What Is Meant By Scavenging? – Importance And Problems


Scavenging is a type of gas exchange process in which combustion product of fuel removed by blowing fresh air into the cylinder. The scavenging takes place during overlapping of valves or ports (when both inlet valve and exhaust valve or both transfer port and exhaust port open at the same time). This process is necessary for smooth running of the both four-stroke and two-stroke engine. However, it is harder to achieve complete scavenging of the two-stroke engine. The main method used in two-stroke is crankcase scavenging.

Read: Port-timing diagram of two-stroke engine

Importance of scavenging in IC engine

There are mainly three types of scavenging methods used in IC engine. In each, the fresh air enters at cylinder during downward movement of the piston, near BDC, and continues till exhaust valve is closed during upward movement of the piston. The flow path of scavenging air depends on the design of engine valves or ports. An efficient scavenging is important to ensure sufficient air supply to the combustion. The more effective scavenging system, the better fuel combustion and output. If the port is not open for enough time, scavenge is incomplete, and the following stroke begins with a mix of burnt gas (exhaust) and fresh charge leading poor performance of the engine.

Problems with scavenging of the engine

In ideal scavenging the air pushing out burnt gases without mix with them and heat transfer between them. Such system, in practice, not possible. The common problem with scavenging is 'short-circuiting loss' and mixing. Short circuiting loss is the expelling of some fresh air directly to exhaust; mixing happens when a small amount of exhaust gets trapped and mix with the fresh charge. There is always a heat transfer between high-temperature burnt gases and low-temperature fresh charge.

Three scavenging models

Perfect scavenging - No mixing, air entirely replace the combustion products.
Short-circuiting - Completely replace exhaust gas and some fresh charge escapes through the exhaust port.
Perfect mixing - Air enters the cylinder, instantaneously mix with burnt gas the expelled by following air flow.

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