Combustion, Timing, Cylinder pressure and Knock

[igo4bmx]

good write up from my tuner, about the whole combustion process...
:super:

Timing and Flame Travel

Under normal operating conditions, the combustion is initiated towards the end of the compression stroke by the spark plug. Following the spark a turbulent flame develops and travels through the pre-mixed fuel, air and burnt exhaust gases from the previous combustion cycle (there is always residual burnt exhaust gas carried through in the combustion chamber) until it reaches the combustion chamber wall, then extinguishes. Pictured below is the spark initiated on the compression stroke by a spark plug:



The following drawings of the piston on the compression/expansion strokes correspond to the pictures above. This is to give you a idea of what is physically happening:



30 degrees BTDC corresponds to the 30 degree picture.








The spark is discharged at 30 degrees BTDC (before top dead center), the flame becomes visible at 24 degrees BTDC. The flame, circular in view, travels outward from the spark plug. The irregular shape of the flame shows the turbulence that the flame is traveling. At TDC (top dead center) the flame is approx. 2/3 of the diameter of cylinder bore, showing that the combustion is close to being finished. The flame reaches the cylinder wall at 15 degrees ATC (after top center), but the combustion continues for approx. another 10 degrees ATC. The highest temperature and cylinder pressure are reached at the 15 ATC, or the extinguishing of the flame front. This is due to the complete combustion of the air/fuel/residual mixture, and in turn having the highest amount of energy released.

In conjunction with the pictures displayed, the following charts show graphically the cylinder pressure increase, mass fraction of fuel burned, and the volume fraction of unburnt/burnt gases in the combustion chamber.



The cylinder pressure vs crank angle shows that the maximum cylinder pressure occurs at 15 degrees ATC, showing that on the power stroke the maximum amount of cylinder pressure and temperature occurs. The variations of the cylinder pressure shown are due to the variations in intake charge and fuel amounts in each intake stroke.



The mass fraction of fuel burned in the combustion chamber vs crank angle shows that the maximum amount of fuel burned in the combustion chamber corresponds to the maximum cylinder pressure and temperature. This is due to the greatest amount of energy being released in the combustion chamber.



The volume fraction of unburnt to burnt gases vs crank angle shows the ratio goes to 1:1, meaning all the fuel is burnt in the combustion chamber releasing the maximum amount of energy.


Timing, Cylinder Pressure and Torque

The next aspect to deal with is adjusting timing in relation to the cylinder pressure developed from doing so. If the compression process is progressively advanced relative to top dead center, the compression stroke work transfer (from the piston to the cylinder gases) increases. If the end of the combustion process is progressively delayed by retarding the spark timing, the peak cylinder pressure occurs later in the expansion/power stroke and is reduced in magnitude. These changes reduce the expansion stroke work transfer from the cylinder gases to the piston. The optimum timing which gives the maximum brake torque, called maximum brake torque, or MBT occurs when the magnitudes of these two opposing trends just offset each other. The optimum spark timing will depend on the rate of the flame development and propagation, and the length of the flame travel path across the combustion chamber. These depend on the engine design and operating conditions, and properties of the fuel, air and burned gas mixture. Below is a graph of cylinder pressure vs crank angle:




From the graph you can see that the sooner the mixture is fired on the compression stroke the higher the cylinder pressure becomes on the power stroke. Note the huge jump in cylinder pressure when the ignition is advance from 30 degrees BDTC to 50 degrees BDTC.

The next graph that is displayed is the measure of torque in relation to the spark advance. You can see at 30 degrees the greatest amount of torque is produced, despite not having the greatest possible timing advance. The torque actually decreases as the spark advance is increased.

Knock and Pre-Ignition

Knock is the most important abnormal combustion phenomenon. Knock is created as the flame front travels across the combustion chamber, and the unburnt mixture ahead of the flame called end gas, is compressed. The compression of the end gas causes its pressure, temperature and density to increase. The product of this event is that the unburnt gases rapidly release its energy up to 25 times that of a normal combustion event. This in turn causes high frequency pressure oscillations inside the cylinder, that in turn produces a sharp metallic noise called knock.

The presence, or absence of knock reflects the outcome of a race between the advancing flame front and the precombustion reactions in the unburned end gas. Knock will not occur if the flame front consumes the end gas before these reactions have time to cause the fuel-air mixture to auto ignite. Knock will occur if the precombustion reactions produce auto ignition before the flame front arrives.

The other significant abnormal combustion phenomenon is pre-ignition, or surface ignition. Surface ignition is ignition of the 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 ignition by any source other than normal spark after normal ignition (post ignition). It may produce a single flame, or many flames. Uncontrolled combustion is most evident and its effects most severe after the spark plug fires (post ignition), the spark discharge no longer has complete control over the combustion process. Surface ignition may result in knock. Knock occurs following normal spark ignition is called spark knock to distinguish if from knock which has been preceded by surface ignition.

Below you can see surface ignition occurs in a combustion chamber:





The arrows indicate the area at which the hot spots are created. These hot spots ignite the mixture prematurely, and the blackish area that occurs is a uncontrolled flame front. Surface ignition can occur anywhere in the combustion chamber. Too much fuel in the combustion chamber (really rich) can initiate surface ignition to occur, and resulting in knock (i.e detonation).

[seneca]

nice writeup by your tuner. who is he?

[2point4DSM]

http://zhome.com/ZCMnL/PICS/detonation/detonation.html

http://auto.howstuffworks.com/engine.htm

[highmilehatch]

Originally Posted by seneca

nice writeup by your tuner. who is he?

www.evans-tuning.com

[TurboTagTeam]

How were them pictures taken? Infra red?