Thursday 3 March 2016

Chokes in Cars

Imagine a cold morning when you are ready to go to work in an old petrol car belonging to 1990s. You crank your engine and it doesn't start due to the low temperature. The engine at this moment requires a lot of raw petrol so that some of its vapors will ignite. This is done with the help of a choke.

A choke is normally used in an engine using carburetor. A carburetor has a choke valve which controls the supply of air to the engine. The choke valve is linked to a cable which can be pulled with the help of a knob in the driver's compartment. By pulling the knob, the cable changes the choke valve position to restrict the flow of air. The choke can be operated manually by pulling a knob or automatically by sensing the engine temperature.

Choke valve in a carburetor

By restricting the air flow, a low pressure region is created in the throat or venturi of the carburetor which causes more amount of petrol to flow from the main jet to the cylinder of an engine. Once the engine warms up and reaches the normal operating temperature, the choke knob can be released to allow normal air flow.

The choke should not be used for a large period of time as it would result in:

  • Higher fuel consumption

  • Black smoke from the exhaust

In an  engine fitted with fuel injection pump instead of a carburetor, a manual choke knob is provided to restrict the air flow. Once the air flow is restricted, the cold starting injector is actuated to inject more fuel in the inlet manifold.

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