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VARIABLE VALVE TIMING Changing valve timing in response to driving conditions improves driveability and lowers fuel consumption and emission levels. There are many different variable valve timing (VVT) systems used on today's engines. Many systems only vary the timing of the intake camshaft, some vary the timing of the intake and exhaust valves, and a few vary the timing and lift of all the engine's valves. VVT systems are either staged or continuously variable designs. Most staged systems allow two different valve timing and lift settings. Continuously variable systems alter valve timing whenever operating conditions change. Continuously variable systems change the phasing or timing of a valve's duration (Fgr__23). These systems provide a wider torque curve, reduction in fuel consumption, improved power at high speeds, and a reduction in hydrocarbon and NOx emissions. On some engines, VVT has eliminated the need for an exhaust gas recirculation (EGR) valve. Fgr__16 This arrangement has three rocker arms per valve. Honda Motor Co., Inc. Fgr__17 Rocker arm shaft assemblies. Chrysler LLC. Section 11 for a discussion on camshafts. Lifter Camshaft Pushrod Oil return Fgr__18 Most pushrods have a hole through the center to allow oil flow from the hydraulic lifter to the rocker arm assembly. Fgr__19 Many OHC cylinder heads are machined to accept one or two camshafts above the valves and have bearing caps to secure the camshaft. Cylinder head Camshaft bearing cap; Camshaft Cylinder Deactivation: Cylinder deactivation works by keeping the intake and exhaust valves constantly closed in a group of the engine's cylinders. This decreases the working displacement of the engine and provides an increase in fuel economy and reduced emissions. The systems are designed to make the deactivation and activation of the cylinders unnoticeable to the driver. This is accomplished by controlling the fuel injectors, ignition timing, throttle opening, and valve timing. The exact system used for cylinder deactivation varies with the engine design and the manufacturer. OHC engines typically have a pair of rocker arms at each valve. One of the rocker arms rides on the camshaft lobe and the other works the valve. When the two rocker arms are locked together, the valve moves according to the rotation of the camshaft. To disable a cylinder, the rocker arms are unlocked. The rocker arm on the cam lobe continues to work but does not transfer its movement to the other. The locking device is simply a pin that moves in response to oil pressure. A solenoid, controlled by the PCM, directs oil pressure to the pin. Honda's variable cylinder management (VCM) system is an example of this. This system is based on the i-VTEC variable valve control system, which is a staged valve timing system. OHV engines also use oil pressure to deactivate the cylinders. High pressure is sent to the hydraulic lifters to collapse them. The lifters then follow the cam lobes but don’t move the pushrods and rocker arms. There are two ways this is done. Chrysler's multi-displacement system (MDS), found in some V8 engines, has an oil circuit controlled by four solenoids and eight unique hydraulic roller lifters. When conditions dictate that the vehicle does not need all eight cylinders, the PCM energizes the solenoids. Oil pressure is sent to the lifters. The pressure pushes on a small pin in the lifters (Fgr__24). As the pin moves, the piston inside the lifter is disconnected from the lifter body. The lifter body continues to move with the cam lobe but no motion is passed on to the rocker arms. General Motors' displacement on demand (DoD) system, now called active fuel management (AFM), uses two-stage switching lifters. The lifters have an inner and outer body connected by a spring-loaded pin. High oil pressure, sent by solenoids, collapses the spring and the two lifter bodies disconnect. === Fgr__23 The effects of changing intake valve timing. Advancing Retarding Begins Intake Event Sooner Delays Intake Event Lengthens Valve Overlap Shortens Valve Overlap Builds More Low-End Builds More High-RPM Torque Power Decreases Piston to Increases Piston-to- Intake Valve Clearance Intake Valve Clearance === Fgr__24 The lifter used in Chrysler's multi-displacement system. When the pin in the center is moved, the piston inside the lifter is disconnected from the lifter body. Courtesy of Chrysler LLC === Intake Exhaust 1st 2nd 3rd 1st 2nd LOBE INTAKE EXHAUST 1st 2nd 3rd 1.1692 in. (29.700 mm) 1.4003 in. (35.568 mm) 1.4196 in. (36.060 mm) 1.1771 in. (29.900 mm) 1.4054 in. (35.699 mm) Fgr__25 The size of the cam lobes for one cylinder for Honda's three-stage VTEC system. === EXHAUST INTAKE Primary rocker arm; Synchronizing pistons Primary rocker arm; Synchronizing pistons; Synchronizing pistons Fgr__26 The synchronizing pin is controlled by oil pressure and locks the rocker arms together. === Fgr__27 The phaser assembly on the intake camshaft of a Lexus' VVT-i