Designs of Variable Valve Timing Actuators

22-05 Evaluate the different designs of camshaft phasers in VVT systems.

VVT systems use one of the several available types of actuator designs to vary camshaft timing in relation to CMP. VVT actuators may be the helical piston (gear) style, the vane type, or lobed rotor phasers. The most common is the vane-type actuator.

Helical Gear-Style and Helical Piston-Style Phasers

The helical gear-type actuator or phaser uses a twisted gear arrangement and a return spring. Several manufacturers have used a helical piston-style phaser, including GM and BMW. The helical phaser/controller is positioned between the camshaft timing pulley and the camshaft. The phaser/controller houses a piston that moves back and forth on helical splines based on oil pressure direction.

The helical gear-type phaser consists of a helical gear (helical gears are cut at an angle and have some curvature to the face of the gear), outer casing, hydraulic cylinder, and a piston. In operation, the helical gear is rotated axially (it moves along the long direction of an object) with a hydraulic cylinder and piston or actuator unit on the end of the camshaft (FIGURE 22-17). Oil pressure varies to move the hydraulic piston, forcing the helical gear to move. The cam sprocket (gear) is located on the end of the camshaft and is part of the phaser. The piston within the cam phaser connects to the camshaft and cam phaser sprocket by helical splines. The piston twists the hub of the camshaft sprocket that rotates the camshaft. Unlike the vane-type phaser, which offers variable control of camshaft timing, the helical phaser-style control is either on or off. In other words, the camshaft is either at rest in its base position or fully applied. The intake camshafts base position is retarded, providing a smooth idle with no overlap. The exhaust camshaft’s base position is advanced. An internal spring aids in returning the camshaft to its original position after the oil pressure has been relieved.

Applying oil pressure to the piston advances timing, causing the camshaft to move clockwise, advancing the timing. Applying oil pressure to the opposite side of the piston drives the camshaft counterclockwise, retarding cam timing. On helical gear-style phasers with return springs, a broken spring will prevent the cam from returning to neutral or base timing after the camshaft has been advanced or retarded (FIGURE 22-18). The use of helical gear-style and helical piston-style phasers has decreased because vane controllers are more reliable and provide a greater range of cam timing adjustment.

FIGURE 22-17 A helical gear-style phaser uses a machined gear and a piston that uses oil pressure to advance the camshaft. This phaser has a lot of moving components, which exposes it to more potential failure than a simpler vane phaser.

FIGURE 22-18 The return spring on the helical phaser is a major component that fails because the amount of actuation will prematurely wear out the material that the spring is made of.

Vane Actuator Phasers

The most common type of actuator/phaser used is the vane-type phaser. The vane-type actuator is lighter and more compact than the helical-spline actuator, resulting in faster response time. Vane-type phasers typically change cam/valve timing up to 20 to 30 degrees, advancing or retarding the cam timing. This phaser uses vanes similar to those found in a vane-type oil pump used in automatic transmissions and power steering systems to move the camshaft. The vane (blade) portion, the rotor (center), of the actuator, attaches to the camshaft by a locating pin and a bolt. The vanes serve as oil seals, positioned inside cavities of the camshaft timing gear. Oil fills the chambers or pockets between the blades and housing, forcing the camshaft to move (FIGURE 22-19).

FIGURE 22-19 The vane phaser is a very simple component in that it has a few springs to return it to start position, a lock pin for start-up, and a rotor attached to the camshaft. The stator housing maintains camshaft timing with the crankshaft while oil pressure and spring pressure move the vane within the housing to adjust for engine performance.

The stator (controller) bolts to the camshaft sprocket and houses the rotor. The timing chain–driven controller housing that encases the blades moves the vane assembly incrementally to advance or retard cam timing. There is no direct mechanical link between the stator and rotor; instead, oil pressure connects the parts. A hydraulic link forms between the rotor and stator when the solenoid applies oil pressure to one side or both sides of the rotors vanes. Using a return spring helps return the rotor to its base/home position of zero degrees advance (FIGURE 22-20).

FIGURE 22-20 The return spring helps to return the rotor to the start position when the engine is turned off.

The design of the actuator rotor is typically four or five vanes; however, three vane designs are also in use. The vanes are located inside their own cavities of the camshaft timing gear and fill with oil during operation, to move the rotor. Oil pressure moves into the phaser and pushes on one side of the vane to advance or retard the cam timing as desired by the PCM. Applying equal pressure to both sides of the vanes maintains the camshaft’s current position. Rotating the rotor inside the phaser advances or retards the camshaft and therefore valve timing. When the PCM controls timing to retard it, the OCV directs oil pressure to the retard side of the cavity. Commanding advance routes oil pressure to enter the opposite side of the chamber, moving the camshaft to a more advanced position. To hold the current CMP, the solenoid applies equal pressure to both sides of the actuator vanes. In addition to directing oil to either the advance or the retard cavities, the spool valve also opens an exhaust port (FIGURE 22-21). When the exhaust port opens, oil that is present on the opposite side of the chamber drains into the oil pan, allowing the phaser to quickly respond to the PCM command.

