Choking Engine: Fuel Injected by Bernoulli's Principle

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In summary, the conversation discusses the concept of choking an engine by restricting the air intake and its effect on the fuel mixture. It is explained that by limiting the air, more fuel is drawn into the mixture, making it richer and easier to start. The conversation also delves into the principles of vacuum and Bernoulli's principle, which play a role in the process of choking. The participants also share their experiences and knowledge about carburators, with some humorous anecdotes thrown in.
  • #1
Rockazella
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Quick question,
when you choke an engine by restricting the carbs air intake..why does this richen the engine? If a carb draws fuel by bernoulli's principal, how could stopping all the air flow allow for any fuel to get pulled in?
 
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  • #2
how could stopping all the air flow allow for any fuel to get pulled in?
I don't believe that all airflow is stopped.
 
  • #3
I don't believe that all airflow is stopped.

Correct.

Choking an engine is used when starting during cold conditions on a carberated engine. Most fuel injection uses what's called a cold start valve.

Basically, the choke works by limiting the amount of air that mixes with the fuel. More fuel and less air makes for a rich mixture. As I'm sure you know, you cannot have combustion without oxygen. BTW, a lean mixture is the exact opposite, more air and less fuel.

But see, by making the mixture richer, it makes for a more combustable fuel, and easier to start. Hence the purpose in choking your engine when it is cold.

FYI, the cold start valve for fuel injection simply sprays gas into ease starting.

Also, depending on vehicle, you may have TBI, or throttle body injection. This looks like a carb, but is essentially a fuel injected carb. It's airflow is controlled by an exhaust sensor and an airflow sensor.

Any specific problems your having or just curious?
 
  • #4
Curious:)

I've been working with small 2 cycles for a while now and just recently thought about what actually takes place when I choke em. Although I've been dubbing with these things for a some time, I'm still no expert. I know that the throttle works by controlling air intake which in turn varies fuel intake. It didn't make sense to me that choking would increase fuel while lowering throttle would decrease fuel.
 
  • #5
Originally posted by Rockazella
I've been working with small 2 cycles for a while now and just recently thought about what actually takes place when I choke em. Although I've been dubbing with these things for a some time, I'm still no expert. I know that the throttle works by controlling air intake which in turn varies fuel intake. It didn't make sense to me that choking would increase fuel while lowering throttle would decrease fuel.
Part of the principal of working the speed of the engine comes from the vacuum principals that operate in the carburator. Higher vacuum sucks harder on any available fuel flow/atomizer. Close the choke and it generates a greater vacuum on the fuel inlet tube/pipe/atomizer, hence more fuel drawn in.
 
  • #6
Originally posted by Rockazella If a carb draws fuel by bernoulli's principal, how could stopping all the air flow allow for any fuel to get pulled in?
The vacuum principles alluded to by Mr. Robin Parsons are, in greater detail, as follows:

Bernoulli's principle states the faster a fluid is moving the more its internal pressure decreases. The faster a fluid is moving the less it is pushing back out on anything that might be exerting pressure on it.

Choking may, incidently, cut down on the amount of air that gets into the cylinder, but restricting the amount of air is not the point of choking. What it does is to require the same amount of air to try to get into the cylinder through a smaller opening, because to do this the air must increase its speed.

The now faster air has a great deal less internal pressure and, like all low pressure systems, it sucks things in from the higher pressure systems it is in contact with. It entrains things into its flow. In the situation under discussion gas vapor is entrained in an amount proportionatly higher to the increase in velocity of the "choked" airflow. Same amount of air, more or less, but a higher proportion of gas vapor, which, as Megashawn said, is more combustable, more prone to igniting, thereby boosting the engine up to smooth operating temperature.
 
  • #7
Colder air is also more dense. Less volume of air is required for proper combustion.

What do you think, does the density have any significance?
 
  • #8
I would think that once the engine is running cold winter air would give higher compression than hot summer air. I have no idea if it would be enough to make a difference.
 
  • #9
Cheese guys, haven't you ever seen a completely frozen solid, in ice, carburator, on a hot summers day??
 
  • #10
Originally posted by Mr. Robin Parsons
Cheese guys, haven't you ever seen a completely frozen solid, in ice, carburator, on a hot summers day??

Carburator? Whuzzat?
 
  • #11
Originally posted by selfAdjoint
Carburator? Whuzzat?
It's from back in the "Neanderthal" days, when cars used to actually mix their gas and air in a premixing manner that was outside of the combustion chamber...but I am dating myself now aren't I!
 
  • #12
Originally posted by Mr. Robin Parsons
It's from back in the "Neanderthal" days, when cars used to actually mix their gas and air in a premixing manner that was outside of the combustion chamber...but I am dating myself now aren't I!

Yup, and me too. I'm sure we both have eye-glazing war stories about the old days and the old ways. Did I ever tell you the one about when I was changing the points and this tall dog came up behind me...?
 
  • #13
Originally posted by selfAdjoint
Yup, and me too. I'm sure we both have eye-glazing war stories about the old days and the old ways. Did I ever tell you the one about when I was changing the points and this tall dog came up behind me...?
No, why? did you have problems in the gapping afterwards?

...or was that pointless (Bad pun!)
 
  • #14
Of course when you change points, you are bending over the engine. So this dog cam up to investigate my rear end. I jumped and dropped the little screw. Went and got another screww, finished the job, and started the engine. SCCCCRUNNCH! The screw had fallen into the transmission. 500 bucks.
 
