02: Exhaust Tech, definitions

Previously we’ve discussed an intro into Exhaust Tech and dropped some fancy words, such as:

  • Volumetric Efficiency
  • Runners and Tuned length
  • P-waves
  • Scavenging
  • Harmonics
  • Choke

Let’s continue by attaining an understanding of what is what, when dealing with exhaust manifolds and systems on a more detailed level than simply grabbing the one stating an XX amount of HP gain.

What is VE?

Strictly speaking: VE, or Volumetric Efficiency is the amount of air trapped in the cylinder when the intake and exhaust valves are closed, note: this does NOT include fuel.

Obviously one needs to add fuel in order to have something combustible.

P-what?

P-waves, pressure waves or pulse waves.

The same waves as referred to in the previous post.

This is a phenomenon, which occurs everywhere in nature.

In an internal combustion engine one needs to be mindful of this phenomenon as it might influence more than you would realise at first glance. P-waves are “created” when there are sudden changes, specifically (in this case) in the exhaust/intake system, for example: the opening and closing actions of valves (intake/exhaust), the rapidly changing of the throttle, a sudden change in diameter in pipe-work, … and so on. These so-called P-waves move freely in a medium, in this case restricted by the pipe walls, at the speed of sound. Something noteworthy: the speed of sound in an exhaust system is different from the speed of sound in normal, ambient, air.  

P-wave propagation

Harmonics? Harmonica? Musical instrument?

When we use the term “harmonics” it isn’t to inform you about an instrument we play, it’s to inform you we’ve taken the system harmonics into account when designing the manifold or system. The reason for this being acoustics.

The previously discussed P-waves have the exact same properties as acoustic waves, or sound waves. Thus the term “harmonics”.

Runners and Tuned Length

Runners: the pipes going from the cylinder head (the flange mounted onto the cylinder head) to the collector (where the pipes “merge” together to continu in one or two pipes).

Tuned Length

When the exhaust valve opens you want the pressure ratio across that valve to be as big as possible, so you want the pressure in the exhaust manifold to be as low as possible in order to extract as much of the burnt gases out of the cylinder to make way for fresh air-fuel mixture.

The term tuned length refers to the length of the runner, which is fabricated so it will reflect a P-wave back up the pipework. This reflection will lower the pressure at the back of the valve and thus increase the pressure ratio across the exhaust valve.

Scavenging

The aiding of the extraction of the burnt gases is commonly referred to as “scavenging”.  

Choke, or Choke-flow

A term you might not have come across, unless you’re sporting a turbocharged engine in your vehicle. In which case choke is in relation to choke-line, basically where the rotational speed of the turbine is too high to actually do anything (in laymen terms).

Something similar applied here. The mass of gas you want/need to move out of the engine is more than your pipework can support and thus becomes a limiter/restrictor.

Why does it become a limiter/restrictor?

Because we can’t maintain a supersonic flow.

Now that we’ve covered some of the basics, the next post will go into more detail.

Have a good one!

VPR-Engineering