MBD Simulation on IC Engine Valve Train

Motion analysis is important because product design frequently requires an understanding of how multiple moving parts interact with each other and their environment. From automobiles and aircraft to washing machines and assembly lines - moving parts generate loads that are often difficult to predict. Complex mechanical assemblies present design challenges that require a dynamic system-level analysis to be met. Accurate modeling can require representations of various types of components, like electronic controls systems and compliant parts and connections, as well as complicated physical phenomena like vibration, friction and noise. Motion analysis enables one to meet these challenges by quickly evaluating and improving designs for important characteristics like performance, safety and comfort.

A valvetrain 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 direct-injected engines) into the combustion chamber, while the exhaust valves control the flow of spent exhaust gasses out of the combusion chamber once combustion is completed.

Components

The valvetrain consists of all the components responsible for transferring the rotational movement of the camshaft into the opening and closing of the intake and exhaust valves. Typical components are listed below in order from the crankshaft to the valves.

Camshaft

The timing and lift profile of the valve opening events are controlled by the camshaft, through use of a carefully shaped lobe on a rotating shaft. The camshaft is driven by the crankshaft and— in the case of a four-stroke engine— rotates at half the speed of the crankshaft.

Motion is transferred from the crankshaft to the camshaft most commonly by a rubber timing belt, a metallic timing chain or a set of gears.

Pushrod

Pushrods are long, slender metal rods that are used in overhead valve engines to transfer motion from the camshaft (located in the engine block) to the valves (located in the cylinder head). The bottom end of a pushrod is fitted with a lifter, upon which the camshaft makes contact. The camshaft lobe moves the lifter upwards, which moves the pushrod. The top end of the lifter pushes on the rocker arm, which opens the valve.

Rocker arm

Depending on the design used, the valves are actuated by a rocker arm, finger or bucket tappet. Overhead camshaft engines use fingers or bucket tappets, upon which the cam lobes contact. Overhead valve engines use rocker arms, which are actuated by a pushrod and pivot on a shaft or individual ball studs in order to actuate the valves.

Valves

Most modern engines use poppet valves type, although sleeve valves, slide valves and rotary valves have also been used at times. Poppet valves are typically opened by the camshaft lobe or rocker arm, and closed by a coiled spring called a valve spring.

Creating a 3D Model

• 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 at one or more points on its circular path.

Pushrod: This is the part to which the lift is provided, which is transmitted to the rocker arm.

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

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.

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

Motion analysis:

To run the motion analysis first go to motion study and establish contact between Cam and push rod, pushrod and rocker arm, rocker arm and valve. Next, give the rotation to the cam with 1500 RPM. Set up a linear spring between the valve and the valve stem with the spring stiffness of 10 N/mm and free length 45mm. Change the frames to 7200 frames per sec and use precise contact. 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 color of the entire assembly. Now click the calculation button. Run the analysis for the required time.

There are 2 cases

The cam lift formulae are given as follows.

CAM 1 :- To lift 3.5mm

CAM LIFT = (L-R1)+R2
3.5=(L-12.5)+5
-1.5=L-12.5
-1.5+12.5=L
L=11

L1=11mm

CAM 2 :- To lift 6mm

CAM LIFT = (L-R1)+R2
6=(L-12.5)+5
1=L-12.5
1+12.5=L
L=13.5

L1=13.5mm

Valve Train Assembly

PLOTS

Case 1 :- 3.5mm Lift Simulation Results

Valve Displacement

Contact Force Plots

Contact Force b/w CAM & Push Rod

Contact Force b/w Push Rod & Rocker Arm

Contact Force(Magnitude) b/w Rocker Arm & Valve

Contact Force(X-direction) b/w Rocker Arm & Valve

Case 2 :- 6mm Lift Simulation Results

Valve Displacement

Contact Force Plots

Contact Force b/w CAM & Push Rod

Contact Force b/w Push Rod & Rocker Arm

Contact Force(Magnitude) b/w Rocker Arm & Valve

Contact Force(X-direction) b/w Rocker Arm & Valve

Observations :-

• With Increased CAM lift, Valve Displacement can be increased.
• Valve lift mainly depends on the Design of CAM and Rocker Arm.
• Selection of Suitable spring (free length & Spring constant) plays a vital role in Valve lift Mechanism.
• Vibrations can be observed during retracing Valve back to to its original Position due to Spring with lower spring constant selected.
• 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 of the fact that an overall net force acting on the systems point of contact is calculated in the Magnitude whereas only the Force/reaction force/ Frictional force/Drag Force between Rocker Arm surface and Valve Surface during sliding is calculated in X direction Evaluation.

Results

1. Explanation: The Magnitude represents net contact force between the parts, while the contact force along the X direction is the frictional force between the bodies that arise due to the rocker arm sliding over the surface of the valve head.
2. The change in direction of the force is because the rocker arm first slides towards origin giving a positive friction force (opposing the motion) and then slides away from the origin giving rise to friction in the negative x-direction. Therefore the plots are opposite to each other.
3. The explanation is same for the cam lift of 6 mm, only it differs in magnitude of contact forces. The movement in this case is quite smoother compared to the cam lift of 3.5 mm as observed. As the cam lift increases more air-fuel mixture can enter into the cylinder of IC engine.