MBD Simulation on IC Engine Valve Train

Objective :

Simulate the IC engine valve train for

Sl.No	CAM Lift (mm)	Speed (RPM)	Material
1	3.5	1500	Cast Carbon Steel
2	6	1500	Cast Carbon Steel

and obtain the following values

• Valve Lift.
• The contact force between
  • Cam and Push Rod
  • Pushrod and Rocker Arm
  • Rocker Arm and Valve

Also, 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.

About IC engine valve train :

Engine valves are used in engines to allow air or air-fuel mixture to enter into the cylinder (through inlet valve) and also to push exhaust gases (through outlet valve) from the cylinder at a specific time during the engine cycle. To have complete contact between the valve and engine surface (head), an accurately faced surface is provided which is known as a valve seat. For inlet valves. the seat surface is directly machined on the engine head and is also known as integral seats. However, special seat inserts are used for exhaust valves as these are continuously exposed to high temperatures and thermal stress.

The stem of the valve needs to move up and down without any deviation from its axis. Since these stems are subjected to side thrust also, the valve guides are provided to maintain its alignment. Sometimes, these guides are the direct holes in the engine block. However, separate valve guides have also been used as inserts which are cylindrical in shape. The clearance between the valve stem and guide is kept optimum/sufficient to maintain lubrication film around the stem surface.

Valve spring (helical) is used to keep the valve in regular contact with the tappet and the tappet with the cam. These valve springs should be fatigue resistant and are made up of high-grade spring steel wire, hard-drawn steel or chrome-vanadium steel.

valve actuation

A complete mechanism is involved in actuating the inlet and exhaust valves (opening & closing) through the motion of different components that come in the valve train. A cam is used as a drive to actuate the valve and this cam rotates at half of the speed of crankshaft in a four-stroke engine. The following are the various components of the valve train.

i) Camshaft

ii) Tappet

iii) Pushrod

iv) Rocker arm

v) Valves

Camshaft: Camshaft is a shaft in which cams are provided as integral parts of the shaft. Separate cam is provided for each valve (inlet and exhaust). Cam has it path profile having a high spot known as the nose and the lowest point termed as a base. These two points nose and base corresponds to fully opened and closing of valves respectively. The cam profile is provided to have a smooth rise (opening) and fall (closing) of valves. The cam is a rotating element that generates the reciprocating motion to the follower which is known as push rod in a plane at a right angle to the cam axis. The camshaft is also used to drive the fuel pumps. The camshaft is made up of forged alloy steel which is case hardened.

Tappet: This is also known as cam follower or valve lifter which follows the shape of the cam to provide reciprocating motion to the valves by conversing angular/rotating motion of the camshaft. The tappet is placed in between the cam and the push rod which helps in preventing the side thrust to be exerted from the cam on the pushrod. Some clearance is provided between the cam and tappet to take care of thermal expansion and this clearance is known as valve lash.

Pushrod: The reciprocating motion of the pushrod actuates the rocker arm which moves in an arc about its pivot. Pushrods are made of carbon-manganese steel.

Rocker arm: The rocker arm is used to convert the upward movement of the pushrod to the downward movement of the valve (opening up) and also the downward movement of the pushrod to upward movement of the valve (closing down). The rocker arm can be made of either hollow or solid material. The rocker arms are pivoted to rocker shaft which also provides lubrication

Valves: Although various types of valves are available poppet valves are the most commonly used engine's valves being used in engines these days. These poppet valves are also known as mushroom valves as their shape resembles the mushroom having head and stem. Generally, inlet valves are larger in size to enhance the air intake and exhaust valves are made relatively harder than the inlet valves as these are exposed to hot exhaust gases always. Exhaust valves are made up of special material i.e austenitic steels and precipitation hardening steel which should have a high strength to resist tensile loads, wear resistance due to heat, corrosion resistance, low coefficient of thermal expansion and high thermal conductivity. Since the valve temperature reaches several hundred (750⁰C), valve cooling is also essential for which valve stems are sometimes filled with sodium (which has high thermal conductivity).

Calculation :

We have taken

R1 = 12.5

R2 = 5

and we know

therefore using this formula

L = cam lift - R2 +R1

• for 3.5 mm lift L = 10mm
• for 6 mm lift L = 13.5mm

Modeling:

Here the 3D model is generated using extrude or revolve command for there respective sketch as shown

• For cam

• Push rod

• Rocker arm

• Valve mount

• valve

Assembly :

• import of cad files
• make a float to the first cad file
• make centerline of rocker coincident with top and the right plane of the assembly
• make take all components in the front plane by coinciding all front planes of all component to assembly front plane
• make a distance of pushrod, cam and rocker arm from the right plane of assembly 20mm 20mm 60mm respectively
• make a distance of 125mm from the top plane to the center of cam
• and make coincident for centerline if valve and valve mount
• use physical collision in move option to set the assembly in contact
• select all and give the material

Motion analysis :

• provide the rotation to the cam at an input speed of 1500 rpm
• create contact between all components
• insert gravity
• insert spring with the values of the stiffness of 15 N/mm and free length of 47 mm for both the cases so the spring will have a pretension
• use precise contact and fps to 10000 for more precise results
• plot the graphs for lift and contact force for the respective contact region

Animation :

Case 1 : https://youtu.be/DdWL856IK7M

Case 2 : https://youtu.be/EWh9473ZKEI

Results :

• For case 1

• For case 2

Conclusion :

By using the same spring for two cases the change in values of contact force increases more than 2 times for the higher lift.

The contact force between the Rocker arm and valve varies while measuring with respect to the X-direction and measuring with respect to magnitude because during turning the distance between the center of rocker to the line of contact varies and have the friction force so with respect to x-direction it also shows the negative values but in magnitude case, it shows the overall force magnitude to push the valve.

Reference : http://ecoursesonline.iasri.res.in/mod/page/view.php?id=672