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Analyzing a Direct Acting Valve Train System

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					                                        Recent Researches in Manufacturing Engineering




                        Analyzing a Direct Acting Valve Train System
                                               LIVIU JELENSCHI
                                             CORNELIU COFARU
                                               GABRIEL SANDU
                              Department D02- High-tech products for automotives
                                      University Transilvania of Brasov
                                     Str. Politehnicii nr.1, 500024, Brasov
                                                  ROMANIA
                                           jelenschiliviu@yahoo.com


Abstract: - This paper presents the research regarding the direct acting valve train system. The main role of the valve
train is to ensure the gases exchange process for all the engine speeds. Knowing the fact that the gases exchange
process influences the engine good functioning, a kinematic and a dynamic analysis are required for the engine valve
train. The model of the valve train was created using 3D software from LMS, Virtual Lab 9A. Then a kinematic and a
dynamic analysis were made considering only a part from entire valve train. The results show that the engine speed is
the most important parameter in valve train dynamics.

Key-Words: - valve train, internal combustion engines, kinematic analysis, dynamic analysis, valve bounce

1 Introduction
The valve train system represents the engine component
which ensures the gases exchange processes. Thus,
depending on engine type, spark ignition engine or
compression ignition engine, and the imposed
requirements, five types of valve train system might be.
Each has its constructive features, advantages and
disadvantages.
Because of constructive simplicity the direct acting valve
train is most common type used. Also the high stiffness
and low variation during operation represent important
characteristics that make it to be used on high speed
engines.                                                                           Fig. 1 Valve displacement
The main purpose of this paper is the development of a
virtual model of the direct acting valve train, a model
that might be used in future research activities.


2 Valve train description
The virtual model is based on an in line four cylinders
engine with four valves per cylinder. For simplifying the
model and for reducing the timing calculation only a
single valve is analyzed.
Cam profile was developed starting from the valve lift
resulted after the measurement made on the test bench.
Thus, knowing the base circle diameter: 34mm, the cam
width: 12.4mm and the maximum cam lift: 9.2mm, a
cam profile was developed using Virtual Lab Motion
software from LMS.
                                                                                         Fig. 2 Cam profile




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                                         Recent Researches in Manufacturing Engineering




The hydraulic tappet and the valve spring have an                   force.
important role in the valve train dynamics.                         The meshed geometry is used in the contact
Besides taking the lateral forces and transmitting the              calculation during the simulation.
motion from camshaft to valve, the hydraulic tappet                 The motion of the camshaft is given by a joint
must take the gap between valve train parts as a result of          position driver which is applied in to the
thermal expansion during engine functioning.                        revolution joint between cam body and
The valve spring has the role to maintain the valve on its          camshaft bearings. The angular speed represents
seat during gases exchange process and to overcome the              the input data for the model.
inertial forces that tend to break the kinematic chain              The tappet guide, the camshaft bearings, the
between cam-tappet-valve.                                           seat and the guide of the valve are fixed to the
                                                                    ground by a bracket joint.


                                                                    3 Kinematic and dynamic analyses
                                                                    After the model was realized, a kinematic and a dynamic
                                                                    analyses were performed. The results were obtained at
                                                                    different engine speeds: 700RPM, 1500RPM, 2500RPM,
                                                                    4000RPM and 7200RPM.
                                                                    The kinematic analysis is used to determine if the valve
                                                                    motion respect the manufacturer specifications,
                                                                    respectively the valve lift, velocity and acceleration
                                                                    without taking into account the bodies mass.

             Fig. 3 Direct acting valve train

In order to simplify the model the valve spring is replace
by a TSDA element which is rigidly fix by the valve
steam and valve guide. A TSDA element represents a
translational spring-damper-actuator and defines a force
element between two bodies. This force is defined by the
combination of a value of stiffness, a value of damping,
and a value of actuation torque.



