The Collaborative International Dictionary
Scavenging \Scav"eng*ing\, p. pr. & vb. n. of Scavenge. Hence, n. (Internal-combustion Engines) Act or process of expelling the exhaust gases from the cylinder by some special means, as, in many four-cycle engines, by utilizing the momentum of the exhaust gases in a long exhaust pipe.
Wiktionary
n. The act of searching through refuse for useful material. vb. (present participle of scavenge English)
WordNet
adj. searching for and salvaging anything useful from discarded material; "scavenging larvae"
searching for and feeding on decaying matter; "scavenging hyenas and vultures"
Wikipedia
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 long between the engine and its cast-iron 'pot' silencer.
Modifying the exhaust system, (i.e. modifying the exhaust gas velocity by changing exhaust tube diameters) can detract from the "ideal" scavenging effects, and reduce fuel efficiency and power if not properly planned out and executed.
To increase scavenging potential, the entire path from intake to exhaust to tailpipe must be tuned in sync with each other. This will ensure that the air flow is never interrupted. The acceleration and deceleration of this exhaust gas is what will hinder the scavenging potential.
For example, fast flowing heads and a tunnel ram intake combined with a poorly planned camshaft and exhaust system will cause the air to "slow down" and "speed up" throughout its journey, thus reducing its scavenging potential. So, to increase scavenging potential, the air must maintain a positive linear acceleration curve.
There are three types of scavenging on the basis of the flow of air:
- Direct, cross or loop scavenging
- Reverse, using Schnuerle porting
- Uniflow scavenging
The term 'loop' scavenging has been applied confusingly to both cross scavenging and reverse scavenging.
Usage examples of "scavenging".
And Barrington Grier had treated a nineteen-year-old Joshua Calvert who had just returned from his first scavenging flight as an equal, as a professional.
First when he was a cocky teenager looking for his nightly fix of spaceflight tales, then when he was scavenging, lying about how much he made and the unbelievable find that had just slipped from his fingers, and now as one of the super-elite, a starship owner-captain, one of the youngest ever.
Predatory and scavenging dinosaurs may have found good hunting for fish or amphibians in streams and lakes, and for small prey on the shores.
This suggestion is based primarily upon the observation that the largest birds today are scavenging vultures and condors.
It had been two months since the Commissioner had declared a moratorium on scavenging and had pulled all ships out of space, but this feeling of a stretched-out vista had not stopped thrilling Long.
Even the thought that the moratorium was called pending a decision on the part of Earth to enforce its new insistence on water economy, by deciding upon a ration limit for scavenging, did not cast him entirely down.
I spent the time scavenging paper and ink as unobtrusively as I might, working on various missives by the light of our campfires at night, and during the day, riding among the women of the zenana and conversing with the Ephesians.
After three days of scavenging for scraps and sleeping wedged in a dark corner of the hold, he was grateful for it.
When humanity had been forced from the sprawling Arcologies, it had tried shrewdly to market its scavenging skills among the mech cities.
No one had ever seen anything like a Mantis scavenging or navvy-policing or hunting the assets of other mech cities.
He had not felt so daunted and humble since the days when he first went out with his father on simple scavenging raids.
They specialized in it, the way Bishops knew scavenging better than anybody, and Pawns could grow food better.
Lori had also gone scavenging and come across a box that had once held some chocolate-covered peppermint candy.
He was referring, not to the boys scavenging coal, but to classes of people doing business on the northern shore of Billingsgate Dock.
New Yorkers threw out the most amazing things, and many city dwellers had furnished entire apartments from curbside scavenging.