Knocking, Detonation or Pinging

[PaykanHunter]

What causes the metallic noise/rattle noise when an engine experiences knocking?

 

Some suggest that the metallic noise is the sound of piston rings rattling, some suggest it comes from valve movements and others say it is the sound of two detonation waves colliding with each other. Can you shed some light on this? Cheers!

 

[conkerman]

Shock wave from uncontrolled combustion hammering at the block.

 

Sent from my MZ604 using Tapatalk

[Junkman]

What causes the metallic noise/rattle noise when an engine experiences knocking?

The big end bearings.

[barefoot]

Pinking?

[Barry Cade]

The correct name is pre ignition, where the boom occurs before the piston reaches TDC, and is trying to be pushed back down before it gets up.. which hammers away at the bearings on the little and big ends. Diesel knock is similar, but a lot more horribler.

 

I had a episode of premature combustion once. Trying to think about how many strokes a Wankel engine has usually delays it long enough so you don't lose compression.

[Uncle Jimmy]

Pinking is caused by the ignition being too far advanced. The fuel/air mix is exploding before the pistons have reached the top of their cycle.

Two ways to get over this; simply retard the ignition by adjusting your distributor; or obtain higher octane fuel.

[Asimo]

The sound is produced by the fuel / air mixture detonating rather than burning: the pressure in the combustion space rises viciously fast, the shock wave so produced being quite audible outside the engine.

 

The detonation happens for at least one of several reasons :

the timing is too advanced and although the burn starts ok, it gets squashed because the piston is still on the way ip, the remaining mixture detonates at some point.

there is something very hot in the combustion chamber, perhaps the wrong sort of spark plug or carbon build-up and the hot spot starts a burn early and that burn has increased pressure to a point where when the spark initiates the "proper" burn the mixture detonates.

The mixture is weak. Weak mixtures are more prone to detonation.

The fuel is more detonation prone.

Modern engines don't normally pink because they have sensors which hear the sound and change something, probably ignition advance, so as to stop detonation occurring. This ability allows them to run at much leaner mixtures and higher pressures hence their improved efficiency.

[the judge]

Its all in the.........

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Timing

[Bren]

A mate had a diagnostic done on his pez astra - knock sensor possibly at fault.

 

The knock sensor was doing a sterling job - the engine was very low on oil.

[alf892]

Oil mist can also cause pinking in worn engines..........

[PhilA]

The manual for my engine states that the knock sensor (a piezo-electric element of some description) is designed to be directional and is tuned to detect knock.

 

Watching the computer output, the ignition is retarded on a somewhat logarithmic scale based upon a percentage activation of the knock sensor.

 

 

However, I misunderstood detonation and pre-ignition, and the difference therein. Expanding a little on what Sorn Me said above- They both are bad for the engine but detonation is generally heard because it's more violent.

Gasoline burns at a particular speed, in fact any burning stuff has a measurable speed (think a line of gunpowder versus a trail of liquid fuel like alcohol in the movies).

 

So, you ideally want the mix to be ignited by the spark at the plug, and imagining in super-slow-motion the fuel starts burning outward, expanding. This brings into the physics of the "moment", that is, force multiplied by time. You can try this moment nonsense for yourself, easily. Stand by a halfway open door and give it a slap and see how far it moves. You might get lucky and close the door, likely though it'll not move very far. Now, move it back to the same place and give it a shove with equal force but keep your hand against the door longer. SLAM! You impart the same force for longer, the door moves with much greater acceleration.

The expanding flame front will push for a set period of time against the piston. The longer it pushes for, the better the engine goes.

 

However, it takes a while for the flame to expand to a useful size, so the mixture is ignited a bit before the piston gets to the top of its' travel. (How many degrees BTDC is your static timing set?) The higher the octane rating of the fuel, the slower it'll burn and expand for longer, giving a greater push. Lower octane rating fuels tend to burn faster and shorter. That's why your engine needed timing adjusting advanced for unleaded fuel. The push from unleaded occurs more quickly and for less time than four-star leaded so you have to start burning later in the cycle. With the timing set too early at "leaded" degrees the unleaded fuel has finished burning (and hence really pushing) against the piston before it's finished going downward- before the middle of the stroke. The tetra-ethyl lead acted as a stabiliser and also slowed down the flame front so you could ignite it earlier and get more torque and power from the engine, also acted to help inhibit compression detonation. Incidentally, oil leaked past the rings and valve seals can also have an effect of slowing the burn. As noted, it can also act as an agent to spur detonation.

