MBD Simulation on IC Engine Valve Train

OBJECTIVE:

To model an IC Engine Valve Train assembly and carry out the MBD simulation. The speed of rotation of the cam is 1500rpm and the cam lift is set as 3.5mm and 6mm for two different cases. The material for the whole assembly is to be assigned as Cast Carbon Steel.

To find:

1. Valve lift
2. Contact forces between:
   1. Cam and Push rod
   2. Push rod and rocker arm
   3. Rocker arm and valve
3. 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

SOLUTION:

An IC Engine valve train is a mechanical system that controls operation of the intake and exhaust valves in an internal combustion engine. The intake valves control the flow of air/fuel mixture (or air alone for CI engines) into the combustion chamber, while the exhaust valves control the flow of exhaust gases out of the combustion chamber once combustion is completed. The IC Engine Valve Train assembly has 5 different parts.

1. Cam profile

The cam is the rotating part in the assembly and it helps in the transfer of intermittent end motion. The same is achieved due to the profile of the cam body, which differs for different applications. The model is shown below:

2. Push rod

The push rod is in contact with the cam and undergoes an intermittent translation motion, depending on the cam profile. The part is shown below:

3. Rocker Arm

The rocker arm undergoes a swivelling motion and it is pivoted along the axis of the hole. The motion is such that, as one end move upwards, the other moves down, transmitting the motion to the body in contact.

4. Valve and Valve stem

The valve is the part which allows the flow of fluid into the engine and it is contact with the rocker arm. As the rocker arm pushes the valve, it is opened and the flow of fluid takes place. It is assembled along with the valve stem and a spring action is provided between these parts, so that the valve returns to the original position when the pushing action of the rocker arm is not present. The parts are shown below:

5. Assembly

The material for all the parts was assigned as cast carbon steel and solid contacts were defined between the adjacent bodies. A spring action was provided between the valve and valve stem, with stiffness value of 10N/mm and a free length of 45mm. The cam was given the rotary motion along its axis, with a speed of 1500rpm. The fps was set to 7200 and the precise contacts was enabled.

The cam lift can be calculated using the formula,

Cam lift = L – R1 + R2,

Where,

L – distance between the centres of the circular profiles in the cam, and

R1 & R2 – radius of the circular profiles.

Case1: Cam lift = 3.5mm

3.5 = L – 12.5 + 5

Hence, L = 11mm

Case2: cam lift = 6mm

6 = L – 12.5 + 5

Hence, L = 13.5mm

Results obtained:

1. Valve Lift

2. Contact force between cam and push rod

3. Contact force between push rod and rocker arm

4. Contact force between rocker arm and valve

5. Contact force between rocker arm and valve (X component)

It can be seen that there is a variation in the contact force values between the magnitude and the X component value. This is because, the magnitude represent the total force or resultant force at the contact, while X component force represents the contact force solely in the X direction. It can also be observed that the magnitude of the valve lift and the contact forces are higher for a cam lift of 6mm. When the cam lift increases, there is an obvious increase in displacement at each of the contact points and this is reflected in the form of higher values.