These conflicting requirements are reconciled by constructing the exhaust pipe with diverging and converging conical sections to create pressure wave reflections which travel back up the pipe and are presented at the exhaust port. The exhaust port opens while there is still significant pressure in the cylinder, which drives the initial outflow of exhaust. As the pressure wave from the pulse of exhaust gas travels down the pipe, it encounters a diverging conical section; this causes a wave of negative pressure to be reflected back up the pipe, which arrives at the exhaust port towards the end of the exhaust phase, when the cylinder pressure has fallen to a low level, and helps to draw the remaining exhaust gas out of the cylinder.