Exchange of Gases
The exchange of burnt mixture for fresh gases in the cylinder is also referred to as exchange of gases. With the two-stroke
engine one speaks of scavenging.
Open exchange of gases
It develops when inlet and outlet ports are open at the same time for a certain period of time. Fresh gas could directly end up in the exhaust gas and vice versa. This problem is less dramatic for the four-stroke
engines because the time frame is limited to the period of the valve overlap. Besides, the valve timing and channels are designed such that mixing of fresh gas and old gas does not take place. It is rather a problem
for two-stroke engines, affecting negatively exhaust gas emissions and mileage.
Closed exchange of gases
Valves open and close the inlet and outlet ports in such a way that as little as possible fresh gas escapes into the outlet port and a good filling without old gases is ensured. The closed exchange of gases is typical
for the four-stroke engine. However, there are also valve-controlled two-stroke engines. 06/08