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What is Engine Knocking?
Spark ignition In internal combustion engines, knocking (also knocking, explosion, spark knocking, pinging or pinking) occurs when the combustion of some air/fuel mixture in the cylinder does not result in flame front propagation by the spark plug, but air / One or more pockets of the fuel mixture burst outside the normal combustion front envelope. The fuel-air charge is only meant to be ignited by a spark plug, and at the precise point in the stroke of the piston. Knock occurs when the peak of the combustion process is no longer at the optimal moment for a four-stroke cycle. The shock wave characteristic produces a metallic "pinging" sound, and the cylinder pressure increases dramatically. The effect of engine knocking ranges from inconsistent to completely devastating.
The knocks are not to be confused with pre-ignition - they are two separate incidents. However, the first-ignition can be knocked out.
In November 1914, a letter from the Lodge Brothers (spark plug manufacturers, and Sir Oliver Lodge's sons) described the explosion incident, settling a discussion about the cause of "conking" or "pinking" in the motorcycle. having had. In the letter, he stated that an initial ignition could give rise to gas detonating rather than normal expansion and that the sound produced by an explosion is the same as if the metal parts were tapped with a hammer. [1] It was further investigated and described by Harry Ricardo during experiments conducted between 1916 and 1919 to find out the cause of failures in aircraft engines.
Normal Engine Combustion
Under ideal conditions, the normal internal combustion engine burns the fuel/air mixture in the cylinder in an orderly and controlled fashion. Combustion is initiated by the spark plug from 10 to 40 crankshaft degrees before the top speed center (TDC), which depends on many factors including engine speed and load. This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from expansion gases.
The electrode of the spark plug has a small kernel approximately the size of the spark plug gap. As it increases in size, its heat production increases, which allows it to grow at an accelerated rate, expanding rapidly through the combustion chamber. This increase is due to the flame traveling through the combustible fuel-air mixture, and due to turbulence that rapidly expands the burning area to a complex area of the fingers of the burning gas, including a simple spherical ball. The flame has a much higher surface area than it would have. In normal combustion, this flame front operates at a rate characteristic for the particular mixture in the fuel/air mixture. The pressure reaches peak smoothly, as almost all available fuel is consumed, then the pressure drops as the piston descends. The maximum cylinder pressure achieves some crankshaft degrees after the piston passes TDC so that the force exerted on the piston (from the increased pressure exerted on the upper surface of the piston) allows the piston to push its hardest when there are speed and mechanical advantage. The best recovery of force occurs from the expanding gases on the crankshaft, thus maximizing the torque transferred to the crankshaft.
Abnormal Engine Combustion
When a mixture of unbalanced fuel/air beyond the flame front limit is subjected to a combination of heat and pressure for a certain period of time (beyond the delay period of the fuel used), an explosion can occur. The blaster is characterized by almost instantaneous, explosive ignition, which contains a fuel/air mixture in at least one pocket. A local shockwave is created around each pocket, and the cylinder pressure will increase rapidly - and possibly beyond its design limits - causing damage.
If allowed to continue to explode under extreme conditions or multiple engine cycles, engine parts may be damaged or destroyed. The simplest detrimental effect is usually due to moderate knocking, which may be caused by the oil filter getting trapped through the engine oil system and on other parts before wear. This type of wear causes erosion, friction, or "sandblasted" form, similar to damage from hydraulic cavitation. Severe knocking can lead to catastrophic failure in the form of physical holes melted and pushed through the piston or cylinder head (ie, the breakdown of the combustion chamber), either of which delineates the affected cylinder and large metal fragments, Introduces fuel and combustion products. In the oil system. Hyperactive pistons are known to break easily from such shock waves.
The explosion can be prevented by any or all of the following techniques:
* Retiring ignition timing.
* Use of fuels with a higher octane rating, which increases the fuel combustion temperature and reduces propagation to explode.
* Enriching the air-fuel ratio that alters chemical reactions during combustion, reduces combustion temperature, and increases the margin of detonation.
* Reducing peak cylinder pressure.
* Reducing manifold pressure by opening throttle or reducing pressure.
* Reduce load on the engine.
Because pressure and temperature are strongly interconnected, it is also knocked down by controlling the peak combustion chamber temperature by compression ratio reduction, exhaust gas recycling, suitable calibration of the engine's ignition timing schedule, and careful design of the engine's combustion chambers and cooling system. May go. As controlling the initial air intake temperature.
