Page 112 - Mechanic Diesel - TT
P. 112

MECHANIC DIESEL - CITS




           When the throttle is closed, a lot of vacuum is created below the throttle and inside the manifold. The jet or
           discharge port of the idle circuit remains below the throttle valve. As mentioned above, there is a lot of vacuum
           below the throttle valve and since this port is below the throttle valve, due to the vacuum on it, petrol keeps getting
           drawn from the chamber and goes into the carburetor. And even when the throttle is slightly open, the engine
           keeps running at idle speed.
           There is an adjusting screw to open the idle port or jet more, so that we can increase or decrease the amount of
           petrol, or adjust it at idle speed.
           Sometimes a hole is made in the idle circuit which opens above the venture, so that air is drawn in along with the
           petrol and is expelled by the jet. This is called air-bleeding of the carburetor
           Cruising Circuit
























           Another disc type valve is fitted inside the carburetor but it is in the lower part of the carburetor. It controls the
           amount of mixture of air and petrol going into the engine
           1   To increase or decrease the speed of the engine.
           2   To maintain the speed when the load on the engine is less or more.
           When the mixture of air and petrol goes in more quantity, more power will be generated. Similarly, when less
           mixture goes in, less power will be generated.
           The throttle valve is connected to the accelerator pedal through a linkage and when the driver presses the pedal,
           this valve opens due to which more mixture goes into the cylinder and the engine gains speed. As the pressure
           on the pedal is reduced, the throttle valve closes and the engine starts running slowly.
           In many engines, automatic governors are fitted between the throttle shaft and the accelerator pedal linkage
           which automatically opens the throttle wider when there is excessive load so that the mixture can enter and the
           engine speed does not decrease.
           High Speed or Full Throttle Circuit

           To run the engine at higher speeds or to get maximum power from it, a rich mixture needs to be supplied. For this,
           the ratio of air and petrol has to be 12:1 as compared to 16:1, when the engine is running at normal speed. The
           following methods are used to supply more quantity of petrol to the engine.

           1  Power jet or vacuum set up,
           2   Metering rod.
           Power Jet: Power jet is operated in two ways. Its mouth opens and closes by a vacuum operated piston. When
           the throttle is closed, the vacuum in the manifold increases due to which this piston is pulled down and closes the
           valve. Due to this, petrol is not able to go out through the jet.  When the throttle is opened, the vacuum inside the
           manifold reduces due to which the vacuum pressure on the piston also reduces and it opens the jet with the help
           of the spring. When the jet opens, more amount of petrol goes into the carburetor and a rich mixture is formed.






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 CITS : Automotive - Mechanic Diesel - Lesson 29 & 30  CITS : Automotive - Mechanic Diesel - Lesson 29 & 30
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