The Basics of Fuel Control

October 6, 2001 Page 1

The Basics of Fuel Control

Presented by: Paul Baltusis

Powertrain Control System Engineering

Diagnostic Systems Department

OBD-II Technical Specialist

Revised: October 6, 2001


Overview

The purpose of the air/fuel ratio control system is to achieve an ideal air/fuel mixture within the combustion chamber.

The goal is to produce maximum power while minimizing emissions and maximizing fuel economy.

To accomplish this goal, the Powertrain Control Module relies on a network of inputs (sensors) and outputs (actuator) to accurately control the air/ fuel mixture.


Basic Fuel Injection System


Air/Fuel RatioThe air/ fuel ratio is the ratio of air to fuel,

by mass.14.7: 1 ( stoichiometric) is the ideal air/

fuel ratio for gasoline, but during normal engine operating conditions, this air/ fuel ratio varies between 12: 1 (rich) to 18: 1 (lean).

The air/ fuel ratio can affect power, fuel economy, and emissions.


Air/Fuel Ratio Factors

The engine fuel system is designed to break the liquid fuel into a vapor of fine fuel particles and mix them with air.

There are many factors involved in the air/ fuel mixing process. – Atomization – Vaporization – Swirl – Condensation– Absorption


Fuel Delivery

The amount of fuel to be delivered by the injector is determined by the fuel control system. Fuel mass depends on:– How much air is entering the engine,

or air mass,– How much fuel is needed to achieve

the desired air/fuel ratio, or fuel mass– and the injector pulse width required

to deliver the correct amount of fuel to the proper cylinder.


Basic Fuel Equation

Fuel Mass = Air Mass Desired A/F Ratio

or

Air Mass Desired Equivalence Ratio (EQ_RAT) *

14.64


Basic Fuel Equation

Because most PCMs use O2 sensors for feedback, the fuel equation includes short and long term fuel trim modifiers:

FUELMASS = AIRMASS * SHRTFT * LONGFT EQUIV_RATIO * 14.64

Now lets see how this equation works!


Measuring Air MassThere are three methods generally used

to measure air mass:– Mass Air Flow (MAF) sensor – mass air

flow system– Manifold Absolute Pressure (MAP)

sensor – speed density system– Vane Air Flow (VAF) sensor – not used

very much (we won’t discuss this system)


Mass Air Systems

The Mass Air Flow (MAF) sensor is a hot wire- sensing element placed directly in the air path to the intake manifold, before the IAC and throttle plates.

As air passes through the MAF sensor and over the hot wire, the wire cools, changing its resistance which in-turn changes the current in the wire. The sensor electronics uses this characteristic to determine air mass.


Mass Air System Characteristics

MAF measures actual air mass - does not need correction for altitude.

MAF does not measure EGR flow - EGR mass calculation is not needed for fuel control.

BARO must be inferred, usually at high throttle openings or is derived from a MAP/BARO sensor.

Unmetered air (vacuum/induction leak) causes a lean error.


Speed Density SystemsA speed density system is more

complicated than the mass air system. Air mass is determined based on MAP and a PCM calculation.

A number of factors are incorporated into the speed-density equation: engine displacement, air density, Manifold Absolute Pressure (MAP), RPM, volumetric efficiency, Engine Coolant Temperature (ECT), and Intake Air Temperature (IAT).


Speed Density Equation

In addition, Exhaust Gas Recirculation mass and/or Purge Flow mass must be independently calculated and subtracted from air mass. Typical formulae are:

Air mass (Ford) = K(constant) * MAP * RPM * Vol. Eff. * ECT correction * IAT

correction – EGR massAir mass (GM) = [Displ * # Cyl/2] * MAP *

RPM * Vol. Eff. / IAT * R – Purge mass – EGR mass


Speed Density System Characteristics

Speed density measures air volume - needs density corrections for altitude, temperature.

S/D needs to know engine volumetric efficiency.

S/D needs to know EGR mass to subtract the proper amount of fuel (EGR is inert and does not burn).

BARO is measure directly from the MAP sensor.Unmetered air (vacuum/induction leak) has no

affect on fuel control.


Desired Air Fuel Ratio

• After the PCM computes air mass, it needs to determine the desired air/fuel ratio (equivalence ratio) to determine fuel mass.

• It then uses the fuel mass to determine the appropriate injector pulse width. The pulse width is the length of time the PCM turns the injector on, and is measured in milliseconds. The actual pulse width depends on the injector flow rate.


Desired Air Fuel Ratio

• Although stoichiometric (EQ_RAT = 1.0) is considered the ideal air/fuel ratio for gasoline (14.64:1), there are many operating conditions where a stoichiometric ratio is not desired. When operating conditions require an air/fuel ratio other than stoichiometric, or the oxygen sensors are not at operating temperature, the fuel system is commanded to open-loop mode.


Open Loop Fuel Control

When the engine is operating open-loop, the PCM commands a rich or lean air/fuel ratio, and uses air mass to calculate the appropriate injector pulse width. It does not use feedback from the oxygen sensor.

The PCM generally commands open-loop operation during the following conditions: Cold engine start-up, high load / wide open throttle catalyst over-temperature protection.



During cold engine start-up, the oxygen sensor does not produce an accurate signal because it has not reached operating temperature. The PCM waits until the O2 sensor is warmed up before attempting to go into closed-loop operation.

