Engine Performance Slides PDF

Four Stroke Combustion Cycle Effects of Compression The decreasing volume of the cylinder causes an increase in pressure This pressure build up raises the temperature Abnormal Combustion Volumetric Efficiency Combustion Chamber Design Engine Vacuum Low pressure created during the intake stroke 15-22...

Four Stroke Combustion Cycle Effects of Compression The decreasing volume of the cylinder causes an increase in pressure This pressure build up raises the temperature Abnormal Combustion Volumetric Efficiency Combustion Chamber Design Engine Vacuum Low pressure created during the intake stroke 15-22” Hg at idle normal Decreases 1” Hg per 1,000 ft above sea level Many engine mechanical conditions can be diagnosed using a simple vacuum test Manifold Vacuum Test Limitations Depending on where you tap the vacuum, your results may be inconclusive. Newer intake manifold designs do not always react like the chart indicates. Intermittent faults may be difficult to detect. Cranking Compression Testing Method used determining cylinder seal. Poor cylinder seal will reduce combustion pressure. Poor compression = poor air/fuel burn = reduced performance Performed with all spark plugs removed and throttle held wide open. Disable Fuel and Ignition systems to avoid personal injury or component damage. Cranking Compression – Cont. Fully charged battery Thread the gauge into a spark plug hole. Crank the engine through one compression stroke “puff” – record PSI Relieve pressure from gauge. Cranking Compression – Cont. Crank the engine for 4 “puffs” – record PSI Repeat for remaining cylinders. All cylinders must be within 70% of the highest reading and above a specified minimum value. Cranking Compression Analysis First “puff” should be at least 50% of the reading after 4 “puffs”. Pressure should rise steadily with each “puff”. Low pressure on the first “puff” and inconsistent pressure build is a good indicator of a piston ring sealing problem. Perform a wet compression test to verify. Cranking Compression Analysis Low compression that doesn’t build up on remaining “puffs” and does not improve with the wet test – Leaking valves, hole in piston, or head gasket leak Low compression in two adjacent cylinders – Suspect leaking head gasket. To further isolate cause of leak proceed to Cylinder Leakage test Cylinder Leakage Test Used to isolate the cause of low compression. Introduce pressurized air and listen for leakage. To be effective the piston must be at TDC compression stroke. Engine should be warm. Results Normal – 12-18% leakage Abnormal – 30% or higher leakage Listen for air escaping – – – – At throttle body: burned intake valve or seat At tailpipe: burned exhaust valve or seat At oil fill cap: worn rings or piston damage Into radiator or surge tank: bad head gasket, cracked head, or cracked block PCV Positive Crankcase Ventilation Vents blow-by gasses to intake Reduces crankcase buildup of moisture, sludge and acids Controls crankcase pressure CAT Catalytic Converter 3-Way catalyst used Main catalyst materials used: Platinum Palladium Rhodium CAT Catalytic Converter Converts CO, HC and NOx into water (H2O), carbon dioxide (CO2) and nitrogen (N) Cerium added for O2 storage Nickel added to reduce sulfur odor AIR Secondary Air Injection System Provides additional oxygen to the exhaust stream to help lower HC & CO emissions Belt driven pump Electric pump AIR / Belt Driven ORVR On-Board Refueling Vapor Recovery Prevents fuel vapors (HC) from escaping to atmosphere during refueling ORVR Vehicles fuel fill pipe is smaller A liquid seal is created Vapors cannot escape PCM Function Takes in incoming information Performs a set of instructions Generates specific outputs PCM Control Voltage Signals Analog: Continuous and variable Voltages on a graph look like a wave Digital: Only has two voltage levels: on and off Only kind of signal the automotive computer understands PCM Components Components Resistors Capacitors Integrated circuits: – Clock circuit – Microprocessor unit (MPU) or central processing unit (CPU) – Memory Other electronic components ROM Non-permanent type of memory Stores temporary data Stored data is erased when PCM loses power (volatile memory) RAM Data cannot be erased or changed Data can only be read Data written from factory is permanent Information randomly accessed from specific location Contains low level instructions to perform tasks of managing engine Does not need applied power to store data (nonvolatile) PROM Can be randomly accessed Is non-volatile When manufactured, all memory locations are empty Electronically written for specific vehicles Many are socketed EEPROM Electronically erasable programmable read only memory (EEPROM): Permanently soldered to PCM circuit boards Can be reprogrammed using Techline scan tool Serial Data String of information transmitted in sequence, one item at a time Consists of voltage signals changing from high to low (on to off) Each individual signal is known as a bit Series of 8 bits makes up a byte (word) Wires that carry serial data messages are called the data bus Serial Data Stream Bits are transmitted at exact intervals Speed at which bits are transmitted is called the baud rate (bits per second) – Early ECMs had baud rate of 160 – Beginning in 1986 model year, ECMs had baud