06-02-2013, 02:50 PM
CAMLESS ENGINE WITH ELECTROMECHANICAL VALVE ACTUATOR
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Abstract
Valve train control is one of the best strategies for optimizing efficiency and emissions of
Internal Combustion (IC) engines. Applications of solenoid valve actuators in (IC)
engines can facilitate operations such as variable valve timing and variable valve lifting
for improved the engine performance, fuel economy and reduce emission, the
electromechanical valve actuator (EMVA) uses solenoid to actuate valve movement
independently for the application of (IC) engine. In this work presents the effects of
design and operating parameters on the system dynamic performances of the actuator and
the proposed an (EMVA) structure by incorporating the hybrid magneto-motive force
(MMF) implementation in which the magnetic flux is combined by the coil excitation and
permanent magnets. A two-degree-of-freedom lumped parameter model is used to
simulate the response of valve actuator system in the opening and closing. The model
and control of an electromagnetic valve (EMV) are described. This is done using
electromagnetic force to open and close the valve and a controller regulates the motion
specifications required. The developments controller is based on a state-space description
of the actuator that is derived based on physical principles and parameter identification.
Linear-quadratic regulator design (LQR) optimal control is designed with the evaluation
reasonable the performance and energy of (EMV) valve are obtained with the design
INTRODUCTION
In recent years camless engine has caught much attention in the automotive industry.
Camless valvetrain offers programmable valve motion control capability. However, it
also introduces valvetrain control issues. There are mainly two types of camless
actuators, electrohydraulic valve (EHV) and electromechanical valve (EMV) actuators.
This paper deals with the EMV type of actuator. The EMV system discussed in this paper
is slightly different from the previous experimental system that the authors had worked
on First of all, stronger springs are used in current system setup in order to get faster
closing. Secondly, there is no physical lash spring between the engine valve and armature
in the new system. The engine valve stem is directly in contact with the armature of the
electromagnet. A lash, the clearance between the valve step and the armature when each
is seated to its own mechanical stops (valve seat and electromagnet) respectively, of 0.15-
0.25 mm is maintained to allow for the valve stem thermal expansion. Thirdly, the
position measurement of armature is used for feedback control. The engine valve position
is still being monitored, but only for the purpose of modeling and performance evaluation
For an EMV system, the control of engine valve seating velocity has been identified to be
a critical problem. The motion of an engine valve on a conventional engine is driven by a
camshaft and constrained by a spring to follow the cam profile. Therefore, small seating
velocity is not difficult to achieve. For a camless valve train, however, a control system is
required to maintain the seating velocity below a given level. In addition to the results
from using the new system and instrumentation described above, in this paper we will
present new modeling analyses that shed light to the adequate control system design
approach. The objective is to find out what kind of control strategy is suitable for this
particular system. Open-loop feed forward control will be the first choice if it is feasible.
Cycle to- cycle iteration upon the open-loop feed forward control could be considered
given the cyclic nature of engine operation and the fact that the system dynamics.
OVERVIEW OF THE EMV ACTUATOR SYSTEM
The actuator consists of a lower electromagnetic coil for opening the valve and an upper
coil for closing the valve. Actuator and valve springs push on the armature and valve
stem through spring retainers. At neutral position the actuator and valve springs are
equally compressed and the armature is centered between the upper and lower coils. At
start-up, a voltage is applied to one of the electromagnets to move the armature from
neutral position to the fully open or fully closed position. A small holding voltage is then
maintained to hold the armature in place against the spring force. Fig. 1 shows valve position
and coil voltages during standard operation. Consider the transition from open toclosed
position. A holding voltage is first applied to the lower coil. Releasing this holding
voltage then allows the compressed spring to push on the armature and initiate the valve
motion. A catching voltage is then applied to the upper coil to capture the armature in the
upper position, and so on. After the initial start-up from the neutral position, the actuator
is principally a spring mass pendulum that is driven by an electromagnetic force. The potential
energy is transferred between the two springs via the armature and valve. A catching
voltage is applied to the appropriate coil to inject enough magnetic energy to overcome
the losses from friction forces, gas flow forces, and possibly magnetic forces from
the releasing coil.
OPERATING PRINCIPLE
A typical construction of EMV consists of two magnets (closer and opener), two springs
(an actuator and a valve spring) and a moving armature that is connected to an engine
valve. Normally, most modern engines incorporate a hydraulic lash adjuster to ensure
proper valve sealing under all thermal operating conditions. The current flowing in the
coil creates a magnetic force on the armature to overcome Compressive spring and
friction forces. Both springs are adjusted such that they are always in compression for
any armature position between the two electromagnets. Pre loading these springs are
ideal for achieving rapid flight time and minimizing electrical energy input. During
normal operation, the spring forces are utilized to accelerate the moving masses while
electromagnetic forces are utilized to attract and dwell the armature.
Equivalent circuit diagram
Fig illustrates a typical operational mode of the EMV actuator. To move the valve from
neutral to closed position, a routine is initialized to impart sufficient armature momentum
for the closer coil magnetic forces to attract the armature. Once contact is established and
quasi-static conditions are reached, a holding current is applied to the upper coil so that
sufficient magnetic force is generated to overcome spring forces while holding the
armature and valve in the fully closed position. When the valve is commanded to open,
the upper coil current is rapidly discharged, allowing magnetic force to decay and the
actuator spring to push the armature down. The lower coil is then activated to capture the
approaching armature. The electromagnetic force generated is proportional to the volume
of the electromagnet. The design volume of the electromagnet is limited by the area in the
cylinder head. Thus a compromise has to be made in the design and operation of an
engine using EMV system. This compromise is usually in the form of a limited
volumetric efficiency and maximum operating speed of the engine