In addition to some materials such as lead and thallium, when some fuel is used the explosion will be suppressed very well. [Citation needed] Addition of tetraethyl lead (TEL), a soluble organohalide compound, was added to gasoline until it ceased for reasons. Of toxic pollution. Lead dust added to the intake charge will also reduce knockdown with various hydrocarbon fuels. Manganese compounds are also used to reduce knockdown with petrol fuel.
Knocking is less in cold weather. As an aftermarket solution, a water injection system can be employed to reduce the combustion chamber peak temperature and thus suppress the explosion. Steam (water vapor) will suppress the knock even if there is no added cooling supply.
Some chemical changes must occur to knock first, so fuels with some structures knock more easily than others. Bronzed chain kinds of paraffin resist knocking while straight chain kinds of paraffin knock easily. It has been theorized [citation needed] that lead, steam, and the like interfere with some of the various oxidative changes that occur during combustion and therefore reduce knockdown.
Turbulence, as stated, has a very significant effect on knocking. Engines with good turbulence knock less than engines with poor turbulence. Turbulence occurs not only when the engine shrinks, but also when the mixture burns and burns. Many pistons are designed to use "squishy" turbulence to violently mix air and fuel together as they ignite and burn, reducing the burn speed and cooling the unbalanced mixture. Reduces a lot. An example of this is all modern side valves or flathead engines. Much of the headspace is made to come close to the piston crown, causing TDC to have a lot of turbulence. This was not done in the early days of the side valve head and a very low compression ratio had to be used for any fuel. Such engines were sensitive to ignition advances and had low power.
Knocking is more or less inevitable in diesel engines, where fuel is injected into highly compressed air at the end of the compression stroke. There is a short interval between fuel being injected and combustion commencing. By this time the combustion chamber already contains the amount of fuel that will ignite in areas of higher oxygen density before full charge combustion. This sudden increase in pressure and temperature causes specific diesel 'knock' or 'clatter', some of which must be allowed in engine design.
Knockdown can be greatly reduced by careful design of injector pumps, fuel injectors, combustion chambers, piston crowns, and cylinder heads, and modern engines using electronic common rail injection have very low levels of knockdown. Due to greater dispersion of oxygen in the combustion chamber and lower injection pressures providing a more complete mixture of fuel and air, engines using indirect injection have a lower level of knockdown than direct injection engines. Diesel does not really suffer as much "knock" as gasoline engines at all because the only reason for this is the very high rate of pressure increase, not unstable combustion. Diesel fuels are actually very prone to knock in gasoline engines but diesel engines do not have the time to knock because the fuel only oxidizes during the expansion cycle. The fuel in a gasoline engine is slowly oxidizing all the time, while it is being compressed before the spark. This allows changes in the structure/makeup of molecules before a very critical period of high temperature and pressure.
Knock detection
Due to major changes in fuel quality, atmospheric pressure, and ambient temperature, as well as the possibility of malfunctions, every modern combustion engine includes mechanisms to detect and prevent knock.
A control loop is permanently monitoring the signal of one or more sensors (typically piezoelectric sensors that are capable of translating vibration into an electric signal). If a knock combustion characteristic pressure peak is detected the ignition timing is dimmed by a few degrees of phase. If the signal indicates controlled combustion, the ignition timing is re-advanced in the same style that the engine is holding at its best possible operating point, the so-called knock limit. Modern knock control loop systems are capable of individually adjusting ignition timing for every cylinder. The simultaneous boost pressure is regulated based on the specific engine. This way performance is kept at its optimum while eliminating the risk of engine damage from most knockdowns. Running on low octane fuel.
Knock prediction
Since it is very important for development engineers to avoid combustion, a variety of simulation techniques have been developed that can identify engine design or operating conditions in which knocking is likely to occur. This then enables engineers to design methods to reduce knockdown combustion to maintain high thermal efficiency.
Since the beginning of combustion is sensitive to the cylinder pressure, temperature, and chemistry associated with local mixing compositions within the combustion chamber, simulations for all these aspects occur, In determining the extent of knock operation in this way Have proven to be the most effective, and enabling engineers to determine the most appropriate operational strategy.
Knock control
The purpose of knock control strategies is to try to optimize the trade-off between protecting the engine from damage events and maximizing the engine's output torque. The knock event is an independent random process. [independent] It is impossible to design knock controllers in a deterministic platform. Using a single time history simulation or knock control methods, the controller is not able to provide duplication of performance due to the random nature of the display. Therefore, the desired trade-offs should be performed in a stochastic framework, which can provide an appropriate environment to design and evaluate the performance of various knock control strategies with rigorous statistical properties.
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