THE BASIC PROCESS
When the exhaust port cracks open, gases still under a considerable pressure burst out into the exhaust tract, forming a wave front that moves away at high speed down the port and headed for less confined quarters. After traveling a comparatively short distance, this wave reaches the first part of the expansion chamber proper-which is a diffuser (commonly called a megaphone). The diffuser's walls diverge outward, and the wave reacts almost as though it had reached the end of the system and is, in the manner of waves explained in the first chapter of this text, reflected back up the pipe toward the cylinder with its sign inverted. In other words, what had been a positive pressure wave inverts, to become a negative pressure wave. The big difference between the action of the diffuser and the open end of a tube is that the former returns a much stronger and more prolonged wave; it is a much more efficient converter (or inverter) of wave energy.
As the initial wave moved down the diffuser, the process of inversion continues apace, and a negative pressure wave of substantial amplitude and duration is returned. Also, overlaid on this is the effect of inertia on the fast-moving exhaust gases, and the total effect is to create a vacuum back at the exhaust port. This vacuum is very much stronger than one might suppose, reaching a value of something like minus-7 psi at its peak. Add that to the plus-7 psi (approximately) pressure in the crankcase working to force the fresh charge up through the transfer ports and you will better understand how the transfer operation is accomplished in such a very short time. Obviously, too, this combined pressure differential of almost one atmosphere is very helpful in sweeping from the cylinder the exhaust residue from the previous power stroke. It's all a lot like having a supercharger bolted on over at the engine's intake side - but without the mechanical complication.
