Put simply, scavenging refers to the process of replacing spent gases in the engine cylinder with a fresh charge of air and fuel. Your engine is basically a large air pump that makes power by rotating pistons through a crankshaft, powered by multiple explosions of air and fuel sprayed in a fine mist. The whole point of the exhaust stroke is to expel the spent gases that remain in the engine cylinder better known as the combustion chamber after the combustion cycle is completed.
When the spent gases are evacuated, it frees up the much-needed space for the next combustion cycle. But have you ever wondered how exactly the exhaust system draws the spent gases out of the engine cylinder?
Allow us to explain — the pressure within the combustion chamber is 6 to 7 times higher than atmospheric pressure, and the pressure outside the combustion chamber in the exhaust manifold is equal to atmospheric pressure.
Naturally, the exhaust gases are going to want to move towards the low-pressure area, that is, out of the engine cylinder, and into the exhaust manifold. Now, remember that exhaust gases travel in pulses; not in a stream. When these gases leave the engine, a negative pressure wave follows each exhaust pulse sent out by individual cylinders. It is this negative pressure wave that helps to draw the next exhaust pulse behind it like a vacuum , and the cycle keeps going.
This is referred to as scavenging. It leads you in a bad direction. That tells me the programs are OK. It helps if you begin to look at engine exhaust as two elements instead of one. First, there are gases, the end result of the combustion of hydrocarbons.
Second, and equally important is the sonic element of exhaust. The two elements have conflicting traits, which likely is one of the reasons exhaust technology is so difficult to pin down. Another phenomenon within the exhaust system is what is known as reversion or a reversion wave.
When the exhaust valve opens, a pressure pulse and wave are released. As soon as it hits the first opening, there is a reflecting wave, which is good because it aids in scavenging. But when the wave travels all the way to the collector, it comes all the way back in the form of a negative wave, which brings with it moving particles, inert and contaminated gases that cannot be burned again.
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. Next to the O2 sensor is the resonator. A resonator is designed to prevent the engine from getting too loud in a particular RPM range.
For example, on the highway, if the car keeps making loud engine sounds, it will be irritating to bear that sound for hours of driving. Lastly, there is a muffler attached to the end of the pipes of the exhaust systems. This muffler is helpful in reducing the noise levels across the entire operation of the engine. This means, that throughout the entire engine RPM range, the levels of noise are controlled by the muffler. These are all the major components of the exhaust system.
Now, let us move on to the clever technologies introduced to make maximum use of exhaust gas energy. The most famous of them all, after turbocharging technology, is exhaust gas scavenging. Let us look at it in detail. When the exhaust gases are pushed out of the cylinder by the piston through the exhaust valves, it creates a certain vacuum or low-pressure area in the exhaust manifold or pipe.
Depending on the diameter of the exhaust manifold and the velocity of the exhaust gases, the vacuum or low-pressure area created behind the exhaust gases is determined. There must be an ideal manifold length and diameter to strike a balance between velocity and back pressure caused by outside conditions.
For example, if the pressure outside is high, the exhaust gases will not move from cylinder to outside and that will cause inefficient combustion in the next cycle. That is not good for the engine at all and can even lead to damaging engine components due to excess heat.
Therefore, to make use of this low-pressure area behind the exhaust wave, the exhaust and intake valves are kept open simultaneously for a very brief amount of time.
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