Fuel Delivery and Air Induction: Description and Operation

Fuel Delivery System:

The fuel metering system consists of the following components:

1. Fuel pump electrical circuit.

2. Fuel supply components, including:

a. Fuel lines and pipes.
b. Fuel pump.
c. Fuel tank.

3. Throttle Body Injection Assembly, including:

a. Fuel injectors
b. Fuel pressure regulator
c. Idle air control valve
d. Throttle position sensor

BASIC SYSTEM OPERATION:

The fuel supply system begins with gasoline in the fuel tank. An electric fuel pump, located in the fuel tank with the fuel level gauge sending unit, pumps fuel to the throttle body assembly through an in-line filter. The pump is designed to supply fuel at a pressure above the pressure needed by the injectors. A pressure regulator located in the throttle body assembly keeps the fuel available to the injectors at a constant pressure. The unused fuel is returned to the fuel tank by a separate line.

MODES OF OPERATION:

The ECM uses voltage inputs from several sensors to determine how much fuel to give the engine. The fuel is delivered under several conditions, referred to as COMPENSATION MODES. All modes are controlled by the ECM, and are described below along with a brief description of injection timing and factors that effect injection operation.

INJECTION TIMING

Ignition Timing At Start:

^ During start up: Fuel is injected at a certain cycle starting immediately after the initial ignition signal is input until the engine starts.

Injection Timing During Normal Driving:

^ In normal driving (Standard Injection Timing): Fuel is injected at every ignition signal synchronously.

^ When accelerating (Additional Injection Timing): Fuel is injected in addition to standard injection timing whenever the throttle valve exceeds a specific opening.

INJECTION TIME

The factors to determine injection time are basic injection time which is calculated on the basis of the engine speed and the intake manifold pressure (amount of intake air) and various compensations which are determined according to the signals from various sensors that detect the state of the engine and conditions.

NOTE: The amount of fuel drawn into the engine is determined by the injection frequency as well as injection time.

BAROMETRIC PRESSURE

At higher altitudes where the barometric pressure is lower than at lower altitudes, compensation is made so as to adjust the air/fuel mixture ratio to accommodate such environmental conditions.

INTAKE AIR TEMPERATURE

As intake air volume varies with the temperature, it is compensated for by an adjustment in injector timing.

Parameter Diagram:

ENRICHING COMPENSATION MODES

^ During Warm Up: When the engine is cold, enrichment compensation is made to ensure good driveability until the engine coolant temperature reaches the specified level. The amount to enrich the air/fuel mixture is decreased as temperature rises.

^ During Warm Up and Engine Loaded: When the engine is cold and a high load is applied to the engine, enrichment compensation is made by the amount according to engine coolant temperature so as to ensure good driveability.

^ After Engine Starts: For a certain time after the engine is started, air/fuel mixture enrichment compensation is made so as to stabilize the engine idle speed. As the amount of compensation depends on the engine coolant temperature, it is the largest immediately after the engine is started and after that, it reduces gradually.

^ During Normal Operation and Engine Loaded: Enrichment compensation is made to make the air/fuel mixture ratio richer than theoretical air/fuel mixture ratio to ensure good driveability under highly loaded driving conditions.

^ During Acceleration: To attain smooth acceleration, enrichment compensation is provided for a certain time according to each accelerating condition, which is obtained through operation using the signal from the pressure sensor (representing variation of pressure in the intake manifold).

LEANING COMPENSATION MODES

^ During Deceleration: To obtain a proper air/fuel mixture ratio during moderate deceleration, leaning compensation is made for a certain time so that the air/fuel mixture leans out to a proper ratio for each deceleration condition, which is obtained through operation using the signal from the Throttle Position Sensor (TPS) (representing variation of the throttle valve opening).

OTHER COMPENSATION MODES

^ During Idle: To stabilize the idle speed, the amount of fuel injection is adjusted according to the varying engine idle speed.

^ Battery Voltage Drop: A power voltage drop delays the mechanical operation of the injector. Then the actual injection time becomes shorter for the time that electricity is supplied to the injector. To compensate for this condition, the electricity supply is made longer when the voltage is lower.

^ Base Air/Fuel Ratio: The air/fuel ratio may vary due to such factors as variations in individual engines and aging. To compensate for such variations, feed back compensation is used and base air/fuel ratio is adjusted to a proper level.

^ Fuel Cut: When decelerating quickly, the fuel supply is cut or decreased to prevent unburned gas from being emitted by making the injector operation time as ineffective injection time. Also, when the engine exceeds 7,200 rpm the fuel supply is cut to protect the engine from damage by making the injector operating time as ineffective injection time. The normal injection is restored when the engine speed is reduced below 6,800 rpm or lower.

Fuel Feed Back Compensation:

FUEL FEED BACK COMPENSATION

It is necessary to keep the air/fuel mixture as close to theoretical (14.7:1) to obtain efficient performance of the three way catalyst and high clarification of CO, HC, and NOx in the exhaust gas. For that purpose, the ECM operates as follows. It first compares the signal from the oxygen sensor with a specified reference voltage and if the signal is higher, it detects that the air/fuel ratio is richer than theoretical air/fuel ratio and reduces fuel. On the other hand, if the signal is lower, it detects that the air/fuel ratio is leaner and increases fuel. By repeating these operations, it adjusts the air/fuel ratio closer to theoretical air/fuel ratio.

a. When oxygen concentration in the exhaust gas is low (air/fuel ratio is smaller or fuel is richer than theoretical) electromotive force of the oxygen sensor increases and a rich signal is sent to the ECM.

b. Upon receipt of the rich signal, the ECM decreases the amount of fuel injection, which causes oxygen concentration in the exhaust gas to increase and electromotive force to the oxygen sensor to decrease. Then a lean signal is sent to the ECM.

c. As the ECM increases the amount of fuel injection according to the lean signal, oxygen concentration in the exhaust gas decreases and the situation is back condition "a" (above condition).

This control process, however, will not take place under any of the following conditions:

^ At engine start and when fuel injection is increased after engine start.

^ When engine coolant temperature is low.

^ When engine load is high and fuel injection is increased.

^ During fuel cut situations.

^ When oxygen sensor is cold.