Explain choke operation to me

[Vikas]

Isn't throttle plate and choke plate both control the amount of air entering in the engine? If so, why does closing the choke has different effect (i.e. engine running rich) versus closing the throttle plate (engine running slow).

The fuel injected car with IACV, idle air is controlled by that valve. When mechanical throttle plate is replaced by electronic throttle, the computer actually controls the throttle plate angle to control the idle speed. That tells me that at least in theory it is possible to use single air control mechanism.

So you can understand why I am confused with the choke plate and throttle plate on traditional carburator.

 

[Gotch]

Choke plate is above venturis. When the choke is closed, it causes a low pressure area in the venturis whichs draws out additional fuel. The rich mixture is easier to ignite when the engine is cold.

In fuel injection, the IAC allows more air to the engine and the ECU compensates by adding additional fuel to make a rich mixture.

 

[Carbon]

On a carburetor, the throttle plate would be after the fuel injection point. So closing that would reduce the amount of suction of fuel. The choke is before the fuel injection point. It would increase the vacuum a the fuel injection point, resulting in more fuel being used compared to the air flow.

 

[UNC017]

Originally Posted By: Gotch
Choke plate is above venturis. When the choke it closed, it causes a low pressure area in the venturis whichs draws out additional fuel. The rich mixture is easier to ignite when the engine is cold.



What he said. If you look at the throttle plate vs. the choke plate, you can see that the throttle plate shuts off the air flow below where the fuel comes into the air stream, so it controls the flow of both fuel and air simultaneously and the A/F ratio stays in proportion. But the choke is upstream of the fuel inlet point, so it shuts off air but not fuel, thus enriching the mixture.

 

[abycat]

You guys all got er right. also the throttle plate does not move unless the pedal is pressed. The idle is controlled by a valve that lets in more or less air.

 

[Gotch]

...everybody under 20yrs. old is now saying "What is a carburetor?" Lol.

 

[ARCOgraphite]

There is No Choke on Lambda FI engines in my experience. Only Carburetors. There may be a "choke cold start simulation" which is not really a choke.

 

[sciphi]

Originally Posted By: abycat
You guys all got er right. also the throttle plate does not move unless the pedal is pressed. The idle is controlled by a valve that lets in more or less air.


Unless it's a DBW throttle with no IAC. Then it's all throttle plate all the time. Both our cars crack the throttle plate open all the time to some degree.

It's neat to see the difference in operation in a lawnmower operated on choke, and at low throttle. Much more smoke on a 2-stroke mower running on choke than on low throttle.

 

[Vikas]

Thank you guys! Indeed, the question was for the lawn-mower operation.