9.14.
Types of Carburettors
Various types of carburettors are used to match fuel flow to engine requirements as follows. 9.14.1. Orientation The carburettor may be
(i) vertical or up-drought,
(ii) inverted or down-drought, and (Hi) horizontal draught (Fig. 9.61).
Fig. 9.61. Carburettor orientation.
In an up-draught carburettor the air-fuel mixture flows upward. This arrangement is convenient for gravity flow of fuel with low overall height. Horizontal carburettor gives higher charge efficiency, because of shorter connections to the intake manifold. It is not suitable for installation of an air filter. The down draught carburettor has flow of the air-fuel mixture in the downward direction. This type of carburettor is most accessible and air filter can be attached easily. Further, there is less chance of the settling out of the heavier fractions of the fuel, because liquid particles flow in the direction of gravity.
9.14.2.
Number of Barrel
Carburettors are usually classified by the number of barrels or venturies used.
One-barrel.
The one-barrel carburettor has a single outlet through which all the systems feed to the intake manifold. This type of carburettor is also known as a single-venturi design. These carburettors are generally used on 4-cylinder and 6-cylinder engines.
Single-stage Two-barrel.
This carburettor has two barrels and two throttles, which operate together. Since the various fuel-discharge passages in each barrel operate at the same time, it can be considered as two numbers of single-barrel carburettors sharing the same body having one air horn. The two throttle plates are mounted on the same shaft, and operate together. The two barrels share a common float, choke, power system, and acceleration pump. Single-stage two-barrel carburettors as used on many 6-cylinder and 8-cylinder engines.
Two-stage Two-barrel.
This carburettor (Fig. 9.62) is relatively a latter development, brought about by emission control requirements. It differs from the single-stage two-barrel design in that it’s two throttle plates operate independently. The primary barrel is generally smaller than the secondary, and handles engine needs at low-to-moderate speeds and loads. The larger secondary barrel opens whenever necessary to meet higher load requirements.
The primary stage usually includes the idle, the ac-
Fig. 9.62. Two-stage two-barrel carburettor.
celerator pump, the low speed, the main metering, and power systems. The secondary stage usually has a transfer, a main metering, and a power system. Both stages draw fuel from the same fuel bowl. Some designs use a common choke for both barrels. In others, only the primary stage is choked. The secondary system supplies large quantities of rich fuel mixture for high engine power.
In moderate driving at legal speed limits, the secondary system normally does not come into operation. The secondary portion comes into operation under approximately the same operating conditions as the primary, but it begins to operate when primary throttle is opened about 60% and throttle plate in this case opens very rapidly so that both secondary and primary throttle plates reach the fully open position at the same time.
Many carburettors incorporate mechanical linkage to operate secondary throttles. In this case an air valve, some what like a choke plate, is installed in the air horn which keeps the secondary air passage closed at full throttle low speed conditions when operation of secondary fuel discharge nozzle is not necessary. In other designs, the secondary throttle is operated
through a spring-loaded diaphragm and linkage, which holds the secondary throttle closed at low speeds. A passage from the narrow portion of the primary venturi senses the venturi vacuum produced by high primary air flow velocity, and this venturi vacuum pulls the diaphragm against the spring, which opens the secondary throttle plate and provides engine with required quantity of mixture to meet the power demand. Vacuum operated secondary throttle is illustrated in Fig. 9.63.
Fig. 9.63. Vacuum operated secondary throttle.
A two-stage two-barrel carburettor is used primarily on 4-cylinder and 6-cylinder engines and a few V-8 engines. A sticking or leaking piston or diaphragm gives problems in the secondary system requiring replacement.
Four-barrel.
The 4-barrel, or quard, carburettor uses two primary barrels and two secondary barrels in a single body. The two primaries operate single stage two-barrel at low-to-moderate engine speeds and loads. The secondary barrels open about half to three-quarter throttle to provide the increased fuel and airflow required for high-speed operation. The primary barrels contain the choke, the idle, the low speed, the high-speed, an accelerator pump and a power system. The secondary barrels have their own high speed and power system, and may use their own acceleration system.
Air flow through the secondary barrels can be provided either by venturi action or air velocity valves. Air velocity valves look like large choke plates located in the secondary barrels. They are opened by the low pressure created in the secondary barrels when the throttles are opened.
Four-barrel carburettor is used on V-8 engines. The primary barrel meets the requirement of all eight cylinders during low-to-moderate speeds and loads. The secondary barrels provide additional fuel and air flow for high speeds and heavy loads.