 

The later the timing is set for crappier fuel, coupled with the less unstable fuel is a bad combination. Pre-ignition may occur where a hotspot (a piece of carbonized oil or gunge) glows with the compression heat and glows brightly enough to start the fuel burning in a smooth flame front, only it's too early and the burn begins to expand, pushing against the piston as it comes upward. If the flame front starts from the piston crown then the flame front will travel upward and hit the head and valves, causing a light plink. That's why it's more audible at lower engine speeds but at wider throttle.

 

Detonation (or "knock"), on the other hand is much worse for the engine. Detonation occurs where the fuel/air mix is unstable enough that towards the peak of compression the fuel doesn't burn, instead undergoes a highly rapid explosion. It ends up exerting almost all of its' energy in a very short period of time compared to a burning flame front. This is heard as the more crackling "TIK" of knock. If the knock sensor (ooh err, posh electronics) or the driver adjusts the timing retarded a little, the flame front is ignited by the spark plug before the fuel reaches critical detonation pressure. This is at the behest of engine performance.

 

 

Muddied the water any?

 

 

--Phil

[Pillock]

Wow. Are you a teacher? You should be a teacher. The slapping a door thing is the best way I've ever had that explained

[PhilA]

Wow. Are you a teacher? You should be a teacher. The slapping a door thing is the best way I've ever had that explained

No, my job mostly involves making the Internet get from place to place. Occasionally I have to explain to semi-technical people why it has broken. I have honed my explaining skills there.

 

 

--Phil

[PhilA]

Actually I was explaining this to a friend earlier tonight. I used another rotational force analogy, that is, the humble bicycle. Ultimately the pedal crank is doing a very similar job to the pistons and crank in the engine. Using a rather tongue-in-cheek method of explaining in more human terms, you easily recognise whilst riding a bicycle that the most effective point in time to push down on the pedal is between about 1 o'clock and 5 o'clock in the crank's revolution. You have the greatest effect at the 3 o'clock position, due to the geometry- the crank is furthest extended horizontally from the centre-line of the crank so you exert the most torque at that point (force x distance- you've read the figures for things in lb/ft). That's where the peak of your push should be. Same for the engine.

 

You want the peak expansion of the flame front to be where it has the most effect- a piston moving up and down spends (comparably) a lot of time at the top and a lot of time at the bottom of the cylinder, and moves rapidly from point to point between times. You need to catch the burn at the right moment for it to impart the greatest force (whilst the piston is moving down halfway between its' zenith and nadir) to get the best torque from the engine.

 

Slow burning fuels (four-star, let's say) may burn effectively from 1 o'clock to 5 o'clock if ignited at the 10 o'clock position in the crank's rotation.

 

Faster burning fuels (unleaded 95, say) may burn effectively from 2 o'clock to 4 o'clock if ignited at the 11 o'clock position in the crank's rotation. (We advanced the ignition for unleaded fuel).

 

 

The problem we see is that when the piston rises up in the compression stroke the temperatures rise and reach a critical point at, say, 10:30 position with unleaded and so the fuel detonates, pushing back against the piston hard (large force, short impact against the door? net result, the door moves only a little but WHAM it makes a big noise), so we need to change the point at which the spark ignites the fuel in a smooth motion.

 

So, we end up having to retard the ignition timing statically by twisting the distributor back a bit. Now, we ignite the unleaded at the 10 o'clock position again, now it burns from 1 o'clock to 3 o'clock. A lot of the push is occurring where it has less mechanical advantage against the crank, so we lose torque. The engine's performance is reduced.

 

This is where electronic ignition with a knock sensor really helps out with overall performance fuel economy and engine longevity. Instead of you hearing the engine pinking on poor fuel, stopping and getting out, twisting the timing back a bit and carrying on, you have the equivalent of someone under the hood while you drive, stethoscope to the engine listening for detonation.. he is trying to keep the distributor as far advanced towards its' peak at all times, backing it off until he hears the pinking noises cease.

When it works, it's effective, also why disconnecting the knock sensor from the engine and isolating it from vibration can provide more performance from the engine as it remains at the pre-programmed optimum timing determined for average fuel all the time... but you do run the risk of pre-ignition or detonation if you do.

 

(Yes, I appreciate this explanation doesn't cover flyweight and vacuum advance and retard, nor the reasons for their existence).

--Phil

[forddeliveryboy]

Great explains, PhilA. I always enjoy cars with remote timing adjustment from the driver's seat, since I'm probably a driving nerd.