OBJECTIVES

2. OBJECTIVES After studying Chapter 16, the reader will be able to: • Explain the purpose and function of fuel trim. • Discuss the difference between speed density and mass air flow fuel control. • Describe how knowing the volumetric efficiency can help diagnose engine performance concerns. • Explain how to tell if a volumetric efficiency concern in an engine is due to a mechanical or an airflow measurement problem.

3. PURPOSE AND FUNCTION OF FUEL TRIM • Fuel trim provides a method that is capable of changing the amount of fuel delivered to the engine based on feedback from the oxygen sensors. • The primary purpose of the fuel trim is to keep the air-fuel mixture as close to 14.7:1 as possible. FIGURE 16-1 The catalytic converter is most efficient when the exhaust ratio is closest to 14.7:1.

4. PURPOSE AND FUNCTION OF FUEL TRIMLambda • Lambda is a Greek letter used to represent ratio, as in air-fuel ratio. • If an engine is operating at exactly 14.7:1, the air-fuel ratio on gasoline, the ratio is called stoichiometric and is assigned a lambda of 1.0. • Air-fuel ratios lower than 1.0 indicate a rich mixture. • Air-fuel ratios higher than 1.0 indicate a lean mixture.

5. PURPOSE AND FUNCTION OF FUEL TRIMEquivalence Ratio • The equivalence ratio (ER) is the inverse of lambda, with 1.0 equal to 1.0 lambda; however, 0.9 ER is equal to 1.1 lambda. • Equivalence ratio (ER) = 1/λ (lambda), which is the inverse of lambda. FIGURE 16-2 Shown is lambda. The equivalence ratio is opposite lambda when the ratio above 1 is rich when any lambda number higher than 1 indicates a lean air-fuel mixture.

6. IMPORTANCE OF LAMBDA • Lambda occurs when the air-fuel mixture is kept at 14.7:1 for gasoline-powered engines. • The primary purpose of keeping the air-fuel mixture at lambda is that it allows the catalytic converter to be the most efficient at reducing all exhaust emissions.

7. BASE PULSE WIDTHPurpose and Function • The base pulse width is the calculated injected pulse width as determined by readings from sensors. • The PCM uses information from the following sensors to determine the base pulse width for the fuel injectors. • RPM (engine speed) • Load • Intake air temperature (IAT) • Engine coolant temperature (ECT) (used mostly when the engine is cold) • Amount of exhaust gas recirculation • Canister purge flow amount • Manifold absolute pressure • BARO (altitude) • Volumetric efficiency calculation

8. BASE PULSE WIDTHPurpose and Function • Mass Air Flow versus Speed Density • Speed Density Modifier • BARO • IAT • RPM • EGR • MAF System Modifiers

9. MEASURING PULSE WIDTH • The PCM determines the base injector pulse width based on the reading from the sensors and the calculations from look-up tables stored in read only memory. • This base pulse width is the best guess as to the correct amount of fuel that the engine needs. • Pulse width is measured in milliseconds (ms) and represents the amount of time the fuel injectors are commanded on. • A typical engine at idle speed will have a pulse width of about 2 to 5 milliseconds, depending on the size of the engine.

10. FUEL TRIM • Based on the oxygen sensor activity, the PCM tries to keep the air-fuel mixture at 14.7:1 under most conditions. • If the vehicle is operating under full load, the oxygen sensor data is ignored and the PCM commands the richer-than-normal air-fuel mixture needed for maximum power based on inputs from the other sensors.

11. THE NEED FOR FUEL TRIM • The purpose of fuel trim is to provide the catalytic converter with a stoichiometric air-fuel mixture, which it needs to reduce NOXexhaust emissions and to help oxidize HC and CO into harmless carbon dioxide (CO2) and water (H2O) vapor. FIGURE 16-3 Scan tool display, showing both long-term and short-term fuel trim. Both LTFT and STFT should be less than 10%.

12. SHORT-TERM AND LONG-TERM FUEL TRIM • Short-term fuel trim (STFT) is a percentage measurement of the amount the computer is adding or subtracting from a calculated value. • Long-term fuel trim (LTFT) is designed to add or subtract fuel for a larger amount of time than short-term fuel trim.

