Back pressure

The exhaust gas back pressure describes the dynamic pressure of a moving stream of gas is caused by fluid friction or geometry changes. For two-stroke engines, the exhaust back pressure is an important functional element of the cylinder charge, the design of the exhaust system is therefore of particular importance.

In slot- controlled two-stroke engines, the exhaust port is opened in time before the spill, but later closed. No exhaust back-pressure part of the fresh gas would enter the unburnt exhaust, also the gas flow would be reduced in the exhaust pressure in the combustion chamber before the exhaust port is closed. The fresh gas loss would lead to increased consumption, the poor degree of filling of the combustion chamber in poor performance yield.

Therefore, the design of the exhaust system is selected so that the gas column at the right moment builds up a back pressure and the intake gas remains in the combustion chamber. Upon opening of the exhaust port, the combustion gases flow with pressure in the exhaust, this surge runs as a pressure wave through the exhaust system. The constructive design of the plant, the pressure wave is reflected by a reflecting surface in the muffler and travels back towards the cylinder. The increase in pressure at the outlet channel at the time at which it is closed again, counteracts the loss of fresh gas and pressure loss.

The duration of the pressure wave from the combustion chamber to the reflecting surface and back again is constant. At low engine speed the cycle time between two is " outlet channel open " longer, the pressure wave is coming back too early. Characterized spent gases are forced back into the combustion chamber. At high speed the cycle time is short, the pressure wave comes too late return. This allows unused escape into the exhaust a portion of fresh gas. These effects are the reason that two-stroke engines have a rough idle and a disproportionate increase in consumption at full load.

The design of the exhaust is thus also depends very much on the intended engine application., A longer distance of the pressure wave to the reflecting surface shift the optimum operating point of the engine in the direction of higher speeds. Sport engines with a design for high performance at high speed with a longer route, Nutzmotoren with a design for torque a shorter route. A corresponding change in the proper exhaust geometry can lead to a noticeable increase in performance. Improper changes lead in most cases to a significant loss of performance and consumption, may also cause engine damage. For modifications to the exhaust system goes out at street-legal vehicles such as mopeds ( scooters, etc.) the approval, unless tested and approved for the vehicle components are used, and the vehicles must not take part in public traffic.