Multi-Body Dyanamics and Simulation of IC Engine's Valve Train Using SolidWorks

AIM : To model the Valve Train components for IC Engine and assemble than to get the simulations in Solidworks According to the Following cases.

OBJECTIVES :

• To get results while performing Motion study simulations for different cam heights.

v  To plot the following quantiies -
(i) Valve Lift. 
(ii) The contact force between

ü  Cam and Push Rod

ü  Pushrod and Rocker Arm

ü  Rocker Arm and Valve

v  Explain why the contact force between the Rocker arm and valve varies while measuring with respect to the X direction and measuring with respect to magnitude. 

THEORY : 

A cam is a common mechanism element that drives a mating component known as a follower. The cam accepts an input motion similar to a crank and imparts a resultant motion to a follower.

The valve train of an automotive engine. In this application, an oblong-shaped cam is machined on a shaft. This camshaft is driven by the engine. As the cam rotates, a rocker arm drags on its oblong surface. The rocker arm, in turn, imparts a linear, reciprocating motion to a valve stem. The motion of the valve must be such that the exhaust pathway is closed during a distinct portion of the combustion cycle and open during another distinct portion. Thus, the application is perfect for a cam because timing and motion must be precisely sequenced.
The rocker arm follower needs to maintain contact with the cam surface to achieve the desired motion. Thus, in most cam applications, the follower is forced against the cam surface through some mechanical means. Springs are very common for this purpose. In cases where the follower is in the vertical plane, the weight of the follower may be sufficient to maintain contact. Some cam designs capture the follower in a groove to maintain contact. The important point is that contact between the cam and the follower must be sustained.

The Fundamental law of Cam Design

Any cam designed for operation at other than very low speeds must be designed with the following constraints:

The cam function must be continuous through the first and second derivatives of displacement across the entire interval (360 degrees).

corollary: The jerk function must be finite across the entire interval (360 degrees).

In any but the simplest of carns, the cam motion program cannot be defined by a single mathematical expression, but rather must be defined by several separate functions, each of which defines the follower behavior over one segment, or piece, of the carn. These expressions are sometimes called piecewise functions. These functions must have third-order continuity (the function plus two derivatives) at all boundaries. The displacement, velocity and acceleration functions must have no discontinuities in them.

If any discontinuities exist in the acceleration function, then there will be infinite spikes, or Dirac delta functions, appearing in the derivative of acceleration, jerk. Thus the corollary merely restates the fundamental law of cam design. Our naive designer failed to recognize that by starting with a low-degree (linear) polynomial as the displacement function, discontinuities would appear in the upper derivatives.

• Cam Shaft and Follower - A cam is a mechanical device used to transmit motion to a follower by direct contact. The driver is called the cam and the driven member is called the follower. In a cam follower pair, the cam normally rotates while the follower may translate or oscillate. A familiar example is the camshaft of an automobile engine, where the cams drive the push rods (the followers) to open and close the valves in synchronization with the motion of the pistons.
  
  
• Push Rod - A metal rod transmitting the reciprocating motion that operates the valves of an internal-combustion engine having the camshaft in the crankcase.
  
  
• Rocker Arm - A rocker arm (in the context of an internal combustion engine of automotive, marine, motorcycle and reciprocating aviation types) is an oscillating lever that conveys radial movement from the cam lobe into linear movement at the poppet valve to open it. One end is raised and lowered by a rotating lobe of the camshaft (either directly or via a tappet (lifter) and pushrod) while the other end acts on the valve stem. When the camshaft lobe raises the outside of the arm, the inside presses down on the valve stem, opening the valve. When the outside of the arm is permitted to return due to the camshafts rotation, the inside rises, allowing the valve spring to close the valve.
  
  
• Valve Guide and Stem - In most types of reciprocating engines, a valve guide is provided for each poppet valve in the cylinder head. Along with the valve spring, it serves to positively locate the valve so that it may make proper contact with the valve seat. A valve guide is a cylindrical piece of metal, pressed or integrally cast into the cylinder head, with the valve reciprocating inside it. Guides also serve to conduct heat from the combustion process out from the exhaust valve and into the cylinder head where it may be taken up by the cooling system.

 PROCEDURE  : -

1.      STEP 1 :- 3D Model of all the Valve-Train components

All the components related to our Assembly has been made Except that of CAM profile because we have to get desired displacement according to the cam design and its parameter, that must be varied.