Most vehicles use a O2 sensor with a heater to allow faster warm-up.



During high load and WOT operation, maximum engine power can be obtained by running about 5% rich.

If inferred catalyst temperature is too high (sustained high rpm and load), running rich (or sometimes lean) can be used to reduce catalyst temperatures.



During open-loop operation, EQ_RAT values come from lookup tables in the PCM. The specific value selected is based primarily on RPM, load, and engine coolant temperature. EQ_RAT values are generally less than 1.0, resulting in a rich air/fuel ratio. Although the O2 sensor is not used for fuel control, it will reflect this rich condition.


Closed Loop Fuel Control

Once the oxygen sensor has reached operating temperature and open loop conditions are not demanded, the PCM commands a stoichiometric air/fuel ratio (EQ_RAT = 1.0) and the engine operates in closed loop.

In closed-loop operation, the PCM calculates air mass and uses feedback from the oxygen sensor to indicate if the mixture is rich or lean. The PCM uses this information to adjust the commanded injector pulse width until a stoichiometric air/fuel ratio is achieved.


Closed Loop Fuel Control• A conventional O2 sensor (not a wide-

range sensor) can only indicate if the mixture is richer or leaner than stoichiometric. During closed loop operation, short term fuel trim values are calculated by the PCM using oxygen sensor inputs in order to maintain a stoichiometric air/fuel ratio.


O2 Sensor Transfer Function



• The PCM is constantly making adjustments to the short term fuel trim, which causes the oxygen sensor voltage to switch from rich to lean around the stoichiometric point. As long as the short term fuel trim is able to cause the oxygen sensor voltage to switch, a stoichiometric air/fuel ratio is maintained.



Short term fuel trim values are displayed on a scan tool as a percentage of fuel added or subtracted. Typically, the SHRTFT value switches above and below zero percent. Zero percent (0%) on the scan tool means there is no adjustment and the PCM multiplies the air mass by 1. If the percentage is positive, the PCM multiplies by a value greater than 1, and if the percentage is negative, the PCM multiples by a value less than 1.


Closed Loop Fuel ControlWhen initially entering closed loop fuel,

SHRTFT starts at zero percent and begins adding or subtracting fuel in order to make the oxygen sensor switch from its current state. If the oxygen sensor signal sent to the PCM is greater than 0.45 volts, the PCM considers the mixture rich and SHRTFT shortens the injector pulse width.



When the cylinder fires using the new injector pulse width, the exhaust contains more oxygen. Now when the exhaust passes the oxygen sensor, it causes the voltage to switch below 0.45 volts, the PCM considers the mixture lean, and SHRTFT lengthens the injector pulse width. This cycle continues as long as the fuel system is in closed loop operation.


Example of Closed Loop Fuel Control


Closed Loop Fuel ControlAs fuel, air, or engine components age or

otherwise change over the life of the vehicle, the PCM learns to adapt fuel control. Corrections are only learned during closed loop operation, and are stored in the PCM as long term fuel trim values (LONGFT). For some manufacturers, LONGFT values are only learned when SHRTFT values cause the oxygen sensor to switch.



If the average SHRTFT value remains above or below 0%, the PCM “learns” a new LONGFT value, which allows the SHRTFT value to return to an average value near 0%. There is normally a different LONGFT value stored for various RPM and load operating conditions. LONGFT is actually stored as a “table”. The LONGFT value displayed on the scan tool is the value being used for the current operating condition.


Example of Learning LONGFT



LONGFT values are displayed on a scan tool as a percentage of fuel added or subtracted. Zero percent on the scan tool means there is no adjustment and the PCM multiplies the air mass by 1. If the percentage is positive, the PCM multiplies by a value greater than 1, and if the percentage is negative, the PCM multiples by a value less than 1. LONGFT values learned during closed loop are used in both open and closed loop modes.


How does this help diagnostics?Vacuum leaks – • Speed density fuel control is unaffected,

mass air systems go lean at idle. Fuel trim can be used to diagnose vacuum leaks on a mass air system.

• Idle speed goes up on a speed density system (a leak is just like opening the throttle), idle speed drops or the engine stalls on a mass air system because the fuel system is lean.


How does this help diagnostics?

Plugged EGR passages• Speed density systems without flow

diagnostics will run lean. The PCM is subtracting fuel for EGR mass even if it’s not there.

• For mass air systems, fuel control is not affected by any EGR errors.



Low Fuel Pressure• Speed density and mass air systems act

alike. Use SHRTFT and LONGFT to see if there is a uniform shift in LONGFT values, on both banks (where applicable).



Contaminated MAF sensor• Use LONGFT to see is idle area is rich,

higher rpm/loads are lean.• Use BARO (where applicable) to see if it

is appropriate for the current altitude.



• Lack of O2 Sensor SwitchesDepends on whether a manufacturer needs

O2 sensor switches to learn LONGFT.If O2 switches are needed, a large fueling

error will set an “lack of O2 switches” code, not a fuel trim code.

If O2 switches are not needed, a large fueling error will set a fuel trim code.


Bottom Line

A basic understand of fuel control systems is essential for proper diagnostics.

Thank you for your attention.

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The Basics of Fuel Control

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