Back pressure

Back pressure refers to pressure opposed to the desired flow of gases in confined places such as a pipe. It is often caused by obstructions or tight bends in a confined space such as an exhaust pipe.

Because of air resistance, friction between molecules, the term back pressure is misleading as the pressure remains and causes flow in the same direction, but the flow is reduced due to resistance. For example, a stock car exhaust system with a particularly high number of twists, bends, turns and right angles could cause lots of back pressure to the gases escaping the car's engine therefore reducing the flow of the gasses.

Back pressure caused by the exhaust system (consisting of the exhaust manifold, catalytic converter, muffler and connecting pipes) of an automotive four-stroke engine has a negative effect on engine efficiency resulting in a decrease of power output that must be compensated by increasing fuel consumption.

In a piston-ported two-stroke engine however, the situation is more complicated due to the need to prevent unburned fuel/air mixture from passing right through the cylinders into the exhaust. During the exhaust phase of the cycle, back pressure is even more undesirable than in a four-stroke engine due to the shorter time available for exhaust and the lack of pumping action from the piston to force the exhaust out of the cylinder. However, since the exhaust port necessarily remains open for a time after scavenging is completed, unburned mixture can follow the exhaust out of the cylinder, wasting fuel and increasing pollution, and this can only be prevented if the pressure at the exhaust port is greater than that in the cylinder.

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. Further along the exhaust pipe, the exhaust pressure wave encounters a converging conical section, and this reflects a positive pressure wave back up the pipe. This wave is timed to arrive at the exhaust port after scavenging is completed, thereby "plugging" the exhaust port to prevent spillage of fresh charge, and indeed may also push back into the cylinder any charge which has already spilled.

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Wikipedia