Definition of Detonation
 

Can one of you knowledgable gentleman please provide a technical definition of "detonation"? So many things cause it and I thought it was just the mixture lighting before the spark plug.

ThanksSmileWavy

Perhaps this will help:
http://www.rennsportsystems.com/2a.html

thanks

Can one of you knowledgable gentleman please provide a technical definition of "detonation"? So many things cause it and I thought it was just the mixture lighting before the spark plug.

Hello Max,

No, that is pre-ignition. "Detonation" is simply an explosion, as the word is defined in the dictionary. In normal combustion, the fuel mixture burns in a consistant pattern (around 2 ms), timed to peak cylinder pressure around 14 degrees ATDC. When detonation occurs, some of the unburnt fuel explodes, in a much smaller period of time, and causes a dramatic pressure spike at the wrong time. Unfortunately many texts repeat the same mistakes confusing knock, detonation, and pre ignition, but most engineering text or Bosch handbook define it correctly.

Paul

Thanks for the reply.

So detonation is basically when the air/fuel mixture is closer to soichometric "optimum" and explodes rather than burns?

So detonation is basically when the air/fuel mixture is closer to soichometric "optimum" and explodes rather than burns?

Hello Max,

No, it is not about mixture strength, it is more a function of octane, temperature, combustion chamber shape, compression ratio and ignition timing. Octane is the measurement of a fuel's resistence to detonate (knock). Knock occurs under load, with the highest probability at peak torque when VE is highest, which is why ignition timing curves have a kink in them. Hemi headed engines with wide VIA's and domed pistons (like 911's) have a lousy combustion chamber shape at TDC (like an orange peel), need more ignition advance for peak output, and are more knock prone for a given octane. Air cooling makes it worse. Water cooled, pent roof engines with narrow VIA's and flat pistons have a better CC shape at TDC and can have higher compression for a given octane. Excess fueling can be used a coolant to reduce knock (nothing below 14:1 actually burns), but it is limited. A recent MIT study shows that knock almost disappears when a small amount of ethanol is injected directly into the cylinder which would allow small high compression engines to run high boost when needed. The alcohol absorbs the heat that would normally cause the fuel to detonate.

Paul

So its not a difference in the rate of flame propagation during combustion (explosion vs. burn)? There are multiple ignition events in the chamber, some caused by excess energy in the outlying mixture supplying the needed activation energy to begin combustion before and without the spark plug firing. Is this more correct?

Thanks for the detailed explanations.

Hello Max,

Detonation is an abnormal explosion vs. a normal burn of the fuel. The "cause" is radiation heat from the advancing flame front reaching the spontaneous ignition point of a given fuel. The rapid pressure increase creates resonances of the CC walls making the Knock sound. Pre-ignition occurs earlier, before the spark event, and is caused by heat from compression and surface temperatures. A comparison of diesel and spark ignition pressure diagrams can help in understanding the difference. There are many variables in the "why" it occurs, octane, temperature (cylinder head and intake air), compression, CC shape, and they need to be isolated to avoid confusion. In 911 terms, the CC shape is not so good, more ignition advance is needed to get optimum burn, temperatures are not well controlled, and the knock margin is slim. The ultimate development of the hemi head N/A engine used fuel chemistry, mainly alcohols, to get around these problems with compression ratios over 14:1 and ignition timing out around 50 BTDC. Despite the corn politics, E85 can be a hemi's best friend.

I think that those were some pretty damn good explanations of "Detonation" and the other types and sources of combustion. Controlled detonation is what is wanted, VIA of proper ignition settings and components, the rest of the sources can be very harmful. Good job!! Tony.

Not my words




"Once detonation becomes serious enough, it disrupts the previously
well-organized thermal boundary layer and allows a greatly increased
rate of heat transfer from the very hot bulk combustion gases
(up around 4,000F) into the cylinder head and the piston. This last
stage in the process is what starts the damage, and drives the CHTs up."

Thanks for the very good information and explanations.