9.17.
Constant-depression Carburettor
This type of carburettor is also known as constant vacuum, variable choke and variable venturi carburettor. They have a variable choke area, which alters to keep the choke air speed constant. Also depression over the jet overcomes the need for a compensating system required to prevent mixture enrichment with increase in engine speed as in case of constant-choke carburettor.
The constant vacuum, variable-choke carburettor (Fig. 9.72) provides a variable quantity of metered and atomised mixture to the cylinders under all running conditions. The constant-pressure carburettor uses a variable-choke piston type air valve fitted at the entrance to the mixing chamber. A tapered needle is attached to the base of the air valve and is submerged in a petrol jet orifice immediately beneath it. The air valve has a central stem, which guides its vertical movement. The upper portion of the valve forms a piston, which seals off a vacuum or suction chamber. The lower annular portion of the piston is subjected to atmospheric pressure. The top side of the piston is exposed to a similar pressure, existing in a mixing chamber, by means of a small transfer hole formed in the air valve.
Fig. 9.72. Constant-vacuum variable-choke carburettor.
When engine is started, air flows through the air valve static position opening, which increases with engine speed and produces a pressure drop over the petrol-jet orifice. This drop in pressure is transmitted to the upper side of the piston through the transfer hole, due to which the pressure in the vacuum chamber becomes almost same as the jet. The higher atmospheric
pressure on the other side of the piston creates a pressure difference across the two sides, causing a upward resultant thrust so that the piston moves up to a equilibrium position where up thrust equals to the downward mass and spring force acting on the piston. Hence piston ensures a constant pressure over the jet irrespective of the engine speed and load conditions.
With a constant size jet and with constant depression at the jet, only a fixed quantity of fuel is always drawn into the air stream and hence no mixture strength correction can take place when air flow increases. To take care of this situation, the effective annular petrol-jet formed between the jet and the tapered needle expands in proportion to the outward movement of the tapered needle as it rises with the air-valve piston. Proper selection of needle profile can provide best mixture ratio over the entire range of speed and load. The mixture strength can also be varied by changing the area of the jet orifice around the needle by an adjustment, which moves the jet nearer or further from the needle; however any alteration influences the discharge throughout the speed range. Figure 9.72 shows such a device in the form of a bi-metal strip lever and a spring loaded L-shaped lever.
The volume of mixture entering the engine during idling is controlled by an idle-adjustment-stop screw, which limits the closing of throttle valve. For cold starting, a hand-controlled cable is pulled to actuate a linkage, which either lowers the petrol jet so that the effective jet orifice is increased or opens a separate cold-start metering valve.
