Post by Scoutpilot on Jan 9, 2015 10:38:35 GMT -5
The choke circuit is used in starting and warming up a cold engine by providing a fuel rich mixture to the engine. The components of the choke circuit include, the choke plate mounted on a shaft and a lever at one end. A spring attached to the lever is actuated by the choke linkage controlled by the choke cable attached to the knob in the driver’s compartment. A connector link is attached to the choke linkage and throttle linkage.
During starting, the engine needs a fuel-rich mixture. When the choke is closed, all vacuum produced by the engine is trapped in the carburetor throat. This enables both the low-speed and high-speed circuits to provide fuel, thus in-richening the mixture. The connector link between the choke linkage and the throttle link opens the throttle slightly to enhance the vacuum above the throttle plate.
Upon starting, the cold engine continues to require a fuel-rich mixture. The choke continues to enhance the vacuum in the carburetor throat where by a fuel-rich mixture continues. A bypass on the choke plate to restricts the vacuum that can be applied.
My thanks to the Mighty Mo.
There is another way to look at it.
During engine crank on a cold start, a number of issues arise due to the temperature of the air and the engine itself, along with the slow speed of the pistons. Gasoline needs heat to vaporize, and as the ambient temperature drops, so does the rate of vaporization. At -45 degrees Fahrenheit, only 20 percent of the fuel will vaporize; under those conditions, a gasoline-powered engine will not start without an external heat source. At 60 degrees, the rate of vaporization is only 50 percent. Additionally, the speed at which the engine cranks is a concern; low piston velocity will not create a significant amount of depression--a low-pressure state--which, in turn, will fail to evoke enough fuel flow to overcome the poor vaporization rate.
Without a way to enrich the mixture, the fuel mixture that reaches the cylinders during crank is too lean to ignite; until the intake manifold reaches a certain temperature, the fuel distribution would suffer. To produce fast cold starts and acceptable engine operation during engine warm-up, the carburetor uses a choke to enrich the air/fuel ratio.
Source:
Tips from Hemmings Muscle Machines
October, 2009 - Ray T. Bohacz
During starting, the engine needs a fuel-rich mixture. When the choke is closed, all vacuum produced by the engine is trapped in the carburetor throat. This enables both the low-speed and high-speed circuits to provide fuel, thus in-richening the mixture. The connector link between the choke linkage and the throttle link opens the throttle slightly to enhance the vacuum above the throttle plate.
Upon starting, the cold engine continues to require a fuel-rich mixture. The choke continues to enhance the vacuum in the carburetor throat where by a fuel-rich mixture continues. A bypass on the choke plate to restricts the vacuum that can be applied.
My thanks to the Mighty Mo.
There is another way to look at it.
During engine crank on a cold start, a number of issues arise due to the temperature of the air and the engine itself, along with the slow speed of the pistons. Gasoline needs heat to vaporize, and as the ambient temperature drops, so does the rate of vaporization. At -45 degrees Fahrenheit, only 20 percent of the fuel will vaporize; under those conditions, a gasoline-powered engine will not start without an external heat source. At 60 degrees, the rate of vaporization is only 50 percent. Additionally, the speed at which the engine cranks is a concern; low piston velocity will not create a significant amount of depression--a low-pressure state--which, in turn, will fail to evoke enough fuel flow to overcome the poor vaporization rate.
Without a way to enrich the mixture, the fuel mixture that reaches the cylinders during crank is too lean to ignite; until the intake manifold reaches a certain temperature, the fuel distribution would suffer. To produce fast cold starts and acceptable engine operation during engine warm-up, the carburetor uses a choke to enrich the air/fuel ratio.
Source:
Tips from Hemmings Muscle Machines
October, 2009 - Ray T. Bohacz