MBD Simulation on IC Engine Valve Train
A valvetrain or valve train is a mechanical system that controls the 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 valvetrain consists of valves, rocker arms, pushrods, lifters, and cam. Valvetrain opening/closing and duration, as well as the geometry of the valvetrain, controls the amount of air and fuel entering the combustion chamber at any given point in time. Valvetrains 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.
A typical Valvetrain is shown below.
The 3D model of the parts of the valvetrain is shown below.
Cam: A part which provides the lift for the pushrod which actuates the valve at regular time intervals.
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 vlave 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 stem: This acts as the stoppage for the valve and limits the movement of the valve.
Final assembly: Assembly is made using the assembly mate option by properly aligning them as shown. Physical dynamics is used here for making contact between valve, rocker arm, pushrod and cam which locks them in position and motion analysis is performed.
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.
Using this we can find the cam lift for the 2 cases.
Case 1: Cam lift = 3.5mm
R1= 12.5mm
R2= 10mm
therefore, L=3.5+R1-R2
L=3.5+12.5-10 =6mm
Case 2: Cam lift = 6mm
R1= 12.5mm
R2= 10mm
therefore, L=6+R1-R2
L=6+12.5-10 =8.5mm
Contact force graphs for case 1, between
Cam and Pushrod
This shows the contact force between the Cam and the Pushrod the maximum force is around 1460 N, this is when the pushrod comes down after the cam lift. The contact force during the cam lift is around 200 N(between 0 & 366 N). The other forces are due to friction while the pushrod slides over the cam.
Pushrod and Rocker arm
This is almost same as that of the graph between the cam and push rod but only the magnitude of the contact force varies.
Rocker arm and Valve (Magnitude)
Rocker arm and Valve (X direction)
Explanation: The Magnitude represents net contact force between the parts, while the contact force along X direction is the frictional force between the bodies that arise due to the rocker arm sliding over the surface of the valve head. 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.
Contact force graphs for case 2, between
Cam and Pushrod
Push rod and Rocker arm
Rocker arm and Valve (Magnitude)
Rocker arm and Valve (X direction)
The explanation is the same for the cam lift of 6mm, only it differs in the magnitude of contact forces. The movement, in this case, is quite smoother compared to the cam lift of 3.5mm as observed. As the cam lift increases more air-fuel mixture can enter into the cylinder of the IC engine.