Valve angles part 1, why the backcut on the valve makes sense.

Okey. I write this piece as much to get a handle on the subject as anything else. By trying to explain something so someone you tend to gain some insight, and boy do I need some here. When you start reading up about cylinder head flow at first you hear about skimming heads, bigger compression values,bigger valves,bigger cams, bigger pistons, bigger ports. BIG is beautifull and is more is even better so too much must be just about right. Then you hear about port velocity, big is good but not too big, no small is better or is it, well it might be if velocities are managed, and then there is pressure recovery, discharge coefficients etc etc . Confused ? Good, join the club and welcome to my world.

Before going overboard on fancy parts I revert to the tried and tested method of going back to square one and starting off with the basics.

First things first,  the valve seat.  It is the most sensitive part of the entire tract with regards to geometry, which is not strange as it presents the major restriction during a very large part of the entire lift cycle.  Simply put ,you have to  squeeze and unsqueeze the air through this small and variable gap in a manner that causes the least amount of energy loss. The squeeze and unsqueeze part will be revisited at a later point in time when I figured out how the formulas work myself 🙂

Reading Professor Blairs book something stood out : the oft mentioned curtain area (valve circumference X valve lift) as a measure for the available cross sectional area is incorrect. The area available is a truncated cone (or fustrum) not a cylinder

note that the curtain area as denoted by (2pi.R)xL is not the same as the x value and the corresponding fustrum shaped area (Copyright  G.P.Blair, The design and simulation of four stroke engines, SAE R-186 1999 ISBN: 978-0-7680-0440-3)

If you look at the above figure it makes quite a bit of sense that the backcut on the valve does what it does.

Note that by doing the back cut the available area has increased a bit.  ( white double arrow between X and L). As well as change the shape of the aperture ( adapted from Blair r-186)

The backcut quite effectively increases the cross sectional area especially at low and mid lift, The arrows are the  back cut transposed as a top cut. Just judging from geometry going from  45 to 90 degree  by halving the 45 top angle you get a 22.5 degree angle. The angle is probably in the right neigbourhood but as the valve lifts and drops, the actual angle that will be the best on average  is something that needs some more investigation.

In part II I will have a small go at the the ”squeeze and unsqueeze” efficiency ( Pressure recovery, Cd’s  and why sinking in an valve is not all that bad)

Tagged , , , , , , , , , , ,

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: