Valve Train

The lightweight valve train provides good economy and noise levels and is chain driven from the crankshaft.

Double overhead camshafts on each cylinder head operate the valves. For each cylinder head, an inverted tooth timing chain transfers drive from the crankshaft to the VCT unit on the front of each camshaft. Graded tappets enable setting of exhaust valve clearances. Switchable tappets with hydraulic lash adjusters are installed on the intake valves.

Each timing chain has a hydraulic tensioner operated by engine oil pressure. The chain tensioners incorporate a ratchet mechanism, which maintains tension while the engine is stopped to eliminate startup noise. The chains are lubricated with engine oil from jets located at the front of the engine block. Nylon chain guides control chain motion on the drive side.

Variable Camshaft Timing 

The VCT system varies the timing of the intake and exhaust camshafts to deliver optimum engine power, efficiency and emissions. The timing of the intake camshafts has a range of 62 degrees of crankshaft angle. The timing of the exhaust camshafts has a range of 50 degrees of crankshaft angle.

In the base timing position:

• The intake camshafts are fully retarded.
• The exhaust camshafts are fully advanced.

VCT Operating Ranges 

The system consists of a VCT unit and a VCT solenoid for each camshaft. The ECM controls the system using PWM (pulse width modulation) signals to the VCT solenoids.

The torsional energy generated by the valve springs and the inertia of the valve train components are used to operate the system.

Variable Camshaft Timing Units 

The VCT units change the position of the camshafts in relation to the timing chains.

Each VCT unit is attached to the camshaft by three bolts. A rotor assembly and a reed plate are installed inside a sprocket housing, which consists of a sprocket, an outer plate and an inner plate held together by six screws.

A reluctor ring, for the CMP (camshaft position) sensor, a center plate and a bias spring are installed at the front of the VCT unit. The ends of the bias spring locate on the center plate assembly and the sprocket housing, to give a turning moment to the camshaft in the advance direction. A snap ring locates the reluctor ring on to a sleeve installed in the center of the rotor assembly. The opposite end of the sleeve locates in a bore in the front face of the camshaft, which contains a filter.

A spring and spool valve are installed in the rotor assembly sleeve and retained by a snap ring. The spring keeps the spool valve in contact with the armature of the related VCT solenoid.

Each VCT unit is supplied with engine oil from an oil gallery in the cylinder head, through the camshaft front bearing cap and a bore in the center of the camshaft.

Variable Camshaft Timing Solenoids 

The VCT solenoids control the position of the spool valves in the VCT units.

The VCT solenoids are installed in the front upper timing covers, immediately in front of their related VCT units. Each VCT solenoid is secured with two screws and sealed with an O-ring. A two pin electrical connector provides the interface with the engine harness.

Each VCT solenoid incorporates a spindle that acts on the spool valve in the related VCT unit to advance and retard the camshaft timing. The VCT solenoids operate independently and are controlled by a PWM signal from the ECM.

Variable Camshaft Timing Operation 

When the engine is running, the compression and expansion of the valve springs causes momentary increases and decreases in the torque acting on the camshafts. These momentary changes of torque are sensed in the VCT units and used to change the camshaft timing.

Camshaft Torsional Energy (For a Single Valve Event) 

Variable Camshaft Timing Unit Schematic - Base Timing 

At engine start-up, once the engine oil pressure in the camshaft is sufficient to open the inlet check valve, engine oil flows across the spool valve, through the advance and retard check valves and into the advance and retard chambers. During the start cycle, the ECM signals the VCT solenoid to move the spool valve into the sleeve and connect the lock pin to inlet oil pressure. The inlet oil pressure causes the lock pin to retract from the inner plate and unlock the rotor assembly and camshaft from the sprocket housing.

There is a constant supply of oil to the VCT to ensure the unit remains filled during operation.

Variable Camshaft Timing Unit Schematic - Advance 

To advance the camshaft timing, the ECM adjusts the signal to the VCT solenoid to move the spool valve so that the advance chamber oil passage is closed and the retard chamber oil passage is connected to inlet oil.

Each momentary increase of the torque acting on the camshaft generates a pressure pulse in the retard chamber. Oil moves from the retard chamber, through the spool valve and the advance check valve to the advance chamber, to equalize the pressures in the two chambers. The displacement of oil from the retard chamber causes the rotor assembly to advance in relation to the sprocket housing. Each momentary decrease of torque acting on the camshaft also generates a pressure pulse in the advance chamber, but, with the advance chamber oil passage closed, no movement of oil between the advance and retard chambers occurs and the rotor assembly cannot move in the retard direction.

