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.

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.

PURPOSE AND FUNCTION OF FUEL TRIM Lambda 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.

PURPOSE AND FUNCTION OF FUEL TRIM Equivalence 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.

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.

BASE PULSE WIDTH Purpose 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

BASE PULSE WIDTH Purpose and Function Mass Air Flow versus Speed Density Speed Density Modifier BARO IAT RPM EGR MAF System Modifiers

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.

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.

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 NOX exhaust 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%.

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.

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.

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.

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

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).

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

VOLUMETRIC EFFICIENCY Definition 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

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).

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.

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).

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.

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.

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.

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?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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.

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

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

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

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