Auto Enthusiast, engine builder, Forced induction, turbo Porsche

Pressure-Wave Superchargers

Forced induction: the replacement for displacement. For years people have been utilizing turbo & supercharger technology to prove that one doesn’t need 502 cubic inches to go fast. In the past decade forced induction has come a long way. Ten years ago a variable geometry turbo or a self-contained centrifugal supercharger would have been quite rare. Now these things are very common and can even be found on a handful of production cars/trucks. This naturally leads to one thing: new ideas. Which brings us to the topic of this little blog:

The Comprex
This Comprex or Pressure-Wave Supercharger is a very simple device in comparison to a turbo or roots-type blower. Having very few parts the Comprex consists of a cylindrical chamber which contains a belt-driven bladed ‘wheel’ and two ports offset from one another on each end. Through those ports ambient air enters and exits on one side and hot exhaust gases enter and exit on the other side. Though it’s construction is very basic, understanding how a pressure-wave supercharger works may not be as simple for some. The single most confusing aspect of this is that physically there is nothing that separates the intake air from the exhaust once inside the compressor, yet only very small amounts of exhaust gases ever return to the cylinder. To better understand how this process works here is a step by step.

First air is drawn into the cylinder like any other internal combustion engine by the downward movement of the piston, and with the Comprex in it’s path the air naturally must pass through it first. Inside the compressor air is turned and the blades are moved past the ports on either end of the chamber. When hot exhaust gases enter the chamber it is at a much higher pressure than that of the air that is already in the Comprex which in turn starts a pressure equalization process. When two compressible mediums change state they change by means of pressure waves. So when each blade passes by the exhaust inlet port air enters that cell (the area between each of the blades) and send a pressure wave towards the intake air at the speed of sound. Now for the tricky part. Since the wheel is turning perpendicular to the movement of the pressure waves and with the help of physics the waves then move in a slanting motion towards the other end of cell, compressing the intake air (which is at atmospheric pressure) to the pressure level of the expanding exhaust gases. The exhaust gas then follows the pressure wave at a much lower velocity. The ports are designed with the timing and speed of the pressure waves factored in so that the wave reaches the intake side at the exact instant the blade passes the leading edge of the outlet or “charge port” leading to the combustion chamber.

Due to the timing of the first pressure wave and the escaping charged intake air a second pressure wave is created which then flows back towards the exhaust side. This compresses the air within the cell again while also slowing it down. By the time the second wave reaches the other side the exhaust port has been passed and the air then has nowhere to go. At this point the charge air port is still open, and because of inertia the exhaust gases are still flowing towards the intake side regardless of the counter pressure wave. Because of this a new element enters the equation: expansion waves. An expansion wave is formed and slows the cell contents down considerably, decreasing pressure in that cell.

Once the wheel has come around to the exhaust exit port yet another expansion wave is created due to the now different pressures inside and outside of the cell. The wave flows towards the intake side, lowering the cell’s pressure and accelerating the air out of the cell and into the exhaust system. This final wave is strong enough to fully evacuate the cell and draw in enough air to completely fill that cell with fresh air. Then, of course, the process repeats. This complete process happens twice per revolution of the wheel and only takes place over the span of three to six milliseconds. However if certain conditions aren’t met and the timing of this process is not tuned perfectly the pressure waves will deteriorate rapidly. Never the less this idea of forced induction could possibly find it’s way under the hoods of production cars one day. Not only is it a very light draw on the engine itself, but NOx gases are partially re-circulated and used to create more energy for the engine. Now if somebody could just apply this technology to performance cars…


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