Back pressure

Back pressure (or backpressure) is a resistance or force opposing the desired flow of fluid through pipes, leading to friction loss and pressure drop. The term back pressure is a misnomer, as pressure is a scalar quantity, so it has a magnitude but no direction. The fluid is what is directed, tending to flow away from high-pressure regions and toward low-pressure regions. If the low-pressure space is more high-pressure than intended (e.g. due to obstructions or tight bends in an exhaust pipe) or the high-pressure space is more low-pressure than intended, this opposes the desired flow and reduces the discharge. Similarly, bending or other operations on a pipe (such as a stock car exhaust system with a particularly high number of twists and bends) can reduce flow rate. Back pressure (or backpressure) is a resistance or force opposing the desired flow of fluid through pipes, leading to friction loss and pressure drop. The term back pressure is a misnomer, as pressure is a scalar quantity, so it has a magnitude but no direction. The fluid is what is directed, tending to flow away from high-pressure regions and toward low-pressure regions. If the low-pressure space is more high-pressure than intended (e.g. due to obstructions or tight bends in an exhaust pipe) or the high-pressure space is more low-pressure than intended, this opposes the desired flow and reduces the discharge. Similarly, bending or other operations on a pipe (such as a stock car exhaust system with a particularly high number of twists and bends) can reduce flow rate. Fluid flows through a pipe because of the pressure difference between the two ends of a pipeline. The fluid will flow from the high-pressure end to the low-pressure end. Consider two systems as shown in the following figure. The flow in each case is caused by a pressure difference between P1 and P2. Pipe 2 has some obstructions (welding remains, a reducer, changes in area, sharp bends, etc.) which will create a pressure drop resulting in less discharge and a reduced rate of flow. The loss of pressure or pressure drop was originally considered to be a result of a pressure exerted in the opposite direction by the obstructions, thereby canceling or reducing the applied pressure. This is the origin of the term backpressure. A common example of backpressure is that caused by the exhaust system (consisting of the exhaust manifold, catalytic converter, muffler and connecting pipes) of an automotive four-stroke engine, which 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, backpressure is even more undesirable than in a four-stroke engine, as there is less 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. This can only be prevented if the pressure at the exhaust port is greater than that in the cylinder.