Scavenging in Two-Stroke Engines

Hannu Jääskeläinen, Magdi K. Khair

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Abstract: The process of simultaneously purging exhaust gas and filling the cylinder with fresh charge for a new cycle is referred to as scavenging. The main scavenging methods are cross scavenging, loop scavenging and uniflow scavenging. The gas exchange process in two-stroke engines can be characterized with a number of parameters including delivery ratio, scavenge ratio, scavenge efficiency, purity of charge and trapping efficiency.

Scavenging Methods

Since one engine cycle in a two-stroke engine is completed in one crankshaft rotation, gas exchange has to occur while the piston is near BDC. There are two important consequences of this:

  1. Since gas exchange commences before and ends after BDC, a portion of the expansion and compression stroke is unusable.
  2. Piston velocity is low during the entire gas exchange phase and is unable to provide a significant pumping effect on the cylinder charge. Hence, gas exchange can only occur when the intake pressure is sufficiently higher than the exhaust pressure to allow the incoming fresh charge to displace the burned gas in the time available. This process of simultaneously purging exhaust gas from the previous cycle and filling the cylinder with fresh charge for a new cycle is referred to as scavenging. To ensure adequate scavenging, two-stroke engines must be equipped with some form of intake air compression and the intake and exhaust ports and/or valves must be open simultaneously for a sufficient period of time.

Both valves in the cylinder head and ports in the cylinder liner are applied as gas exchange control elements. In the case of ports, the piston also assumes the function of a control slide.

Scavenging in two-stroke engines is performed mainly by one of three methods: