Engine Choke Throttle

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Contents

Description

The choke and throttle are used to control the amount of air and fuel that mix in the carburetor. Within the carburetor there is a choke plate at the opening, a main gas jet at the narrowest part of the venturi, a throttle plate, and some idle jets after the throttle. The venturi is a throat between the choke and throttle that has a smaller diameter than the rest of the carburetor. This narrowing causes the air flow to increase through this part of the tube. The choke on the lawnmower has two settings - "run" and "choke" - while the throttle has three settings - "stop", a picture of a turtle, and a picture of a rabbit. These settings are used in varying combinations to adjust the fuel and air mixture, thereby regulating engine power and speed.


How it works

Basic Carburetor Cross Section [1]

When starting a cold engine, a richer mixture of fuel in air is required because it is more difficult for the fuel to vaporize in cold air. To account for this, the choke is placed in the "choke" position, thus cutting off the air to the carburetor. In contrast, the throttle is usually turned wide open (or set to the rabbit picture). Having the throttle open while the choke is shut creates a vacuum within the carburetor. This vacuum causes more fuel from the main jet in the venturi to be sucked in, making a fuel-rich mixture. After the engine gets started, the choke can be set to "run", which lets the air flow in and create a more lean mixture.


As the throttle is closed, airflow through the venturi decreases until the pressure drops to a point that is unable to support the flow of the fuel. At this point the carburetor enters idle operation. With the throttle closed, there is a significant vacuum behind the plate, enough to pull fuel and air through small holes placed after the plate. This mixture generates enough force to keep the engine running.


As the throttle is opened, the vacuum is reduced. The venturi now becomes effective because it increases the air flow through that section of the carburetor. This increased air flow drops the pressure (by Bernouli's Principle) and thus sucks fuel into the airstream via the main gas jet. It is easier for the engine to operate in this state than in the idle state. As the air and gas flow increase, the engine's power also increases.



Evolution

Before the mid 1960's, older carbureted cars had a pull-knob on the dashboard that the driver could use to control the choke, which was connected by a cable. After this period, they were automatically controlled by a thermostat with a bimetallic spring exposed to engine heat. Later, sensors were used to detect engine heat. The sensors used varying electrical current connected to a heating element to change the tension in the spring which controlled the choke. However, around 1990 the last car utilizing a carburetor was manufactured. The next year carburetors in cars were completely replaced by fuel injectors because of their better fuel efficiency and reduced emissions. In this present-day system, the "gas pedal" controls the throttle valve which actually controls the air flow. When the throttle is open, letting more air in, the engine control unit (the computer controlling all the electronic components in the engine) responds by increasing the fuel rate. Although, cars today have switched away from the carburetor, other machines like lawnmowers and chainsaws still use it because it is relatively cheap.


References

Wikipedia: Carburetor

PowerPedia: Carburetor

Answers.com: Carburetor

How Stuff Works

3D parts

Subassembly Bill of Materials
Part Name # Req'd Function Mfg Process Material CAD File Image
Throttle Pin 1 1 Used to adjust throttle. Bent Metal Steel Throttle Pin 1
Choke pin 1.jpg
Throttle Pin 2 1 Connects governor to carburetor. Bent Metal Steel Throttle Pin 2
Choke pin 3.jpg
Choke Pin 1 Used to adjust amount of air allowed into carburetor. Bent Metal Steel Choke Pin
Choke pin 2.jpg
Choke Arm 1 Lever that connects to choke pin. Press Molded Steel Choke Arm
Choke arm.jpg
Throttle Arm 1 Lever that connects to throttle pin. Press Molded Steel Throttle arm
Throttle arm.jpg
Knob 2 Allows for movement of choke and throttle arms. Injection Molded Plastic Knob
Throttle knob.jpg
Throttle/Coke Plate 1 Attaches choke and throttle arms. Press Molded Steel Throttle/Choke Plate
Throttle plate.jpg
Throttle/Choke Assembly 1 Controls choke and throttle. Various Various Throttle/Choke Assembly
Throttle choke assem.jpg


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