VARIABLE CHOKE – CONSTANT-DEPRESSION CARBURETTORS

2.27.1 Constant-choke limitations

The varying depression which acts on the jet of a constant-choke carburettor makes it necessary to fit some form of compensating device to prevent mixture enrichment with increase in engine speed. Also, the size of the venturi (choke) is a compromise that gives neither maximum economy nor maximum power.

Carburettors which operate using a ‘variable choke’

or ‘constant-depression’ principle do not suffer these drawbacks because the size of the choke alters to keep the air speed through the choke constant.

The air speed can therefore be set to that required for good atomization of fuel over the full engine speed range. Also, the constant depression over the jet overcomes the need for a compensating system.

2.27.2 Principle of the constant-depression carburettor

Figure 2.244 indicates the basic construction of a constant-depression carburettor (which is also known as variable-choke and variable-venturi carburettor).

The venturi (choke) is formed by a movable piston which alters the size of the venturi to suit the quantity of air being drawn in by the engine. An air vent maintains atmospheric pressure in the space below the piston and a communicating passage transfers the depression from the mixing chamber (the space between the choke and throttle) to the space above the piston.

When air is flowing through the carburettor, there is a difference in pressure between the air intake and the mixing chamber. This difference in pressure acts on the piston and gives an upward force to oppose the downward force caused by the piston weight and the light spring.

When the upward force due to air pressure difference is increased by opening the throttle, the piston rises and the choke area enlarges. Similarly, when the throttle closes, the mixing chamber depression reduces, the piston falls and the choke area decreases.

Whatever the airflow, the piston assembly always takes up a position that maintains a constant air speed through the choke to ensure that the petrol jet is acted upon by a constant depression – hence the name given to this type of carburettor.

A tapered needle is attached to the piston, the rise or fall of the piston will therefore vary the effective area of the petrol jet. By altering the taper of the needle, it is possible for the carburettor manufacturer to vary the fuel flow to suit the quantity of air being supplied at any speed.

2.27.3 SU-type

Figure 2.245 shows a section through an SU constant-vacuum set in the cruising position. The main features of its operation are:

● Slow-running

The piston will be lifted very slightly from the lowered position and the depression caused by the air rushing over the petrol jet will give a small fuel supply to suit Figure 2.244 Principle of the constant-depression carburettor

the conditions. A separate slow-running jet system is usually unnecessary unless emission restrictions are strict.

● Throttle opening

When the throttle is opened, the mixing chamber depression is increased. The increase in depression causes the piston to rise to a point where the mixing chamber depression is just sufficient to support the piston. The higher the piston moves, the larger the area of the jet and the greater the flow of petrol.

● Cold starting

A lever, operated by a cable control, lowers the fuel jet in relation to the needle. This enlarges the jet opening and increases the amount of fuel which is mixed with the air. The initial movement of the control cable also slightly opens the throttle to give ‘fast idle’ action.

● Mixture adjustment

An adjusting nut acts as a stop to limit the upward movement of the jet. Unscrewing the nut lowers the jet and richens the mixture throughout the entire speed and load range of the engine. Note that later SU carburettors were fitted with a mixture-adjusting screw located on the side of the body of the carburettor. This adjusting screw acted on the jet.

● Hydraulic damper

The damper restricts the rate of upward movement of the piston during acceleration. This provides a slight enrichment of the mixture. The damper also reduces piston flutter caused by the irregular flow of air through the induction systems.

● Over-run limiting valve

The valve (poppet valve) which is spring-loaded, is situated in the throttle butterfly. When the vehicle is

decelerating (i.e. road wheels drive the engine) with the throttle closed, the valve opens to supply a fuel mixture, which helps to reduce exhaust pollution.

2.27.4 The Zenith–Stromberg constant-depression (CD) carburettor

This is a very similar carburettor to the SU type, but whereas the SU type uses a solid piston, the Zenith–Stromberg CD carburettor has a synthetic rubber diaphragm (Figure 2.246).

Figure 2.245 The SU carburettor

Figure 2.246 The Zenith–Stromberg carburettor

2.27.5 Ford Variable-Venturi (VV) carburettor

The addition of the Ford VV carburettor to the range of variable-venturi carburettors introduces many features which help to reduce harmful exhaust emissions. It also overcomes the two main disadvantages of the fixed-venturi type: the need for mixture correction devices and the choke air speed problem that gives poor atomization of fuel at low engine speeds and breathing restriction at high engine speeds.

Figure 2.247 shows a diagram of this type of carburettor. An air valve, operated by a diaphragm, is used to vary the area of a large, fixed venturi. A tapered needle, attached to the air valve, fits into a main jet to control the petrol flow.

The operating principle of the Ford VV carburettor is similar to that of other variable-venturi carburettors.

