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EFI Operation

EFI Operation . ECM Controls. Theory of Operation. The ECM seeks to maintain the proper air fuel ratio during all modes of engine operation to ensure optimal: Fuel mileage Power Drivability Emissions. Modes of Operation. Starting Clear flood Warm up (Open Loop) Run (Closed Loop)

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EFI Operation

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  1. EFI Operation ECM Controls

  2. Theory of Operation • The ECM seeks to maintain the proper air fuel ratio during all modes of engine operation to ensure optimal: • Fuel mileage • Power • Drivability • Emissions

  3. Modes of Operation • Starting • Clear flood • Warm up (Open Loop) • Run (Closed Loop) • Acceleration • Deceleration • WOT • Fuel cutoff • Limp-in These modes are not necessarily separate “programs” the ECM operates within, rather they are meant to be representative of the different engine operating conditions the ECM must adjust fuel delivery to.

  4. Starting Mode • When you turn the key on the ECM will energize the fuel pump relay for two seconds • This ensures the proper fuel pressure exists at the fuel injectors for quick starting • To determine A/F ratio the ECM looks at: • ECT • IAT • TPS • RPM

  5. Clear Flood Mode • If the ECM sees TPS voltage above a certain level (approx 4.5 V), the ECM assumes the throttle blades are WFO • If the ECM sees WOT by looking at the TPS and the engine RPM are below a certain level (approx. 600 RPM) it will enter “clear flood mode” • In clear flood mode, the ECM will either totally cut fuel off, or adjust it to an extremely lean A/F ratio (approx. 20:1)

  6. Warm Up Mode (Open Loop) • When the engine is cold the ECM will enrichen the A/F ratio just like the choke circuit on a carburetor would do • When the engine is cold the ECM will also increase the idle speed just like the fast idle cam on a carburetor would do • During warm up the ECM ignores the O2 sensor • During open loop operation the ECM calculates fuel delivery by looking at: • ECT • MAP • TPS • RPM

  7. Run Mode (Closed Loop) • When the engine has warmed up enough that certain conditions are met the ECM will enter closed loop operation • O2 sensor voltage varying (cross counts) • ECT above a certain level • Certain amount of time elapsed since startup • During closed loop the computer tries to maintain a 14.7:1 A/F ratio

  8. Acceleration Mode • When the ECM sees a rapid increase in TPS and MAP (or MAF) it knows that the driver is accelerating quickly • Just like the acceleration circuit in the carburetor, the ECM will add an extra amount of fuel during rapid acceleration

  9. Deceleration Mode • When the ECM sees a rapid decrease in TPS and MAP (or MAF) it knows that the driver is decelerating quickly • Under rapid deceleration the ECM will either drastically lean out the A/F ratio or completely cut fuel off • This prevents backfiring and excessive emissions

  10. WOT Mode • When the ECM sees TPS voltage increase to a certain level (approx 4.5 V) and engine RPM is above 600 RPM the ECM will enter WOT mode • During WOT conditions the ECM will go into open loop • O2 sensor will signal, but the ECM ignores it • For maximum power production the ECM will enrichen the A/F ratio (approx 13:1)

  11. Fuel Cutoff Mode • When the ignition is turned off the ECM will stop pulsing the injectors • Prevents dieseling • Positive side of the injector windings may also be connected to a circuit through the ignition key • Speed limiter – under certain conditions the ECM will intermittently cut fuel to limit engine power • Excessive engine RPM • Excessive road speed (tire rating)

  12. Limp-in Mode • If the ECM sees a problem with engine operation it will take steps to correct it • Overheat • Sensor out of acceptable range • Possible sensor failure • The ECM will turn the MIL on • If it is flashing, the ECM has detected a fault that could damage the catalytic converter • The ECM may enter open loop

  13. Limp-in Mode • Sensor failure • In the event of a sensor failure the ECM will ignore the data coming from that sensor and calculate a likely data value to use in fuel calculations • Lookup tables (O2) • Other sensors (TPS – MAP)

  14. Closed Loop ECM Input • Crankshaft position sensor (RPM) • Oxygensensor (O2) • MAP sensor • MAF sensor • CTS • IAT sensor • TPS

