Electronic Carburettors (Automobile)


Electronic Carburettors

Every year more and more stringent anti-pollution regulations are being introduced. Also there is a constant need for improvements to obtain better fuel economy from an engine. To meet these two requirements, it is necessary for a fuel system to sense the engines operating conditions accurately and then to use these information to provide a near-ideal mixture. Such a system must be very sensitive and quick in operation, and so electronic control systems have come into prominence. The features covered in this text apply to a constant-depression carburet­tor, however, many aspects considered also apply to other types of carburettor.
Figure 9.78 illustrates the layout of a typical electronic control system fitted to a constant-depression carburettor. This system uses four sensing devices to measure engine parameters and 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). The ECU is programmed during manufacture to execute in response to a given set of conditions, which enables the carburettor to operate efficiently over a wide speed and load range.
SU carburettor with electronic control. The electronic system can be used to control i) The mixture for cold-starting, (ii) Slow-running speed, and(Hi) Fuel cut-off when the vehicle is on over-run or the ignition is switched-off.
Fig. 9.78. SU carburettor with electronic control. The electronic system can be used to control
(i) The mixture for cold-starting, (ii) Slow-running speed, and
(Hi) Fuel cut-off when the vehicle is on over-run or the ignition is switched-off.


Cold-starting Mixture Control

Many mechanically operated automatic choke lack sensitivity because of the limitations of the sensor system. This has resulted in unreliable operation, high emissions and high fuel consumption, especially when the vehicle is operated on short journeys.
Figure 9.79 represents an SU carburettor in which the cold-starting mixture is supplied by an auxiliary carburettor. Petrol-air metering and mixing take place in a separate chamber, which is activated by a cylindrical valve called a rotary choke. This valve is rotated by a stepper motor using a 9:1 reduction gearbox. The stepper motor is energised by an ECU. A permanent-magnet-type stepper motor is used having a step angle of 7.5 degrees and it works through a range of three revolutions.
Carburettor with auxiliary cold starting system (simplified).
Fig. 9.79. Carburettor with auxiliary cold starting system (simplified).
The ECU receives signals for ambient air temperature, engine coolant temperature and engine speed (Fig. 9.79). Temperatures are measured by thermistors and engine speed is sensed from pluses generated by the ignition primary circuit. When these signals indicate the need for choke operation, then pulse signals are sent from the ECU to the stepper motor. This feature enriches the mixture to suit the initial start conditions. This also weakens-off the mixture as the engine warms up.
Stepper-motor control of the choke works in conjunction with a vacuum valve that enriches the mixture, when a cold engine is accelerated (Fig. 9.80). This valve controls the quantity of air entering the choke system. With this feature, the mixture supplied by the choke can be set closer to the ideal providing better economy. Plus signals to the motor are supplied from the ECU by five cables; one supply and four returns. This circuit arrangement enables the direction of flow through the motor windings to be controlled by earthing out the appropriate return.
Vacuum valve to vary mixture during engine acceleration.
Fig. 9.80. Vacuum valve to vary mixture during engine acceleration.


Speed Control

Maintenance of the throttle opening for a satisfactory idling speed is critical especially when the engine is cold and the oil is thick. Slight change in the throttle, or variation of the engine load, causes the engine to either race or stall, and hence it is difficult to set a fixed throttle stop required for all operating conditions. Mechanically operated chokes, both manual and automat­ic, normally use a throttle jack to slightly open the throttle when the engine is cold. These arrangements provide a wide throttle opening to prevent stalling during the warm up period.
Electronically controlled carburettors, such as the SU type, have a provision for adjusting the idling speed suiting to the engine conditions. Since the speed is continually monitored, the throttle stop can be reset at one-minute intervals. This arrangement permits a lower idling speed, and hence lower fuel consumption is obtained.
In the idling-speed control system a stepper motor operates the automatic choke. The first revolution of the motor controls the throttle stop setting for fast idle and the subsequent movement controls the mixture enrichment. Once the engine is warm and the ECU detects that a stall is likely, the stepper motor receives a pulse signal and increases the engine speed to 100-200 rpm. By controlling the engine speed every 60 seconds, the ECU maintains the engine at a low speed.


Fuel Cut-off Arrangement

Fuel Cut-off Valve. Improvements in fuel consumption and exhaust emission levels are achieved by cutting-off the fuel supply when the engine speed exceeds about 1200 rpm with the throttje closed. In the SU carburettor this is attained by reducing the air pressure in the float chamber. A solenoid valve, controlled by an ECU, is positioned in an air passage connecting the top of the float chamber with a vacuum region between the venturi and the throttle valve (Fig. 9.81).
Fuel cut-off by varying float chamber pressure.
Fig. 9.81. Fuel cut-off by varying float chamber pressure.
When the solenoid is energized and the vacuum valve is opened, a depression is created above the fuel in the float chamber. This reduction in pressure stops the flow of fuel to the venturi. Solenoid energisation takes place at half-second intervals to prevent stall­ing and extend for a maximum duration of 9 seconds. The ECU is programmed to energize the solenoid only when the air and coolant temperatures reach 279 K and 353 K respectively.
An alternative arrangement used on a Bosch-Pierburg electronic carburettor closes the throttle valve completely during over-run at speeds above 1100-1400 rpm. This design incorporates a slow-run-
ning fuel outlet on the air-intake side of the throttle, and hence the fuel supplies are cut-off completely when the throttle is closed. Throttle actuation is carried by a diaphragm, which is controlled electronically from an ECU (Fig. 9.82).
Anti-run-on Valve. Now-a-days many carburettors are fitted with a solenoid-operated valve to cut-off the idling mixture supply when the engine is switched-off. This prevents the engine running on or dieseling due to the residual heat in the combustion chamber. Lean-burn engines are prone to this problem. Carburettors that close the throttle to cut-off the fuel during overrun often include this throttle closing provision to prevent run-on.

Mixture Control

Constant-choke carburettors require a compensation system to arrest the air-fuel mixture becoming too rich with the increase of the engine load. Although these system have met requirements in past, the current stricter emission and economy regulation, together with the need for extra carburettor refinements, make electronic control an attractive solution. The alternative, however, is to use petrol injection system which is generally more expensive.
In most electronic carburettors, the basic layout resembles a simple carburettor in which an ECU dictates the final adjustment of the air-fuel ratio. The output from the ECU controls a separate metering system. This supplements the fuel provided by the basic system and gives an air-fuel ratio to suit the conditions as sensed by the various transducers. The main sensing
Fig. 9.82. Fuel cut-off by throttle actuation.
Fuel cut-off by throttle actuation.
system signals the engine speed and load. The extra transducers sense other important variables that affect the air-fuel ratio requirements (Fig. 9.83).
 Mixture control system.
Fig. 9.83. Mixture control system.
Normally the ECU is a computer, which is pre-programmed for the mixture requirements of a particular engine. The tests on the engine during its development provide the air-fuel ratio requirements to suit the various operating conditions. This data is then entered and locked into ECU memory. During normal operation of the engine, the computer compares the input information to its stored data. It then gives an output signal to the carburettor, which operates the metering system to provide a particular air-fuel ratio.
Digitals or analogue output signals are converted by a vacuum regulator or stepper motor to physically control the rate of flow of fuel. Various arrangements are used. One system in use controls the flow by a tapered needle (Fig. 9.84). Another system uses the choke plate to vary the depression in the venturi (Fig. 9.85).
Electronic Carburettor tapered needle control.
Fig. 9.84. Electronic Carburettor tapered needle control.
Electronic carburettor choke-plate control.
Fig. 9.85. Electronic carburettor choke-plate control.

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