Variable Valve Timing Actuator: Description and Operation

VCC System

Introduction
the variable intake valve timing system on the V8 is known as Variable Camshaft Control (VCC).

the V8 VCC system is a new system providing stepless VCC functionality on each intake camshaft. the system is continuously variable within its range of adjustment providing optimized camshaft positioning for all engine operating conditions.

While the engine is running both intake camshafts are continuously adjusted to their optimum positions. This enhances engine performance and reduces exhaust emissions.

Both camshafts are adjusted simultaneously within 20° (maximum) of the camshafts rotational axis.







This equates to a maximum span of 40° crankshaft rotation. the camshaft spread angles for both banks are as follows.







the design of a camshaft for a non adjustable valve timing system is limited to the required overall performance of the engine.
An intake camshaft with an advanced (early profile will provide a higher performing power curve at a lower engine speed. But at idle speed the advanced position will create a large area of intake/exhaust overlap that causes a rough, unstable idle.

An intake camshaft with a retarded (late) profile will provide a very smooth stable idle but will lack the cylinder filling dynamics needed for performance characteristics at mid range engine speeds.

the ability to adjust the valve timing improves the engines power dynamics and reduces exhaust emissions by optimizing the camshaft angle for all ranges of engine operation. VCC provides the following benefits:
- Increased torque at lower to mid range engine speeds without a loss of power in the upper range engine speeds
- Increased fuel economy due to optimized valve timing angles
- Reduction of exhaust emissions due to optimized valve overlap
- Smoother idle quality due to optimized valve overlap.

Variable Camshaft Control Electronic Control

the following describes the electronic control of the VCC system.

Electronic Control
the engine control module is responsible for activating a VCC variable position solenoid valve based on EMS program mapping. the activation parameters are influenced by the following input signals:
- Engine speed
- Load (intake air mass)
- Engine temperature
- Camshaft position.

Mechanical Control
the position of the solenoid valve directs the hydraulic flow of engine oil. the controlled oil flow acts on the mechanical components of VCC system to position the camshaft.
the hydraulic engine oil flow is directed through advance or retard activation oil ports by the VCC solenoid. Each port exits into a sealed chamber on the opposite sides of a control piston.
In its default position the oil flow is directed to the rear surface of the piston. This pulls the helical gear forward and maintains the retarded valve timing position.
When the oil flow is directed to the front surface of the piston the oil pushes the helical gear in the opposite direction which rotates the matched helical gearing connected to the camshaft.
the angled teeth of the helical gears cause the pushing movement to be converted into a rotational movement. the rotational movement is added to the turning of the camshaft providing the variable camshaft positioning.

System Components
the V8 VCC components include the following for each cylinder bank:
- Cylinder heads with oil ports for VCC
- VCC transmission with sprockets
- Oil distribution flange
- Oil check valve
- PWM controlled solenoid valve
- Camshaft position impulse wheel.

Control Solenoid and Check Valve







the VCC solenoid is a two wire, pulse width modulated, oil pressure control valve. the valve has four ports;
1 Input Supply Port Engine Oil Pressure
2 Output Retard Port, to rear of piston/helical gear (retarded camshaft position)
3 Output Advance Port to front of piston/helical gear (advanced camshaft position)
4 Vent released oil pressure.

A check valve is positioned forward of the solenoid in the cylinder head oil gallery. the check valve maintains an oil supply in the VCC transmission and oil circuits after the engine is turned off. This prevents the possibility of piston movement (noise) within the VCC transmission system on the next engine start.

VCC Transmission
the primary and secondary timing chain sprockets are integrated with the VCC transmission. the transmission is a self contained unit.
the adjustment of the camshaft occurs inside the transmission controlled oil pressure then moves the piston axially.
the helical gear cut of the piston acts on the helical gears on the inside surface of the transmission and rotates the camshaft to the specific advanced or retarded angle position.
Three electrical pin contacts are located on the front surface to verify the default maximum retard position using an ohmmeter. This is required during assembly and adjustment.