  15. ECM Injector Control • To alter the A/F ratio the ECM alters the pulse width of the fuel injector • Pulse width refer to the “on time” of the injector

  16. Injector Control The “On” time of the injector is referred to as the injector pulse width

  17. ECM Pulse Width Calculation • The ECM looks at the current airflow to calculate the required amount of fuel • It then factors in adjustments based on O2 and ECT readings • HowStuffWorks • Fuel Injection Simulation

  18. Short and Long Term Fuel Trim • Fuel Trim is a ECM adaptive strategy to compensate for variations in fuel or air flow between engines • Fuel Trim is expressed in percent change from the base pulse width • + 4%, - 8% etc. • Short term fuel trim is based on the O2 sensor • If the O2 is reading lean the short term fuel trim will adjust the injector pulse width (I.e +8%) • Long term fuel trim is based on the short term fuel trim • If the short term fuel trim is consistently adding fuel, the long term fuel trim will increase (I.e 8%) allowing the short term fuel trim to return to 0

  19. ECM Pulse Width Calculation • The following computations are theoretical and simplified • We will only look at MAP, RPM, ECT, short term fuel trim and long term fuel trim.

  20. ECM Pulse Width Calculation High Pressure Airflow Calculation Low Pressure MAP Voltage R P M

  21. ECM Pulse Width Calculation Base Pulse Width Calculation Airflow E C T

  22. ECM Pulse Width Calculation

  23. ECM Pulse Width Calculation High Pressure Long Term Fuel Trim Low Pressure MAP Voltage R P M

  24. ECM Pulse Width Calculation • Sample Engine • RPM - 1600 • MAP Voltage – 3 Volts • ECT – 150 degrees • O2 Voltage – .35

  25. Speed Density Step 1 – Calculate Airflow High Pressure Airflow Calculation Low Pressure MAP Voltage R P M

  26. ECM Pulse Width Calculation • Sample Engine • RPM - 1600 • MAP Voltage – 3 Volts • ECT – 150 degrees • O2 Voltage – 3.5 • Airflow = 22 grams per second

  27. Step 2 – Calculate Base Injector Pulse Width Base Pulse Width Calculation Airflow E C T

  28. ECM Pulse Width Calculation • Sample Engine • RPM - 1600 • MAP Voltage – 3 Volts • ECT – 150 degrees • O2 Voltage – 3.5 • Airflow = 22 grams per second • Base Injector Pulse Width 6 ms

  29. Step 3 – Calculate Short Term Fuel Trim

  30. ECM Pulse Width Calculation • Sample Engine • RPM - 1600 • MAP Voltage – 3 Volts • ECT – 150 degrees • O2 Voltage – 3.5 • Airflow = 22 grams per second • Base Injector Pulse Width = 6 ms • Short Term Fuel Trim = +5%

  31. Step 4 - Lookup the Long Term Fuel Trim High Pressure Long Term Fuel Trim Low Pressure MAP Voltage R P M

  32. ECM Pulse Width Calculation • Sample Engine • RPM - 1600 • MAP Voltage – 3 Volts • ECT – 150 degrees • O2 Voltage – 3.5 • Airflow = 22 grams per second • Base Injector Pulse Width = 6 ms • Short Term Fuel Trim = +5% • Long Term Fuel Trim = -3%

  33. ECM Pulse Width Calculation • Sample Engine • RPM - 1600 • MAP Voltage – 3 Volts • ECT – 150 degrees • O2 Voltage – 3.5 • Airflow = 22 grams per second • Base Injector Pulse Width = 6 ms • Short Term Fuel Trim = +5% • Long Term Fuel Trim = -3% 6ms x 1.05 x .97 = 6.11 ms

  34. Integrator • Used on OBD I GM ECM controls • Serves the same function as short term fuel trim • Ranges from 0 to 255 • 0% fuel trim = 128 integrator • Less than 128 integrator = fuel removed • Greater than 128 integrator = fuel added

  35. Block Learn • Used on OBD I GM ECM controls • Serves the same function as long term fuel trim • Ranges from 0 to 255 • 0% fuel trim = 128 block learn • Less than 128 block learn = fuel removed • Greater than 128 block learn = fuel added

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