In Internal combustion engines, abnormal combustion is a significant phenomenon associated with the combustion processes on which the life and performance of the engine depends. The two important abnormal combustion phenomenons are 1. KNOCK and 2.SURFACE IGNITION. These abnormal combustion phenomenons are of concern because (1) when severe, they can cause major engine damage; and (2) even if not severe, they are regarded as an objectionable source of noise by the engine.
Knock is the most important abnormal combustion phenomenon. It important because it puts a limit on the compression ratio at which an engine can be operated ,which in turn controls the efficiency and to some extent power output. It got the name
“Knock” because of the noise that results from the auto ignition of a portion of fuel air mixture ahead of the advancing flame. As the spark is ignited there is a formation of flame front and it starts propagating. As the flame propagates across the combustion chamber, speed of flame front is about 15-30 m/s ; the unburned charge ahead of the flame called the END GAS is compressed, raising its pressure, temperature and density. In case of abnormal combustion the end gas fuel air mixture undergo fast chemical reactions, which results in auto ignition prior to normal combustion (i.e. the flame front reaching it). During auto ignition a large portion of end gas releases its chemical energy rapidly and spontaneously at a rate 5 to 25 times as in case of normal combustion. This spontaneous ignition of the End gas raises the pressure very rapidly and causes high frequency oscillations inside the cylinder resulting in a high pitched metallic noise characterized as KNOCK. During this knocking phenomenon pressure waves of very large amplitudes propagate across the combustion chamber and very high local pressures are produced which are as high as 150 to 200 bars . Local 5% ofthe total charge is sufficient to produce a very violent serve knock. The velocity reached during knock is of the order of 300 to 1000 m\s.
Basically knock depends on the outcome of shorter of two different processes (i.e. Least time taken by one of the two processes) they are:
1. The advancing flame front grabbing all the fuel air mixture.
2. The pre combustion reaction in the unburned end gas. The time taken in this preparative phase of auto ignition (i.e. pre combustion reaction) is called “Ignition delay". Knock will not occur if the ignition delay is so long that the flame front consumes all the end gas and auto ignition takes place i.e. normal combustion occurs. Knock will occur if the pre combustion reaction produce auto ignition before the flame front arrives. Auto ignition when occurs repeatedly, the phenomenon is called “Spark Knock" .Spark knock is controllable by spark advance: advancing the spark increases the knock intensity and retarding the spark decreases the knock.
CHARACTERISTICS OF KNOCK :
v Knock in S.I. engine generally occurs at the end of combustion process during which the end gas charge is trapped between the advancing flame front and the engine. Cylinder periphery.
v Knock primary occurs under wide open throttle operating condition thus it is a direct constrain on engine performance.
v It also constrains engine efficiency, as it limits the temp and pressure of the end gas and thus limits the compression ratio of the engine
v The impact of knock depends on its intensity and duration. If knock is short duration usually called "Acceleration Knock" it is unlikely to cause damage. But “Constant Speed Knock" however can lead to engine damage.
The other important abnormal combustion phenomenon is surface ignition. Surface ignition is ignition of fuel air charge by overheated valves or spark plugs, by glowing combustion chamber deposits, or by any other hot spot in the engine combustion chamber. It is the initiation of flame front by a hot surface other than the spark plug.
Mostly surface ignition is due to carbon deposits.
Surface ignition may occur before the spark plug ignites the charge (pre ignition) or after normal ignition (post ignition). It may produce a single flame or multiple flames. Uncontrolled combustion is most evident and the results are most severe in case of pre ignition. Surface ignition may also causeknock . As due surface ignition there is a rapid and high rise of pressure and temperature than in case of normal combustion because the flame either starts propagating sooner, or it does from more than one sources, which likely results in Knocking.
Knocking is also an outcome of Surface ignition. To identify whether surface ignition causes knock or not, the term "knocking surface ignition" and "non-knocking s01urface ignition" are used. Knocking surface ignition usually occurs due to pre ignition caused by glowing combustion chamber deposits and can't be controlled by retarding the spark timing since knock is not due to spark ignited flame. While the non-knocking surface ignition occurs late in the operating cycle.
Different surface ignition phenomenon is wild ping, run on, run away, rumble, etc.
Knocking surface ignition may give rise to "Wild Ping” and non-knocking surface ignition to "Rumble". While both knocking and non-knocking surface ignition may give rise to "Run-On" and "Run-Away".
Wild ping is a variation of knocking surface ignition, which produces sharp cracking noise in bursts. Probably it results from early ignition of the fuel air mixture in the combustion chamber by glowing loose deposit particles. It disappears when the particles are exhausted and reappears when fresh particles break loose from the combustion chamber surfaces.
Rumble is a relatively stable low frequency noise (600-1200 Hz) phenomenon associated with deposit -caused surface ignition in high compression ratio engines. The pressure rises rapidly to a high value and resulting in engine vibration. Rumble and knock can occur together.
Run-on occurs when the fuel air mixture within the cylinder continues to ignite when the ignition system is switched off. During run-on the engine usually produces knock like noises. It is probably caused by the compression ignition of fuel air mixture (assisted by surface ignition).
Run-away is surface ignition which occurs earlier and earlier in the cycle. It is usually caused by overheated sparkplugs or valves. It the most destructive type of surface ignitions which may lead to disastrous results—seizure or melting of piston or the engine catching fire. It can lead to serious overheating and structural damage to the engine.
As discussed earlier, pre ignition is the phenomenon of surface ignition before the passage of spark. The usual cause is an overheated spot, which may occur at spark plugs, combustion chamber deposits, or exhaust valves. Mostly it is due to spark plug. Exhaust valve usually run hot and sometimes when there is increase in heat load for these valves there will be an increase in the temperature and may cause pre ignition. Heat transfer principles indicate that the surface of the deposits is hotter than the metal surface to which the deposits are attached. Hence, sufficient deposits result in hot enough surfaces to cause pre ignition.
Pre ignitionis potentially the most damaging surface ignition phenomenon. The effect of pre ignition is same as very advanced ignition timing. Any process that advances the start of combustion that gives maximum torque will cause higher heat rejection because of the increased burned gas pressures and temperatures (due to the negative work done during the compression stroke). Higher heat rejection causes higher temperature components thus the pre ignition damage is largely thermal which is evidenced by the fusion of spark plugs, piston and destruction of piston rings.