MBD Simulation on IC Engine Valve Train using SolidWorks.
Aim:
To perform MBD Simulation on IC Engine Valve Train in.
Objective:
- Model the parts and assemble them in SolidWorks.
- Run the simulation using the below parameters
|
Sl. No |
CAM Lift (mm) |
Speed (RPM) |
Material |
|
1 |
3.5 |
1500 |
Cast Carbon Steel |
|
2 |
6 |
1500 |
Cast Carbon Steel |
Obtain the Plot of the following:
- Valve Lift.
- The contact force between
- Cam and Push Rod
- Pushrod and Rocker Arm
- Rocker Arm and Valve
Explain 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.
Basic Definitions:
Valve Train
A valve train is a mechanical system that controls the 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 combustion chamber once combustion is completed.
Camshaft
A camshaft is a rotating object— usually made of metal— that contains pointed cams, which converts rotational motion to reciprocal motion.
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.
Cam Lobe
The cam lobe is the basis of valve opening and closing rates, amount of lift, and operating RPM ranges. The lobe profile can directly act on the valve, as in some over-head camshaft applications, or be amplified by rocker arms.
Radius of Lower/Base Circle = R1
Radius of Upper Circle = R2
Distance between the two centres = L
Cam Lift formula = (L-R1) + R2
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
A rocker arm is an oscillating lever that conveys radial movement from the cam lobe into linear movement at the poppet valve to open it. One end is raised and lowered by a rotating lobe of the camshaft (either directly or via a tappet (lifter) and pushrod) while the other end acts on the valve stem.
Poppet Valve
A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into an engine.
Poppet valves are typically opened by the camshaft lobe or rocker arm, and closed by a coiled spring called a valve spring.
Valve Spring
Valve springs take on the task of controlling the valvetrain. Spring pressure is the major factor that forces its components to stay in contact with the camshaft lobe. Other forces play a role, but without the proper spring pressure the valvetrain becomes uncontrollable.
“The goal is to ‘control’ the valve and prevent ‘valve float,’ which robs the engine of power and can ultimately cause failures.”
Valve spring pressure must be enough to shut the valve and keep it from bouncing off of the valve seat. When valve bounce occurs, dynamic compression is lowered resulting in a loss of power. Bouncing the valve off the seat also can cause valve breakage or piston to valve contact. The result is catastrophic engine failure.
Valve float will occur when the valvetrain gear loses contact with the camshaft lobe. Lifters, pushrods, rocker arms, and valves may lose contact if the open pressure of the spring is not sufficient. Open pressures are critical because the spring force is opposing the inertia of the valvetrain. If inertia of the opening valve overcomes spring force, the spring has lost the ability to keep the valvetrain following the cam lobe profile.
Weak springs will allow the lifter/push rod to loft over the nose of the camshaft. What goes up must come down. The landing causes damage to the bearings and axle. This will eventually pound the axle out of the lifter body and cause catastrophic failure.”
The opening and closing rates and lift at the valve depends on:
- The Cam Lobe
- Rocker arm ratios
Higher rocker arm ratios will amplify the opening and closing rates and lift at the valve.
- Valve Spring
Model of all the Components:
- Cam
- Push Rod
- Rocker Arm
- Valve
- Valve mount
- Complete Assembly
Calculations:
In order to get desired Cam lift, the centre distance between the upper and lower circle (L) in a cam can be varied.
- Case 1: To obtain cam lift of 3.5mm.
R1 = 12.50mm
R2 = 5mm
Cam Lift = (L-R1) + R2
3.5 = (L-12.5) + 5
L = 11mm
- Case 2: To obtain cam lift of 6mm.
R1 = 12.50mm
R2 = 5mm
Cam Lift = (L-R1) + R2
6 = (L-12.5) + 5
L = 13.50mm
Note:
Simulation is performed without taking into account the valve open and closing time.
Material Applied to all the components, Cast carbon steel.
Motor rotates at 1500RPM, clockwise, hence in 1second cam will rotate 25 times or it will complete 9000degrees.
Use 9000FPS with Precise contact.
Spring Length = 45mm.
Spring Stiffness = 20N/mm.
Establish contacts between components and put material as steel(dry).
Motion Analysis is run for 1second.
Motion Analysis:
|
Case No. |
CAM Lift (mm) |
Speed (RPM) |
Material |
|
1 |
3.5 |
1500 |
Cast Carbon Steel |
|
2 |
6 |
1500 |
Cast Carbon Steel |
Case 1: Cam Lift 3.5mm.
Case 2: Cam lift 6mm
Conclusion:
- The contact force between the valve rocker and valve varies, while measuring with respect to x-coordinate and magnitude because the magnitude represents the resultant of all the acting forces and is greater than the constituent forces in either of the direction or co-ordinate axis.
- As seen from the graph the maximum magnitude of force is transferred by the cam to Push rod.
- The force transferred by the Rocker Arm to Valve is reduced significantly due to the Rocker Lever mechanism or higher rocker ratio.
References: