Motion Analysis of a Valve Train

OBJECTIVE

To perform motion analysis of a valve train.

INTRODUCTION

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 injection engine) into the combustion chamber, while the exhaust valves control the flow of spent exhaust gasses out of the combustion chamber once combustion is completed. Thus motion analysis is significant to study the motion and the forces involved during the opening and closing action of the valve.

THEORY

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.

Push Rod: 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.

METHODOLOGY

Modelling: The cam, Push Rod, Rocker Arm and the valves are modelled and assembled using SOLIDWORKS

Cam dimensions:

The diameter of the bigger circle= 35 mm

The diameter of the smaller circle= 20 mm

Cam Lift= Distance between the centres-Radius of the bigger circle+Radius of the smaller circle

Distance between the centres= Cam Lift + Radius of the bigger circle - Radius of the smaller circle

Case 1:

Cam lift= 3.5 mm

Distance between the centres= 3.5+17.5-10= 11 mm

Case 2:

Cam lift= 6 mm

Distance between the centres= 6+17.5-10= 13.5 mm

Material: Carbon Cast Steel

Cam Speed: 1500 rpm

RESULTS

Case 1: Valve Lift= 3.5 mm

Valve Lift v/s Time

Push Rod lift v/s Time

Contact Force Plots:

Cam and Push Rod

Push Rod and Rocker Arm

Rocker Arm and Valve

X component of Contact Force between Rocker arm and Valve

Case 2: Valve Lift= 6 mm

Valve Lift v/s Time

Push Rod lift v/s Time

Contact Force Plots:

Cam and Push Rod

Push Rod and Rocker Arm

Rocker Arm and Valve

X component of Contact Force between Rocker arm and Valve

CONCLUSIONS:

• Cam Lift can be confirmed by the difference in the highest and lowest position of the Push Rod from the Linear displacement graph of Push Rod.
• X component of the contact force differs from the magnitude of Contact force Because the X component is the value of the force in the x-direction whereas magnitude gives the resultant of the x, y and z component of contact force.