CHAPTER 11 Variable Valve Timing Systems
OBJECTIVES After studying Chapter 11, the reader will be able to: • List the reasons for variable valve timing. • Describe how the valve timing is changed. • Discuss the various types of variable valve timing. • Explain how to diagnose variable valve timing faults.
Active fuel management (AFM) Cylinder cut off system Displacement on demand (DOD) EVCP Ground side switching MDS Oil control valve (OCV) Power side switching PWM Spline phaser Vane phaser Variable displacement system VTEC VVT KEY TERMS
PRINCIPLES OF VARIABLE VALVE TIMINGPURPOSE OF VARIABLE VALVE TIMING • There are three basic types of variable valve timing used on vehicles: • Exhaust camshaft variable action only on overhead camshaft engines, such as the inline 4.2 liter engine used in Chevrolet Trailblazers. • Intake and exhaust camshaft variable action on both camshafts used in many General Motors engines. • Overhead valve, cam-in-block engines use variable valve timing by changing the relationship of the camshaft to the crankshaft.
PRINCIPLES OF VARIABLE VALVE TIMINGPARTS AND OPERATION • The camshaft position actuator oil control valve (OCV) directs oil from the oil feed in the head to the appropriate camshaft position actuator oil passages. • There is one OCV for each camshaft position actuator. • The OCV is sealed and mounted to the front cover. • The ported end of the OCV is inserted into the cylinder head with a sliding fit. • A filter screen protects each OCV oil port from any contamination in the oil supply.
FIGURE 11–1 Camshaft rotation during advance and retard. PRINCIPLES OF VARIABLE VALVE TIMINGPARTS AND OPERATION
What Are The Various Names Used for Variable Valve Timing Systems? • BMW-VANOS (Variable Nockenwellen Steuerung) • Ford-VVT (Variable Valve Timing) • GM-DCVCP–(Double Continuous Variable Cam Phasing) if used for both intake and exhaust camshafts • Honda-VTEC - Variable valve Timing and lift Electronic Control • Hyundai-MPI CVVT (Multiport Injection Continuously Variable Valve Timing) • Mazda-S-VT (Sequential Valve Timing) • Mitsubishi-MIVECC (Mitsubishi Innovative Valve timing Electronic Control system)
What Are The Various Names Used for Variable Valve Timing Systems? • Nissan-N-VCTT (Nissan Variable Control Timing) • Nissan-VVL (Variable Valve Lift) • Porsche-variocam–(Variable camshaft timing) • Suzuki-VVT (Variable Valve Timing) • Subaru-AVCS (Active Valve Control System) • Toyota-VVT-i (Variable Valve Timing-intelligent) • Toyota-VVTL-I (Variable Valve Timing and Lift-intelligent) • Volkswagen-VVT (Variable Valve Timing) • Volvo-VVT (Variable Valve Timing)
OHV VARIABLE TIMING • The variable valve timing system uses electronically controlled, hydraulic gear-driven cam phaser that can alter the relationship of the camshaft from 15 degrees retard to 25 degrees advance (40 degrees overall) relative to the crankshaft. • By using variable valve timing (VVT), engineers were able to eliminate the EGR valve and still be able to meet the standards for oxides of nitrogen (NOX). • The VVT also works in conjunction with an active manifold that gives the engine a broader torque curve.
FIGURE 11–2 The camshaft is rotated in relation to the camshaft by the PCM to provide changes in valve timing. OHV VARIABLE TIMING
OHV VARIABLE TIMING • There are two types of cam phasing devices used on General Motors engines: • Spline phaser • Vane phaser
OHV VARIABLE TIMINGSPLINE PHASER SYSTEM • The spline phaser system is also called the exhaust valve cam phaser (EVCP) and consists of the following components: • Engine control module (ECM) • Four-way pulse-width-modulated (PWM) control valve • Cam phaser assembly • Camshaft position (CMP) sensor
FIGURE 11–3 Spline cam phaser assembly. OHV VARIABLE TIMINGSPLINE PHASER SYSTEM
FIGURE 11–4 A spline phaser. OHV VARIABLE TIMINGSPLINE PHASER SYSTEM OPERATION
Check the Screen on the Control Valve If There Are Problems • If a NOX emission failure at a state inspection occurs or a diagnostic trouble code is set related to the cam timing, remove the control valve and check for a clogged oil screen. A lack of regular oil changes can cause the screen to become clogged, thereby preventing proper operation. A rough idle is a common complaint because the spring may not be able to return the camshaft to the idle position after a long highway trip.
