more fuel vapors floating in the chamber creating more of an ideal case for ignition.

If I go out in the middle of winter and pour gasoline on a pile of hay, it will light no matter how cold. That’s where my confusion/lack of clarity was. Was was it important to stop the air flow to ignite.

themanwithsauce - has as many vehicles as job titles > PartyPooper2012 05/19/2017 at 11:11

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Ha ha.....well.....I’m pretty sure stories like that would deviate far too greatly from the SFW nature of this board so let’s not go there here.

Something else I thought of after I hit post is a better way to visualize it. You get it, and understand it, but in case future people read this post, think of it this way - Upon cold start, your combustion has reduced efficiency. Say when it’s warm, 10 units of fuel combusts 100% to generate 100 units of energy. But when cold, you only get 40% efficiency. 10 units of fuel only generates 40 units of energy. You need at least 80 to run the motor. 40 is no good. BUT! You choke the engine, force in more fuel, and then even at 40% efficiency, you make enough energy to run. So if you choke the carb, force in 25 units of fuel, even at 40% efficiency, you still get 100 units of energy.

....Hope that makes sense to future people.

PartyPooper2012 > themanwithsauce - has as many vehicles as job titles 05/19/2017 at 11:28

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Stop the violence.... Think of the future people...

Possible slogan for next anti war protest or something

TheBimmerGuyWhoNowOwnsAChevy > PartyPooper2012 05/19/2017 at 17:33

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Not going to lie, I expected a conversation about chokers, but considering this is oppo, a misdirection towards chokes doesn’t suprise me

PartyPooper2012 > TheBimmerGuyWhoNowOwnsAChevy 05/22/2017 at 04:39

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I do that... often. See other posts