engine. Toyota. === Staged Valve Timing: Most staged valve timing systems switch between two or more different camshaft profiles based on operating conditions. An example of this is Honda's VTEC system used on multivalve engines. The camshaft has three lobes for each pair of intake valves. The third lobe is shaped for more valve lift and different open and close times (Fgr__25). There is a rocker arm over each of the three lobes. At low engine speeds, only the second lobe's rocker arms move the valves. At high speed, a solenoid valve sends pressurized oil through the rocker shaft to a piston in the outer rocker arms (Fgr__26). This pushes the piston partly into the center rocker arm, locking the three rocker arms together. The valves now open according to the shape of the third lobe. When the solenoid valve closes, a spring pushes the pistons back into the outer rockers, and the engine runs with normal valve timing. This system has been modified to include VCM, also called the cylinder idling system, on some engines. This system is used on Honda's hybrid to increase the regenerative braking capabilities of the vehicle and to minimize fuel consumption. The sys tem allows normal and high-output valve timing, plus cylinder idling at all or some cylinders. The system is based on three rocker arms per valve. One rocker arm is used to activate the valve. The other two ride on camshaft lobes. A hydraulically controlled pin connects and disconnects the rocker arms. When there is no connection between the two cam-riding rockers and the valve rocker, the cylinder is idling or deactivated. There are three separate oil passages leading to the pin. As the pressure moves through a passage, it moves the pin. The PCM controls a spool valve that directs the pressurized oil to the appropriate passage. It also controls solenoids that control the amount of pressure. Operation In a typical system, when the brake pedal is released and the accelerator depressed, the vehicle moves by both electric and engine power. At this time the engine is running in the economy mode with the valves opening by the low lift camshaft profile. When the driver is maintaining a very low cruising speed, the engine shuts off and the electric motor powers the car by itself. During this time, the engine's rocker arms are not opening the valves. During acceleration from a low speed, the engine runs in the economy mode. During heavy acceleration, the engine runs in its high-output mode and the electric motor assists the engine. During deceleration, the motor begins to work as a generator and the engine's valves close and remain closed. This allows for maximum regenerative braking and reduces fuel consumption. Continuously Variable Timing--To provide continuously variable value timing, cam shafts are fitted with a phaser. The phaser is mounted where a timing pulley, sprocket, or gear would be (Fgr__27). The phaser allows camshaft-to-crankshaft timing to change while the engine is running. Phasers can be electronically or hydraulically controlled. In a hydraulically controlled system, oil flow is controlled by the PCM. Electronic systems rely on stepper motors. A few engines have electronic/electric intake cam phasers with hydraulic exhaust cam phasers. A phaser assembly is a sealed unit comprised of a hub and an internal vane assembly (Fgr__28). Around the hub is a timing gear that is connected by chain or belt to the crankshaft. The vane assembly is attached to the camshaft. As the vane assembly moves, the phasing of the camshaft changes. At the base of the hub are oil ports. Oil from control solenoids enters and exits through these ports. When the PCM determines a need to change valve timing, oil is sent to the correct port. The pressurized oil then pushes on the vanes and causes a change in valve timing. Changes in valve timing are made by changes in oil pressure on either side of the vane. On a few engines, phasers are connected to the intake and exhaust camshafts, therefore altering the timing of both. It’s important to realize that by altering the timing of both the intake and exhaust valves, valve overlap is also changed. Ford's 5.4-liter Triton V8 engine has a single camshaft in each cylinder bank. The engine also has VVT. This means the timing of the intake and exhaust valves are shifted in equal amounts. When more low-speed torque is required, the PCM orders earlier valve opening and closing. When more high-speed power is needed, the cam timing is retarded. Prius VVT-i System The engine in a Prius, like other hybrid vehicles, operates on the Atkinson cycle. How ever, it also runs with a conventional four-stroke cycle. The switching between the two cycles is done by valve control. Toyota's VVT-i system is reprogrammed to allow the intake valve to close later for the Atkinson cycle. The Atkinson cycle effectively reduces the displacement of the engine and is in operation when there is low engine load. The VVT-i system is controlled by the PCM. The PCM adjusts valve