FIGURE 11–5 The screen(s) protect the solenoid valve from dirt and debris that can cause the valve to stick. This fault can set a P0017 diagnostic trouble code (crankshaft positioncamshaft position correlation error.) Check the Screen on the Control Valve If There Are Problems
OHV VARIABLE TIMING • VANE PHASER SYSTEM ON AN OVERHEAD CAMSHAFT ENGINE • MAGNETICALLY CONTROLLED VANE PHASER • CAM-IN-BLOCK ENGINE CAM PHASER
FIGURE 11–6 A vane phaser is used to move the camshaft using changes of oil pressure from the oil control valve. OHV VARIABLE TIMING
FIGURE 11–7 A magnetically controlled vane phaser. OHV VARIABLE TIMING
FIGURE 11–8 When the PCM commands 50% duty cycle, the oil flow through the phaser drops to zero. OHV VARIABLE TIMING
FIGURE 11–9 A camshaft position actuator used in a cam-in-block engine. OHV VARIABLE TIMING
What Happens When the Engine Stops? • When the engine stops, the oil pressure drops to zero and a spring-loaded locking pin is used to keep the camshaft locked to prevent noise at engine start. When the engine starts, oil pressure releases the locking pin.
VARIABLE VALVE TIMING AND LIFT • Many engines use variable valve timing in an effort to improve high-speed performance without the disadvantages of a high-performance camshaft at idle and low speeds. • There are two basic systems including: • Variable camshafts such as the system used by Honda/Acura called Variable Valve Timing and Lift Electronic Control or VTEC. • Variable camshaft timing is used on many engines including General Motors 4-, 5-, and 6-cylinder engines, as well as engines from BMW, Chrysler, and Nissan.
FIGURE 11–10 A plastic mock-up of a Honda VTEC system that uses two different camshaft profiles; one for low-speed engine operation and the other for high speed. VARIABLE VALVE TIMING AND LIFT
FIGURE 11–11 Engine oil pressure is used to switch cam lobes on a VTEC system. VARIABLE VALVE TIMING AND LIFT
FIGURE 11–12 A typical variable cam timing control valve. The solenoid is controlled by the engine computer and directs engine oil pressure to move a helical gear, which rotates the camshaft relative to the timing chain sprocket. VARIABLE VALVE TIMING AND LIFT
COMPUTER CONTROL OF VARIABLE VALVE TIMING • Variable valve timing is controlled by the Powertrain Control Module (PCM) and can be one of two different circuits: • Ground side switching • Power side switching
FIGURE 11–13 The schematic of a variable valve timing control circuit, showing that battery power (+) is being applied to the variable valve timing (VVT) solenoid and pulsed to ground by the PCM. FIGURE 11–14 A variable valve timing solenoid being controlled by applying voltage from the PCM. COMPUTER CONTROL OF VARIABLE VALVE TIMING
DIAGNOSIS OF VARIABLE VALVE TIMING SYSTEMS • The diagnostic procedure as specified by most vehicle manufacturers usually includes the following steps: • STEP 1 Verify the customer concern. • STEP 2 Check for stored diagnostic trouble codes (DTCs). • STEP 3 Use a scan tool and check for duty cycle on the cam phase solenoid while operating the vehicle at a steady road speed. The commanded pulse width should be 50%. If the pulse width is not 50%, then the PCM is trying to move the phaser to its commanded position and the phaser has not reacted properly. A PWM signal of higher or lower than 50% usually indicates a stuck phaser assembly.
DIAGNOSIS OF VARIABLE VALVE TIMING SYSTEMS • STEP 4 Check the solenoid for proper resistance. • STEP 5 Check for proper engine oil pressure. Low oil pressure or restricted flow to the cam phaser can be the cause of many diagnostic trouble codes. • STEP 6 Determine the root cause of the problem and clear all DTCs. • STEP 7 Road test the vehicle to verify the fault has been corrected.
The Case of the Wrong Oil • A 2007 Dodge Durango was in the shop for routine service, including a tire rotation and an oil change. Shortly after, the customer returned and stated that the “check engine” light was on. A scan tool was used to retrieve any diagnostic trouble codes. A P0521, “oil pressure not reaching specified value at 1250 RPM” was set. A check of service information showed that this code could be set if the incorrect viscosity engine oil was used. The shop had used SAE 10W-30 but the 5.7 liter Hemi V-8 with multiple displacement system (MDS) required SAE 5W-20 oil. The correct oil was installed and the DTC cleared. A thorough test drive confirmed that the fault had been corrected and the shop learned that the proper viscosity oil is important to use in all vehicles.