ü  Design of Rocker Arm :-

ü  Design of Valve –

ü  Design of Valve Stem –

ü  Design of Pushrod -

ü  Design of CAM :-

• The diameters of both circles are kept according to the image shown above, the distance between the centre of the Two Lobs were Designed by using following calcualtions.
• For Case 1,
  i.e., cam lift 3.5mm, ⇒3.5=(L1−17.5)+10⇒3.5=(L1-17.5)+10 , ⇒L1=11mm⇒L1=11mm,   

• Assembly for Case 1 -
1. All components sketched were imported into assembly and therefore the first component inserted (cam) was made to float from being fixed. Temproary axes were switched on for better viewings. All components were bought to front plane and made coincident to the present plane. The cam was positioned by also making it's axis coincident to top plane and right plane. Distance mate were provided to other component in order that they are doing not obstruct one another during the motion and therefore the axes of valve and valve stem were made coincident. Top plane of valve rocker and assembly top plane were provided a further angle mate of 0 degree in order that it remains parallel to the present plane.
2. After successfull assembly, it had been made to maneuver using the physical dynamics in order that necessary clearance and get in touch with are provided between components.
3. Motion Study and Analysis –
   Case 1 (for cam lift 3.5 mm) :- After successful assembly, Solidworks motion is chosen from Solidworks add-ins and time and time and motion study is made active. For case 1, a motor/torque of 1500 rpm is provided to the cam axis. Solid contact between cam and pushrod, pushrod and rocker arm , rocker arm and valve got and material provided was Steel(dry). A linear spring was provided between upper fringe of valve stem and lower stem of valve with spring constant k as 15 N/mm. The spring was pre-tensioned by changing it's natural length to 45 mm. For solving the study more accurately 9000 fps got precise contact. By selecting the whole assembly the material specified was Cast steel .
•                             
•  
•                               Plot for linear displacement of valve 
  
1. 
2. Plot for contact force between Cam and Push Rod -
   
   
   Plot for contact force between Pushrod and Rocker Arm 
3. 
   
4. Plot for contact force between Rocker Arm and Valve-
5. 
   . :Similarly

• For Case 2,
  i.e, cam lift 6 mm, ⇒6=(L2−17.5)+10⇒6=(L2-17.5)+10 , ⇒L2=13.5mm⇒L2=13.5mm
• Assembly for Case 2 -
      All components sketched were imported into assembly and therefore the first component inserted  (cam) was made to float from being fixed. Temproary axes were switched on for better viewings. All components were bought to front plane and made coincident to the present plane. The cam was positioned by also making it's axis coincident to top plane and right plane. Distance mate were provided to other component in order that they are doing not obstruct one another during the motion and therefore the axes of valve and valve stem were made coincident. Top plane of valve rocker and assembly top plane were provided a further angle mate of 0 degree in order that it remains parallel to the present plane.After successfull assembly, it had been made to maneuver using the physical dynamics in order that necessary clearance and get in touch with are provided between components.
• Motion Study and Analysis –
  After successful assembly, Solidworks motion is selected from Solidworks add-ins and motion study is made active. For case 2, a motor/torque of 1500 rpm is provided to the cam axis. Solid contact between cam and pushrod, pushrod and rocker arm, rocker arm and valve were given and material provided was Steel(dry). A linear spring was provided between upper edge of valve stem and lower stem of valve with spring constant k">kk as 15 N/mm. The spring was pretensioned by changing it's natural length to 50 mm. For solving the study more accurately 9000 fps were provided with precise contact. By selecting the complete assembly the material specified was Cast Carbon Steel.
  

               1.Plot for linear displacement of valve 

                  2.Plot for contact force between Cam and Push Rod

                  3.Plot for contact force between Pushrod and Rocker Arm 

                  4..Plot for contact force between Rocker Arm and Valve-

Q . Explain why the contact force between the Rocker arm and valve varies while measuring with respect to the X direction and measuring with respect to magnitude ?

ANS :-The contact force between the valve rocker and valve varies,while measuring with reference to the X direction and measuring with reference to magnitude because The line of action of contact force is changing continuously and along X direction the surfaces of valve and valve rocker rubs over one another and produces friction between them and is both positive and negative as obtained from the plots. This frictional force will tend to losses in energy and can dissipate as heat. Whereas the contact force magnitude calculates resultant of forces altogether three directions and can be greater than forces in either directions.

For Exmaple in case 2, "comparison of contact forces" 

REFERENCES :- 

• "DESIGN OF MACHINERY"  by ROBERT.L.NORTON
• "MACHINES AND MECHANISMS" BY David H. Myszka
• INTRODUCTION of CAMS and Types of CAM
• Valve train mechanism of internal combustion engine