                                                                                     Fig. 5 Kinematic results

                                                                    The dynamic simulation is use to establish the behavior
                                                                    of the valve taking into account bodies masses and
                                                                    inertial forces. Also with a dynamic analysis can be
                                                                    determinate de contact forces, the bodies’ deformation or
                                                                    the reactions from joints.

          Fig. 4 Scheme of the dynamic model

As it can be seen in above figure, the contact
between valve train elements is made using
CAD Contact elements. This method allows the
simulation of the contact between rigid bodies
with arbitrary geometry. The bodies for which
contact is to be calculated are selected, and the
solid geometry associated with each body is
tessellated based on user-specified parameters.
So must be defined the spring damping and the                                   Fig. 6 Valve bounce phenomenon
friction coefficients that characterize the contact



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                                        Recent Researches in Manufacturing Engineering




                                                                   Also those spikes can occur as a result of the type of
                                                                   contact used between components. Since the cad contact
                                                                   element uses the triangles for approximation the surfaces
                                                                   in contact and thus their size too large can cause
                                                                   vibration at high engine speed, while an exaggerated
                                                                   finesse of the tessellation leads to increasing
                                                                   computational time without obtaining results with
                                                                   greater accuracy.


                                                                   4 Conclusion
                                                                   This paper presents a preliminary model of a direct
                                                                   acting valve train system. The virtual model contain
                                                                   seven rigid bodies between are established different
                 Fig. 7 Valve velocties                            kinds of relations.
                                                                   Between the moving bodies like cam, tappet and valve
                                                                   contact force elements were used.
                                                                   Once the engine speed increases the inertial forces rise
                                                                   and the behavior of the valve worsens. Also an unwanted
                                                                   phenomenon appears, the valve's bouncing, which
                                                                   causes the valve and valve seat wear.
                                                                   The valve bounce phenomenon influences the shapes of
                                                                   the velocities and acceleration curves.
                                                                   Based on the presented result we can conclude that this
                                                                   simplified model needs to be improved. The next step, in
                                                                   the analysis, will be the introduction of flexible bodies in
                                                                   the model structure.


                                                                   ACKNOWLEDGMENTS
               Fig. 8 Valve accelerations                          This paper is supported by the Sectoral Operational
                                                                   Programme Human Resources Development (SOP
As can be seen in the figures above, at higher engine
                                                                   HRD), financed from the European Social Fund and by
speeds, the performance of the direct acting valve train
                                                                   the Romanian Government under the contract number
becomes critical. Figure 6 presents a comparison
                                                                   POSDRU/88/1.5/S/59321.
between the valve lift at 700rpm and 7200rpm (the
maximum engine speed). When the valve returns on its
seat, an unwanted phenomenon appears the valve
                                                                   References:
bounce. The valve bouncing phenomenon will accelerate
                                                                   [1] C.M. Taylor, Engine tribology, Elsevier, 1993.
the seat wear and possible to lead to valve failure. The
                                                                   [2] D. Moreno, E. Mucchi, G. Dalpiaz, A. Rivola,
influence of the valve bounce might be observed in the
                                                                       Multibody analysis of the desmodromic valve train
valve velocity and acceleration (fig.7, fig.8). The spikes
                                                                       of the ducati motogp engine, Proceedings of the
that appear on those graphs are due to this phenomenon.
                                                                       Multibody dynamics - ECCOMAS Thematic
                                                                       Conference, Milano, Italy, 2007.
                                                                   [3] Gheorghe Al. Radu, Analiza în mediu virtual a
                                                                       procesului de admisie în motorul cu ardere internă,
                                                                       Transilvania University Publishing House, 2004.
                                                                   [4] Radek Tichánek, David Fremut, Robert Čihák, The
                                                                       Over-Head Cam (OHC) Valve Train Computer
                                                                       Model, 2005.
                                                                    [5] LMS International, Virtual Lab 9A, 2010.
          Fig. 9 Tessellation of CAD Contact
            element between cam and tappet




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posted:8/1/2011
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