Dynamic Simulation:
Piston Assembly Example
Objective




The objective of this module is to show how an example problem is
solved using the Dynamic Simulation environment within Autodesk
Inventor software.
This kinematic analysis problem involves imposing a rotational motion
on the rotating assembly of a boxer style engine.
The Output Grapher is used to plot computed moments,
displacements, velocities, and accelerations.
Friction in a prismatic joint is also illustrated.
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Section 4 – Dynamic Simulation
Components
Module 5 - Piston Assembly Example
Page 2
Crank Shaft
Pistons x 4
Cylinder
Liner x 4
Crankshaft
Bearings x 3
Connecting rods
and piston pin
bearing assemblies
x4
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Section 4 – Dynamic Simulation
Sub-assemblies
Module 5 - Piston Assembly Example
Page 3


The connecting rod
components and the piston
components were modeled as
sub-assemblies.
Piston
Assembly
This will cause Inventor and
Dynamic Simulation to treat
them as rigid bodies with no
relative motion between the
sub-assembly parts.
Connecting Rod
Assembly
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Assembly Constraints
Ground Joints
Section 4 – Dynamic Simulation
Module 5 - Piston Assembly Example
Page 4
The cylinder liners and crankshaft bearings are fixed in the engine block
and cannot move. They were positioned using the cylinder block and
then grounded in Inventor. The ground is carried over into Dynamic
Simulation.
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Assembly Constraints
Revolute Joint
Section 4 – Dynamic Simulation
Module 5 - Piston Assembly Example
Page 5
A mate is placed between
the centerline of the
crankshaft and the
centerline of the crankshaft
bearing.
This constraint will be
converted to a revolute joint
in Dynamic Simulation.
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Assembly Constraints
Axial Crankshaft Constraint
Section 4 – Dynamic Simulation
Module 5 - Piston Assembly Example
Page 6
A mate is placed
between the two
surfaces shown to
position the crankshaft
along the axis of the
engine.
 This constraint removes
the translation DOF
between the crankshaft
and bearings.
 A 2mm offset is
required.

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Assembly Constraints
Revolute Joints
Section 4 – Dynamic Simulation
Module 5 - Piston Assembly Example
Page 7
Mates are placed between the centerlines of the piston bearings
and the crankshaft journals. These will automatically be converted to
revolute joints by Dynamic Simulation.
Crankshaft
Journals
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Section 4 – Dynamic Simulation
Assembly Constraints
Module 5 - Piston Assembly Example
Axial Constraint
Page 8
 A mate is placed between
the machined connecting rod
surface and the face of the
crankshaft journal.
 This removes the
translational DOF between
the connecting rod assembly
and the crankshaft.
 This constraint is applied to
each connecting rod
assembly.
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Crankshaft
journal face
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Machined
connecting
rod surface
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Assembly Constraints
Revolute Joints
Section 4 – Dynamic Simulation
Module 5 - Piston Assembly Example
Page 9
In a manner similar to the other revolute joints, centerline
mates are placed between the pistons and the connecting
rod assemblies. This is called “Joint 1” in Module 3.
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Assembly Constraints
Prismatic Joints
Section 4 – Dynamic Simulation
Module 5 - Piston Assembly Example
Page 10
The final set of constraints is
between the cylinder liners
and the pistons. Centerline
mates are again used. The
centerline constraints will
allow translational motion of
the piston along the axis of
the cylinder liner.
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Section 4 – Dynamic Simulation
Completed Assembly
Module 5 - Piston Assembly Example
Page 11
The complete assembly is shown in the figure. The crankshaft can be
rotated and all other parts move in accordance to the assembly
constraints.
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Section 4 – Dynamic Simulation
Dynamic Simulation Environment
Module 5 - Piston Assembly Example
Page 12
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Section 4 – Dynamic Simulation
Dynamic Simulation Environment
Module 5 - Piston Assembly Example
Page 13
3 Grounded Crank Bearings (one bearing has two parts)
4 Grounded Cylinder Liners
Crankshaft
4 Piston Assemblies
4 Connecting Rod Assemblies
Kinematic constraints are automatically
generated by Dynamic Simulation from
the assembly constraints.
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Section 4 – Dynamic Simulation
Mobility
Module 5 - Piston Assembly Example
Page 14

The mobility of the
mechanism is checked
using the Mechanism
Status feature on the
ribbon.

The mechanism has a
mobility of one as
expected.

The free DOF is the
rotational DOF of the
crankshaft.
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Section 4 – Dynamic Simulation
Motion Groups
Module 5 - Piston Assembly Example
Page 15
The parts that
can move are
shown in a
solid color.
The parts that
cannot move
are shown in
transparent
mode.
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Section 4 – Dynamic Simulation
Motion Constraint
Module 5 - Piston Assembly Example
Page 16
 A mobility of one
requires that one
motion constraint
be specified.
Right click on properties
for Revolution Joint 1
 A rotational velocity
of 3600 deg/sec is
applied to the
crankshaft rotational
degree of freedom.
 This is equivalent to
600 rpm.
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Section 4 – Dynamic Simulation
Input Motion
Module 5 - Piston Assembly Example
Page 17
As a check, the computed angular velocity is constant and is 3,600 deg/sec
which agrees with the input angular velocity.
Velocity
Position
Acceleration
The angular position of the crank shaft should be a linear function of time
with a slope of 3,600 deg/sec. The computed position is correct. The
angular acceleration of the crank shaft should be zero since the velocity is a
constant. The computed acceleration is correct.
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Section 4 – Dynamic Simulation
Input Torque
Module 5 - Piston Assembly Example
Page 18
The computed torque that is required to impose a constant angular
velocity of 3,600 deg/sec on the crankshaft is shown in the Output
Grapher plot. Notice that the input torque is sinusoidal.
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Section 4 – Dynamic Simulation
Input Torque
Module 5 - Piston Assembly Example
Page 19

Positive values of the torque mean that energy is being put into
the system so that the constant crankshaft angular velocity is
maintained.

Negative values of the torque mean that energy is being taken out
of the system so that the constant crankshaft angular velocity is
maintained.

Since there is no friction in the system the net energy required to
maintain a constant angular velocity of the crankshaft is zero.

The average torque is zero which indicates that the average input
power is zero.
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Section 4 – Dynamic Simulation
Friction
Module 5 - Piston Assembly Example
Page 20
Friction can be added by editing the properties of a joint. In
this case we are adding a coefficient of friction of 0.2 to the
translational degree of freedom of the prismatic joint
between the piston and cylinder liner.
Friction is
added to all
joints using
this approach.
The properties dialog
box is obtained by right
clicking on the joint.
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Section 4 – Dynamic Simulation
Input Torque with Friction
Module 5 - Piston Assembly Example
Page 21
The addition of a coefficient of friction of 0.2 increased the average
torque from zero to 491 Nm. The input power required to overcome
the friction in the system is
P  T    491Nm  62.83rad / sec  554 Watt
Max = 6,267 Nm
Average = 491 Nm
Min = -5,776 Nm
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Section 4 – Dynamic Simulation
Module Summary
Module 5 - Piston Assembly Example
Page 22

This module provided an example of how to perform a dynamic
simulation of a mechanism using Autodesk Inventor’s Dynamic
Simulation environment.

This kinematic analysis problem involved imposing a rotational
motion on the rotating assembly of a boxer style engine.

Correlations were made between the theory presented in previous
modules and this example.

Although the mathematics is hidden from the user, the input
information associated with joints and constraints can clearly be
seen in the user interface.
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