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Choking an Engine

  1. Jun 11, 2003 #1
    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?
  2. jcsd
  3. Jun 11, 2003 #2
    I don't believe that all airflow is stopped.
  4. Jun 11, 2003 #3


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    Choking an engine is used when starting during cold conditions on a carberated engine. Most fuel injection uses whats 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 in to 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?
  5. Jun 11, 2003 #4

    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 sence to me that choking would increase fuel while lowering throttle would decrease fuel.
  6. Oct 7, 2003 #5
    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.
  7. Oct 8, 2003 #6

    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.
  8. Oct 9, 2003 #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?
  9. Oct 9, 2003 #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.
  10. Oct 9, 2003 #9
    Cheese guys, haven't you ever seen a completely frozen solid, in ice, carburator, on a hot summers day??
  11. Oct 10, 2003 #10


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    Carburator? Whuzzat?
  12. Oct 11, 2003 #11
    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!!!
  13. Oct 11, 2003 #12


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    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...?
  14. Oct 12, 2003 #13
    No, why? did you have problems in the gapping afterwards?

    ......or was that pointless (Bad pun!)
  15. Oct 17, 2003 #14


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    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.
  16. Oct 17, 2003 #15
  17. Apr 13, 2004 #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...
  18. Apr 14, 2004 #17
    Would cars without hoods cost less

    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. […]
  19. Apr 14, 2004 #18

    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.

  20. Apr 15, 2004 #19
    This is worded incorrectly. Internal combustion engines are positive displacement machines. Lets 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.
  21. Apr 18, 2004 #20
    Is it just air being drawn into your cylinder then?
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