Continuously Variable Valve Lift Assembly
The Continuously Variable Valve Lift (CVVL) assembly is a non-serviceable assembly for control of the intake valve lift. The CVVL assembly temperature sensor can be removed from the CVVL assembly when required. The CVVL assembly is installed to the top of the camshaft carrier by 10 bolts.
There is no mechanical connection between the inlet valves and the camshaft. Instead of direct operation, the inlet camshaft operates 4 small oil pumps which charge 4 hydraulic accumulators.
The CVVL assembly ultimately allows the control of air into the cylinder by delaying the valve opening, or by closing the valve early. The CVVL assembly also allows a combination of both.
| ITEM | DESCRIPTION |
|---|---|
| 1 | Intermediate pressure chamber |
| 2 | Solenoid valve |
| 3 | Pressure accumulator |
| 4 | Cam follower |
| 5 | Camshaft |
| 6 | Inlet valve |
| 7 | Brake unit |
| 8 | Temperature sensor |
| 9 | High pressure chamber |
| 10 | Pump 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 (2175 psi).
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 opens some of the oil pressure releases from the high pressure chamber back in to the intermediate pressure chamber. By releasing the pressure, there is effectively a '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. The relieved oil makes sure that the chamber has a constant de-aerated oil supply. The brake unit, as well as acting as a hydraulic valve lash adjustment, 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. The brake action makes sure that there is a gentle, controlled seating of the valve.
The solenoid switching time is controlled by the Powertrain Control Module (PCM) based on calculated engine load values from existing sensors. The Continuously Variable Valve Lift (CVVL) temperature sensor is the only additional sensor for the system.
Continuously Variable Valve Lift Hydraulic Circuit Schematic
| ITEM | DESCRIPTION |
|---|---|
| 1 | Normally open solenoid |
| 2 | Engine oil supply |
| 3 | Check valve |
| 4 | Intermediate chamber |
| 5 | Pressure accumulator |
| 6 | Oil return |
| 7 | Continuously Variable Valve Lift (CVVL) oil temperature sensor |
| 8 | Brake unit |
| 9 | Intake valve |
| 10 | Engine oil supply |
| 11 | Roller finger cam follower |
| 12 | Camshaft |
| 13 | Hydraulic tappet |
| 14 | Pump unit |
There are 4 Continuously Variable Valve Lift (CVVL) operating modes.
The CVVL assembly ultimately allows the control of air into the cylinder by delaying the valve opening, or by closing the valve early. The CVVL assembly also allows a combination of both.
CVVL Operating Modes:
- Full lift mode - The valves are fully opened and closed as during conventional control by the camshaft. The full lift mode is used at high engine speeds to obtain maximum engine power.
- Late intake valve opening mode - 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 - Activated during low to medium engine speeds. The intake valves are closed hydraulically before the camshaft profile would normally allow. The mode reduces pumping losses, increases the engine output and prevents an undesirable backflow of the fuel mixture into the intake ports.
- Composite mode is used at very low engine speeds and loads. Composite mode is used as it provides a stable combustion.
Solenoid
There are 4 solenoids in use in the Continuously Variable Valve Lift (CVVL) system on every cylinder. Each solenoid is supplied with a dedicated live and ground from the Powertrain Control Module (PCM). The PCM regulates the position of the solenoid through the Pulse Width Modulated (PWM).
To enable the rapid action of the solenoid valve, a special operating strategy was developed with the lowest possible current requirements. The strategy 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 make sure that a rapid and precise energising procedure, an increased current is applied at the exact time of switching. The current 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. The engine enters 'limp-home' mode, running on the 3 remaining cylinders. The driver experiences misfires and a reduction in performance.
Continuously Variable Valve Lift Brake Unit
The brake unit is a slave cylinder that converts the hydraulic pressure of the pump into the movement of the intake valve. The conversion is through a hydraulic valve lash adjustment element. The brake unit design not only gives a controlled closure of the valve but also enables a rapid opening speed. The rapid opening is because as the braking element of the unit is by-passed by the use of a check valve during the opening phase.
It is a critical part of the unit set up that the valve lash adjustment is reset. The valve lash adjustment must be completed when the unit is removed from the cylinder head.
Where it is necessary to remove and reinstall the Continuously Variable Valve Lift (CVVL) unit, the brake pistons must be reset. The brake pistons are reset by pressing down all CVVL brakes to make sure that they are all in the reset position. The reset position is confirmed by using the Jaguar Land Rover (JLR) special tool (CVVL guide pins - JLR-303-1639). When the unit has been reset and the tool removed a visual check that the piston is sitting inside the sleeve should be made. The tool resets the brake unit position from A to position B as in figure E202344.
| ITEM | DESCRIPTION |
|---|---|
| A | Piston prior to reset |
| B | Piston correctly reset |
| 1 | Brake unit piston |
| 2 | Reset tool |
Continuously Variable Valve Lift Oil Temperature Sensor
The Continuously Variable Valve Lift (CVVL) oil temperature sensor provides feedback to the Powertrain Control Module (PCM). The temperature is provided through the High Pressure (HP) oil in both CVVL units.
The PCM uses the temperature to determine oil viscosity and allows accurate solenoid switching time compensation across a wide engine temperature range. The temperature range is -40°C (-40°F) to 150°C (302°F).
The sensor has a 2-pin connector which provides a temperature signal input to the PCM and a ground connection. The sensor has a Negative Temperature Coefficient (NTC) element and is specifically calibrated for use at low temperatures. It is possible to remove the CVVL temperature sensor from the assembly when required.
When the CVVL oil temperature sensor fails, there is reduced accuracy in the control of the CVVL system. The reduced accuracy causes a slight reduction in performance and fuel economy.