13. USING FUEL TRIM AS A DIAGNOSTIC AID • Fuel trim values can only be observed with a scan tool. • A scan tool will display both short-term and long-term fuel trim. • For system diagnosis, refer to the long-term fuel trim because it represents a longer amount of time (history) and a greater amount of mixture correction.

14. FUEL TRIM CELLS • Both STFTs and LTFTs react to oxygen sensor voltage to modify fuel delivery. • Most vehicles set aside different fuel trim cells for each combination of engine speed (RPM) and load. • The computer can then correct for slight differences in fuel mixture separately for each cell.

15. FUEL TRIM CELL DIAGNOSIS • To use fuel trim as a diagnostic aid, the data should be observed during the same condition as the problem.

16. MAF SENSOR ACCURACY • In an engine equipped with a MAF sensor, the accuracy of the sensor is critical for the PCM to provide the current pulse-width command to the fuel injectors. • Factors that can affect the accuracy of the MAF sensor readings include: • Vacuum leaks. • False air. • PCV airflow. • Airflow disturbance (disruption).

17. MAF SENSOR ACCURACY FIGURE 16-4 Any fault in the air cleaner assembly can disrupt the airflow through the MAF sensor.

18. VOLUMETRIC EFFICIENCYDefinition of Volumetric Efficiency • Volumetric efficiency (VE) is the percentage of air entering the engine compared to the theoretical airflow. • Typical normally aspirated engines will test having a VE of 75% to 90%. • Older two-valve cylinder head engines will test lower than newer engines equipped with four valves per cylinder

19. NEGATIVE FUEL TRIM BANK #1; POSITIVE FUEL TRIM BANK #2 • If one bank of a V-6 or V-8 engine has a restricted exhaust on one bank, the fuel trim numbers will be negative on the bank that is restricted and positive on the bank that is not restricted. FIGURE 16-5 This properly operating engine is drawing in 48 grams per second of air for all eight cylinders. This indicates that each cylinder will be receiving 6 grams per second (GPS).

20. NEGATIVE FUEL TRIM BANK #1; POSITIVE FUEL TRIM BANK #2 FIGURE 16-6 If the exhaust system on the left bank (bank #1) were to become restricted, the total airflow through the MAF sensor would also decrease. The cylinders on the right bank (bank #2) would draw the same 6 GPS as before and the cylinders on bank #1, which have a restricted exhaust, would draw just 4 GPS.

21. NEGATIVE FUEL TRIM BANK #1; POSITIVE FUEL TRIM BANK #2 FIGURE 16-7 If all cylinders were equal and showed the 40 grams per second, then each cylinder will be drawing 5 grams per second (5 x 8 cylinders = 40 GPS).

22. NEGATIVE FUEL TRIM BANK #1; POSITIVE FUEL TRIM BANK #2 FIGURE 16-8 As a result of the restricted exhaust on bank #1,the restricted bank will operate too rich and bank #2 will operate too lean. The long-term fuel trim will be negative for bank #1 and positive for bank #2.

23. SUMMARY • Lambda is a Greek letter used to represent air-fuel ratio. Lambda of 1.0 is equal to an air-fuel rate of 14.7:1. • Equivalence ratio is the inverse of lambda. • Base pulse width is determined by the PCM based on input from many sensors. • Speed density fuel control uses calculations based on the input from various sensors such as the TP and MAP sensor to determine the amount of fuel needed. • Mass air flow systems use a mass air flow sensor to measure the mass of the air entering the engine directly.

24. SUMMARY • Fuel trim uses the oxygen sensor data to fine tune the airfuel mixture to ensure lowest emissions. • Short-term fuel trim (STFT) is capable of quickly adding or subtracting fuel, but only a limited amount. • Long-term fuel trim (LTFT) is capable of adding or subtracting more fuel than STFT, but is slower to react. • Volumetric efficiency is the percentage of air entering the engine compared to the theoretical airflow.

25. REVIEW QUESTIONS • What is the difference between lambda and equivalence ratio? • How is base pulse width determined? • Why is fuel trim needed? • What is the difference between short-term and long-term fuel trim?

