Continuous Variable Valve Lift Assembly
| Item | Description |
|---|---|
| 1 | Temperature sensor |
| 2 | Solenoid valve (cylinders 1, 2 and 3) |
| 3 | Solenoid valve (cylinders 4, 5 and 6) |
| 4 | Rear CVVL unit |
| 5 | Front CVVL unit |
The 6 cylinder engine utilises two separate CVVL units which are fixed to the top of the camshaft carrier by eight bolts per unit. Each unit controls the inlet valves of three cylinders, the front unit controls cylinders 1, 2 and 3 and the rear unit controls 4, 5 and 6.
The CVVL assemblies are a non-serviceable with the exception of the Temperature Sensor fitted to the front CVVL unit.
There is no mechanical connection between the inlet valves and the camshaft, instead of direct operation, the inlet camshaft operates six small oil pumps which charge six hydraulic accumulators. These act as hydraulic buffers between the camshaft and the inlet valves. If less than full valve lift is required, oil is released from each accumulator by an electronic solenoid, reducing the lifting effect of the camshaft.
| Item | Description |
|---|---|
| 1 | Temperature sensor |
| 2 | Solenoid valve |
| 3 | Intermediate pressure chamber |
| 4 | Pressure accumulator |
| 5 | High pressure chamber |
| 6 | 6 Pump unit |
| 7 | Cam follower |
| 8 | Camshaft |
| 9 | Inlet valve |
| 10 | Brake unit |
As the camshaft rotates the cam lobe lift translates to pump piston movement and creates the hydraulic pressure achieved within the unit. The high pressure chamber is the hydraulic connection between the pump, brake unit and solenoid valve where pressures of up to 150 bar can be reached.
When the solenoid valve is closed the oil operates as a 'hydraulic pushrod' through the brake unit opening the intake valve. When the solenoid valve is opened some of the oil pressure is relieved from the high pressure chamber back in to the intermediate pressure chamber, effectively 'shortening' the length of the 'hydraulic pushrod' and the amount of valve lift.
When the solenoid valve is opened the pressure accumulator feeds the relieved oil back to the high pressure chamber ensuring the chamber has a constant de-aerated oil supply. The brake unit, as well as acting as a Hydraulic valve Lash Adjustment (HLA), also regulates the valve closing speed.
When the solenoid is opened for early intake valve closing, the valve spring causes the valve to enter a 'ballistic flight phase'. An 'uncontrolled' period of travel as the valve does not close by following the cam profile. To prevent excessive closure speeds that can cause damage to the valve, the brake unit acts as a hydraulic brake to ensure a gentle, controlled seating of the valve.
The solenoid switching time is controlled by the PCM based on calculated engine load values from existing sensors. The CVVL temperature sensor is the only additional sensor for the system.
| Item | Description |
|---|---|
| 1 | Normally open solenoid |
| 2 | Engine oil supply |
| 3 | Check valve |
| 4 | Intermediate chamber |
| 5 | Pressure accumulator |
| 6 | 6 Oil return |
| 7 | CVVL oil temperature sensor |
| 8 | 8 Brake unit |
| 9 | Intake valve |
| 10 | Engine oil supply |
| 10 | Engine oil supply |
| 11 | Roller finger cam follower |
| 12 | Camshaft |
| 13 | Hydraulic tappet |
| 14 | Pump unit |
| A | Front CVVL unit |
| B | Rear CVVL unit |
The CVVL assembly ultimately allows the control of air in to the cylinder by delaying the valve opening, or by closing the valve early. There are four operating modes.
CVVL Operating Modes:
- Full lift mode - the valves are fully opened and closed as during conventional control by the camshaft. This mode is used at high engine speeds to obtain maximum engine power.
- Late Intake Valve Opening Mode (LIVO) - the opening of the intake valves are delayed when starting the engine and during idling. The valves open for a shorter period and at a lower lift providing precise control of the exact amount of air entering the cylinder. As a result fuel economy is improved during idling periods. During a cold start, only a small amount of cold air enters the cylinder, meaning the engine starts more easily.
- Early Intake Valve Closing Mode (EIVC) - is activated during low to medium engine speeds. The intake valves are closed hydraulically before the camshaft profile would normally allow. This reduces pumping losses, increases the engine output and prevents an undesirable backflow of the fuel mixture into the intake ports.
- Composite mode - at very low engine speeds and loads, a combination of the LIVO mode and the EIVC mode is used as it provides a stable combustion.
There are six solenoids in use in the CVVL system; one per cylinder. Each solenoid is supplied with a dedicated live and ground from the PCM and is Pulse Width Modulated (PWM) controlled to regulate its position.
To enable the rapid action of the solenoid valve, a special operating strategy was developed with the lowest possible current requirements. This resulted in a current profile consisting of several phases.
At rest the solenoid valve has no current supply and is in the open position.
At the first phase of activation the solenoid valve is supplied with a current, which pre-magnetises the valve but does not switch it.
In order to ensure a rapid and precise energizing procedure, an increased current is applied at the exact time of switching. This is determined by the PCM depending on sensor input for the current operating conditions.
After the solenoid valve has been fully activated, the current is reduced to a holding current, which maintains the solenoid valve in the closed position. Again depending on operating conditions, the PCM software controls the point in time at which the solenoid opens by completely switching off the holding current.
In the event of a CVVL solenoid failure, a total loss of valve lift occurs on the related cylinder and the engine will enter 'limp-home' mode, running on the five remaining cylinders. The driver will experience misfires and a reduction in performance.