COMPUTER-CONTROLLED CARBURETORS

A computer-controlled carburetor uses a solenoid-operated valve to respond to commands from the microcomputer (electronic control unit). The system uses various sensors to send information to the computer that calculates how rich or lean to set the carburetor air-fuel mixture. The system is also known as a computer controlled emission system which consists of the following: oxygen sensor, temperature sensor, pressure sensor, electromechanical carburetor, mixture control solenoid, computer, and idle speed actuator. The function of each is as follows:

• The oxygen sensor, or exhaust gas sensor, monitors the oxygen content in the engine exhaust. The amount of oxygen in the exhaust indicates the richness (low oxygen content) or leanness (high oxygen content) of the air- fuel mixture. The sensor voltage output changes with any change in oxygen content in the exhaust gases.
• The temperature sensor detects the operating temperature of the engine. Its resistance changes with the temperature of the engine. The change in resistance allows the computer to enrich the fuel mixture during cold engine operations.
• The manifold pressure sensor (MAP) measures intake manifold vacuum and engine load.
• High engine load or power output causes intake manifold vacuum to drop. The pressure sensor then signals the computer with a change in resistance and current flow. As manifold pressure drops, the computer increases the air-fuel mixture for added power. As the manifold pressure increases, the computer makes the carburetor setting leaner for improved economy.
• The electromechanical carburetor has both electrical and mechanical control devices. It is commonly used with a computer control system.
• The mixture control solenoid alters the air-fuel mixture in the electromechanical carburetor. Electrical signals from the computer activate the solenoid to open and close air and fuel passages in the carburetor..
• The computer, also called the electronic control unit (ECU), uses sensor information to operate the mixture control solenoid of the carburetor.
• The idle speed actuator is a tiny electric motor and gear mechanism that allows the computer to change engine idle speed by holding the throttle lever in the desired position.

Many of the components and sensors are also used in gasoline fuel injection systems, which we will discuss later in this lesson.

In a computer-controlled carburetor, the air-fuel ratio is maintained by cycling the mixture solenoid ON and OFF several times a second. Control signals from the computer are used to meter different amounts of fuel out of the carburetor. When the computer sends a rich command to the solenoid, the signal voltage to the mixture solenoid is in the OFF position more than it is ON, causing the solenoid to stay open more. During a lean signal the mixture solenoid has more ON time, causing less fuel to pass through the solenoid valve and the mixture becomes leaner.

NOTE

Computerized carburetor systems vary. For exact detail on a particular system, refer to the manufacturer’s service manual, which will explain how the specific system functions.