MBD Simulation on IC Engine Valve Train

AIM: MOTION ANALYSIS OF IC ENGINE VALVE TRAIN MECHANISM

THEORY:

Valve Train is a mechanical system in an IC engine that is responsible for controlling the intake and the exhaust of the combustion process. During the intake process, the intake valve opens to let air enter the combustion chamber and during the exhaust process, the exhaust valve opens to let the gas mixture move out of the chamber to complete the combustion process. This mechanism of intake and exhaust is controlled by Valve Train System that is mounted above the cylinder block in an IC engine.

Valve Train Mechanism consists of Cam, pushrod, rocker arm, Valve, spring, valve mount. The cam rotates which makes the pushrod to perform the reciprocatory motion, The rocker arm also rotates which pushes the valve against the loaded spring to open the port. The spring pushes the valve back to close the port.

A timing belt or timing chain links the crankshaft to the camshaft so that the valves are in sync with the pistons. The camshaft is geared to turn at one-half the rate of the crankshaft. Many high-performance engines have four valves per cylinder (two for intake, two for exhaust), and this arrangement requires two camshafts per bank of cylinders, hence the phrase "dual overhead cams."

OBJECTIVE:

• To perform multi-body dynamics analysis of valve train mechanism and obtain the valve lift and the contact force between components for the following two cases:

CALCULATION:

For the following cam design:

We have, R1 (lower circle)=12.5 mm and R2 (upper circle)= 5 mm

From the cam lift formula, CAM Lift = (L-R1) + R2

For Cam Lift = 3.5:                 3.5 = L - 12.5 + 5           , L=11 mm

For Cam Lift = 6:                     6 = L - 12.5 + 5             , L=13.5 mm

Now we have all the geometry parameters needed to model the cam geometry.

3-D MODELLING AND ASSEMBLY:

All the assembly parts namely cam, pushrod, rocker arm, valve and valve stem were designed according to the given dimensions. Two different cam designs were created according to the design parameters. Then, all parts were assembled to create the assembly of the valve train mechanism.

                                        Cam 1 (lift=3.5)                                 Cam 2(lift=6)

                                    Rocker Arm                                           valve mount

                                       Valve                                                Pushrod

ASSEMBLY:

                                    Valve Train 1                                            Valve Train 2

CAD MODEL FILES: https://drive.google.com/drive/folders/1PTkyVXkRQIhKWdsD1IKASn5f82q-gJDl?usp=sharing

MOTION ANALYSIS:

Two different assemblies of valve train created for two different cam designs. Proper mates were given in order to run motion analysis of the mechanism. All the parts were given material properties of Cast Carbon steel.

Case 1: Cam lift = 3.5, Rpm = 1500, material = Cast carbon steel

                                                                     Valve Lift

Case 2: Cam lift = 6, Rpm = 1500, material = Cast carbon steel

                                                                      Valve Lift

EXPLANATION OF RESULTS:

From the valve displacement plots of both cases, we can observe that we get greater valve displacement for the second case in which cam lift is 6. This provides us with the capability of adjusting how much the valve opens by changing the cam design. 

We can observe the distribution of load which develops on different contacts during motion of the mechanism. The peeks in load time plot occurs when the cam pushes pushrod upwards and through rocker arm, the valve moves downward i.e. when pushrod slides on the varying profile of the cam. The load remains constant when pushrod slides over the circular profile of the cam and valve is at its initial position. For cam lift=3.5, the max load of 850 Newton occurs between the cam and the pushrod at max. lift of the valve.  For cam lift=6, the max load of 3150 Newton occurs between the Rocker arm and the pushrod at max. displacement of the valve. 

The magnitude of the contact force is more in the second case when the cam lift is 6. This is because the cam pushes pushrod more due to increase in cam lift. Also, spring is more compressed in this case, therefore, the contact force is large.

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 if we look the x-component force, it achieves negative force because the rocker arm slides both forwards and backward during one complete rotation of the CAM and hence the friction force acts in both directions. The magnitude of the contact force between the rocker arm and the valve pushes the valve downwards, but its X component gives us only the friction force of the rocker arm sliding over the face of the valve head.

CONCLUSION:

Motion analysis of Valve Train System was performed which tells us that the proper design of the valve train is very important for optimum engine performance. There are mainly two factors: valve lift and Timing. These parameters should be controlled very efficiently for better engine output. From the results, we can determine the loads which develop on different parts during operation and optimize the valve train design.