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Knocking (also called pinking or pinging)— colloquially detonation—in internal combustion engines occurs when air/fuel mixture in the cylinder detonates or ignites prior to the timed pre-set conditions in the engine's cylinder(s). In non-diesel combustion engines, the fuel-air charge is supposed to be ignited by the spark plug only, and at a precise time in the piston's stroke cycle designed for the engine. In engine knocking the smooth burning of the fuel-air mixture is interrupted by the unburned mixture in the combustion chamber detonating before cylinder's piston has reached the designed position. The design intent of the combustion process ceases, because of the explosion, before the optimum moment for the four-stroke cycle. The resulting shockwave reverberates in the combustion chamber, creating a characteristic metallic "pinging" sound, and pressures increase catastrophically. It can range from hardly noticeable to complete engine destruction. A weapons cache is detonated at the East River Range on Bagram Airfield, Afghanistan Detonation is a process of supersonic combustion in which a shock wave is propagated forward due to energy release in a reaction zone behind it. ...
The internal combustion engine is an engine in which the combustion of fuel and an oxidizer (typically air) occurs in a confined space called a combustion chamber. ...
Air-fuel ratio (AFR) is the mass ratio of air to fuel present during combustion. ...
This article or section should include material from Spark gap A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. ...
Today Internal combustion engines in cars, trucks, motorcycles, construction machinery and many others, most commonly use a four-stroke cycle. ...
Normal combustion
Under ideal conditions the common piston internal combustion engine burns its fuel air mix in the cylinder in an orderly and controlled fashion. The combustion is started by the spark plug some 15–40 crankshaft degrees prior to TDC (top dead center, the point of maximum compression). This ignition advance allows time for the combustion process to develop peak pressure at the ideal time for maximum recovery of work from the expanding gases. This point is typically 14–18 crankshaft degrees ATDC (after top dead center). ...
The spark plug produces an electrical spark that jumps a small gap from its center electrode to its ground electrode. This spark, if the air/fuel mix is within the flammable range for the fuel, initiates combustion. The initial phase forms a small kernel of flame approximately the size of the spark plug gap. For the first few milliseconds of the combustion process, this flame kernel is struggling to survive, producing only slightly more heat than is necessary to continue the combustion process. As it grows in size its heat output increases allowing it to grow even faster. A combustion reaction taking place in a igniting match Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames. ...
After this early slow burn phase passes, the flame kernel grows much faster expanding rapidly across the combustion chamber. This growth is due to the travel of the flame front through the combustible fuel air mix itself and due to turbulence rapidly stretching the burning zone into a complex of fingers of burning fuel air that have a much greater surface area than a simple spherical ball of flame would have. This greatly accelerates the combustion process.
In normal combustion, this flame front moves throughout the fuel air mix at a rate characteristic for the fuel-air mixture. Pressure rises smoothly to a peak, burning nearly all the available fuel then falls as the piston descends. In normal combustion this produces a rapid increase in cylinder pressure as the piston passes TDC and begins to move down the cylinder. As mentioned above in a properly tuned engine the maximum cylinder pressure is achieved a few crankshaft degrees after the piston passes TDC, so that the increasing pressure can give the piston a hard push when its speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases.
Detonation The fuel/air mixture is normally ignited slightly before the point of maximum compression to allow a small time for the flame-front of the burning fuel to expand throughout the mixture so that maximum pressure occurs at the optimum point. The flame-front moves at roughly 33.5 m/second (110 feet/second) during normal combustion[citation needed]. It is only when the remaining unburned mixture is heated and pressurized by the advancing flame front for a certain length of time that the detonation occurs. It is caused by an instantaneous ignition of the remaining fuel/air mixture in the form of an explosion. The cylinder pressure rises dramatically beyond its design limits and if allowed to persist detonation will damage or destroy engine parts. This article is about a foot as a unit of length. ...
A non conventional engine that makes use of detonation to improve efficiency and decrease pollutants is the Bourke engine. The Bourke engine was designed by Russell Bourke in the late 1930s, who endeavored to improve upon the Otto cycle engine. ...
Detonation can be prevented by:
- The use of a fuel with higher octane rating
- The addition of octane-increasing "lead", methylcyclopentadienyl manganese tricarbonyl (MMT), isooctane, or other antiknock agents.
