MBD Simulation on IC Engine Valve Train

AIM:

to apply motion analysis study on I.C.Engine valve train and obtain valve displacement as well as analyze contact forces exerted within various components of the assembly.

DESIGN OF CAM :

to obtain the desired cam lifts of 3.5 mm and 6 mm, we have to modify parameters of the cam. the design considerations of cam used to achieve desired lifts are as below:

PARAMETERS FOR CAM:

for 3.5 mm lift:

• diameter of the lower circle(D1) = 25 mm
• diameter of the upper circle(D2) = 10 mm
• distance between two centers(L) = 11 mm

HENCE, 

cam lift = L + D2 / 2 - D1/2

            = 11 + 5 - 12.5

            = 3.5 mm

for 6 mm lift:

• diameter of the lower circle(D1) = 25 mm
• diameter of the upper circle(D2) = 15 mm
• distance between two centers(L) = 11 mm

HENCE, 

cam lift = L + D2 / 2 - D1/2

            = 11 + 7.5 - 12.5

            = 6 mm

MOTION ANALYSIS:

for setting up motion study analysis, the parameters used for each setup is given below:

CASE 1: 3.5 mm cam lift:

• motor rpm: 1500 c.w
• material: cast carbon steel (all components)
• gravity: on (y-dir)
• precise contacts: on
• fps: 1500 * 6 * 2 = 18000 (solver takes reading at each 0.5 degree cam rotation) 
• spring stiffness constant = 15 N/mm
• free length of spring = 45 mm

   we'll run the simulation for 0.2 seconds for which, we'll have displacement plots for pushrod and rocker arm. along with it, the contact force between desired pairs will be achieved.  

 furthermore, the relation of arising contact forces with respect to various pre-compression and stiffness constants of spring will be tested while justifying the changes in these forces with respect to magnitude and x-direction.

 PLOTS:

1) pushrod displacement:

by observing the plot, we can see that the overall displacement is between 3.5 mm. the displacement here is calculated with respect to initial position of the origin of pushrod. point1 of sketch1 is a point drawn on origin of the push rod.

another method of obtaining the displacement is by selecting bottom face of pushrod only; here solver will calculate the distance with respect to the origin of assembly and with linear displacement option, we have to select the y-direction option to get the accurate results.

2) valve displacement (valve lift):

 by observing the above plot, we can conclude that there is at max. 8 mm of valve displacement.

3) cam-pushrod (bearing):

according to the plot, contact force peaks when pushrod is at maximum cam lift and due to the momentum of the pushrod, the contact breaks between cam and the pushrod which causes the contact lost and subsequently the zero contact force.

the proof for this can be shown as the spreadsheet value of pushrod displacement being higher than the 3.5 mm which indicates that pushrod is being overshot due to the inertia. higher the rpm of cam higher is overshoot. similarly, higher the spring stiffness lower is the overshoot.  

the steep rise and falls occurring between the mean value of 330 N shows the bouncing of the pushrod for a while between rocker arm and the cam.

hence, a similar fashion can be observed in bearing between the pushrod and the rocker arm.

4) pushrod - Rocker arm (bearing):

 5)  Rocker arm - valve (bearing):

    in this case, the contact force between the rocker arm and the valve is two dimensional      (x-y plane). this situation is shown as the figure given below:

as shown in figure, the net resultant contact force Fr is made up of two components; namely, Fx & Fy.

hence, the plots obtained while plotting magnitude (Fr vs. time) and with x-direction (Fx vs. time) will give different results. 

however, we can have column operations on the values of Fx and Fy and by taking the resultant out of it, we can plot the obtained values vs time and can get similar results. 

below plots mention the discussion held above:

Fr and Fx comparison: 

Fr and Sqrt[Fx^2 + Fy^2] comparison:

we can clearly observe the graph of Fr and resultant magnitude are the same. hence this dissolves the question of discrepancy between Fr and Fx.

CASE 2: 6 mm cam lift:

• motor rpm: 1500 c.w
• material: cast carbon steel (all components)
• gravity: on (y-dir)
• precise contacts: on
• fps: 1500 * 6 * 2 = 18000 (solver takes reading at each 0.5 degree cam rotation) 
• spring stiffness constant = 15 N/mm
• free length of spring = 45 mm

   we'll run the simulation for 0.2 seconds for which, we'll have displacement plots for pushrod and rocker arm. along with it, the contact force between desired pairs will be achieved.  

PLOTS:

1) PUSHROD DISPLACEMENT:

 we can verify the displacement of the pushrod is consistent with the cam lift and is approximately 6 mm.

2) valve displacement (valve lift):

 we can assess the valve lift obtained being around 14 mm from the plot given above.

 3) cam-pushrod (bearing):

4) pushrod - Rocker arm (bearing):

5)  Rocker arm - valve (bearing):

EFFECT OF PRE-SPRING COMPRESSION ON CONTACT FORCES:

we will take 2 cases of 50 mm (5 mm more than initial) free spring length and 40 mm (5 mm less than initial) of free spring length; for which, there would be a difference of:

 K * change in free length = 15 N/mm * 5 mm = 75 N.

however, one thing to consider is that the difference obtained is a change in y-direction, hence the component getting affected is Fy (and not the Fr).

plots:

 the above plot shows the approximate spacing of 75 N (ignore the sign) between each successive spring lengths.

similarly, we can also show such a relation by changing the stiffness coefficient of the spring.

more stiffness coefficient will lead to higher contact forces, and lower stiffness coefficient will result in lower contact forces.