In book it is written:
In SI engine, the charge that autoignites is homogeneous and therefore intensity of knocking or the rate of pressure rise at explosive autoignition is likely to be more than that in CI engines where the fuel and air are not homogeneously mixed even when explosive autoignition of charge occurs. Therefore, it is often called detonation in SI engines.
But so far goes my knowledge, due to heterogeneous mixture of CI engine, fuel will ignite faster in area where oxygen is rich. The fuel around oxygen enriched areas combust much faster than other areas. Two (or many) different flame fronts from two (or many) different oxygen enriched areas may propagate simultaneously and hence the area between the flame fronts will be compressed, and this area will release energy all of sudden, at higher pressures and higher noises leading to detonation. Hence, my thinking is that detonation is more pronounced in CI engine rather than SI engine.
So, which one is true?
In SI engines, air fuel ratio is between 0,8 and 1,2. CI engines are able to run with a large exeed of air. Due to exhaust emission regulation, most modern SI engines run with stoechiometric air - fuel ratio = 1, which means, that the amount of air (oxigen) corresponds with the amount of fuel for a (theoretical) complete combustion. Knocking occures in SI engines because the (homogenous) mixture is self-ignited in advance to the spark-caused ignition, often at the outer diameter area of the combustion chamber. Once ignited, the homogenous mixture burns with high velocity, causing pressure waves. In CI engines, fuel is injected to initiate combustion start. There is no homogenous mixture, and combustion starts at the surface area of injected fuel droplets. In this way, no knocking occurs in CI engines.
Kind regards,
Mario
No knocking accors in CI engines .... it occurs only in SI engines... that's the reason why we are not going for the higher compression rations in case of SI engines.....
It is possible to produce knocking in CI engines: you just have to inject the fuel early enough. Because engine manufacturers know this, they ensure that the fuel injection doesn't occur too many °BTDC. You only have to look for "diesel HCCI knock" to see that the phenomenon is being observed (HCCI injects the diesel VERY early to produce a homogeneous mixture).
To answer your original question, because an SI engine is homogeneous, then the flame front can progress without obstacle through the chamber (premixed flame). The CI engine is non-homogeneous, so any flame front has to drive evaporation and mixing in order to propagate (non-premixed flame). Under the same conditions, the premixed flame will always be faster, therefore more devastating. However, the CI engine is operating at higher pressures and temperatures, so that would accelerate the non-premixed flame. My feeling is that the much larger size of the premixed flame would be worse.
On a side note, you're asking whether a bad phenomenon is "merely bad" or "very bad". Knocking will break your engine regardless of its mode of operation!
In CI engines, very early fuel injection may cause knocking, but this is not intended. Some type of early injection, so-called pilot injection, targets a more smooth combustion (lower noise), but due to the inhomogenous mixture distribution in combustion chamber, knocking does not occure - or is avoidable. In HCCI combustion, a controlled self-ignition is the target. Its possible in both, CI and SI engines (see publications from HONDA, more than 10 years ago). This controlled self-ignition has some characteristics of knocking, but it does not lead to the high pressure waves in combustion chamber, because controlled self-ignition starts in numberous areas at more or less the same time (in case of "real" knocking, the engine would be destroyed).
Kind regards,
Mario
Knocking in SI engine occurs due to the pre-ignition of the air fuel mixture. This means the charge is already heated to the self ignition temperature of the fuel before the spark plug ignites the mixture by generating spark. Hence the power is generated before completion of the stroke.
Whereas in case of CI engines, there is no spark plug and the fuel is ignited due to the temperature of the compressed air. When the temperature of the compressed air doesn't reach the self ignition temperature of diesel before reaching TDC of the cylinder, more fuel will accommodate in the cylinder and when it is ignited it results in the abrupt pressure rise causing audible knocking.
Case-1:
For increasing engine speed (if load remains same), there will be less detonation in an SI engine compared to a CI engine. Since the flame front speed will increase at higher turbulence & more amount of fuel (diesel) will be accumulated in combustion chamber, resulting in longer ignition delay period.
Case-2:
For higher engine loads, the tendency of knocking will increase in SI engines compared to CI engines. When load increases, more fuels will be admitted in combustion chamber of SI engines raising the compression, thus resulting in higher pressure & temperature of the end charge. This leads to auto-ignition of the end charges before the flame reaches there. But higher compression ratio in CI engines enhances the quality of combustion, i.e. reduction of delay period.
Case-3:
For same engine speed & load (i.e. power) the In SI engine, the charge that auto-ignites is homogeneous and therefore intensity of knocking or the rate of pressure rise at explosive auto-ignition is likely to be more (SI engines) than that in CI engines, where fuel and air are not homogeneously mixed even when explosive auto-ignition of charge occurs. The auto-ignition in SI engines, if takes place, can not be compensated by any means (even after increasing the turbulence of air-fuel mixture in the cylinder). But in CI engines since a multiple flame (different strength) exist in the charge, any sudden pressure rise after a long ignition delay period can be compensated by the air-fuel charge which are either too rich or too lean (non-favorable condition for the flame to sustain).
For all these reasons.... Detonation / knocking is more pronounced in SI engines compared to CI engines.
Dear Parag,
Compared to SI engines knocking in CI engines have less occurrence. This is due to time delay or ignition delay.
Detonation is a kind of knocking. Generally, it is an abnormal combustion in SI and CI engines. But, this is a very important phenomenon which occurs in both Spark Engines (SI) and Compression Ignition Engines (CI). Kind of knocking noise comes from the engine. The other names of knocking are knock, pinging, detonation etc. It reduces engine life. It is caused by fuel with a low octane rating, tendency for the fuel to pre-ignite or auto-ignite in an engine's combustion chamber.
In four strokes Spark ignition (SI) Engines air and fuel mixture i.e Petrol or Gasoline is used and this mixture is compressed and spark plug is used to produce combustion, which pushes the piston, resulting heat energy in mechanical energy. What exactly happens here, we can understand the mechanism by the air fuel mixture that self ignites in SI engines is homogeneous. In S.I. engines detonation is caused by the auto-ignition of end gas towards the end of combustion whereas knocking in CI engines is due to auto-ignition of the first charge at the start of combustion.
In CI Engines Only Air is taken in Combustion Chamber, Which is then compressed, Compression of air raises the temperature and fuel i.e, Diesel is injected through fuel injector, which ignites and combustion takes place. This fuel injection takes place same as SI Engines, But there is a phenomena called as Time delay or Ignition Delay, i.e, fraction of seconds, before piston reaching TDC(top dead Center) and fuel injection takes place which spread over piston head uniformly and temperature raises till the auto ignition point, and combustion takes place and expansion/power stroke occurs.
But, If we compare knocking, knocking in CI engines have less occurrence. Knocking mainly occurs because in earlier compression stroke temperature of air does not reach the auto ignition point resulting in combustion failure and huge amount of fuel concentrates in combustion chamber, during the next compression stroke the temperature raises before the critical point of piston movement and ignition takes place which knocks the piston, and produces knocking.
Ashish
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