MBD Simulation on IC Engine Valve Train
AIM: To model an IC Engine Valve Train and run a simulation to obtain:-
Obtain the
- Valve Lift.
- The contact force between
- Cam and Push Rod
- Pushrod and Rocker Arm
- Rocker Arm and Valve
| Sl.No | CAM Lift (mm) | Speed (RPM) | Material |
| 1 | 3.5 | 1500 | Cast Carbon Steel |
| 2 | 6 | 1500 |
Cast Carbon Steel |
To determine 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.
CAM Lift Formula
CAM Lift can be derived from the sketch
● The diameter of lower circle (D1)- 35 mm
● The diameter of upper circle (D2)- 20 mm
● The distance between two centres (L)- 30 mm
CAM Lift = (L-R1)+R2
INTRODUCTION:
A valvetrain or 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 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.
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 / bucket tappet
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, side valves and rotaryvalves 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.
valve float occurs when the valve spring is unable to control the inertia of the valvetrain at high engine speeds (RPM).
MOTION ANALYSIS:
In Solidworks Simulation set motion analysis and set the motor speed to 1500 rpm .
Add spring of stiffness 10 N/mm for case 1 and 15 N/mm for case 2.
Define the contact between the different parts and set the material as Cast Carbon Steel.
Set the number of frames per second to be 9000 so to get a better simulation by capturing each angle.
Run the simulation
Case 1: Cam lift at 3.5mm
R1 = 12.5mm (dia - 25mm)
R2 = 5mm (dia - 10)
CAM LIFT = (L - R1) + R2
3.5 = (L - 12.5) + 5
3.5-5 = L-12.5
-1.5+12.5 = L
L = 11Liner Displacement of Valve lift at 3.5mm
The contact force between
- Cam and Push Rod
The contact force between
- Pushrod and Rocker Arm
The contact force between
- Rocker Arm and Valve
Explanation: The contact force between Rocker Arm and Valve, 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.
Case 2: Cam lift at 6mm
R1 = 12.5mm (dia - 25mm)
R2 = 5mm (dia - 10)
CAM LIFT = (L - R1) + R2
6 = (L - 12.5) + 5
6-5 = L-12.5
1+12.5 = L
L = 13.5Liner Displacement of Valve lift at 6mm
The contact force between
- Cam and Push Rod
The contact force between
- Pushrod and Rocker Arm
The contact force between
- Rocker Arm and Valve
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.
Result:
- With increasing the CAM lift, Linear displacement of valve also increased.
- Valve lift mainly depend on the design of CAM.
- Contact force b/w the rocker arm and valve with respect to the X direction force gives the contact force only in X direction.
- But in x direction force has both negative and positive when the valve is moving upwords and downwords.
- Contact force b/w the rocker arm and valve with respect to magnitude because the magnitude gives net contact force in all direction. hence the valve is much higher then x direction.
Conclusion
1) Valve lift for cam lift of 3.5 mm is 0.9 mm
2) Valve lift for cam lift of 6 mm is 2.1 mm
3) Valve lift is less than that of cam lift and it depends upon the design of mechanism.
4) Contact force is not constant throughout due to absence of damper in system, which causes some unwanted vibrations in system.
5) Contact force between the Rocker arm and valve varies while measuring with respect to the X direction and measuring with respect to magnitude due to varing contact point and line of action of contact force between rocker arm and valve. Due to this reason a reaction force perpendicular to valve motion develops which causes losses in system and produces sound.
6) Free length of spring should be carefully selected to maintain a precmpression in spring. This precompresion should be sufficient to keep continuous contact between cam and push rod. (refer animation 3)