MBD simulation on IC engine valve train

AIM

MBD Simulation on IC Engine Valve Train

OBJECTIVES

• To create 3D models of all the parts(i.e.Cam, Push-Rod, Rocker Arm, Valve stem and Valve) and assemble them in SolidWorks.
• To carry out the motion analysis in two different conditions-

1. Cam Lift- 3.5 mm, Speed 1500 RPM
2. Cam Lift- 6 mm, Speed 1500 RPM

• To plot the following Plots-

1. Valve Lift
2. Contact Force between Cam and Push-Rod
3. Contact Force between Push-Rod and Rocker Arm
4. Contact Force between Rocker Arm and Valve(Magnitude)
5. Contact Force between Rocker Arm and Valve(X direction)

INTRODUCTION

A valve train mechanism of an Internal Combustion engine controls the operation of the intake and exhaust valves and hence controlling the inflow of air-fuel mixture(or air) and outflow of exhaust with respect to the cylinder. It comprises of:

• A valve for opening or closing the intake port and the exhaust port formed in the cylinder head of the engine
• A cam mounted to the camshaft so as to be rotated together
• A Push-Rod to transmit the motion from the cam
• A rocker arm supported on a rocker shaft to be swingable
• The valve stem and Valve spring to guide and retain the valve in its required position

wherein: the valve is opened or closed by swinging the rocker arm by rotation of the cam.The rocker arm includes a bearing portion supported on the rocker shaft, the bearing portion is formed with an oil injection hole for injecting lubricating oil from the oil outlet to a sliding contact portion side between the rocker arm and the cam.

The timing of the opening and closing is controlled by the arrangement of cams mounted on the camshaft, the motion of which is synchronised with the crankshaft by the help of gear, chain or belt drive.

This simulation will result in understanding the kinematics of motion of valve train components, the force between components and the required motion transmission starting from the cam to the Valve.

THEORY

The valve-train 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. The components are described below in order from the Cam to the Valves.

Cam

The part which provides the lift for the Push-Rod and actuates the valve at required intervals. It is mounted on a shaft called the Camshaft.Cams are precisely machined into shapes which determine when the valve opens in relation to piston position, how far the valve is displaced, and the length of time the valve remains open. The distance between the end point of the base radius and the nose controls valve displacement. The geometry of the sides (flank) and nose determine how long the valve remains open. The Cam profile is responsible for the displacement pattern of the valve.

Some of the terms associated with the Cam are-

Cam Follower- The cam follower is seated in between the Push-Rod and cam to smoothen the transmission of motion.

Cam Lift- Camshaft lift is the distance the cam lobe rises away from the base circle. The lift determines how far the valve will be displaced. Increasing the lift increases valve displacement.

It is given by the formula-

Cam Lift= (L-R1)+R2 ,where

L-Distance between the centres

R1-Radius of the Larger circle

R2-Radius of the Smaller circle

Using the Cam profiles for lift 3.5 mm and 6mm are designed as given below-

Push-Rod

Push-Rods 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 Push-Rod is fitted with a lifter, upon which the camshaft makes contact. The cam moves the lifter upwards, which moves the Push-Rod. The top end of the lifter pushes on the rocker arm, which opens the valve.

Three types of lifters are used: hydraulic valve lifter, mechanical lifter, and roller lifter. Some pushrods are hollow, providing a means to feed oil from the lifters to the rocker arms. This reduces wear on the pushrod tip and rocker arm.

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 Push-Rod and pivot on a shaft or individual ball studs in order to actuate the valves.

Valve and Valve Stem

Valves- Each cylinder has at least one intake valve and one exhaust valve. 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.

Valve Springs- Valve springs provide a resisting force that returns displaced valves to their closed position.

The valve moves inside the Valve stem which guides its motion in and out of the cylinder.A spring retainer is designed to hold in place the valve stem tip. This allows the rocker arm to act directly on the valve.

MOTION STUDY SETUP

• The parts(i.e.Cam, Push-Rod, Rocker Arm, Valve stem and Valve) were modelled in SOLIDWORKS with required dimensions.
• All the parts are assembled using the following Mates-

1. ‘Coincident’ mates for Axis of Rocker Arm with Top Plane and Right Plane.
2. ‘Coincident’ mates of Front Plane of Assembly with Front Planes of Rocker Arm and Cam respectively.
3. ‘Coincident’ mates of Front Plane of Assembly with axes of Push-Rod, Valve and Valve Stem respectively.
4. ‘Coincident’ mate between axes if Valve and Valve Stem.
5. ‘Distance’ mate between the following pairs:

• Axis of Push-Rod-Right Plane of Assembly(20 mm)
• Axis of Cam-Right Plane of Assembly(20 mm)
• Axis of Valve-Right Plane of Assembly(50 mm)
• The top face of Valve Stem-Top Plane of Assembly(60 mm)
• Axis of Cam-Top Plane of Assembly(125 mm)

• Then the ‘SOLIDWORKS Motion’ option is enabled and Motion Analysis is started.
• The Cam is provided rotation by means of a Rotary Motor with constant speed of 1500 RPM.
• Then Solid Body Contact was established between the pairs of parts by using ‘Contact’ option: Cam-Push-Rod, Push-Rod-Rocker Arm, Rocker Arm-Valve
• Then a Linear Spring was set up between the Valve and Valve stem with stiffness 10 N/mm and Free Length 45 mm(for pre tension).
• In the ‘Motion Study Properties’ the Frames per second was selected to 9000( for 1500 RPM=1500*6=9000 deg/sec )
• The whole assembly was selected and Material was changed to Cast Carbon Steel.
• The time key was set to 0.2 sec.
• The ‘Calculate’ option was used and Motion Analysis was done.
• The required plots are plotted.

RESULTS

Case1: 3.5 mm Cam Lift and Speed 1500 RPM

Plot for Valve Lift

Plot for Contact Force between Cam and Push-Rod

Plot for Contact Force between Push-Rod and Rocker Arm

Plot for Contact Force between Rocker Arm and Valve(Magnitude)

Plot for Contact Force between Rocker Arm and Valve(X direction)

Case2: 6 mm Cam Lift and Speed 1500 RPM

Plot for Valve Lift

Plot for Contact Force between Cam and Push-Rod

Plot for Contact Force between Push-Rod and Rocker Arm

Plot for Contact Force between Rocker Arm and Valve(Magnitude)

Plot for Contact Force between Rocker Arm and Valve(X direction)

OBSERVATIONS & EXPLANATION

• As the Valve lift is directly proportional to Cam Lift, Valve lift for 6 mm Cam Lift is more than that for 3.5 mm
• As Cam Lift increases, the Contact Forces between the Parts also increases as a reaction force is being applied by the Spring provided.
• For a given Cam Lift the Contact Forces follow the order- Contact Force between Push-Rod and Rocker Arm > Contact Force between Cam and Push-Rod > Contact Force between Rocker Arm and Valve
• As the Rocker Arm pushes the valve downwards, it travels along a arc around the pivot point, so the contact Force between Rocker Arm and Valve, when measured with respect to X-direction, is positive for a short while and then becomes negative.The force along X-direction is small as the contact force is mostly used to push the Valve in Y-direction.
• But when the Contact Force between Rocker Arm and Valve is measured with respect to magnitude, it is the resultant of the forces in X and Y-directions i.e. (Force in Z direction is nearly zero as the rotation is in the X-Y plane)

CONCLUSION

So, the study and simulation of the IC Engine Valve Train was successfully completed.