Exhaust and back pressure

There back pressure refers to the pressure opposite the desired flow of gas in confined places such as a pipeline. It is often caused by blockages or tight bends in a narrow space such as a drain pipe .

Due to air resistance , the friction between molecules, the term "back pressure" is misleading as the pressure remains and causes flow in the same direction, but flow is reduced due to resistance. For example, an automobile exhaust system with a particularly high number of twists, bends, curves, and right angles could cause a lot of backpressure to the gases escaping from the car's engine, thus reducing the flow of gases.

Back pressure in the exhaust of two-stroke engines

In a two-stroke engine, however, the situation is more complicated due to the need to prevent the fuel/unburned air mixture from passing into the exhaust through the cylinders. During the exhaust cycle, back pressure is even more undesirable than in a four-stroke engine due to the 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 some time after scavenging is completed, the unburned mixture can follow the exhaust out of the cylinder, wasting fuel and increasing pollution, and this can only be avoided if the pressure at the exhaust is greater than that in the cylinder.

These conflicting requirements are accommodated by constructing the exhaust pipe with diverging and converging conical sections to create pressure wave reflections which travel up the pipe and are supplied to the discharge port. The exhaust port opens while there is still significant pressure in the cylinder, which drives the initial outflow of the exhaust. As the pressure wave from the exhaust gas pulse travels down the tube, it encounters a divergent conical section; this causes a negative pressure wave to reflect off the pipe, which arrives at the exhaust port towards the end of the exhaust stroke, when the cylinder pressure has dropped to a low level, and helps to push the exhaust gas out residue from the cylinder. Further along the exhaust pipe, the exhaust pressure wave encounters a converging cone section, and this reflects a positive pressure wave back up the pipe. This wave is timed to arrive at the exhaust port after scavenging has been completed, thereby "plugging" the exhaust port, to prevent new charge from escaping, and can also push back into the cylinder any charge that is already been paid.

Since the timing of this process is mainly determined by the geometry of the exhaust system, which is extremely difficult to make variable, correct timing and thus optimal engine efficiency can typically only be achieved over a small part of the operating speed range of the engine.

Back pressure in automotive exhaust (four-stroke engine)

The back pressure caused by the exhaust system (consisting of the collector exhaust, from the catalytic converter , from the muffler and by collectors connection) of a four-stroke automotive engine has a negative effect on the thermal efficiency , with a consequent decrease in power which must be compensated by increasing fuel consumption.