Scavenging

from Wikipedia, the free encyclopedia

Under scavenging (dt. Dispel) refers to the operation of a reciprocating engine , the opening times of at the intake and exhaust valve overlap partially. As a result, some of the cold fresh air drawn in flushes the hot exhaust gas in the cylinder into the exhaust manifold , which significantly improves the filling compared to conventional operation.

The increased mass flow in the exhaust system improves the response of the turbocharger, especially at low speeds. Scavenging therefore helps to reduce the so-called “turbo lag”.

requirements

The prerequisites for exploiting the scavenging effect are variable valve timing , gasoline direct injection and high-temperature-resistant materials for the exhaust manifold and turbocharger .

Variable control times are necessary because the scavenging effect is only desired at low engine speeds. At higher speeds, the amount of exhaust gas is sufficient to bring the turbocharger to the desired speed even without this effect. The variable control times are achieved by cam phasers , which reduce the overlap of the control times with increasing engine speed. They are controlled by the engine electronics.

The direct injection ensures that the fresh gas is not mixed with fuel when it enters the cylinder, as would be the case with carburettor engines or engines with manifold injection. This avoids undesirable flushing losses , i.e. unburned gasoline overflowing into the exhaust manifold.

Effects

Scavenging has two desirable effects: The increase in the turbocharger speed (due to the increasing exhaust gas enthalpy) at low engine speed leads to higher boost pressure and thus to better filling of the cylinders with fresh gas. At the same time, the improved internal cooling reduces the knocking tendency so that an earlier ignition angle (gasoline engine) or injection angle (diesel) can be used. This increases the effectiveness of the combustion and thus the torque and the power of the engine. This makes it possible to achieve good acceleration behavior even with downsizing gasoline engines from low speeds.

The exhaust gas aftertreatment is made more difficult by the air pulses . If the amount of scavenging air is large, the corresponding amount of fuel cannot be injected for the optimal exhaust gas lambda = 1 operation, and increased NO x emissions can result.

Meaning in the English-speaking world

In English, scavenging has another, original meaning in engine technology: It describes the gas exchange in the cylinder of an internal combustion engine in general, in particular the flushing of a two-stroke engine with fresh gas.

Web links