03: Exhaust Tech, flow and velocity

By now we understand a quite a bit.  

But what does this boil down to?  

    A lot. A lot of time and effort in order attain balance.  

With balance I don’t mean some random B.S. about some Yin Yang stuff, but balance in terms of the goals to be attained.

Bear in mind that statements made here aren’t blanket statement!!! These are but factors to be weighed in when designing your manifold.

First things first.

Let’s start just before the exhaust valve opening. The ignition has happened and the cylinder is being pushed down by the expanding combustion, converting the chemical energy into thermal (and kinetic) energy. After the piston has reached the bottom dead centre the exh.valve opens (assumption). Let’s freeze right here.

At this stage the pressure inside cylinder is much higher than the pressure in the exhaust manifold, whether it’s a turbo, N/A or supercharged platform. The opening of the valve creates a massive pulse, a sudden motion of the gas particles due to the pressure differential across the exh.valve, to such a point even that the particles can reach supersonic speeds. Obviously this supersonic bit isn’t sustained, but is relevant to some degree. P-waves are also created due to this particle motion, much like the image to the right. But it isn’t all Pulses. The average particle velocity is much lower.

When designing the exhaust manifold one needs to take these factors into account. When using a too small runner diameter the velocity increases, but only until you choke the flow (reach the maximum mass flow rate for that given pipe diameter).

Let’s recap right here.

The runner diameter dictates how much mass can be flowed. Too narrow and you won’t be able to flow enough for your HP-goals, too large and the gas velocity will end up being lower than desired resulting in more boost lag than necessary in a turbo application. In a non-turbo application the same effect occurs and to a degree lowers the dynamic pressure in the runner and thus increases the static pressure, resulting in a less efficient extraction of exhaust gases from the cylinder.

This static and dynamic pressure is explained by the Bernoulli Equation.

Keep in mind Bernoulli doesn’t explain Pulses and the propagation of these through a medium, this effect is superimposed upon steady state behaviour such as described by Bernoulli.

Pulse waves propagate through air or exhaust gases much like waves travel through water or acoustic (sound) waves traveling through air.

As you know Pulse waves generate reflections, but when does this reflection phenomenon occur?

This is the interesting bit and where relatively big performance gains can be found. P-wave reflections are generated when sudden changes occur, for example:

  • diameter changes
  • sharp angular changes
  • obstructions
  • and so on

Knowing how P-waves behave and what causes reflections allows us to time the reflections in order to lower the pressure at the exhaust valve opening. This will benefit VE, because the amount of residual gas in the cylinder is reduced.

Do realise the speed of sound in an exhaust system is NOT equal to the speed of sound in normal air due to density and temperature differences. One must compensate for this.

More next time!