The present invention relates to electromechanical valves actuators and, more particularly, to electromechanical valve actuators that reduce noise, vibration, and harshness issues associated with the opening of the valve.
As engine technology advances and manufacturers strive to increase engine power, improve fuel economy, decrease emissions, and provide more control over engines, manufacturers are developing electromechanical valve actuators to replace cam shafts for opening and closing engine valves. Electromechanical valve actuators allow selective opening and closing of the valves in response to various engine conditions.
Electromechanical valve actuators generally include two electromagnets formed from a core having an embedded power coil. A spring loaded armature located between the electromagnets is movable between the electromagnets as the power coils are selectively energized to create a magnetic force to attract the armature. The surface of the electromagnets to which the armature is attracted when the power coil of an electromagnet is energized is generally referred to as a pole face. The armature abuts to the valve so that as the armature moves between pole faces in pole-face-to-pole-face operation, the valve is opened and closed.
When the valve is in a closed position, the armature plate is generally held against or near the armature electromagnet and a gap is created between the armature stem and the valve stem. This gap is commonly referred to as a lash gap. The lash gap allows thermal movement of metal parts during engine operation and is necessary to ensure that under all conditions the valve is fully closed, while the armature is seated on or near the armature electromagnet pole face. One problem with traditional electromechanical valves is that noise, vibration, and harshness issues occur when the armature stem contacts the valve stem as the valve is opened. More specifically, as the armature plate is released from the armature electromagnet, the armature spring pushes the armature assembly, specifically the armature stem, toward the valve stem so that the armature stem, contacts the valve stem, typically a lash cap on the valve stem, at a high velocity to move the valve from the closed position to the open position. The impact between the valve stem and armature stem may cause noise, vibration, and harshness issues. These noise, vibration, and harshness issues may be amplified due to the excitement of other components within the electromechanical valve actuator, such as the armature plate which may act as a radiator to amplify the noise.
While some manufacturers have attempted to alleviate these problems by combining traditional hydraulic lifters with electromechanical valve actuators, problems may occur from this combination. Hydraulic lifters typically increase the friction experienced by electromechanical valve actuators and add mass to the moving parts. Any increase in mass or friction requires additional power consumption by the electromechanical valve actuator to move the armature and valve and such additional power consumption is particularly acute in opening the valve during the exhaust cycle. Additional power consumption raises additional issues such as undesirable excess heating of the electromagnet power coils and consumption of additional electrical power from the vehicle's generating system which reduces fuel economy and adds to the cost of the generating and distribution system. Any requirement of additional power puts increased demand on today's already overloaded vehicle electrical systems. Another disadvantage to using traditional hydraulic lifters is that hydraulic lifters are relatively expensive and add to the overall cost of the vehicle.
To avoid additional expense, increased friction, increased mass, and increased power consumption associated with hydraulic lifters, some manufacturers have attempted to limit the impact force between the valve stem and armature stem by controlling the current profile supplied to the electromagnets. One method of controlling the current profile includes slowly decreasing the current supplied to the armature electromagnet. For example, the electromechanical valve actuator control system slowly lessens the amount of magnetic attraction of the armature electromagnet. As the force of the armature spring surpasses the combined force of the valve spring and magnetic attraction force of the armature electromagnet, the armature accelerates away from the armature electromagnet. The armature electromagnet continues to apply a magnetic attraction force to the armature plate, thereby slowing the acceleration away from the armature electromagnet. While current profile control reduces noise, vibration, and harshness issues associated with the contact between the armature stem and valve stem during the opening of the valve, current profile control requires additional power consumption as the armature electromagnet works against the force provided by the armature spring. The power consumption of the valve electromagnet is also significantly increased due to the increased magnetic force needed to attract and retain the armature plate against the valve electromagnet. More specifically, because the force applied by the armature spring was inhibited by the armature electromagnet during the initial portion of the open cycle, the valve electromagnet must compensate for this inhibition by increasing the magnetic attraction force of the valve electromagnet. As stated above, to increase magnetic attraction, increased power consumption is required which raises additional problems. Further, the increased power consumption is particularly acute due to the distance between the valve electromagnet and armature plate at which the valve electromagnet must apply the magnetic force especially due to the exponential decrease in magnetic force as the distance increases. Another problem associated with current profile control include the necessity of additional and expensive position sensors and microprocessors to accurately determine and control the position and movement of the armature assembly. Yet another problem with current profile control is that as the armature plate approaches the pole face, the gap between the pole face and armature plate, generally referred to as the air gap, decreases, causing the magnetic force acting on the armature to exponentially increase. This exponential increase is particularly acute due to the additional power being applied by the valve electromagnet to the armature in response to reduced force from the armature spring to ensure attraction and retention of the armature plate as desired. Any increase in magnetic force causes the armature to increase in velocity as it approaches the pole face of the energized electromagnet thereby increasing the force of the impact of the armature. This impact causes noise, vibration, and harshness concerns, also making quiet operation of electromechanical valve actuators challenging to achieve.
In view of the foregoing, there is a need for an electromechanical valve actuator having reduced noise, vibration, and harshness issues during opening of the valve, especially during the exhaust cycle, without having increased power consumption.