My good friend www.google.com has this to say:

http://en.wikipedia.org/wiki/Engine_knocking

Thought I would give you a couple of nice examples of what can happen if you allow "knocking" to go on too long.

Hi DA.ansari,

What type fuel this IC engine is designed to use?

Do you mean mechanical knock, or the pinging as already explained?. If mechanical, is due to increased clearances in working parts- & can result as the 2nd pic shows- thrown rods- but the 1st pic shows ping results- due to over advanced ign timing/weak mixture= ping= hole in piston/s. The 2nd rod throw is due usually to wear on rod bearings/crankshaft, resulting in knock, & in extreme cases, throw- usually caused by inadequate lubrication/ abuse/ over revving. There can also be knock from pistons, rockers, any rotating parts etc if worn- there can also be crankcase echo- where piston blow-by sounds as a knock.

And a loose connecting rod will sound like a little guy in there with a ball-peen hammer for awhile - then the rod will let go and it will sound like he touched off a stick of dynamite!

Spark knock can be caused by improper ignition timing, piston/head getting carbon build up raising compression ratio, low octane/cheap gasoline, improper spark plug and probably a few others but that's off the top of my head.

Mechanical knock, some already covered (rod bearings worn, loose rod bolts). Can range anywhere from a slight tick from a collapsed/weak hydraulic lifter or improperly adjusted valves to the little guy in there with a hammer, loose torque converter bolts and a host of other things.

Looking for something in particular?

One of the best examples of "knock" was on the drag strip I worked at many years ago. A tin indian wanted out: front half of the crank landed in the middle of the drag strip, back half locked up caused the drive shaft to break, it beat the heck out of the under side of the car, engine dropped down broke the distributor. I think they may have salvaged $5 bucks worth of nuts and bolts. When I got there the young driver/owner (who later became rather famous) and his buddies were standing around wondering why the indian got out when a younger kid (10 or 11) asked " Do you think it could have had anything to do with that noise we always thought was the tappets?" -- JHF

Rod bearings both crank and wrist pin, main bearings, wrong adjustment on the push rods, a bad throw-out bearing, loose exhaust components, pinging (wrong grade of gasoline, wrong timing, carbon buildup), Harmonic about to fly off.

just a few things!

It's broke and needs to be fixed. -- JHF

Threre are three reasons that I have heard to explain the "Knocking" sound generated from inproper combustion in internal combustion engines. One explination is from my personel experience with a steam engine!

While investigating a Camero equipped with a steam converted Mercury in-line six cylinder 2 cycle engine, a terrible "Knocking sound" was evident when operating the engine in reverse rotation. ( which was necessary in this unusual design to accomplish reverse gear in the Camero)This was explained to me as the sound of "piston slap" that occurred as a result of wrist pin offset, as the Mercury marine engine was not designed to run counter rotation when operating on gasoline.

This Knocking noise was identical to the noise that I sometimes heard in high compression high performance gasoline engines running on too low an octaine fuel- yet this was running with no combustion-only steam pressure!

Most all piston engines have "piston wristpin offset" that assures that the piston's downward thrust is dominant on the levered side of the center line axis of the wristpin. This design characteristic keeps the piston from "slapping its skirt" against the "non-thrust side" of the cylinder, or even "wobbling" in the cylinder. The offset is very slight, and the wobble would only be a few thousansths of an inch on a new engine, yet this slapping is audible under certian circumstances.

As the gasoline/air mixture is ignited by a spark plug, a "flame front" is generated that expands in the cylinder as the piston is thrust downward by the expanding oxidizing gases. Ignition spark is timmed to start this flame front about 20 degrees before top dead center (TDC) of the crankshaft rotation- so that the relative small movement of the piston at the top of its stroke can take advantage of "the traveling teather ball hit" momentium of the expanding gasses pushing the piston to where the crankshaft travels to its maximum leverage oppurtunity of right angular thust at halfway down the cylinder.

Hi octaine gasoline contains an ignition supressent to keep pre-igniting the air/fuel charge until the correctly timmed sparkplug fires.

Ping, or pre-ignition is a spark and flamefront generated from some soarce other than the spark plug. It could be a glowing ember of carbon, a high-compression anomly, an unusually hot sharp edge of a cylinder head casting ot sparkplug thread, too low an octaine fuel for your vehicles designed compression ratio,--whatever the inniation- now there are two flamefronts in the cylinder-- one created PRE-sparkplug ignition, and the other is the properly timed spark plug firing.

My understanding is that the knock noise is generated by one or more of these three happenings:

1. Piston slap as the two flamefronts push and "wobble" the piston in its cylinder due to an impballance of pressure on the top of the piston.

2. Piston and crank bearing noise as the working clearances of the components are abruptly jarred due to the two flamefronts prematurely burning up the fuel and thus prematurely release pressure on the piston/rod/crank assemblies.

3. The actual "supersonic boom" generated by colliding flame fronts.

Comments appreciated.

Answer 7 is correct in most all respects. However, modern engines are MUCH MORE FRAGILE that the old cast iron work-horses of old. But, most do have excellent engine management systems that prevent any significant combustion knock. If the 'knocking' is loud enough for you to hear in the passenger compartment, the engine is being stressed beyond design limits. Whether or not it will suffer mechanical breakage depends on duration, severity, and how long an observation period following the knocking. Combustion knock will break pistons and piston rings, pound out the rod bearings, crack cylinder heads and valve seats etc.etc.

MOST knocking experienced by motorists is 'detonation', which is caused when a residual portion of the fuel-air mixture (heated by the compression of the piston and main flame front expansion, self ignites and starts burning ahead of the spark ignited flame front. This second flame front is usually VERY FAST (almost explosive), unlike the main one which burns smoothly across the chamber. When the two flame fronts collide, very high pressures and pressure oscillation result--heard as KNOCK.

Sometimes it is light clicking, sometimes pinging, sometimes loud lower pitched thud thud. This is because different components in the gasoline sound different under knocking conditions. Light engine loads typically have light knock intensities and modest excess pressure peaks. Knocking under heavy loads generates much more destructive pressure waves. SOME engines can tolerate light to moderate detonation for long periods without apparent damage (the Jeep cast iron 6 is one) while most newer aluminum alloy designs will not. Aluminum loses strength rapidly as temperatures rise above about 300F.

Far more destructive is a different kind of 'knocking' called 'pre-ignition'. This results when a hot spot causes the fuel-air mixture to self ignite BEFORE the ignition spark. Peak pressure in the cylinder can reach 3-5 times HIGHER than normal, and peak temperatures are much higher. THis type of knocking will usually quickly result in piston failure and worse. Unfortunately, one usually cannot hear preignition because other normal engine noise when under high power usually masks it. Pre-ignition causes significant loss of power, increased water (or cylinder head) themeratures, lower exhaust temperatures, high emissions. A typical scenario is 'foot to the floor' up a grade (or race) for many seconds or minutes, and then pre-ignition starts suddenly, If power is not reduced engine destruction follows in seconds. Passenger cars seldom are driven this way. Pick-ups and SUV's towing heavy loads or high wind resistance loads ARE prime candidates for this kind of damage if high octane fuel is not used when driven under these conditions. EVEN if normal driving finds 87 ON fine, towing often requires the 92+ stuff to avoid detonaion and progression to pre-ignition.