The simple carburettor as explained above, would work well if the engine were to run at one particular speed and load. The nozzle and the venturi sizes then can be set once and the carburettor will then work satisfactorily at that particular speed and load. But in actual practice, the engine has to run at different speeds and loads ranging from the lowest to the highest and, therefore, certain irregularities creep in the functioning of this simple carburettor. The two basic reasons for defects are:
If the carburettor is designed to work at high speeds at full throttle, it will not work at low speeds and part throttle, reason being that in the later case the suction created at the venturi will not be sufficient to draw fuel from the nozzle. Similarly if the carburettor is designed to work at low speeds and part throttle, it would deliver richer mixture at high speeds and full throttle.
2. The coefficients of discharge for air and fuel vary in different amounts with the change of depression. Whereas the coefficient of discharge for air becomes almost constant at certain value of depression, the coefficient for fuel increases gradually. This means that at higher heads, e.g., at higher engine speeds, the air fuel mixture goes on becoming richer and richer. Therefore, if the carburettor is set at one particular speed, it will give richer mixture at higher speeds and leaner mixture at lower speeds.
The various defects- thus experienced and their remedies are discussed in the following paragraphs.
1. Starting difficulty
For starting, it is seen that very rich mixture is required, but actually the mixture provided by the simple carburettor will be very lean, because of the reasons already explained. The different methods to provide for enrichment of fuel at very low speeds are :
(a) Ticklers. These are devices used to cause flooding of carburettor at the start. By depressing the tickler, the float is depressed, thereby providing more fuel. These were commonly used in the old motor cycle carburettors.
(b) Choke. This is a simple butterfly valve fitted at the top of air horn. It may be operated by hand or it may be automatic. For starting, choke is closed so that very small amount of air gets past it and the throttle valve is open due to which whole of the suction is applied to the nozzle, which delivers sufficient fuel to provide a mixture rich in quality, though small in quantity.
The choke valve must be opened immediately when the engine starts, otherwise engine cylinder wilt be flooded with fuel which would cause it to stall. This is helped by any of the following methods:
(i) by mounting the choke eccentrically. With this, when the engine starts, the forces due to pressure on the two sides of the choke spindle are unequal, their difference producing a turning moment to open the choke.
(ii) by the use of strangler valve fitted on the choke. As soon as the engine is started, the air pressure forces open the strangler against the spring pressure and the air enters the carburettor avoiding the mixture being over-rich. By this time the driver also presses the choke knob to open the choke valve. An automatic choke employed in modern automobiles. The choke valve is mounted eccentrically. A bimetal thermostatic coil located on the exhaust manifold to quickly sense the heat, is linked to the choke valve. A vacuum diaphragm unit in which the diaphragm is subjected to inlet manifold vacuum is provided. The diaphragm itself is connected to the choke valve. A fast-idle cam is provided to maintain a small throttle opening when the engine is cold. This cam is linked to the choke valve and the idle adjusting screw rests in the cam profile.
When the engine is cold, the thermostatic spring holds the choke closed. In this position, the fast-idle cam keeps the throttle valve from fully closing. Moreover the idle adjustment screw now rests on the high point of the cam due to its rotation by the linkage. Thus the throttle valve is held partly open for fast idle. This causes the engine suction to draw additional fuel form the
main nozzle and also from the idle system at fast idle speed. Moreover while cranking, the manifold vacuum also opens the choke slightly to allow some air to mix with fuel.
After the engine is started the vacuum diaphragm pulls the choke valve open somewhat. This tendency is helped by eccentric mounting of the choke valve. As the engine warms, the thermostatic spring gradually winds up and allows the choke valve to open. As the choke opens the fast-idle cam is released step by step till when the engine is warm, the choke is open wide and the fast-idle cam is fully released.
(c) Adjustable area jet. A long tapered needle is used which is screwed into the jet. For starting, the screw is loosened, so that the jet area providing fuel is increased and thus rich mixture is provided for the start. This method has been used on some constant vacuum carburettors.
2. Idling difficulty
During idling, when the engine runs at low speed and the throttle is closed, the suction on the main nozzle is still insufficient to draw fuel from there. Therefore a separate supply-of fuel must be provided on the engine side of the throttle valve during idling. This is done by providing a separate idle jet and an air bleed hole, so that metered quantities of air and fuel are drawn into the idle passage where they form an emulsion. This mixture then comes out through the idle part which is located on the engine side of the throttle. The volume of the mixture and hence the idle speed is controlled by the idle adjustment screw. To avoid a 'flat spot' when the engine is accelerated from idling to the main running condition, changing from the idle circuit to the main nozzle circuit, transfer parts are provided as shown. When the throttle is opened slightly, the transfer port is also exposed to the manifold depression which causes additional quantity of mixture to enter the engine. This provides a progressive smooth transition from the idling to the main circuit. For this reason, the transfer parts are also called progression holes. When, however, the throttle valve is opened further the idle circuit stops supplying the mixture which is now supplied by the main nozzle only.
3. Operation at different speeds
As explained earlier, a simple carburettor would provide richer mixture at high speeds and weaker mixture at low speeds. The process of adjusting the mixture strength at all speeds so that throughout the whole range correct proportions of air fuel mixture are maintained is called Compensation.
Various devices used for providing compensation are:
(a) Extra air valve. This is a spring-loaded valve arranged to open by means of the engine suction. It is thus controlled by the spring stiffness and the engine suction and therefore, its operation has been found to be satisfactory.
(b) Compensating jet. In this an additional fuel supply nozzle. A is the main jet and B is the compensation jet. C is the well which is vented to atmosphere so that when the engine is not running, the fuel level in C is same as in the float chamber and under this condition no flow of fuel takes place through the compensating jet. The main nozzle D is connected directly to float chamber and it will supply richer mixture at larger throttle openings. But the case is different of
the compensating jet. When the engine is just started and the throttle valve is open a bit, small engine suction is applied, which draws fuel from the well C through delivery nozzle D, till the well c becomes empty. At this point, the flow through D is minimum. Further opening the throttle, therefore, does not increase the fuel flow through the compensating jet, it remains constant. However, the air flow is nevertheless increasing. As a result the higher speeds result in weaker mixtures through the compensating jet. Thus a uniform mixture of nearly constant strength is obtained at different throttle openings. The mixture strength when the petrol well is emptied depends mainly on the size of the petrol jet. This method, however, tends to overcompensate at large throttle openings. Zenith carburettor uses this method for compensation.
(c) Air bleed compensation. the detailed construction of a carburettor nozzle with provision of air bleed compensation. A-A is the fuel level in the nozzle under no load conditions. B is the jet tube containing air bleed holes around its periphery as shown. C are the holes communicating the air bleed holes to the atmosphere when the nozzle is partly empty. As the throttle is opened gradually, due to depression at the venturi, the fuel in the tube and the nozzle around is consumed, providing gradually richer mixture. Simultaneously the level of fuel in the jet tube and nozzle drops, exposing the uppermost holes in the jet tube to put them in direct communication with the atmosphere through holes C. This destroys the depression to some extent and the fuel flow is thus decreased. As the fuel demand is increased by the engine, the fuel level is the jet tube and around drops further, so as to expose the next row of air bleed holes, thus increasing the amount of air bleed to compensate the increased fuel flow from the jet. The fuel level ultimately gets stabilized. The degree of compensation depends upon the size and number of the air bleed holes and their location. The method is employed in the Solex carburettor.
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