Fuel Delivery and Air Induction: Description and Operation

Fuel And Emission Systems Diagram:

DESCRIPTION
This vehicle is equipped with a 1.6 liter, SOHC 4 cylinder gasoline engine, and a computer controlled electronic fuel injection system (referred to as EGI - Electronic Gasoline Injection). The system maintains an air/fuel mixture as close as possible to the ideal stoichiometric ratio, for all operating conditions. During normal operation, the computer monitors various engine sensors, including an exhaust gas oxygen sensor, and operates the injectors for varying lengths of time depending on the amount of fuel required. Ignition timing is also computer controlled. The computer monitors engine sensors and calculates crankshaft position, engine speed and load, to determine the correct spark timing.

FUEL INJECTION
Fuel is supplied to the fuel rail and injectors at high pressure by the fuel pump in the tank. A fuel pressure regulator varies fuel pressure slightly depending on manifold vacuum. When fuel demand is high as indicated by low manifold vacuum, the pressure is increased slightly. The injectors are electric solenoid valves that get power when the engine is cranking or running and turn "ON" when the computer (Engine Control Unit - ECU) completes their circuits to ground. Injectors spray fuel into the intake port when they are turned "ON". The amount of fuel sprayed depends on the injector "pulse width" (length of time the injectors are turned "ON" during each injection cycle). Injectors are operated in pairs, so that injection occurs at each cylinder at about the same time the intake valves for those cylinders are open.

STARTING
During cranking, all the injectors energize at the same time once for each ignition pulse (twice for each revolution of the crankshaft). This provides a richer mixture for easy starting. Once the engine is started, injectors operate in pairs.

DE-CHOKE CONTROL
If the engine won't start or is hard to start due to excess fuel in the manifold or cylinders (spark plugs wet with fuel), the ECU will stop injection under the following conditions. If all these conditions exist at the same time, no fuel will be injected. This allows air to be pumped through the cylinders, purging excess fuel.
^ Ignition switch is in the "START" position (engine cranking signal to the ECU).
^ Engine speed is less than 500 rpm (tells the ECU the engine is not running).
^ Throttle sensor signal indicates throttle wide open.

WARM-UP
When the engine is running cold, the fuel injection system operates in what is known as "OPEN LOOP" mode. In this mode, the ECU determines the amount of fuel needed depending on the engine coolant temperature, throttle position sensor signal, and the amount of air flow as measured by the air flow meter. A specific pulse width is assigned for certain air flow rates at various temperatures and throttle openings. These are the "base" pulse width values stored in the computer memory, which provide a slightly rich air/fuel ratio for good driveability while the engine is cold and during warm-up.

NORMAL WARM ENGINE OPERATION
When the engine warms up, the fuel injection system switches to "CLOSED LOOP" operation. In this mode, the computer monitors several sensors, including an exhaust gas oxygen sensor. Signals from the oxygen sensor tell the ECU if the air/fuel ratio is rich or lean. The ECU then modifies the base pulse width accordingly to increase or decrease the amount of fuel being injected, depending on the need at that moment. Injector pulse width is adjusted several times each second to maintain the air/fuel ratio as close as possible to ideal.

IDLE SPEED CONTROL
There are two valves that regulate idle speed. The Air valve and the ISC valve.

Air Valve
The Air valve is a temperature sensitive valve that is open when cold. This allows additional measured air into the engine, bypassing the throttle valve, for cold start fast idle. In the air valve, a temperature sensitive wax pellet is heated by engine coolant. As the engine warms up, the pellet expands against the air valve, closing it. When the engine is warm, idle speed is controlled by the ECU through signals to the ISC valve (see below).

Idle Speed Control (ISC) Valve
When the engine is warm, idle speed is controlled by the Idle Speed Control (ISC) valve. The ISC valve regulates how much measured air is allowed to bypass the throttle valves, depending on signals from the ECU. The ISC valve is electrically operated by the ECU through duty cycle signals (the ratio of "ON" time vs."OFF" time).

DECELERATION FUEL CUT-OFF
The ECU recognizes when the engine is decelerating by monitoring the engine speed and an idle switch in the throttle position sensor that tells the ECU when the throttle is closed. When the throttle is closed and the engine rpm is above a certain speed the ECU will leave the injectors turned "OFF" and no fuel in injected. This reduces emissions, prolongs catalytic converter life, and improves fuel economy.

COMPONENTS
The primary fuel and intake components are covered in this section. For information regarding electronically controlled devices, the computer, and its functions, refer to Computers and Control Systems.
^ Accelerator pedal and cable
^ Air cleaner, intake air ducts and associated parts
^ Air valve
^ Fuel tank
^ Fuel pump
^ Fuel pump relay
^ Fuel filter
^ Fuel pressure regulator
^ Fuel rail
^ Fuel hoses
^ Fuel injectors
^ Idle speed control (ISC) valve
^ Resonance chamber
^ Resonance duct
^ Throttle body