  • #15
Originally posted by selfAdjoint
Of course when you change points, you are bending over the engine. So this dog cam up to investigate my rear end. I jumped and dropped the little screw. Went and got another screww, finished the job, and started the engine. SCCCCRUNNCH! The screw had fallen into the transmission. 500 bucks.
OUCH!
 
  • #16
2 cycle motorcycle engines commonly use a "fuel enrichening" device that, when activated, adds fuel directly from the float bowl (through a small metering orifice) to the inlet air stream on the engine side of the carburetor slide valve. If you were to open the throttle, there by breaking the high vacuum around the fuel enrichening orifice located in the throttle bore, it would negate any effect this system has on the fuel delivery to the engine. In other words, the carburetor will mix the fuel/air normally at higher speeds at fully open throttle positions even with the fuel enrichening device activated.
Please do not ask how it is that I am aware of this, it is rather embarrassing...
 
  • #17
Would cars without hoods cost less

selfAdjoint said:
Of course when you change points, you are bending over the engine. So this dog cam up to investigate my rear end. I jumped and dropped the little screw. Went and got another screww, finished the job, and started the engine. SCCCCRUNNCH! The screw had fallen into the transmission. 500 bucks.
Hence, automobile manufacturers are considering selling cars with hoods that are welded shut -- yielding lower average ownership costs.



  • “We learned that if you meet women's expectations, you exceed those for men,” Volvo CEO Hans-Olov Olsson said at the unveiling. Poretto writes:

    The result: A car that's designed to be nearly maintenance-free, requiring an oil change every 50,000 kilometers (31,000 miles.) When it's time for an engine inspection, the car sends a wireless message to a local service center, which notifies the driver. The vehicle has no hood, only a large front end primarily suited for opening by a mechanic. It also features a race-car-like fueling system with a roller-ball valve opening for the nozzle, but no gas cap. […]
 
  • #18
Hi,

yep, cold winter mornings certainly seem to give more power. Don't quote me, but i ran through the figures once out of interest (just using basic PVT relationship) and presuming pressure stays constant, and working around about the 20-50 degrees celcius range, there should be approx 3% power gains from every 10 degree reduction in temperature due to air density increase (working in kelvin, 10/293... 10/313... 10/323 etc)
But of course this is assuming everything else stays constant.

Martin
 
  • #19
zoobyshoe said:
Choking may, incidently, cut down on the amount of air that gets into the cylinder, but restricting the amount of air is not the point of choking. What it does is to require the same amount of air to try to get into the cylinder through a smaller opening, because to do this the air must increase its speed.

This is worded incorrectly. Internal combustion engines are positive displacement machines. Let's look at my Pontiac 455 for example:

8 cylinders 57.875 ci volume per cylinder.

When any single piston goes down on the intake stoke 57.875 ci of air WILL be drawn in. It doesn't matter where the throttle is 57.875 ci will always be displaced. If said 57.875 ci is not displaced on an intake stroke, the engine will seize. The throttle alters the density of air filling the cylinder not the volume.

A couple of decent articles on how carbs work in general terms.
http://auto.howstuffworks.com/question377.htm
http://www.musclecarclub.com/library/tech/carburetor.shtml
http://www.rodandcustommagazine.com/techarticles/135_0402_carb/
 
  • #20
Origionally posted by Faust9
When any single piston goes down on the intake stoke 57.875 ci of air WILL be drawn in. It doesn't matter where the throttle is 57.875 ci will always be displaced. If said 57.875 ci is not displaced on an intake stroke, the engine will seize. The throttle alters the density of air filling the cylinder not the volume.

Is it just air being drawn into your cylinder then?
 
  • #21
Andy said:
Is it just air being drawn into your cylinder then?
No, its A/F; however, A/F ratio is a mass ratio not a volume ratio. An A/F of 10:1 (about 7 to 9:1 for a choked engine) means 10 mass units of air to 1 mass unit of fuel (i.e. 10kg of air to 1 kg of fuel). The volume of fuel is insignificant when compared to the volume of air because the atomized/vaporized fuel constitutes only a tiny tiny portion of the overall A/F volume drawn into the cylinder.
 

1. What is a choking engine?

A choking engine is a term used to describe an engine that is not receiving enough fuel flow to operate optimally. This can lead to a decrease in engine power and performance.

2. How does Bernoulli's Principle relate to fuel injection in a choking engine?

Bernoulli's Principle states that as the velocity of a fluid (such as fuel) increases, the pressure decreases. In fuel injection systems, this principle is used to create a low-pressure area that allows for the fuel to be injected into the engine at a high velocity, resulting in better combustion and engine performance.

3. What causes a choking engine in fuel-injected vehicles?

A choking engine can be caused by a variety of factors, such as a clogged fuel filter, a malfunctioning fuel pump, or a faulty fuel injector. These issues can restrict the flow of fuel to the engine, resulting in a decrease in engine performance.

4. How can a choking engine be diagnosed and fixed?

To diagnose a choking engine, a mechanic may use a fuel pressure gauge to check the fuel flow to the engine. If the pressure is lower than the manufacturer's specifications, the mechanic may then inspect the fuel system components to determine the cause of the issue. Depending on the cause, the choking engine may be fixed by replacing a clogged filter, repairing a faulty fuel pump, or replacing a malfunctioning fuel injector.

5. Can a choking engine cause any long-term damage to the vehicle?

If not addressed promptly, a choking engine can cause damage to the engine over time. This is because a decrease in fuel flow can lead to incomplete combustion, which can result in a buildup of carbon deposits on the engine's components. This can reduce engine efficiency and potentially lead to more serious issues in the long run.

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