MBD SIMULATION ON IC ENGINE VALVE TRAIN

MBD SIMULATION ON IC ENGINE VALVE TRAIN

CONTENTS

1. AIM
2. INTRODUCTION
3. CALCULATIONS
4. PART MODELLING
5. ASSEMBLY MODELLING
6. SIMULATION SETUP
7. RESULTS ANS PLOTS
8. ANIMATIONS
9. OBSERVATIONS AND CONCLUSIONS

AIM

1. To DESIGN and ASSEMBLE the various components of an IC Engine Valve Train in SOLIDWORKS.

2. To run the Motion Analysis (MBD) for the following different cases of CAM Lift and,

	CAM Lift (mm)	Speed (rpm)	Material
Case 1	3.5	1500	Cast Carbon Steel
Case 2	6	1500	Cast Carbon Steel

3. Obtain the results for Valve Lift, the Contact Force between: Cam and Push Rod, Pushrod and Rocker Arm and Rocker Arm and Valve.

INTRODUCTION

A valve-train or valve train is a mechanical system that controls operation of the valves in an internal combustion engine, whereby a sequence of components transmits motion throughout the assembly. A conventional reciprocating internal combustion engine uses valves to control the flow of the air/fuel admix into and out of the combustion chamber.

A typical valve-train as shown below consists of valves, rocker arms, push-rod, lifters, and cam. Valve-train opening/closing and duration, as well as the geometry of the valve-train, controls the amount of air and fuel entering the combustion chamber at any given point in time. Valve-trains are built in several configurations, each of which varies slightly in layout but still performs the task of opening and closing the valves at the time necessary for proper operation of the engine.

CAM Lift

The 'CAM Lift' is the resultant net rise of the valve from its seat. The farther the valve rises from its seat the more airflow can be provided, which is generally more beneficial. But, greater lifts also have some limitations. Firstly, lift is limited by the increased proximity of the valve head to the piston crown and secondly, greater effort is required to move the valve springs to a higher state of compression. Increased lift can also be limited by lobe clearance in the cylinder head casting. Higher valve lift can have the same effect as increased duration where valve overlap is less desirable.

The 'CAM Lift' is given by:

`L=C-R2-R1`

;

`C to`CAM Lift.

CALCULATIONS

Since, we now know the CAM Lift formula, let us now calculate the distance between the two centres for the given two cases. This will result in creation of two different geometries for the CAM as shown in the preceding (part modelling) sections.

The diameters taken here are as follows,

The diameter of lower circle, D1 = 25 mm. Therefore, R1 = 12.5 mm
The diameter of upper circle, D2 = 10 mm. Therefore, R2 = 5 mm

Case 1: CAM Lift, C = 3.5 mm		Case 2: CAM Lift, C = 6.0 mm
`L=C-R2-R1`		`L=C-R2-R1`
`L=3.5-5+12.5`		`L=6-5+12.5`
`L=11mm`		`L=13.5mm`

Thus, The centre distance between two circles in Case 1, L1 = 11 mm
And, The centre distance between two circles in Case2, L2 = 13.5 mm

PART MODELLING

1. Cam:

A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion. It is often a part of a rotating wheel or shaft that strikes a lever (Push Rod) at one or more points on its circular path.

Based on the above two calculation for the centre distances, the two geometries for the cam are as shown below.

2. Push Rod:

It is a rod that’s used to connect the camshaft located in the engine block with the rocker arms to open the valves.

3. Rocker Arm:

This is part which is connected to push rod in one end and valve on the other end.

4. Valve:

This allows the air-fuel mixture to enter the cylinder or exhaust the gases out from the cylinder by opening and closing of the inlet/exhaust port.

5. Valve Mount:

This acts as the stoppage for the valve and limits the movement of the valve.

The final preview of all these parts in their respective order is as shown below:

Note that we can use the first CAM in an assembly and den simultaneously replace it with another CAM either by changing its geometry internally or importing externally. Thus, we can avoid creating multiple assemblies for minor changes in the part geometries and the same results.

ASSEMBLY MODELLING

Assembly is made using the assembly mate option by properly aligning them as shown.

Coincident2, Coincident3: The centre axis of the Rocker Arm is made coincident with the Top and the Right planes of the Assembly respectively.

Coincident4, Coincident5: The front plane of the Rocker Arm and the Front plane of the CAM is made coincident with the Front plane of the Assembly respectively.

Coincident6, Coincident7, Coincident8: The centre axis of Push Rod, Valve Mount and the Valve is made coincident with the Front plane of the Assembly respectively.

Distance1, Distance2: The centre axis of the push rod and the CAM is given a distance mate of 20 mm with respect to the Right plane of the assembly as shown.

Distance3: The centre axis of the cam is given a distance mate of 125 mm below the Top plane of the assembly.

Distance4: The centre axis of the Valve Mount is given a distance mate of 50 mm with respect to the Right plane of the Assembly as shown.

