Variable Camshaft Timing
| ITEM | DESCRIPTION |
|---|---|
| 1 | Intake camshaft Variable Camshaft Timing (VCT) actuator |
| 2 | Exhaust camshaft VCT actuator |
| 3 | Exhaust VCT solenoid |
| 4 | Camshaft Position (CMP) sensors |
| 5 | Intake VCT solenoid |
The timing of the intake and exhaust camshafts can be adjusted independently by an electro-hydraulic controlled Variable Camshaft Timing (VCT) system. The Powertrain Control Module (PCM) controls the system using information from the Camshaft Position (CMP) sensors and the Crankshaft Position (CKP) sensor.
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.
| CAMSHAFT | VALVE OPENS | VALVE CLOSES |
|---|---|---|
| Intake | 33 degrees Before Top Dead Center (BTDC) to 29 degrees After Top Dead Center (ATDC) | 197 to 259 degrees ATDC |
| Exhaust | 236 to 186 degrees BTDC | 6 to 56 degrees ATDC |
The system consists of a VCT actuator and a VCT solenoid for each camshaft. The PCM controls the system using Pulse Width Modulation (PWM) 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 Actuators
| ITEM | DESCRIPTION |
|---|---|
| 1 | Bolt (quantity 3) |
| 2 | Variable Camshaft Timing (VCT) actuator |
| 3 | Filter |
| 4 | Camshaft |
| 5 | Inner plate |
| 6 | Housing and sprocket |
| 7 | Rotor assembly |
| 8 | Reed plate |
| 9 | Spring and lock pin |
| 10 | Spring (quantity 3) |
| 11 | Seal (quantity 3) |
| 12 | Spring (quantity 2) |
| 13 | Seal (quantity 2) |
| 14 | Spring |
| 15 | Dowel pin |
| 16 | Spring |
| 17 | Snap ring |
| 18 | Sensor wheel |
| 19 | Center plate |
| 20 | Snap ring |
| 21 | Screw (6 off) |
| 22 | Spool valve |
| 23 | Outer plate |
Each VCT actuator is attached to the camshaft by 3 bolts. A rotor assembly and a reed plate are installed inside a sprocket housing. This consists of a sprocket, an outer plate and an inner plate and is held together by 6 screws.
A sensor wheel, for the Camshaft Position (CMP) sensor, a center plate and a spring are installed at the front of the VCT actuator. The ends of the 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 sensor wheel 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 actuator 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
| ITEM | DESCRIPTION |
|---|---|
| 1 | Electric connector |
| 2 | O-ring |
The VCT solenoids control the position of the spool valves in the VCT actuators.
The VCT solenoids are installed in the front upper timing covers, immediately in front of their related VCT actuators. Each VCT solenoid is secured with 2 screws and sealed with an O-ring seal. A 2 pin electrical connector attaches to the engine wiring harness.
Each VCT actuator incorporates a spindle that acts on the spool valve to advance and retard the camshaft timing. The VCT solenoids operate independently and are controlled by a Pulse Width Modulation (PWM) signal from the Powertrain control Module (PCM).
Variable Camshaft Timing Operation
When the engine is operating, 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 actuators and used to change the camshaft timing.
| ITEM | DESCRIPTION |
|---|---|
| A | Camshaft torque |
| B | Camshaft rotation (degrees) |
| C | Valve opening |
| D | Peak lift |
| E | Valve closing |
| 1 | 1000 rev/min |
| 2 | 4000 rev/min |
| 3 | 7000 rev/min |
| 4 | Inertia effects from valve train rotating components |
| 5 | Force caused by valve spring |
| 6 | Bias torque from friction |
Variable Camshaft Timing Actuator Schematic - Base Timing
| ITEM | DESCRIPTION |
|---|---|
| 1 | Advance chamber |
| 2 | Retard chamber |
| 3 | Sprocket housing |
| 4 | Rotor assembly |
| 5 | Lock pin |
| 6 | Sleeve |
| 7 | Engine oil supply from camshaft |
| 8 | Intake check valve |
| 9 | Lock pin drain |
| 10 | Spool valve |
| 11 | Advance check valve |
| 12 | Retard check valve |
At engine start-up, once the engine oil pressure in the camshaft is sufficient to open the intake 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 Powertrain Control Module (PCM) signals the VCT solenoid to move the spool valve into the sleeve and connect the lock pin to intake oil pressure. The intake 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 actuator to make sure that it remains filled during operation.
Variable Camshaft Timing Actuator Schematic - Advance
To advance the camshaft timing, the PCM 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 the intake supply of engine 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 2 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 generates a pressure pulse in the advance chamber. With the advance chamber oil passage closed, no movement of oil between the advance and retard chambers occurs. The rotor assembly cannot move in the retard direction.
Variable Camshaft Timing Actuator Schematic - Null
Once the camshaft has reached the required timing position the PCM 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 Actuator Schematic - Retard
To retard the camshaft timing, the PCM adjusts the signal to the VCT solenoid to move the spool valve to close the retard chamber oil passage. During this operation the advance chamber oil passage is connected to the oil intake.
Each decrease of torque acting on the camshaft causes engine oil to transfer from the advance chamber, through the spool valve and the retard check valve to the retard chamber. This retards the camshaft timing.
For additional information, refer to: ELECTRONIC ENGINE CONTROLS