Variable Valve Actuation Assembly - Operation

OPERATION





Variable valve actuation controls the operation of the intake valves. By using a hydraulic link between the camshaft and the intake valves, the valve lift and timing can be adjusted infinitely.

The main components of variable valve actuation are the Powertrain Control Module (PCM) programming and the variable valve actuation module. The variable valve actuation module contains one set of the following components for each cylinder:

- Upper Pumping Element (3)
- Solenoid Valve (1)
- Hydraulic Brake Pumping Element/Hydraulic Lash Adjusters (2)
- Oil Accumulator (4)





The upper pumping elements (1) in the variable valve actuator are filled with oil from the engine lubrication system. The upper pumping elements (1) are driven by the rocker arms through camshaft motion (9) to create high-pressure oil inside the oil chamber (2). As the camshaft continues to rotate and the camshaft lobe ramps down, a spring in the upper pumping element returns the piston and the rocker arm to their home positions.

The high-pressure oil in the oil chamber passageway is delivered to the solenoid valve (8) from the upper pumping element. The solenoid valve (8) is a normally open hydraulic control valve. Therefore, the high-pressure oil is vented from the oil chamber to the accumulator (7) unless the solenoid valve is powered. When the solenoid valve is supplied 12 volts, the valve closes and high pressure oil instead acts upon the hydraulic brake pumping elements (6) to operate the intake valves (5).

The hydraulic brake pumping element (6) is essentially a piston that is operated by the high-pressure oil to open the intake valves (5). It also functions as a brake to stop the movement of the pumping element and intake valves. The braking function is the result of vent holes around the perimeter of the element's cylinder. When the piston approaches full stroke, the lower perimeter vent holes bleed off oil. This stops the piston from moving any farther. When the pressure from the solenoid is taken away, the upper supply holes in the cylinder become bleed holes that allow the piston to return to the home position. The piston is pushed back to the home position by the force of the intake valve springs. As the piston approaches the home position, some of the bleed holes are covered and the speed of the piston is regulated as it approaches the final resting position. This also controls the valve speed as it seats in the cylinder head. The hydraulic brake pumping element also functions as a hydraulic lash adjuster (3) because the valve stem clearance is hydraulically taken up by the piston. This occurs because the hydraulic brake pumping element/hydraulic lash adjuster is always under some oil pressure from the engine oil lubrication system. This creates enough pressure to take up valve clearance.

The variable valve actuator features three areas that function together as an oil reservoir so that the actuator always has a ready supply of engine oil. The engine lubrication system provides oil to the lower reservoir area and keeps it full of oil. To flow from the lower reservoir to the upper reservoir, the oil must pass through very small holes. This helps to purge air from the lower reservoir and to maintain engine oil pressure in the lower reservoir. The third reservoir area, the oil accumulator (7), is filled by oil vented from the oil chamber by the solenoid valve (8). The oil accumulator is a spring loaded accumulator that absorbs the hydraulic shock that would otherwise be created when the solenoid valves are opened. This helps to increase the durability of the system, maintains higher pressure in the lower pressure side of the system and lowers the overall load on the engine oil pump.

Variable valve actuation provides five possible phases of operation. Each phase offers unique advantages compared to normal camshaft operation. The five phases are:





- Full Lift. When variable valve actuation functions in the full lift phase, all of the camshaft lobe lift is transferred to the intake valves. The intake camshaft lobe is designed with a very aggressive lift and duration profile. This results in good power in the upper RPM ranges with high loads. This profile would rarely be used in everyday driving.





- Early intake valve closing (EIVC). When variable valve actuation functions in the EIVC phase, the camshaft lobe lift is transferred to the intake valves at the beginning of the lift duration cycle. However, the hydraulic connection between the camshaft lobe and the valves is taken away before the lobe reaches full lift. The exact timing and lift can be infinitely varied to meet driver requirements. EIVC provides smooth engine performance and more torque at lower engine speeds.





- Late intake valve opening (LIVO). When variable valve actuation functions in the LIVO phase, the camshaft lobe lift is NOT transferred to the intake valves at the beginning of the lift duration cycle. The hydraulic connection between the camshaft lobe and the valves is completed after the rocker arm has already begun riding the ramp of the camshaft lobe. When the hydraulic connection is completed, the intake valves will begin to open. The valve lift timing can be varied infinitely within the full profile of the camshaft lobe. Therefore, as long as the hydraulic connection is completed before the camshaft lobe reaches its maximum lift, some valve lift will result. The lift profile will follow the camshaft lobe profile for the time that the hydraulic link is complete. Like EIVC, the exact timing and lift can be infinitely varied to meet driver requirements. LIVO provides lower emissions and a higher efficiency at lower loads or idle conditions.





- Multi-Lift. Multi-Lift is a combination of EIVC and LIVO because the hydraulic connection between the camshaft lobe and the intake valves is closed early and then re-opened later in the cycle. This creates a longer duration valve lift with a smaller amount of lift. The result is a higher velocity of air flow into the cylinder over a longer period of time. Multi-Lift may be used in mixed driving of acceleration and deceleration with moderate engine speeds.





- Closed. The closed phase simply leaves the intake valves closed by not utilizing the camshaft lobe to lift the intake valves.