Disance5: The top face of the Valve Mount is given a distance mate of 60 mm below with respect to the Top plane of the Assembly.

Coincident9: the centre axis of the Valve and the Valve mount are made coincident with each other.

Finally, physical dynamics is used here for making contact between valve, rocker arm, push rod and cam which locks them in position and the motion analysis is performed.

SIMULATION SETUP

1. Introduction to Motion Simulation / MBD

Suppose an engineer is designing an elliptic trammel meant for tracing different ellipses. When he has defined mates in the CAD assembly, he can animate the model to review how the components of the mechanism move. Although assembly animation can show the relative motion of assembly components, the speed of motion is irrelevant and timing is arbitrary. To find velocities, accelerations, joint reactions, power requirements, etc., the designer needs a more powerful tool. This is where motion simulation comes in.

Motion simulation provides complete, quantitative information about the kinematics—including position, velocity, and acceleration, and the dynamics—including joint reactions, inertial forces, and power requirements, of all the components of a moving mechanism. Often of great additional importance, the results of motion simulation can be obtained virtually at no additional time expense, because everything needed to perform motion simulation has been defined in the CAD assembly model already, and just needs to be transferred to the motion simulation program.

In the case of the elliptic trammel described, the designer needs only to decide the speed of the motor, the points to be traced, and the motion results he wishes to see. The program does everything else automatically, without the user’s intervention. The motion simulation program uses material properties from the CAD parts to define inertial properties of mechanism components, and translates CAD assembly mating conditions into kinematic joints. It then automatically formulates equations that describe the mechanism motion.

Apart from this, motion simulation conducts interference checks in real time, and provides the exact spatial and time positions of all mechanism components as well as the exact interfering volumes.
Product developers can also use motion simulation for mechanism synthesis by converting trajectories of motion into CAD geometry. Designers can also use trajectories of motion to verify the motion of an industrial robot.

Thus, SOLIDWORKS CAD program together with SOLIDWORKS Simulation (FEA) and SOLIDWORKS Motion (i.e., the motion simulation) as add-ins represents the state of the art in integrated simulation tools.

2. Steps to set up the Motion Analysis:

(i) Enable SOLIDWORKS Motion, in the Add-Ins tab of the SOLIDWORKS. If the Add-Ins tab is not visible on the screen, It can be made visible by right-clicking any of the tabs in the SOLIDWORKS. The same goes with hiding and making any of the other tabs visible as per the convenience.

(ii) Set the 'Motor'. Select the Centre Axis of the CAM as the reference and give the input as '1500rpm'. Note that for clarity, the other parts of the assembly are hidden as shown below.

(iii) Change the no. of frames to '9000' frames per sec and use 'Precise Contact'. Note that we have used 9000 fps because for 1500 rpm, 9000 fps will give us result for each degree rotation of the camshaft. Thereby resulting in a very smooth and regular plots. This value can be calculated as follows:

1500 rotations/minute = (1500x360) degrees/ (60) seconds = 9000 degrees/second.

(iii) Add a linear spring between valve and valve mount with a spring constant of 25 N/mm and free length of 45 m.

(iv) Set the 'Solid Body Contact' between the various parts. There are three Contacts as shown below.

(v) Select the entire assembly and assign the material as follows, go to Material-Edit material and select Cast Carbon Steel and click apply. There will be an indication of the material assigned with a change in colour of the entire assembly.

(vi) Click calculate and run the analysis for the required time.

RESULTS AND PLOTS

For Case 1: CAM Lift = 3.5 mm

For Case 2: CAM Lift = 6 mm

ANIMATIONS

For Case 1: CAM Lift = 3.5 mm

For Case 2: CAM Lift = 6 mm

OBSERVATIONS AND CONCLUSIONS

For Case 1: CAM Lift = 3.5 mm

1. The linear displacement of the valve i.e. the valve lift is around (52.3-50.2) = 2.1 mm.

2. The contact force between the cam and the push rod is 1755 N.

3. The contact force between the push rod and the rocker arm is 1411 N.

4. The contact force between the rocker arm and the valve is 442 N.

For Case 2: CAM Lift = 6 mm

1. The linear displacement of the valve i.e. the valve lift is around (54-50) = 4 mm.

2. The contact force between the cam and the push rod is 2834 N.

3. The contact force between the push rod and the rocker arm is 1885 N.

4. The contact force between the rocker arm and the valve is 587 N.

Comparison and some other conclusions:

1. The valve lift and the contact forces increase as the CAM Lift increases.

2. The spring free length and the spring constant must be carefully selected such that their is a continuous contact maintained between the rocker arm and the valve and also the other components of the valve train assembly.

3. The contact force between the rocker arm and the valve varies while measuring along the x-direction and measuring with respect to the magnitude because the contact force between the rocker arm and the valve along x-direction is frictional force which is caused by sliding of the rocker arm on the valve but the contact force between the rocker arm and the valve with respect to the magnitude considers the magnitude of both the friction force occurring in x-direction and the force in the y-direction to move the valve.