Variable Camshaft Timing Unit Schematic - Null 

Once the camshaft has reached the required timing position the ECM adjusts the signal to the VCT solenoid to set the spool valve in the null position. In the null position, the advance and retard chamber oil passages are both closed by the spool valve and the rotor assembly is hydraulically locked to the sprocket housing.

Variable Camshaft Timing Unit Schematic - Retard 

To retard the camshaft timing, the ECM adjusts the signal to the VCT solenoid to move the spool valve to close the retard chamber oil passage and connect the advance chamber oil passage to the inlet oil.

Each momentary decrease of the torque acting on the camshaft causes oil to transfer from the advance chamber, through the spool valve and the retard check valve to the retard chamber, and so retard the camshaft timing.

Camshaft Profile Switching 

The CPS (camshaft profile switching) system switches the intake valves between two cam profiles which have different lift and period. The low lift profile improves driveability and emissions at lower engine speeds. The high lift profile improves power and torque output at higher engine speeds.

The intake camshafts have three lobes for each valve. The two outer lobes have identical profiles that produce the high lift of 10.53 mm (0.415 in.). The central lobe produces the low lift of 5.50 mm (0.217 in.). Switching between cam profiles is performed by a switchable tappet on each intake valve. The switchable tappets are operated by engine oil controlled by a CPS solenoid on each cylinder head. Operation of the CPS solenoids is controlled by the ECM.

The CPS solenoids control the supply of engine oil pressure to the locking pins in the switchable tappets, to switch the tappets between the two cam profiles.

A CPS solenoid is installed on the rear of each cylinder block. Each CPS solenoid has a pintle installed in a sleeve, which incorporates oil inlet and outlet holes. The sleeve is installed at the junction of oil galleries in the cylinder head, with the oil inlet and outlet holes aligned with the galleries. Movement of the pintle in the sleeve controls a connection between the oil galleries. When the CPS solenoid is energized, the pintle connects an oil supply gallery to the gallery along the outboard side of the switchable tappets. When the CPS solenoid is de-energized, the oil gallery along the outboard side of the switchable tappets is connected to drain.

The CPS solenoids receive a fused battery supply from the main relay. The ECM switches a ground connection to operate the solenoids.

The switchable tappets are installed on the intake valves, in bores in the cylinder heads. The cylinder heads incorporate engine oil galleries along the inboard and outboard sides of the bores. The inboard oil galleries (between the tappets and the spark plug/fuel injector bores) supply an oil feed to the locking pins in the switchable tappets. The outboard oil galleries (over the inlet ports) supply an oil feed to the hydraulic lash adjusters in the switchable tappets.

Each switchable tappet consists of inner and outer tappets, which can operate independently or be locked together by locking pins. A hydraulic lash adjuster on the bottom of the inner tappet locates on the intake valve stem.

In low lift, the intake valve lift is controlled by the inner tappets, which run on the center lobes of the intake camshafts. The outer tappets run on the outer lobes of the intake camshafts, and move up and down the inner tappets without affecting the valve lift. The lost motion springs keep the outer tappets in contact with the outer lobes. Movement of the inner tappets is transferred to the intake valves through the hydraulic lash adjusters.

In high lift, engine oil is supplied to the locking pins, which lock the outer tappets to the inner tappets. Intake valve lift is controlled by the outer tappets, which run on the outer lobes of the intake camshafts. Movement of the outer tappets is transferred to the intake valves through the locking pins, the inner tappets and the hydraulic lash adjusters.

Camshaft Profile Switching Operation 

The switching point is speed and load dependent. This strategy ensures that switching occurs at air flow neutral points in the engine's operation and is imperceptible to the driver.

At engine speeds from idle up to the range of 2825 - 4250 rev/min (depending on engine load), the CPS solenoids are de-energized and the switchable tappets are set to low lift. At engine speeds above the 2825 - 4250 rev/min range, the CPS solenoids are energized by the ECM and the switchable tappets are set to high lift. There is a 200 rev/min hysteresis when switching from high lift to low lift with decreasing engine speed. Switching between lift settings occurs within one revolution of the camshaft.

Switching is only enabled at engine oil temperatures of 20 °C (68 °F) and above. At oil temperatures below 20 °C (68 °F), CPS operation is disabled and the switchable tappets remain in the low lift setting. CPS operation is also disabled if a CPS solenoid fails. When CPS operation is disabled, engine speed is limited to 5000 rev/min.

The ECM can diagnose the operation of the CPS solenoids and store fault related DTC (diagnostic trouble code) if it detects a failure.