Scavenging (automotive)

In automotive usage, scavenging is the process of pushing exhausted gas-charge out of the cylinder and drawing in a fresh draught of air or fuel/air mixture for the next cycle.

This process is essential in having a smooth-running internal combustion engine. If scavenging is incomplete, the following stroke will begin with a mix of exhaust fumes rather than clean air. This may be inadequate for proper combustion, leading to poor running conditions such as four-stroking.

Scavenging is equally important for both two- and four-stroke engines. However it is more difficult to achieve in two-stroke engines, owing to the proximity, or even overlap, of their induction and exhaust strokes. Scavenging is also equally important to both petrol and diesel engines.

The first engines deliberately designed to encourage scavenging were the Crossley 'scavenging engines' designed by Atkinson in the early 1890s. These were soon after Crossley's gas engines had adopted poppet valves rather than their earlier slide valves, which allowed more flexible control over valve timing events. They had exceptionally late exhaust valve closing, over 30° after that of earlier engines, giving a long overlap period when both valves were open together. As these were gas engines they did not require much compression or a long period of valve closure during the compression stroke. To drive the waste gases through the cylinder during this period they assumed the 'vacuum piston' model of exhaust, where a slug of exhaust gas moving down an exhaust pipe was expected to suck further gasses after it. This model assumed that an exhaust pipe needed to be long enough to contain the gas plug for the entire duration of the stroke. As the Crossley engine was so slow-revving, this needed an exhaust pipe of a considerable 65 feet (20 m) between the engine and its cast-iron 'pot' silencer.

There are three types of scavenging on the basis of the flow of air:

This method was used for early crankcase compression two-stroke engines, as used for small motorcycles.

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