- Increasing the amount of fuel injected/inducted (resulting in lower Air to Fuel Ratio)
- Reduction of cylinder pressure by increasing the engine revolutions (lower gear), decreasing the manifold pressure (throttle opening) or reducing the load on the engine, or any combination.
- Reduction of charge (in-cylinder) temperatures (such as through cooling, water injection or compression ratio reduction).
- Retardation of spark plug ignition.
- Improved combustion chamber design that concentrates mixture near the spark plug and generates high turbulence to promote fast even burning.
- Use of a spark plug of colder heat range in cases where the spark plug insulator has become a source of pre-ignition leading to detonation.
Correct ignition timing is essential for optimum engine performance and fuel efficiency. Modern automotive and small-boat engines have sensors that can detect knock and retard (delay) the ignition (spark plug firing) to prevent it, allowing engines to safely use petrol of below-design octane rating, with the consequence of reduced power and efficiency. This article needs additional references or sources for verification. ...
Tetra-ethyl lead (also known as TEL, lead tetraethyl and tetraethyllead) is a toxic organometallic chemical compound, with formula (CH3CH2)4Pb, which was once used as a gasoline (petrol) additive. ...
Methylcyclopentadienyl manganese tricarbonyl (MMT) is an organometallic compound with the formula (CH3C5H4)Mn(CO)3. ...
Octane is an alkane hydrocarbon with the chemical formula CH3(CH2)6CH3. ...
An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuels octane rating. ...
Air-fuel ratio (AFR) is the mass ratio of air to fuel present during combustion. ...
Bold text The compression ratio is a single number that can be used to predict the performance of any engine (such as an internal-combustion engine or a Stirling Engine). ...
This article or section should include material from Spark gap A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. ...
The ignition system of an internal-combustion engine is an important part of the overall engine system that provides for the timely burning of the fuel mixture within the engine. ...
This article or section should include material from Spark gap A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed aerosol gasoline by means of an electric spark. ...
Fuel efficiency sometimes means the same as thermal efficiency, that is, the efficiency of converting energy contained in a carrier fuel to kinetic energy or work. ...
Not to be confused with censure, censer, or censor. ...
A knock sensor consists of a small piezoelectric microphone, on the engine block, connected to the engine's ECU. Spectral analysis is used to detect the trademark frequency produced by detonation at various RPM. When detonation is detected the ignition timing is retarded, reducing the knocking and protecting the engine. See also Automatic Performance Control (APC). Piezoelectricity is the ability of some materials (notably crystals and certain ceramics) to generate an electric potential[1] in response to applied mechanical stress. ...
“Microphones†redirects here. ...
In automotive electronics, an electronic control unit (ECU) is an embedded microcomputer that controls one or more of the electrical subsystems in a vehicle. ...
A spectrum analyzer is a device used to examine at the spectral composition of some electrical, acoustic or optical waveform. ...
Automatic Performance Control (APC) is a system that was introduced on turbo charged Saab H engines in 1982. ...
Consequences of Knocking Engine knocking has disastrous consequences for the engine, since it leads to the catastrophical wear of the combustion chamber walls, through particle wear for moderate knocking, to welding for serious knocking. This is due to the contact between those walls and high temperature gases resulting from the unwanted explosion. The processes also lead to a 'knocking' noise for the engine, that give its name to the phenomenon.
Pre-ignition Pre-ignition is a different phenomenon from detonation, explained above, and occurs when the air/fuel mixture in the cylinder (or even just entering the cylinder) ignites before the spark plug fires. Knocking (also called pinking or pinging)—technically detonation— in internal combustion engines occurs when fuel in the cylinder is ignited by the firing of the spark plug but burns too quickly, combusting completely before the optimum moment during the compression phase of the four-stroke cycle. ...
References - Charles Fayette Taylor, Internal Combustion Engine in Theory and Practice: Vol. 2, Revised Edition, MIT Press, 1985, Chapter 2 on "Detonation and Preignition", pp 34-85. ISBN 0-262-20052-X
- http://naca.central.cranfield.ac.uk/reports/1942/naca-report-727.pdf
- http://naca.central.cranfield.ac.uk/reports/1940/naca-tn-774.pdf
- http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930091978_1993091978.pdf
- http://www.avweb.com/news/pelican/182132-1.html
External links - Knock Knock. Who's There? Detonation! Understanding Detonation & How Water/Alcohol Injection Can Help Prevent It
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