Engine knocking

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Knock (also beats , detonation , spark knock , ping or pinking in internal combustion internal combustion engines occurs when the combustion of some air/fuel mixtures in the cylinder is not generated from propagation of the flame front being ignited by the spark plug, but one or more air bags/fuel mixes explode outside envelope from the front of the normal combustion.

The air-fuel is meant to be ignited by the spark plug only, and at the right point in the piston stroke. Knock occurs when the peak burning process no longer occurs at the optimum for the four-stroke cycle. Shock waves create a characteristic "ping" metal sound, and the cylinder pressure increases dramatically. The effects of a tap of a machine range from unimportant to utterly damaging.

Knocks should not be confused with pre-ignition - they are two separate events. However, pre-ignition is usually followed by a tap.

The detonation phenomenon was first observed and described by Harry Ricardo during experiments conducted between 1916 and 1919 to discover the reasons for failure in aircraft engines.

Video Engine knocking

Pembakaran normal

Under the ideal conditions the internal combustion engine burns the fuel/air mixture in the cylinder regularly and under control. The combustion is started by a spark plug about 10 to 40 degrees crankshaft before dies over the center (TDC), depending 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 working recovery of the expanding gas.

Sparks in the spark plug electrode form a small flame core approximately the size of a spark plug gap. As it grows in size, its heat output increases, allowing it to grow at an accelerating rate, growing rapidly through the combustion chamber. This growth is caused by a front flame journey through a mixture of combustible fuel air itself, and because of the turbulence that quickly stretches the combustion zone into a complex of radius of burning gas that has a much larger surface area than a simple ball of fire will have. In normal combustion, the flame front moves across the fuel/air mixture at a speed characteristic for a particular mixture. Pressure rises smoothly to the top, as almost all available fuel is consumed, then the pressure drops as the piston falls. The maximum cylinder pressure is achieved by several degrees of crankshaft after the piston passes the TDC, so the force applied to the piston (from the increased pressure applied to the top surface of the piston) can give the hardest impulse just as the piston speed and mechanical advantage of the crankshaft provide best of expanding gas, thus maximizing the torque transferred to the crankshaft.

Maps Engine knocking

Abnormal combustion

When an unburned fuel mixture outside the front of the flame is subjected to a combination of heat and pressure for a given duration (beyond the period of fuel delay used), detonation may occur. Detonation is characterized by almost instantaneous explosive ignition, at least one bag of fuel/air mixture outside the front flame. Local shockwaves are made around each pocket and the cylinder pressure will increase sharply, and possibly beyond the design limit causing damage.

If detonation is allowed to withstand extreme conditions or over many machine cycles, machine parts may be damaged or destroyed. The simplest destructive effect is usually the wear of particles caused by moderate beats, which can then occur through the engine oil system and cause wear on the other part before being trapped by the oil filter. Such clothing gives the appearance of erosion, abrasion, or "sandblasted", similar to the damage caused by hydraulic cavitation. Severe knocking can cause catastrophic failure in the form of a melted physical hole and push through the piston or cylinder head (ie, breakage of the combustion chamber), both of which depressurizes the exposed cylinder and introduces large metal fragments, fuels, and combustion products into the oil system. Hypereutectic pistons are known to break easily from shock waves.

Detonation can be prevented by any or all of the following techniques:

• high octane fuel usage, which increases fuel burning temperatures and reduces explosive tendencies
• enriches the air-fuel ratio that alters the chemical reaction during combustion, reduces combustion temperature and increases the margin above detonation
• reduces peak cylinder pressure
• reduces manifold pressure by reducing throttle opening or increasing pressure
• reduce the load on the machine
• slows (decreases) ignition time

Because the pressure and temperature are strongly linked, the tap can also be attenuated by controlling the temperature of the top combustion chamber by reducing the compression ratio, the exhaust gas recirculation, the exact calibration of the engine ignition timing, and the careful design of the combustion chamber and engine cooling system as well controller of initial air intake temperature.

The addition of certain ingredients such as lead and thallium will suppress detonation very well when certain fuels are used. The addition of tetraethyllead (TEL), a soluble organolead compound added to gasoline is common until it is stopped for reasons of toxic pollution. Lead dust added to the intake charge will also reduce the tap with various hydrocarbon fuels. Manganese compounds are also used to reduce beats with gasoline.

Beats are less common in cold climates. As an aftermarket solution, the water injection system can be used to reduce the peak temperature of the combustion chamber and thus suppress detonation. Steam (moisture) will hit a tap even though no additional cooling is provided.

A particular chemical change must first occur for a tap to occur, so a fuel with a particular structure tends to knock more easily than the other. Branched chain paraffins tend to withstand taps while straight chain paraffins knock easily. It has been theorized that lead, steam, and the like disrupt some of the oxidative changes that occur during combustion and hence reduction of taps.

Turbulence, as stated, has a very important influence on tapping. Machines with good turbulence tend to knock less than machines with bad turbulence. Turbulence occurs not only when the machine inhales but also when the mixture is compressed and burned. Many pistons are designed to use "squish" turbulence to mix the air roughly and coalesce simultaneously as they are ignited and burned, which reduces large beats by accelerating combustion and cooling the unburned mixture. One example is all modern side valves or flathead machines. Most of the headroom is made close to the piston crown, making a lot of turbulence near the TDC. In the early days the valve head is not performed and a much lower compression ratio should be used for each given fuel. Such machines are also sensitive to the progress of ignition and have less power.

