Exhaust back pressure

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The exhaust back pressure describes the back pressure of a moving gas flow, which is caused by fluid friction or changes in geometry. For two-stroke engines , the exhaust back pressure is an important functional element for the cylinder filling, so the design of the exhaust system is of particular importance.

In slot-controlled two-stroke engines, the exhaust port is opened before the transfer port, but closed later. Without exhaust back pressure, part of the fresh gas would get unburned into the exhaust , and the gas flow in the exhaust would reduce the pressure in the combustion chamber before the exhaust port is closed. The loss of fresh gas would lead to increased consumption, and the poor degree of filling of the combustion chamber would result in poor power output.

The exhaust is therefore designed so that the gas column builds up a counterpressure at the right moment and the fresh gas remains in the combustion chamber. When the exhaust port is opened, the combustion gases flow under pressure into the exhaust, this pressure surge runs as a pressure wave through the exhaust system. Due to the structural design of the system, the pressure wave in the silencer is reflected on a reflective surface and travels back towards the cylinder. The increase in pressure at the outlet channel at the point in time at which it is closed again counteracts the loss of fresh gas and pressure loss.

The travel time of the pressure wave from the combustion chamber to the reflection surface and back is constant. If the engine speed is low, the cycle time between two "exhaust port opening" is longer, the pressure wave comes back too early. This pushes used gases back into the combustion chamber.
At high speed the cycle time is short, the pressure wave comes back too late. This allows a portion of the fresh gas to escape unused into the exhaust. These effects are one of the reasons why two-stroke engines have a bumpy idle and a disproportionate increase in consumption at full load.

The design of the exhaust is therefore also very dependent on the intended engine application. A longer travel distance of the pressure wave to the reflection surface shifts the optimal operating point of the motor in the direction of lower speeds. Sports engines designed for high performance at high speed have a shorter running distance, as the cycle times are significantly shorter. Utility engines designed for torque have a longer running distance. Appropriate changes to the exhaust geometry can lead to a noticeable increase in performance. In most cases, improper changes lead to a considerable loss of performance and increased consumption, and under certain circumstances to engine damage. In the event of changes to the exhaust system , the operating license for street-legal vehicles such as small motorbikes ( scooters, etc.) expires , provided that no tested and approved components are used for the vehicle, and the vehicles are no longer allowed to drive on public roads.

literature

  • Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden, 2005, ISBN 3-528-23933-6