FIGURE 22-21 The OCV controls the flow of oil to either side of the phaser, which causes the phaser to advance or retard the camshaft. The ports on this valve are very precise and can easily become restricted, which requires that the oil be very clean to operate the VVT system.

The vane-type phaser consists of several components. Servicing it, however, requires replacing it as an assembly; individual parts are not available from OEM or aftermarket sources.

Vane-type actuators have many components:

Oil flows through a solenoid that in turn controls a spool valve in a phaser system. The majority of systems are fed by oil galleys in the head and then through the camshaft bearing journals. The oil then flows through machined passages in the camshaft and phaser-retaining bolt before it enters the phaser assembly. Other systems may have the oil fed directly into the front of the phaser from a valve body.

A locking pin locks the actuator in place, so the assembly turns as one unit. The pin engages during times of no or low oil pressure or low-volume conditions. The majority of systems actuate the pin during idle and after a cold start. The lock pin fits inside the locking collar and vane assembly. During start-up, the pin locks the VCT assembly in the default (base cam timing) position. A spring under the lock pin pushes the pin outwards, at which point it enters a hole in the controller housing. When the pin engages the two separate components, the vane and controller become one assembly, securing the entire controller to the camshaft. Locking the parts together serves two purposes: First, it prevents a potential knocking noise following a start-up, or low oil pressure or volume conditions. Second, it locks the camshaft in its base or default position, eliminating any unwanted camshaft timing changes. The default for an intake camshaft(s) is fully retarded. The default position for a camshaft that includes both intake and exhausts lobes (single-camshaft engine, OHV) is at full advance. After meeting all the enabling criteria, the PCM controls oil flow to the base of the lock pin, unlocking the camshaft from the phaser, allowing VCT operation to begin.

There are primarily two control designs used for the vane-type actuator. The most popular type uses a solenoid that is remotely mounted and usually found on the cylinder head near the actuator(s). Oil is supplied through galleys/passages in the cylinder head, through the camshaft, and then to the actuator. Since the camshaft and actuator are spinning, oil is delivered through the camshaft bearing caps. The camshaft has separate channels for advance and retard oil paths. These pathways are also reversible. In other words, the retard supply path becomes the bleed path for the retard side of the vanes when the solenoid/spool valve assembly routes oil to the advance path.

TECHNICIAN TIP

Any hydraulic system can leak oil pressure, resulting in incorrect VVT operation. Pressure loss and oil leak possibilities include the usual gasket, O-ring, and sealant failure. Plugged or restricted VVT filters and screens also reduce oil flow. Another often-overlooked possibility is worn camshaft bearing journals that result in a loss of oil pressure. Worn cam bearings or caps increase the very tight clearances required for proper oil supply, reducing pressure and flow to the actuators. Replacing an actuator that fails to work/respond, without checking for a cause, may lead to a failed repair attempt. Before replacing any part, always perform a visual check of the cam bearings, caps, and any accessible screens/filters for damage.

Other variations of this system include a solenoid directly mounted to the bearing caps. Placing the solenoid on the bearing cap removes oil flow through the cylinder head. Another option for oil control is routing it through the timing cover into the center of the actuator from the spool valve (FIGURE 22-22).

FIGURE 22-22 The OCV must be near the engine camshafts so that it can control the phaser for that camshaft. The valve could be in the timing cover, valve cover, or cylinder head to control the phaser for the camshaft nearest it.

TECHNICIAN TIP

As technology continues to improve, some manufacturers have transitioned to now use electronic VVT systems. An electric motor inside the phaser completely removes hydraulic control. Electric phasers offer an immediate response to the constantly changing operating conditions that occur while driving. Electronic control removes the faults associated with poor or incorrect maintenance and the use of oil pressure and its related faults.

Lobed Rotor Phasers

The lobed rotor phaser design is similar to the vane style. The phaser housing generally has four chambers and a movable four-lobed rotor inside. When routing oil pressure into the chambers, it pushes the rotor to either advance or retard the camshaft timing. Many of these systems provide incremental timing changes, moving the camshaft from its base installation point to a fully advanced or retarded position. The amount of timing change available is application/engine-dependent and can range from 20 degrees to nearly 60 degrees (FIGURE 22-23).

FIGURE 22-23 The lobed phaser works in the same way as the vane phaser; it is just a different shape that must be oil fed. It can operate the camshaft in a wider range than the vane phaser can.