Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why engine choke works

  1. Sep 1, 2012 #1
    Why does the choke help an engine start when it is cold? I know that the choke restricts the air flow and thus makes the mixture of air/gas entering the engine more concentrated with fuel, but why would that make it easier for the engine to start on a cold day? Shouldn't the engine be easiest to start when you send in the ideal mixture of air and gas (since combustion needs both) and not when you send in a extra-concentrated mixture of gas?
  2. jcsd
  3. Sep 1, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    The "ideal mixture" changes with air temperature. However - when you are starting the car, you are not interested in clean-efficient burning. You don't care about ideals - you just want it to go bank please please pretty please and you'd got to get to work and your hands are freezing ... <ahem>.

    of course, too much will flood the engine.

    Once you've started the car you have to let out the choke.

    [edit]may not be clear ...
    On a cold day - less of the fuel is useable: the fuel is a liquid - you need it to evaporate to ignite it.
    Last edited: Sep 1, 2012
  4. Sep 1, 2012 #3
    So its not really that it needs more fuel, it actually needs the same exact amount of fuel (in vapor form), it is just that it takes extra liquid fuel to produce an equivalent amount of evaporated fuel on a cold day?
  5. Sep 1, 2012 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    That's probably the best way to think about it.

    Cold air is also denser - so you have more oxygen in the same volume to burn... so you can burn more fuel, but the lower proportion of the fuel sufficiently evaporated (you need small droplets) to ignite will be the biggest effect. Once ignited, burning fuel helps the cold fuel evaporate better and you can let the choke out.
  6. Sep 2, 2012 #5
    When the choke plate is closed, the vacuum signal is brought into the throttle bore of the carburetor. This vacuum pulls extra fuel into the intake manifold.
  7. Sep 2, 2012 #6
    As a auto tech in a prior life, I like to think about it like this....................

    There is no guarantee that a carbureted (note that fuel injection vehicles operate similarly, but automatically without operator intervention) vehicle will start every time. This can be dangerous on a very cold day. Ironically, this temp also makes it harder to ignite fuel. The fuel is now much further from it's ignition point temp. The fuel will not vaporize and fill the chamber as fast as in a hot engine. Remember, its the vapors that burn, not the liquid itself. So, in order to maximize the chances of getting your vehicle started, it is best to put a little extra fuel in the chamber. By closing the choke, a higher vacuum is created. This causes extra fuel to squirt in. Hopefully, some of it gets close enough to the spark plug when it fires!!
    Last edited: Sep 2, 2012
  8. Sep 2, 2012 #7

    Hmmmm..........you might want to double check that Simon. I'm pretty sure that "2C8H18 + 25O2 ~> 16CO2 + 18H2O" is not a function of temperature.

    However, I do agree that clean burning is not a concern when starting. Why engines run dirty under non-ideal stoic. ratios is a whole other discussion though..
  9. Sep 3, 2012 #8

    jack action

    User Avatar
    Science Advisor
    Gold Member

    From Advanced engine technology:
    So 2 things happen when the choke is on:
    1. The throttle is slightly opened to increase the air stream velocity to help support the fuel droplets;
    2. The mixture is enriched to compensate for the fuel that wets the engine parts. Once the engine parts are hot enough, the fuel evaporates and go back into the air stream, which is why some manufacturers heat up the intake manifold to make the process faster.
  10. May 5, 2014 #9
    Sorry Rob, but the ratio or fuel is air is very much so a function of temperature. Fuel to air ratio is a matter of weight: therefore as air temperature decreases, air density increases, and the amount (weight) of fuel required increases as well. As a pilot who has to manually adjust the mixture in his aircraft I can assure you I burn more fuel AND produce better power on colder days (pressure and humidity not withstanding)
  11. May 5, 2014 #10

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    oops i see somebody beat me to it..

    Think of it in terms of partial pressures.
    Partial pressure due to the gasoline vapor is a strong function of temperature.
    Warmer fuel/air mix can hold more fuel,,,

    so it comes back to your "easier to evaporate" statement - it's easier to get a suitable mix into the combustion chamber when it's warm.

    Stone cold i would guess a goodly fraction your evaporation takes place on surface of the fuel droplets during compression stroke as air/fuel mix is being heated by compression. That is, whetever fuel droplets made it that far and aren't laying in a pool just below carburetor.
    That's why intake manifolds have a "hot spot" right under carburetor that gets heated by exhaust, it lets you ease off the choke earlier ...

    Ahh nostalgia ! How well i remember my '53 Ford's manual choke.
  12. Apr 24, 2015 #11
    When you start the car you need to let it work for a while to reach a working temperature. Then you can let out the choke. If you push it back to soon the engine will turn off or it will run unevenly at low rpm.

    When I start my car I must use choke even at summer. The instruction manual for Yugo says that when you want to start a cold engine you must pull the choke and do not press the gas pedal. When the engine is warmed or hot it will not start if the choke is pulled. You need to push the choke down and turn the key with a slight touch of the gas pedal.

    Normally the engine in Yugo runs on 900 rpm but when you pull the choke it increases to 1400. Some people use the choke while driving for additional power when they are trying to overtake a faster car. The author of the book called ''Internal combustion engines'' form the Faculty of mechanical engineering in Belgrade says that usage of the choke in driving causes removal of oil on engine block. On lower rpm the choke usage increases the fuel consumption at lower temperatures.
  13. Apr 24, 2015 #12

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF,
    ... thats a lot more detail than was needed to answer the question but fair enough.
    "Use of the choke" does indeed have many effects.
    There are a lot of engineering questions that could use your experience.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook