joey nathan
Petty Officer 2nd Class
- Joined
- Mar 17, 2009
- Messages
- 185
So my curiosity was really piqued with this discussion. And now I know "the rest of the story". Thanks for helping!
FWIW, and I know it really helped me? I couldn't, for the life of me, figure this out - but know now that it's really very logical, like all things mechanical, eh?
12VDC Automatic Choke Explained
There is a flat coil spring under the black plastic cap on the choke mechanism. One end of the spring is anchored to the plastic cap and the other end of the spring is attached to the choke shaft. The cap can thus be rotated to open or close the choke. The spring is a special material called 'bi-metal'. It is made of two metal alloys that are bonded to each other and have widely different thermal expansion values. All metals expand when they are heated and these two metals expand at different rates so as the spring gets hot, it tends to unwind, as one surface gets longer than the other. When the spring cools, it winds back. If controlled, this winding and unwinding could be used to rotate a shaft; as in a carburetor?s choke. This is exactly how your automatic choke is working. The 12V wire you connected to the choke heats a resistance heater that is enclosed in the choke housing next to the bi-metal spring. When you switch your key to 'RUN', the resistance heater begins to warm the spring and the choke slowly opens. You can adjust the opening time by turning the cap and loading or unloading the springs closing force on the choke.
Bi-metals are used in many every day items. Hair dryers and heat guns use them as a safety switch. If you have ever been using one of these tools and it suddenly turned off, and then began to work again after a wait, that was a b-metal safety switch opening the power circuit when it got to a potentially dangerous temperature.
The click, click, click you hear when you actuate the turn signal in your car is another bi-metal switch in action. Rather than being a slow responding switch or spring as in the previous two examples, it is a rapid acting device so it will cycle fast enough to act as a flashing switch. There is a resistance wire coil wrapped around the bi-metal and the turn signal current flows through it heating it but when the switch opens, it breaks the heating current (and lamp current) and the bi-metal quickly cools and the switch closes, restoring the heating and lighting circuit. If you have a taillight burned out; there is less current flowing through the heating coil. That is why the flasher will slow or sometimes even quit flashing when a bulb is burned. Have you ever changes a flasher thinking it was bad to find the new flasher didn't work either? Problem wasn't the flasher; it was the burned out taillight that starved the flasher of proper heating current!
Further, you can see that the main weakness of an electric choke is normally the operator not understanding that the choke is sort of on a timer. If you do not crank the engine immediately after turning the key, you can run out of time and are trying to start an engine with the choke partially to fully open. This is not an uncommon occurrence with mothers that have small children distracting them, if you get my drift - hehe.
And so, as usual, the old time engineers did it better - they either embedded the bi-metallic spring in a pocket in the exhaust x-over in the intake manifold with an actuator rod that ran up to the choke, or they ran a small exhaust tube from the exhaust manifold to the spring in the black plastic cap on the carburetor so the spring didn't heat up until the motor was warm. Thus the choke was in sync with the engine. With the electric choke, the timer starts when you turn the key, regardless whether the engine starts or not.
Some choke covers have a bi-metal switch in them between the wire connector and the spring that is heated by the hot air from the exhaust tube, then when it gets hot enough the bi-metal switch closes, and so current can flow to the spring coil. That is used primarily in very cold weather climates to allow extra time for the engine to warm up so the choke doesn't come off prematurely. That type cover can be tested by putting the cover in the freezer for an hour, then putting a jumper wire from battery +, thru a test light, then to the choke cover electric wire connector, then ground the choke cover case to battery - . Hold a match on the cover until the cover warms up enough to trip the bi-metal switch, and you will see the light come on and current will flow thru the switch. (the test light will be off until the switch closes.)
Most vehicles supply power to the choke from the alternator. That way the cover only gets power when the engine is running therefore eliminating the chance of someone turning on the key and not starting the engine quickly enough and letting the choke come off when the engine is not running.
Some Ford, GM and MoPar's use a spring mounted in the manifold and allow the engine heat to unwind the spring.
FWIW, and I know it really helped me? I couldn't, for the life of me, figure this out - but know now that it's really very logical, like all things mechanical, eh?
12VDC Automatic Choke Explained
There is a flat coil spring under the black plastic cap on the choke mechanism. One end of the spring is anchored to the plastic cap and the other end of the spring is attached to the choke shaft. The cap can thus be rotated to open or close the choke. The spring is a special material called 'bi-metal'. It is made of two metal alloys that are bonded to each other and have widely different thermal expansion values. All metals expand when they are heated and these two metals expand at different rates so as the spring gets hot, it tends to unwind, as one surface gets longer than the other. When the spring cools, it winds back. If controlled, this winding and unwinding could be used to rotate a shaft; as in a carburetor?s choke. This is exactly how your automatic choke is working. The 12V wire you connected to the choke heats a resistance heater that is enclosed in the choke housing next to the bi-metal spring. When you switch your key to 'RUN', the resistance heater begins to warm the spring and the choke slowly opens. You can adjust the opening time by turning the cap and loading or unloading the springs closing force on the choke.
Bi-metals are used in many every day items. Hair dryers and heat guns use them as a safety switch. If you have ever been using one of these tools and it suddenly turned off, and then began to work again after a wait, that was a b-metal safety switch opening the power circuit when it got to a potentially dangerous temperature.
The click, click, click you hear when you actuate the turn signal in your car is another bi-metal switch in action. Rather than being a slow responding switch or spring as in the previous two examples, it is a rapid acting device so it will cycle fast enough to act as a flashing switch. There is a resistance wire coil wrapped around the bi-metal and the turn signal current flows through it heating it but when the switch opens, it breaks the heating current (and lamp current) and the bi-metal quickly cools and the switch closes, restoring the heating and lighting circuit. If you have a taillight burned out; there is less current flowing through the heating coil. That is why the flasher will slow or sometimes even quit flashing when a bulb is burned. Have you ever changes a flasher thinking it was bad to find the new flasher didn't work either? Problem wasn't the flasher; it was the burned out taillight that starved the flasher of proper heating current!
Further, you can see that the main weakness of an electric choke is normally the operator not understanding that the choke is sort of on a timer. If you do not crank the engine immediately after turning the key, you can run out of time and are trying to start an engine with the choke partially to fully open. This is not an uncommon occurrence with mothers that have small children distracting them, if you get my drift - hehe.
And so, as usual, the old time engineers did it better - they either embedded the bi-metallic spring in a pocket in the exhaust x-over in the intake manifold with an actuator rod that ran up to the choke, or they ran a small exhaust tube from the exhaust manifold to the spring in the black plastic cap on the carburetor so the spring didn't heat up until the motor was warm. Thus the choke was in sync with the engine. With the electric choke, the timer starts when you turn the key, regardless whether the engine starts or not.
Some choke covers have a bi-metal switch in them between the wire connector and the spring that is heated by the hot air from the exhaust tube, then when it gets hot enough the bi-metal switch closes, and so current can flow to the spring coil. That is used primarily in very cold weather climates to allow extra time for the engine to warm up so the choke doesn't come off prematurely. That type cover can be tested by putting the cover in the freezer for an hour, then putting a jumper wire from battery +, thru a test light, then to the choke cover electric wire connector, then ground the choke cover case to battery - . Hold a match on the cover until the cover warms up enough to trip the bi-metal switch, and you will see the light come on and current will flow thru the switch. (the test light will be off until the switch closes.)
Most vehicles supply power to the choke from the alternator. That way the cover only gets power when the engine is running therefore eliminating the chance of someone turning on the key and not starting the engine quickly enough and letting the choke come off when the engine is not running.
Some Ford, GM and MoPar's use a spring mounted in the manifold and allow the engine heat to unwind the spring.
