Continuously Variable Valve Lift Assembly
| ITEM | DESCRIPTION |
|---|---|
| 1 | CVVL oil temperature sensor |
| 2 | CVVL assembly |
| 3 | CVVL solenoid valve (quantity 4) |
The 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 if required. The CVVL assembly is fixed to the top of the camshaft carrier by 10 bolts.
There is no mechanical connection between the intake valves and the camshaft. Instead of direct operation, the intake camshaft operates 4 small oil pumps which charge 4 hydraulic accumulators.
| ITEM | DESCRIPTION |
|---|---|
| 1 | Intermediate pressure chamber |
| 2 | Pump unit |
| 3 | Solenoid valve |
| 4 | Pressure accumulator |
| 5 | Cam follower |
| 6 | 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 can be reached up to 150 bar (2, 175 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 is opened, some of the oil pressure is relieved from the high pressure chamber back in to the intermediate pressure chamber. This is 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. 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'. That is 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 provide a gentle, controlled seating of the valve.
The solenoid switching time is controlled by the PCM , based on calculated engine load, engine speed and oil temperature values from existing sensors. The CVVL oil temperature sensor is the only additional sensor for the system.
Schematic View - Continuously Variable Valve Lift Hydraulic Circuit
| ITEM | DESCRIPTION |
|---|---|
| 1 | Solenoid valve |
| 2 | Engine oil supply |
| 3 | Check valve |
| 4 | Intermediate chamber |
| 5 | Pressure accumulator |
| 6 | Oil return |
| 7 | 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 |
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 4 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 - The opening of the intake valves are delayed when starting the engine and during the engine idle. 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 - This mode is activated during low to medium engine speeds. The intake valves are closed hydraulically before the camshaft profile would normally allow. This mode reduces pumping losses, increases the engine output and prevents an undesirable backflow of the fuel mixture into the intake ports.
- Composite mode - This mode is activated at very low engine speeds and loads. Composite mode is a combination of the late intake valve opening and the early intake valve closing modes, and it provides a stable combustion.
Continuously Variable Valve Lift Solenoid Valve
There are 4 solenoid valves used in the CVVL system, 1 per cylinder. Each CVVL solenoid valve is supplied with power supply and ground from the PCM . The PCM controls the solenoid position, using Pulse Width Modulated (PWM) signals.
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.
The solenoid normally has no power supply and is in the open position. At the first phase of activation the CVVL solenoid valve is supplied with a current, which pre-magnetise the solenoid but does not switch the valve.
In order to allow a rapid and precise energizing 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 CVVL solenoid valve is fully activated, the current is reduced to a holding current, which maintains the solenoid valve in the closed position. Depending on operating conditions, the PCM software controls the point in time at which the solenoid opens by completely switch 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 enters a 'limp-home' mode, running on the 3 remaining cylinders. In this case, the driver can experience misfires and a reduction in engine performance.
Continuously Variable Valve Lift Brake Unit
The CVVL brake unit is a slave cylinder that converts the hydraulic pressure into the intake valve movement. The pump unit hydraulic pressure is converted through a hydraulic valve lash adjustment element in the CVVL brake unit. 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 CVVL assembly, the brake pistons must be reset. After the CVVL assembly installation the CVVL software calibration must be completed using the JLR approved diagnostic equipment.
For additional information, Refer to: CONTINUOUS VARIABLE VALVE LIFT .
Continuously Variable Valve Lift Oil Temperature Sensor
The CVVL oil temperature sensor is located at the front of the CVVL assembly. The sensor provides feedback to the PCM on the temperature of the high pressure oil in the CVVL assembly.
The PCM determines the oil viscosity and allows accurate solenoid switching time compensation across a wide engine temperature range -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.
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.