The Basics of Fuel Control

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The Basics of Fuel Control
Presented by
Paul Baltusis Powertrain
Control System Engineering Diagnostic
Systems Department OBD-II Technical
Specialist Revised October 6,
2001
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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.

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Basic Fuel Injection System
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Air/Fuel Ratio

• The 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.

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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

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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.

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Basic Fuel Equation

• Fuel Mass Air Mass
• Desired A/F Ratio
• or
• Air Mass
  
• Desired Equivalence Ratio (EQ_RAT) 14.64

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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!

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Measuring Air Mass

• There 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 wont discuss this system)

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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.

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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.

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Speed Density Systems

• A 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).

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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 mass
• Air mass (GM) Displ Cyl/2 MAP RPM
  Vol. Eff. / IAT R Purge mass EGR mass

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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.

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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.

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Desired Air Fuel Ratio

• Although stoichiometric (EQ_RAT 1.0) is
  considered the ideal air/fuel ratio for gasoline
  (14.641), 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.

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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.

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Open Loop Fuel Control

• 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.

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Open Loop Fuel Control

• 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.

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Open Loop Fuel Control

• 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.

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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.

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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.

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O2 Sensor Transfer Function
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Closed Loop Fuel Control

• 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.

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Closed Loop Fuel Control

• 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.

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Closed Loop Fuel Control

• When 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.

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Closed Loop Fuel Control

• 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.

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Example of Closed Loop Fuel Control
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Closed Loop Fuel Control

• As 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.

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Closed Loop Fuel Control

• 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.

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Example of Learning LONGFT
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Closed Loop Fuel Control

• 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.

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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.

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

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How does this help diagnostics?

• 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).

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How does this help diagnostics?

• 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.

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How does this help diagnostics?

• Lack of O2 Sensor Switches
• Depends 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.

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Bottom Line

• A basic understand of fuel control systems is
  essential for proper diagnostics.
• Thank you for your attention.