Valve Train: A means to open and close valves in engines

INTRODUCTION

In my earlier projects

https://skill-lync.com/projects/flowbench-simulation-for-valve-flow,

https://skill-lync.com/projects/wrist-pin-offset-piston-slap-and-internal-friction

we had looked on how the flow is affected by the valve lift and how piston moves in IC engine. However, to bring about this motion, a mechanical component called cam is used. This is more like the timing device which controls when the valve should open, how long should it stay open, when should it close and what is the rate of opening and closing of valves.

The cam design is used to address all these questions. Cam mechanism works by direct contact. Cam operates on the cam shaft. Camshaft is designed to operate at half the crankshaft speed in 4 stroke engines. The movement of cam is translated into linear movement by the pushrod. The pushrod is a simple cylindrical component which comes in direct contact with the cam. The pushrod in turn contacts rocker arm, which works on lever arm principle. The other end of rocker arm contacts the valve in discussion. The valve guide are used to ensure that valve moves in vertical axis alone and other directions are constrained. Spring is used to hold the valve in position.

Cam, as we have noticed is the most important object which is at the heart of the mechanism. The smallest circle which is tangential to cam contour drawn from center of the rotation is called as the base circle. Pitch curve is the locus of trace points where the follower would move if cam was stationary. Cam lift is the distance from the base circle to prime circle where the pressure angle is maximum.

Changing shape of the contour would have effect on rate of rise and fall of valve, period of dwell, changing the cam lift would have consequences on the valve lift profile and maximum lift the configuration can produce.

GEOMETRY

Cam, Pushrod, Rocker arm, valve, Valve guide is modelled as follows and are asssembled. The study is carried for two cam configuration.

3.5mm Cam lift 6mm cam lift

The components are assembled as shown

MOTION STUDY

Motor is applied to Cam axis and constant speed of 1200RPM is applied. Contacts are established between cam and pushrod, pushrod and rockerarm, rockerarm and valve, valve and valve stem

Cast Carbon steel is assumed to be the material

The linear displacement of the valve is recorded and contact forces are plotted.

RESULTS

The Valve lift for the both the cams are plotted, Higher the cam lift, higher the valve lift and its increase is not linear.

For an increase in the cam lift by 2.5mm, max valve lift is less than 2mm.

Contact forces are not constant, With change in the angular movements, spikes of contact forces are seen during the rise of the valve and fall of the valve. In between, forces stay constant. However, if dwell is introduced, spikes are seen before entering dwell and then exiting dwell. Hence, this is predominantly due to change in the profile being non linear.

DISCUSSION ON RESULTS

1) Maximum valve lift increases with increase in cam lift

2) Cam profile plays a major role in controlling the mechanism rate and hence forms the heart of mechanism

3) Contact forces variesn and spikes are noticed because of non linearties in the profile and angular movement changes.

4) Contact force between valve and rocker arm is least, owing to lever effect induced by the rocker arm

5) The mechanism can be visualised in the link: https://www.youtube.com/watch?v=GIP2rCtv-XY