Starter flap

from Wikipedia, the free encyclopedia

A starter flap (also known as choke [ t͡ʃoʊk ]) is a component in carburetor engines that enables the air flowing into the carburettor to be limited in a targeted manner .

Because a large part of the fuel in the intake air in a cold engine is deposited in the intake tract and on the cylinder wall, the proportion of fuel in the mixture is increased during a cold start . One speaks of enrichment or enrichment of the mixture.

This is achieved by further limiting the air flowing into the carburetor with the help of an adjustable starter flap. This is usually located in front of the nozzles at the beginning of the carburetor and has one or two notches on the edge. Since the fuel outlet opening of the main nozzle is located between the choke valve and the throttle valve, a little more fuel flows into the air flow due to the slightly increased negative pressure in front of the throttle valve. In figurative terms, this flap partially cuts off the air supply to the engine, hence the English term choke (from English to choke stifle, strangle).

There are manual, semi-automatic and automatic versions of the choke.

Manual choke

With manual choke, the starter flap is operated by hand using a button or lever using a linkage or Bowden cable : Before starting, the button must be pulled out completely, then the engine is started and, after a short warm-up phase, the button is pushed back in. This design was standard for carburetors in cars and motorcycles until the early 1980s. Since the driver was responsible for operating the choke, the choke was often operated too long or was forgotten, which causes unnecessarily high fuel consumption and can clog the spark plugs if the mixture is too rich . For this reason, a manual choke is practically only used in small aircraft for the exact setting of the mixture , as well as in simple power tools such as chainsaws , lawn mowers or outboard motors . In many lawnmower engines, instead of a separate choke actuation, there is a lever construction that closes the choke when the throttle lever is brought to the maximum position and automatically opens it again when it is turned back.

Semi-automatic choke (automatic start)

Carburetor with automatic starter (top right) in maximum position

With a semi-automatic choke, the flap is coupled to the accelerator pedal and a bimetal spring . Before starting, the accelerator pedal must be fully depressed to activate the choke. The engine is then started. After the warm-up phase, the expanding bimetal spring automatically deactivates the choke. Semi-automatic chokes were common in the 1970s and 80s.

There were versions with an electrically heated bimetal spring, e.g. B. in air-cooled VW engines. This largely ruled out incorrect operation of the choke by the driver.

Fully automatic choke

The fully automatic choke is used in modern carburettors and only has the name in common with the original choke. It is a system of diaphragm pumps, electrical preheating and bimetal strips.

In engines with manifold injection, there is no longer such a flap; the mixture is enriched, among other things, via increased injection quantities.

literature