In Figure 2.247 the carburettor is shown in the cruising position. A depression in the mixing chamber

(A) is communicated through the air valve chamber (B) to the diaphragm (C). When engine load is steady, the difference in air pressure on the diaphragm balances the force given by the diaphragm spring.

When the throttle is opened a given amount, the following events take place in quick succession:

1 Depression will become more intense in the mixing chamber (A) because insufficient air passes the air valve. This depression is transferred to the diaphragm chamber (C).

2 Atmospheric pressure now pushes the diaphragm against the spring. As the diaphragm deflects, the linkage will move the air valve and increase the venturi opening. This prevents the venturi air speed

from increasing and will restore the depression in the mixing chamber (A) to the original value. At this point the various forces on the diaphragm are again balanced.

Closing the throttle gives a reverse action – the depression in regions (A) and (C) diminish and the spring closes the air valve to prevent a decrease in the venturi air speed. Operating in this manner, the air valve will keep the venturi air speed constant irrespective of throttle opening.

Idle speed

Due to stricter emission legislation, carburettors were required to provide improved atomization and a better Figure 2.247 Ford variable-venturi carburettor (shown in diagrammatic form)

distributed idle mixture. The VV design achieves this by using a separate slow-running system. This is similar to that used in constant-choke units except that in the VV carburettor the slow-running system only supplies about 70% of the total mixture; the remainder is provided by the main jet.

Acceleration

A slightly rich, well-atomized mixture is obtained by using a vacuum-operated accelerator pump. The mixture provided by this pump compensates for the sudden drop in venturi air speed caused by the rapid opening of the air valve.

Cold starting

When the engine is cold either a manual or automatic choke, in the form of a miniature auxiliary carburettor, supplements the mixture provided by the main system.

The choke control acts on a needle in a fuel jet which opens when the engine is cold. Fuel from this jet is mixed with air and the resultant mixture is discharged into the main system at a point beneath the throttle valve.

2.27.6 Electronically controlled carburettors

With the introduction of increasingly stringent emissions legislation, a provision for improved fuelling was required to provide lower emissions and an increase in fuel economy.

These two requirements can only be met by a fuel system that is able to accurately monitor the engine’s

operating conditions, and so provide a near-ideal air/fuel mixture. Such a system must be very sensitive and quick to react; electronic control systems were used in conjunction with the carburettor to meet these requirements.

The features covered here applied to a constant-depression carburettor, but many aspects considered also apply to other types of carburettor.

Figure 2.248 shows the layout of a typical electronic control system fitted to a constant-depression carburettor. In this system four sensors are used to monitor the engine and ambient (surrounding) conditions that affect the operation of the carburettor.

Electrical signals from these sensors are passed to a computer called an electronic control unit (ECU), in effect the ‘brain’ of the system. From these input signals, the ECU responds to the given set of conditions and provides output signals to various components (actuators). This enables the carburettor to operate efficiently over a wide speed and load range.

The electronically controlled fuel system provides:

● mixture for cold-starting

● idle speed

● fuel cut-off when the vehicle is on over-run or the ignition is switched-off.

Cold starting

Accurate measurement of ambient and engine temperature conditions via electronic sensors ensures that the mixture supplied during cold-starting is set to suit the engine temperature.

Figure 2.248 SU carburettor with electronic control

Figure 2.249 shows a carburettor with an auxiliary starting system that is brought into operation when the cylindrical ‘choke’ is rotated by an electrical stepper motor.

The diagram shows the choke in operation and supplying an extra-rich mixture to supplement that delivered by the main system. Air that enters the starting carburettor flows around the rotary choke and mixes with fuel coming from the float chamber. The quantity of air/fuel mixture is determined by the position of the rotary choke, which in turn is controlled by the stepper motor. As the engine warms up, the associated movement of the rotary choke gradually reduces the mixture supplied by the system until the fuel and air ports are eventually cut off and the auxiliary fuel system closes.

Idle speed control

Many modern generation carburettors use a stepper motor to control the idle speed or engine slow-running.

The stepper motor acts directly on the throttle linkage and effectively ‘jacks’ open the throttle when the ECU detects that the engine speed is too low when at idle.

The engine idle speed can be set lower without the engine stalling when this type of control system is used.

Fuel cut-off

Economy can be improved by cutting off the fuel supply when engine power is not required. The fuel cut-off can be achieved by using an electrical solenoid valve to reduce the air pressure in the float chamber.

Mixture control

Elaborate compensation systems that are required on constant-choke carburettors can be eliminated with variable-choke designs by using an electronic system to control the mixture strength.

The system sets the air/fuel ratio to suit operating conditions such as speed, load, temperature and throttle position of the engine, as well as the ambient temperature.

Figure 2.249 Carburettor with auxiliary starting system

This type of rotary motor has a range of about three revolutions and is capable of rotating in either direction through small angles. The motor responds to an electrical pulse, so when a series of pulses is applied, it rotates through a larger angle.