timing according to engine speed, intake air volume, throttle position, and water temperature. In response to these inputs, the ECM sends commands to the camshaft timing oil control valve (Fgr__29). A change in oil pressure changes the position of the camshaft and the timing of the valves. The camshaft timing oil control valve is duty cycled by the ECM to advance or retard intake valve timing. === Intake camshaft Timing chain sprocket Port closed Lock pin Outer housing Timing rotor At stop In operation Oil pressure Vane assembly PHASER ASSEMBLY OIL CONTROL SOLENOID Oil passage Port open Fgr__28 An exploded view of a phaser. == Valvetronic System: Many BMW engines have infinitely variable intake and exhaust valve lift and timing control. Valvetronic is used with variable intake and exhaust valve timing to regulate the flow of air into the cylinders by controlling valve lift. By doing this, the engine has no need for a throttle plate. In fact, this is one of the biggest advantages of the system. A throttle plate has a tendency to rob the engine of power, especially at low engine speeds. In conventional engines the throttle plate regulates the flow of incoming air, while the lift and duration of the intake valve remain constant. During low speeds, the throttle plate is almost totally closed and blocks most of the air available for the cylinders. This results in a pumping loss. Pumping loss is a term used to describe the difficulty a piston has in moving air into the cylinder and moving it out on the exhaust stroke. Pumping losses are a major reason why engines consume a disproportionately large amount of fuel in city driving. Fgr__29 The spool valve in the camshaft timing oil control valve is duty cycled by the PCM. This allows oil pressure to be applied to the advance or retard side of the phaser. Coil Plunger Spring Oil pressure Sleeve To VVT-i controller (Advance side) To VVT-i controller (Retard side) Spool valve --Drain -- Drain Fgr__30 The secondary eccentric shaft and stepper motor assembly for the Valvetronic system. BMW of North America, LLC Fgr__31 The position of the eccentric shaft to provide maximum lift in a Valvetronic system. BMW of North America, LLC Fgr__32 The position of the eccentric shaft to provide minimum lift in a Valvetronic system. BMW of North America, LLC Valvetronic solves this problem by controlling the flow of incoming air at the intake valves. This system uses a conventional camshaft with a secondary eccentric shaft and a series of levers and roller followers that are activated by a stepper motor (Fgr__30). A computer changes the phase of the eccentric cam to change the action of the valves. At high engine speeds the system dials in maximum lift, opening the ports for maximum flow to guarantee rapid filling of the cylinder (Fgr__31). At low engine speeds, the system reverts to minimal valve lift (Fgr__32). This reduces the amount of air entering the cylinder. The action of the valves becomes the engine's throttle plates. Other VVT Systems: A unique setup for controlling intake and exhaust valve timing and lift relies on a conventional cam shaft, which is ground for high performance, with hydraulic lifters fitted with ultra-high-speed valves to bleed off the fluid. The PCM changes the pressure in the lifter to delay valve openings, change valve duration, or prevent valves from opening. A solenoid is used to control the flow of oil into a piston in each lifter that effectively determines the tappet's height. Another design uses pressurized oil to allow a four-valve-per-cylinder engine to operate as a three valve engine at low speeds. At high speeds the engine uses the four valves. Below 2,500 rpm, each intake valve follows a separate camshaft lobe. The primary valve opens and closes normally, while the secondary intake opens just enough to keep the engine running smoothly. As the engine reaches 2,500 rpm, the PCM allows pressurized oil to move small pins that lock each pair of rocker arms together, causing both intake valves to follow the normal cam lobe. When the engine slows down, the pressurized oil is bled off and the pin releases, separating the two rocker arms. The Volvo 3.2-liter engine uses a mix of staged and continuously variable cam timing. It’s also unique in the fact that the camshaft is driven at the flywheel end of the engine. The camshaft's phasers are chain driven. The lift of the intake valves varies with the cam lobe that the rocker arms are riding on. This change is done with a locking pin controlled by oil pressure. Fgr__33 On some OHC heads, the valve springs must be slightly compressed to remove the rocker arms. Remove rocker arm Compress the spring with a special tool or prybar Fgr__34 Rocker arm assemblies should always be removed by following the prescribed order for loosening the mounting bolts. Fgr__35 Measure valve spring height before disassembling the cylinder head. Ford Motor Company === Next: part 4 Prev.: Intake and Exhaust Valves Home Article Index top of page |