VARIABLE DISPLACEMENT SYSTEMSPURPOSE AND FUNCTION • Some engines are designed to be operated on four of eight or three of six cylinders during lowload conditions to improve fuel economy. • The powertrain computer monitors engine speed, coolant temperature, throttle position, and load. • It also determines when to deactivate cylinders. Systems that can deactivate cylinders are called: • Cylinder cutoff system • Variable displacement system • Displacement on Demand (DOD) (now called Active Fuel Management) for General Motors • Multiple Displacement System (MDS) for Chrysler
VARIABLE DISPLACEMENT SYSTEMSPARTS AND OPERATION • The key to this process is the use of two-stage hydraulic valve lifters. In normal operation, the inner and outer lifter sleeves are held together by a pin and operate as an assembly. • When the computer determines that the cylinder can be deactivated, oil pressure is delivered to a passage, which depresses the pin and allows the outer portion of the lifter to follow the contour of the cam while the inner portion remains stationary, keeping the valve closed. • The electronic operation is achieved through the use of lifter oil manifold containing solenoids to control the oil flow, which is used to activate or deactivate the cylinders
FIGURE 11–15 Oil pressure applied to the locking pin causes the inside of the lifter to freely move inside the outer shell of the lifter, thereby keeping the valve closed. VARIABLE DISPLACEMENT SYSTEMSPARTS AND OPERATION
FIGURE 11–16 Active fuel management includes many different components and changes to the oiling system, which makes routine oil changes even more important on engines equipped with this system. VARIABLE DISPLACEMENT SYSTEMSPARTS AND OPERATION
FIGURE 11–17 The driver information display on a Chevrolet Impala with a 5.3 liter V-8 equipped with active fuel management. The transition between 4-cylinder mode and 8-cylinder mode is so smooth that most drivers are not aware that the switch is occurring. CYLINDER DEACTIVATION SYSTEM DIAGNOSIS • A cylinder deactivation system, also called cylinder cutoff system or variable displacement system, often displays when the system is active on the driver information display.
CYLINDER DEACTIVATION SYSTEM DIAGNOSIS • The diagnostic procedure specified by the vehicle manufacturer usually includes the following steps: • STEP 1 Verify the customer concern. With a cylinder deactivation system, the customer concern could be lower than expected fuel economy. • STEP 2 Check for any stored diagnostic trouble codes (DTCs). A fault code set for an emission-related fault could cause the PCM to disable cylinder deactivation. • STEP 3 Perform a thorough visual inspection, including checking the oil level and condition. • STEP 4 Check scan tool data for related parameters to see if any of the sensors are out of the normal range. • STEP 5 Determine the root cause and perform the repair as specified in service information. • STEP 6 Test drive the vehicle to verify proper operation.
SUMMARY • Variable valve timing is used to improve engine performance and reduce exhaust emissions. • Intake cam phasing is used to improve low-speed torque and high-speed power. • Exhaust cam phasing is used to reduce exhaust emissions and increase fuel economy by reducing pumping losses. • Variable valve timing or overhead valve, cam-in-block engines are used to reduce NOX emissions. • Variable valve timing faults are often the result of extended oil change intervals, which can clog the screen on the cam phaser. As a result of the clogged screen, oil cannot flow to and adjust the valve timing, thereby setting valve timingrelated diagnostic trouble codes (DTCs).
SUMMARY • Oil flow to the phasers is controlled by the Powertrain Control Module (PCM). If a 50% duty cycle is shown on a scan tool, this means that the phaser has reached the commanded position. • If the duty cycle is other than 50% while operating the vehicle under steady conditions, this means that there is a fault in the system because the cam phaser is not able to reach the commanded position. • Variable valve timing and lift electronic control (VTEC) is used on most Honda/Acura vehicles to improve performance.
SUMMARY • Control of the variable valve timing (VVT) solenoid can be either ground side switching or power side switching. • Common variable valve timing diagnostic trouble codes include P0011, P0021, P0012, and P0022. • Cylinder deactivation systems improve fuel economy by disabling half of the cylinders during certain driving conditions, such as steady speed cruising.
REVIEW QUESTIONS • What is the advantage of varying the intake camshaft timing? • What is the advantage of varying the exhaust camshaft timing? • Why must the engine oil be changed regularly on an engine equipped with variable valve timing? • What sensors does the PCM monitor to determine the best position for the camshaft(s)? • What diagnostic trouble codes are associated with the variable valve timing (VVT) system?
CHAPTER QUIZ 1. Variable valve timing can be found on which type of engines? • Cam-in-block • SOHC • DOHC • All of the above
CHAPTER QUIZ 2. To reduce oxides of nitrogen (NOX) exhaust emissions, which camshaft is varied? • Exhaust camshaft only • Intake camshaft only • Both the intake and exhaust camshaft • The exhaust camshaft is advanced and the intake camshaft is advanced.
CHAPTER QUIZ 3. To increase engine performance, which camshaft is varied? • Exhaust camshaft only • Intake camshaft only • Both the intake and exhaust camshaft • The exhaust camshaft is advanced and the intake camshaft is advanced.
CHAPTER QUIZ 4. What is the commanded pulse width of the camshaft phaser that results in the desired position? • 0% • 25% • 50% • 100%
CHAPTER QUIZ 5. What sensors are used by the PCM to determine the best position of the camshafts for maximum power and lowest possible exhaust emissions? • Engine speed (RPM) • Crankshaft position (CKP) sensor • Camshaft position (CMP) sensor • All of the above
CHAPTER QUIZ 6. How is the camshaft actuator controlled? • On only when conditions are right • Pulse-width-modulated (PWM) signal • Spring-loaded to the correct position based on engine speed • Vacuum-controlled valve
CHAPTER QUIZ 7. If the engine oil is not changed regularly, what is the most likely fault that can occur to an engine equipped with variable valve timing (VVT)? • Low oil pressure diagnostic trouble code (DTC) • A no-start condition because the camshaft cannot rotate • The filter screens on the actuator control valve become clogged • Any of the above