rate of 8,192 OBD II vehicles use data stream with baud rate of 10.4K (10,400 bits per second) Modes of Operation Starting Mode Clear Flood Open Loop Closed Loop Acceleration Mode Deceleration Mode Fuel Cut Off Mode Check Mode Starting Mode Energizes fuel pump relay to pressurize fuel system Determines initial air/fuel ratio Delivers one injector pulse per RPM reference pulse Adjusts injector pulse width Clear Flood Accelerator pedal pressed to floor (WOT) to assure 80% TP attained RPM below 600 PCM pulses injectors for 20:1 air/fuel ratio In some applications, PCM completely cuts off fuel Open Loop Does not use O2 sensor information to control air/fuel mixture Calculates ratio based on inputs from ECT, IAT, TP, MAP or MAF, and CKP sensors Open Loop Remains in open loop until: – O2 sensor warms up – Coolant temperature reaches specific temperature – Specified time has elapsed Closed Loop O2 sensor warmed up Engine at specified temperature Predetermined time has elapsed Air/fuel is controlled to 14.7:1 based on O2 feedback Semi-Closed Loop Semi-closed loop mode: – Highway driving, light engine load – PCM corrects fuel leaner than 14.7:1 for improved fuel economy Converter protection mode: – PCM determines converter overheat condition, returns to open loop – Enriches air/fuel mixture to cool converter Acceleration Mode Simultaneous increase in TP and MAP PCM increases injector pulse width PCM provides additional pulses timed between base pulses Prevents engine stumble during hard acceleration Deceleration Mode Reduces emissions, prevents backfire Light throttle deceleration - PCM shortens injector ontime Close throttle deceleration - PCM may cut off fuel entirely Fuel Cut Off Mode Fuel cut-off mode: PCM shuts off fuel at predetermined MPH and RPM Fuel cut-off when ignition OFF Prevents dieseling or runon Selective fuel cut-off mode: Used for engine torque management protection Used to reduce torque during transmission & transaxle shifts Used to reduce torque in conjunction with brakes applying Protects engine from overheating when coolant is low Typical Parameters Sensors monitor the operating environment and vehicle conditions The microprocessor then uses this information to calculate desired powertrain operation Signal Types Analog signal have continuously varying voltage Digital signals consist of two conditions, HI and LO Temperature Input PCM supplies a 5 volt reference and ground temperature low - the theromistor’s resistance is high temperature high - the thermistor’s resistance is lower voltage dropped across the PCM’s internal resistor Position/Pressure Input Three-wire sensor (potentiometer) 5 volt reference Ground circuit back to the PCM Signal voltage circuit Depending on the position/pressure - signal voltage varies between a low voltage (0.5v) and a high voltage (4.5v). Voltage Generator sensors produce voltage signal PCM looks for quantity or voltage level of the signal Signal Generator Signal generator generate a timed voltage signal PCM is looks at frequency or timing of the signal Engine Coolant Temperature (ECT) Sensor Fuel delivery Ignition control Knock sensor system Idle speed Torque converter clutch application Canister purge Exhaust gas recirculation, Cooling fan operation. Intake Air Temperature (IAT) Sensor Adjust the air-fuel ratio in accordance with air density Modify spark advance and acceleration enrichment Determine when to enable EGR Throttle Position (TP) Sensor fuel delivery ignition timing transmission shifting schedule EGR torque converter clutch application evaporative emission control Heated Oxygen Sensor (HO2S) Circuit HO2S Operation HO2S Voltages Intake Air Flow Measurement Manifold Absolute Pressure (MAP) Sensor Changes in applied vacuum causes a corresponding change in the sensor's resistance Low manifold pressure - sensor voltage is low High manifold pressure - sensor voltage is high Mass Air Flow (MAF) Sensor Greater volume of air - more current is required to maintain the constant temperature of the conductor Air more humid, denser, or cooler - more current to maintain the temperature of the sensor Current translates into a voltage signal, telling the PCM how much airflow Ignition Reference Signals The PCM requires ignition reference pulses in order to control: Spark timing Triggering and synchronization of fuel injectors Idle Air Control (IAC) valve operation Fuel pump relay EGR Canister purge (EVAP) CKP & CMP Crankshaft Position Sensor The crankshaft position sensor (CKP), identifies cylinder pairs at top-deadcenter Camshaft Position Sensor CMP sensor identifies cylinder stroke The CMP (sync pulse) triggers injectors in proper sequence CMP signal indicates position of the number one piston during its intake stroke synchronize the ignition system and calculate true Sequential Fuel Injection Vehicle Speed Sensor (VSS) Permanent magnet generator AC voltage Voltage level & number of pulses increase with vehicle speed AC voltage converted to digital signal Knock Sensor System The Knock Sensor sends an AC voltage signal PCM modifies the ignition timing to control knock Fuel Tank Pressure Sensor Sensor is used to detect leaks in the evaporative emissions system Sensor is a three-wire strain gauge sensor Sensor measures the difference between the air pressure, or vacuum, in the fuel tank and the outside air