CVCP

CVCP

CVCP, general
continous variable cam phasing (CVCP) is a system that makes continual variation of the intake camshafts' position, i.e. phasing, possible. The duration (the number of degrees the values are hold open) remains constant. It is the position of the cam lobes, i.e. the camshaft angles when the intake values open or close, that are different. This is controlled by ECM via hydraulic valves in order to obtain the optimal intake camshaft setting at any given operating point.





Camshaft, general
The camshafts control the movements of the valves. The profile of the camshaft lobes is determined by which properties were prioritized for the engine type in question. Normally, there are many conflicting requirements that must be met. High peak output often leads to uneasy idling and a weak low rpm register. On the other hand, if good low rpm properties are prioritized, high peak output suffers.

Use of supercharging with a turbocharger makes it possible to make design limits a little fuzzy. You can choose a camshaft with good properties during idling and in the low rpm register. In the high rpm register, where the camshaft is a limitation, you can chan allow boost pressure to increase in order to compensate for the output the engine loses due to the mild camshaft profile.

Following this line of argument, fixed camshaft times are a limitation. With stricter environmental requirements and increased demands for soft engine running and high output, it is becoming increasingly difficult to get these conflicting requirements to come together. The solution is variable cam phasing. By equipping the intake camshafts with a pinion containing a hydraulic mechanism, the intake camshafts can assume the position most beneficial to the specific operating point.

Camshaft, idling
When idling, the camshaft is let go later, which reduces the overlap between the exhaust phase and intake phase. When the engine is idling, pressure in the intake manifold is low (vacuum). If the camshafts have a large overlap, i.e. the exhaust valves and intake valves are wide open at the same time, exhaust gases are sucked into the cylinder due to the negative pressure in the intake manifold. This results in a high residual gas content in the cylinder, which in turn leads to uneven idling and increased emissions. Note that at a slightly higher engine load, the engine can tolerate a higher residual gas content without uneven running. This is used to reduce emissions and fuel consumption.





Camshaft, moderate load
In order to reduce both exhaust emissions, especially NOx and CO2, as well as fuel consumption, the intake camshafts are set rather early when engine load is moderate, corresponding to the load of the most common driving conditions, such as motorway driving. The purpose is to increase internal EGR. By increasing overlap, the residual gas content in the cylinders increases. The increased residual gas serve as a standard EGR system - adding exhaust gases to the charge air reduces the NOx content of the exhaust gases.

Increased internal EGR also reduces pump loses during the engine intake phase. Pump loses are the work used for engine gas exchange. For example, when driving with a light load, the engine's air supply must be choked using the throttle valve. The pressure in the intake manifold is then low (vacuum). Because of this low pressure, the piston needs more energy to move down during the intake phase. If it is possible to increase pressure in the intake manifold without increasing the intake air mass (which would generate the wrong torque), pump loses would be reduced. In this case, the cylinders would be partially filled with residual gases, which would cause the pressure in the intake manifold to increase in order to take in the same air mass as would be required if there was not a high residual gas content.





Camshaft, high engine speed and high load
When the engine is running with a high load at a high engine speed, the capacity is increased in that the intake camshafts are allowed to go later. By filling the cylinder when the piston is down low (creating a large volume), there is room for more air mass at the same given intake pressure. Since engine speed is high, there is minimal backflow of air when the piston is on its way up in the cylinder even through the intake valves are open.





CVCP mechanism
The CVCP mechanism consists of a housing with a gear for the timing chain. The walls on the inside of the housing contain four chambers. A rotor with four wings is situated in the center of the housing and is mounted on the camshaft. A guide pin ensures correct positioning between CVCP and the camshaft during installation. Oil is led from the CVCP solenoid valve through ducts up to the camshaft bearing cap. A number of radial holes bored into the camshaft bearing surface lead oil in the camshaft to the CVCP mechanism's rotor and into the chambers.








Regulation of oil flow affects the position of the wings in the chambers. The rotor (camshaft) position can thus be displaced in relation to the housing (timing chain). This way, the camshaft position can be displaced 50 crankshaft degrees. An aperture disc is mounted to the side of the rotor to enable ECM to measure the position of the camshaft.

In order to ensure the camshaft position upon start and in the event of a system fault, the CVCP mechanism is equipped with a lock consisting of a spring-loaded pistons actuated by oil pressure to the "pre-setting side". Upon start or, for example, in the event of an open circuit on the lead to the solenoid valve, pressure on the pre-setting side becomes zero. The pistons locks the housing with the rotor and the camshaft position is now in the starting position. When the engine is then started and camshaft control begins to work, pressure on the pre-setting side increases and the spring force for the piston is overcome. Locking is released and the oil pressure actuates the wings in the chambers to turn the CVCP mechanism.





ECM regulates a PWM voltage to the solenoid valve, causing the valve plunger to move. The position of the plunger is determined by the balance between the return spring and the power from the coil. When the coil is de-energized, the plunger resumes its home position through the force of the return spring. The valve closes the oil flow to "pre-setting" and opens it to "pre-setting". The CVCP mechanism then assumes its starting position.





The valves have four ports, three of which have filters. The return port does not need a filter. The oil pressure from the engine's lubrication oil pump is supplied to the center port. The plunger assumes different positions depending on the size of the PWM voltage. When no voltage is supplied, there is a connection between oil pressure and "post-setting". The port for "pre-setting" is connected to "oil return". By varying the PWM voltage, the plunger can assume any number of positions in order to achieve the desired camshaft setting.





ECM regulates camshaft position based primarily on engine speed and load. A number of other values are used for minor compensations. Based on these values, ECM calculates requested camshaft position, converts it to the necessary PWM and sends it out to the valves. A Hall sensor reads the position of the aperture discs and thereby the current position of the camshafts. ECM compares the requested and actual positions. If there is deviation, the PWM voltage to the valves is corrected.