Oil Distribution Flanges:
the oil distribution flanges are bolted to the front surface of each cylinder head. They provide a mounting location for the VCC solenoids as well as the advance-retard oil ports from the solenoids to the intake camshafts.

Camshafts
Each intake camshaft has two oil ports separated by three sealing rings on their forward ends.
the ports direct pressurized oil from the oil distribution flange to the inner workings of the VCC transmission.
Each camshaft has REVERSE threaded bores in their centres for the attachment of the timing chain sprockets on the exhaust cams and the VCC transmissions for each intake camshaft as shown.

Camshaft Position Impulse Wheels:
the camshaft position impulse wheels provide camshaft position status to the engine control module via the camshaft position sensors. the asymmetrical placement of the sensor wheel pulse plates provides the engine control module with cylinder specific position ID in conjunction with crankshaft position

V8 VCC Control
As the engine camshafts are rotated by the primary and secondary timing chains, the ME7.2 control module activates the VCC solenoids via a PWM (pulse width modulated) ground signal based on a program map. the program is influenced by engine speed, load, and engine temperature.
In its inactive or default position, the valves direct 100% engine oil pressure tow to achieve maximum retard VCC positioning.







As the Pulse Width Modulation (PWM) increases on the control signal the valve progressively opens the advance oil port and proportionately closes the retarded oil port.






Oil pressure pushes the piston toward the advance position. Simultaneously the oil pressure on the retarded side (rear) of the piston is decreased and directed to the vent port in the solenoid valve and drains into the cylinder head.

At maximum PWM control, 100% oil flow is directed to the front surface of the piston pushing it rearward to maximum advance.







Varying the pulse width (on time) of the solenoids control signals proportionately regulates the oil pressures on each side of the pistons to achieve the desired VCC advance angle.


VCC Timing Procedures
Always refer to RAVE for complete Valve Timing Procedures. the V8 valve timing adjustment requires the setting of the VCC transmissions to their maximum retard positions with an ohmmeter and attaching the camshaft gears to each camshaft with single reverse threaded bolts.

the process is as follows:
- After locking the crankshaft at TDC the camshaft alignment tools are placed on the square blocks on the rear of the camshafts locking them in place
- the exhaust camshaft sprockets and VCC transmission units with timing chains are placed onto their respective camshafts
- the exhaust camshaft sprockets and VCC transmissions are secured to the camshafts with their respective single reverse threaded bolt. Finger tighten only at this point. Install the chain tensioner into the timing chain case and tension the chain
- Connect an ohmmeter across two of the three pin contacts on the front edge of one of the VCC transmissions. Twist the inner hub of transmission to the left (counter clock wise). Make sure the ohmmeter indicates closed circuit. This verifies that the transmission in the default maximum retard position
- Using an open end wrench on the camshaft to hold it in place torque the VCC transmission centre bolt to specification.

Camshaft Impulse Wheel Position Tools
the camshaft impulse wheels require a special tool set to position them correctly prior to torquing the retaining nuts.
the impulse wheels are identical for each cylinder bank. the alignment hole in each wheel must align with the tools alignment pin. Therefore the tools are different and must be used specifically for their bank. the tool rests on the upper edge of the cylinder head and is held in place by the timing case bolts.

VCC Solenoid Replacement
Refer to the appropriate RAVE section for complete solenoid replacement procedure.
the solenoids are threaded into the oil distribution flanges through a small opening in the upper timing case covers.

VCC Transmission Retard Position Set Up Tools
A special tool (see RAVE for correct tool number) is used to rotate the transmission to the full retard position when checking the piston position with an ohmmeter.
This tool engages the inner hub of the transmission provides an easy method of twisting it to the let for the ohmmeter test.

Diagnostics
the VCC is fully compatible with the diagnostic software providing specific fault codes and test modules. Additionally diagnostic requests section provides status of the PWM of the VCC solenoids and camshaft position feedback via the camshaft position sensors. the Service Functions section of the TestBook/T4 also provides a VCC system test.