26. CHAPTER QUIZ • If the air-fuel ratio is 14.7:1, what is lambda? • 1.0 • 0.9 • 1.1 • 14.7

27. CHAPTER QUIZ • If the air-fuel ratio is 14.7:1, what is lambda? • 1.0 • 0.9 • 1.1 • 14.7

28. CHAPTER QUIZ 2. If lambda is 0.98, this means the _____. • Mixture is lean • Air-fuel mixture is within 2% of stoichiometric • Air-fuel mixture is slightly rich • Both b and c

29. CHAPTER QUIZ 2. If lambda is 0.98, this means the _____. • Mixture is lean • Air-fuel mixture is within 2% of stoichiometric • Air-fuel mixture is slightly rich • Both b and c

30. CHAPTER QUIZ 3. Base pulse width is determined by _____. • Oxygen sensor data • Computer calculations • Input from many sensors, except the oxygen sensor • Both b and c

31. CHAPTER QUIZ 3. Base pulse width is determined by _____. • Oxygen sensor data • Computer calculations • Input from many sensors, except the oxygen sensor • Both b and c

32. CHAPTER QUIZ 4. The air at high altitude has _____. • 21% oxygen • Less than 21% oxygen • A higher pressure • A higher density

33. CHAPTER QUIZ 4. The air at high altitude has _____. • 21% oxygen • Less than 21% oxygen • A higher pressure • A higher density

34. CHAPTER QUIZ 5. In a speed density system, what does not need to be corrected for? • Air temperature • Oxygen in the exhaust from the oxygen sensor • Amount of EGR • BARO sensor data

35. CHAPTER QUIZ 5. In a speed density system, what does not need to be corrected for? • Air temperature • Oxygen in the exhaust from the oxygen sensor • Amount of EGR • BARO sensor data

36. CHAPTER QUIZ 6. In a MAF system, which is correct? • The system measures the actual mass of the air entering the engine. • The amount of EGR flow must be subtracted from the MAF sensor reading. • Altitude correction is not needed. • Temperature of the air correction is not needed

37. CHAPTER QUIZ 6. In a MAF system, which is correct? • The system measures the actual mass of the air entering the engine. • The amount of EGR flow must be subtracted from the MAF sensor reading. • Altitude correction is not needed. • Temperature of the air correction is not needed

38. CHAPTER QUIZ 7. Injector pulse width is measured in _____. • Percentage (%) • Milliseconds (ms) • Duty cycle (%) • Frequency (Hz)

39. CHAPTER QUIZ 7. Injector pulse width is measured in _____. • Percentage (%) • Milliseconds (ms) • Duty cycle (%) • Frequency (Hz)

40. CHAPTER QUIZ 8. What is not true about short-term fuel trim? • It is able to react quickly to add or subtract fuel. • It can add or subtract a large amount of fuel. • It uses the oxygen sensor. • It is expressed in percentages.

41. CHAPTER QUIZ 8. What is not true about short-term fuel trim? • It is able to react quickly to add or subtract fuel. • It can add or subtract a large amount of fuel. • It uses the oxygen sensor. • It is expressed in percentages.

42. CHAPTER QUIZ 9. A Nissan is being checked, using a scan tool, and Alpha is 107. This means _____. • The PCM is adding fuel • The PCM is subtracting fuel • The PCM represents STFT only • The PCM represents LTFT only

43. CHAPTER QUIZ 9. A Nissan is being checked, using a scan tool, and Alpha is 107. This means _____. • The PCM is adding fuel • The PCM is subtracting fuel • The PCM represents STFT only • The PCM represents LTFT only

44. CHAPTER QUIZ 10. A contaminated or defective MAF sensor is indicated if _____. • The fuel trim number follows the airflow • The VE is bad • The fuel trim numbers are within ±2% • The BARO reading is lower than normal

45. CHAPTER QUIZ 10. A contaminated or defective MAF sensor is indicated if _____. • The fuel trim number follows the airflow • The VE is bad • The fuel trim numbers are within ±2% • The BARO reading is lower than normal