What is Abnormal Combustion?

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Abnormal Combustion:

When unburned fuel/air mixture beyond the boundary of the flame front is subjected to a mixture of warmth and pressure for a specific duration (beyond the delay period of the fuel used), detonation may occur. Detonation is characterized by an almost instantaneous, explosive ignition of a minimum of one pocket of fuel/air mixture outside of the flame front. An area shockwave is made around each pocket, and also the cylinder pressure will rise sharply – and possibly beyond its design limits – causing damage.


If detonation is allowed to persist under extreme conditions or over many engine cycles, engine parts are often damaged or destroyed. the best deleterious effects are typically particle wear caused by moderate knocking, which can further ensue through the engine’s oil system and cause go down other parts before being trapped by the filter. Such wear gives the looks of abrasion, abrasion, or a “sandblasted” look, just like the damage caused by hydraulic cavitation. Severe knocking can cause catastrophic failure within the style of physical holes melted and pushed through the piston or plate (i.e., rupture of the combustion chamber), either of which depressurizes the affected cylinder and introduces large metal fragments, fuel, and combustion products into the oil system. Hypereutectic pistons are known to interrupt easily from such shock waves.


Detonation may be prevented by any or all of the subsequent techniques:

• Retarding ignition timing
• The use of a fuel with high octane number, which increases the combustion temperature of the fuel and reduces the proclivity to detonate
• Enriching the air-fuel ratio which alters the chemical reactions during combustion reduces the combustion temperature and increases the margin to detonation
• Reducing peak cylinder pressure
• Decreasing the manifold pressure by reducing the throttle opening or boost pressure
• Reducing the load on the engine

Because pressure and temperature are strongly linked, knock can even be attenuated by controlling peak combustion chamber temperatures by compression ratio reduction, exhaust gas recirculation, appropriate calibration of the engine’s ignition timing schedule, and careful design of the engine’s combustion chambers and cooling system additionally as controlling the initial air intake temperature.

The addition of certain materials like lead, and thallium will suppress detonation extremely well when certain fuels are used. The addition of lead tetraethyl (TEL), a soluble organolead compound added to gasoline, was common until it absolutely was discontinued for reasons of toxic pollution. Lead dust added to the intake charge also will reduce knock with various hydrocarbon fuels. Manganese compounds also are accustomed to reduce knock with petrol fuel.



Knock is a smaller amount common in cold climates. As an aftermarket solution, a water injection system will be employed to scale back combustion chamber peak temperatures and thus suppress detonation. Steam (water vapor) will suppress knock although no added cooling is supplied.

Certain chemical changes must first occur for knock to happen, hence fuels with certain structures tend to knock more easily than others. Branched-chain paraffin tends to resist knock while open-chain paraffin knocks easily. it’s been theorized that lead, steam, and also the like interfere with a number of the varied oxidative changes that occur during combustion and hence reduce knock.

Turbulence, as stated, encompasses an important effect on knocking. Engines with good turbulence tend to knock but engines with poor turbulence. Turbulence occurs not only while the engine is inhaling but also when the mixture is compressed and burned. Many pistons are designed to use “squish” turbulence to violently mix the air and fuel together as they’re ignited and burned, which reduces knock greatly by speeding up burning and cooling the unburnt mixture. One example of this is often all modern side valve or flathead engines. a substantial portion of the headspace is created to come back in close proximity to the piston crown, making for much turbulence near TDC. within the time period of side valve heads, this wasn’t done, and away lower compression ratio had to be used for any given fuel. Also, such engines were sensitive to ignition advance and had less power.



Knocking is more or less unavoidable in diesel engines, where fuel is injected into highly compressed gas towards the top of the compression stroke. there’s a brief lag between the fuel being injected and combustion starting. By this point, there’s already a quantity of fuel within the combustion chamber which is able to ignite first in areas of greater oxygen density before the combustion of the whole charge. This increase in pressure and temperature causes the distinctive diesel ‘knock’ or ‘clatter’, a number of which must be allowed for within the engine design.

Careful design of the injector pump, fuel injector, combustion chamber, piston crown, and plate can reduce knocking greatly, and modern engines using electronic common rail injection have very low levels of knock. Engines using indirect injection generally have lower levels of knock than direct injection engines, thanks to the greater dispersal of oxygen within the combustion chamber and lower injection pressures providing a more complete mixing of fuel and air. Diesel actually doesn’t suffer precisely the same “knock” as gasoline engines since the cause is thought to be only the in no time rate of pressure rise, not unstable combustion.

Diesel fuels are literally very liable to knock in gasoline engines but within the diesel motor, there’s no time for knock to occur because the fuel is just oxidized during the expansion cycle. within the internal-combustion engine, the fuel is slowly oxidizing all the time while it’s being compressed before the spark. this enables changes to occur within the structure/makeup of the molecules before the very critical period of high temperature/pressure.

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