Tapping more or less can not be avoided in diesel engines, where fuel is injected into high-pressure air toward the end of the compression step. There was a short pause between fuel injected and burning started. There is already a quantity of fuel in the combustion chamber that will fire first in areas with greater oxygen density before complete load loading. This sudden increase in pressure and temperature leads to typical diesel-type 'tapping' or 'tinkling', which must be partially permitted in machine design.

Careful design of the injector pump, fuel injector, combustion chamber, piston crown and cylinder head can reduce large beats, and modern machines that use electronic rail injection electronics have a very low level of tapping. Machines that use indirect injection generally have lower tap rates than direct injection machines, because greater oxygen deployment in the combustion chamber and lower injection pressure provide a more complete mixing of fuel and air. Diesels actually do not experience the exact same "beats" as gasoline engines because the cause is known to be just a very fast rate of rising pressure, not unstable combustion. Diesel fuel is actually very susceptible to tapped gasoline engines but in diesel engines there is no time for beats to occur because the fuel is only oxidized during the expansion cycle. In a gasoline engine, the fuel slowly oxidizes all the time when it is compressed before the spark. This allows for changes in molecular structure before the period of very high temperature/pressure.

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Detection knock

Due to large variations in fuel quality, a large number of engines now contain mechanisms for detecting taps and adjusting time or increasing pressure to offer improved performance on high-octane fuel while reducing the risk of engine damage caused by taps while running on low octane fuel.

The earliest example of this is a charged Saab H turbo engine, where a system called Automatic Performance Control is used to reduce push pressure if causing the engine to knock.

Various monitoring devices are commonly used by tuners as a method of viewing and listening to machines to ascertain whether the tuned vehicle is safely under load or used to realign the vehicle safely.

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Tap prediction

Because avoiding knocking fires is very important for development engineers, a variety of simulation technologies have been developed that can identify machine design or operating conditions where a tap may be expected to occur. This then allows engineers to devise ways to reduce combustion while maintaining high thermal efficiency.

Since the onset of the beats is sensitive to in-cylinder pressure, temperature and chemical autoignition are associated with local mix compositions in the combustion chamber, a simulation that takes into account all these aspects has proved most effective in determining the limit of knock operations and allowing engineers to determine the most appropriate operating strategy.

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Knock Control

The purpose of the knock knock strategy is to try to optimize the trade-off between protecting the engine from damaging knock events and maximizing engine output torque. Knock events are independent random process. It is impossible to design a tapping controller in a deterministic platform. A single time history simulation or experimental tapping control method can not provide repeatable performance measurements of the controller because of the random nature of the tap event. Therefore, the desired trade-off should be done in a stochastic framework that can provide an appropriate environment for designing and evaluating the performance of different knock control strategies with strict statistical properties.

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References

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Further reading

• Identification of combustion and detonation in spark plug ignition engines using ion current signals Fuel 227, 469-477 (2018), DOI: 10.1016/j.fuel.2018.04.080
• In-cylinder pressure oscillation modeling under knocking conditions: General approach based on damped wave equation , FIRE Journal, DOI: 10.1016/j.fuel.2012.07.066, ISSN 0016-2361, February, 2013.
• Experimental Evaluation of Reduced Kinetic Model for Simulation Tapping on SI Machine , SAE Technical Paper n. 2011-24-0033, DOI: 10.4271/2011-24-0033, ISSN 0148-7191, Sep., 2011.
• Stress Oscillation Modeling Under Tapping Condition: Partial Differential Equation Equation Approach , SAE Technical Paper n. 2010-01-2185, DOI 10.4271/2010-01-2185, ISSN 0148-7191, October, 2010.
• Predictive combustion simulation for directional directional-spark-ignition engines: solutions for pre-ignition ("mega-tapping"), shootout, extinction, conventional fire propagation and "knocking", innovation cmcl, accessed June 2010.
• Machine Basics: Detonation and Pre-ignition , Allen W. Cline, accessed June 2007.
• Experimental Investigation of Current Ion Usage on SI Machines for Knock Detection , SAE Technical Paper n. 2009-01-2745, DOI 10.4271/2009-01-2745, ISSN 0148-7191, November, 2009.
• Charles Fayette Taylor, Internal Combustion Engine In Theory And Practice, Second Edition, Revised, Volume 2 , MIT Press, 1985, Chapter 2 on "Detonation and Preignition", pp 34-85, ISBNÃ , 9780262700276.

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External links

• Bob Hewitt Pre-ignition and Detonation (Misterfixit) Accessed June 2007
• NACA - Burning and tapping in the spark-ignition engine
• NACA - Ionization in the tap zone of the internal combustion engine
• NACA - Interdependencies of different types of autoignition and tapping
• Avweb - Detonation Myth
• Misterfixit - What is detonation?
• Gasoline FAQ

Source of the article : Wikipedia