Knock, detonation and pre-ignition
That metallic rattle when accelerating in a high gear, which people have always called pinking, is one of the things that can do most damage to a petrol engine. Here is what happens inside the cylinder, why knocking is not the same as pre-igniting and how the car defends itself.
Good combustion
In a petrol engine, the spark plug fires a little before the piston reaches the top, and from there the flame travels across the chamber in an orderly way, like a wave, pushing the piston as it starts to go down. It is fast combustion, but controlled.
Knock, or detonation
Sometimes, before the flame reaches the far side of the chamber, the mixture still waiting to burn, squeezed and hot, explodes on its own all at once. Two flame fronts collide and the pressure jumps sharply, hitting the piston like a hammer: that is knock, or detonation, and it is the metallic noise you hear. Illustration 1 shows it in red, in the second cylinder: a disorderly flare instead of a wave.
It happens more with an engine under heavy load at low revs, with fuel of a lower octane than specified, in heat, with carbon in the chamber or with too lean a mixture. Now and then, the engine copes; continuously, it erodes the pistons, breaks rings and can damage the head gasket.
Pre-ignition
Pre-ignition is even worse, and it is not the same thing. Here the mixture lights before the spark fires, from a hot spot: a glowing bit of carbon, a spark plug that runs too hot, the edge of a valve. The piston rises compressing a mixture that is already burning, and the pressure goes through the roof. It can hole a piston in very little time. Modern direct injection turbo petrol engines have a version of it that happens at low revs and high load, which is why the newest oils have formulations designed to prevent it.
Do not confuse this with diesel ignition: a diesel lighting itself through compression is exactly how it is supposed to work.
How the car defends itself
The engine has one or more knock sensors, small microphones bolted to the block. The engine control unit listens to each cylinder just when it fires and, if it hears knock, retards the ignition on that cylinder by a few degrees until it stops. Then it advances it again little by little. That protects the engine at the cost of a little power and a little more fuel.
The most common faults
- Knock sensor or its wiring
Without it, the engine control unit plays safe: it retards the ignition on every cylinder and the car goes slower and uses more.
Typical codes: P0325, P0327, P0328, P0330 - Knock caused by the fuel
Using fuel with a lower octane than the manufacturer specifies in a turbo or high-compression engine.
- Carbon in the chamber
Raises compression and leaves hot spots. More knock and a risk of pre-ignition.
- Lean mixture
An air leak or an injector delivering too little make the mixture burn hotter and more prone to knock.
Typical code: P0171
What you can see with Final OBD
On some cars the engine control unit reports how much it is retarding the ignition on each cylinder because of knock, and Final OBD shows it cylinder by cylinder: if one always takes more retard than the rest, something is going on there (carbon, a spark plug, an injector). On our own Abarth we are fine-tuning it so it shows its true values. On other cars you see the ignition advance from standard OBD and the knock sensor faults, explained.
Open the bonnetthe in-depth explanation
Octane: what it really means
The octane rating does not measure the energy in the fuel, but its resistance to detonating on its own. A 98 octane fuel withstands more compression and heat without knocking than a 95. That is why, in a car designed for 95, using 98 almost never gives more power: it only does if the engine control unit was retarding the ignition because of knock, for instance with heat and heavy load. In an engine that calls for 98, using 95 makes the control unit retard the ignition often, and you feel it in power and consumption.
Ignition advance
The spark fires before the piston reaches the top because the mixture takes a moment to burn, and maximum pressure has to arrive just as the piston starts going down. There is an ideal advance for each speed and load: with less, the engine loses power and heats the exhaust; with more, it knocks. Modern engines run very close to the knock limit to get the most out of them, relying on the sensors to back off in time.
Ignition advance is one of the standard OBD data items. At idle it is usually between 5 and 15 degrees, and it rises with revs. If, with the engine warm, you see that under load it stays well below what it shows in the same conditions at other times, the engine control unit may be retarding it because of knock.
How the control unit hears one specific cylinder
A knock sensor is a piezoelectric element that turns block vibrations into voltage. Knock has a characteristic frequency, several kilohertz, which depends on the cylinder bore. The control unit only listens in the time window in which each cylinder fires and filters that frequency: that way it knows which one knocked even if the sensor sits between two. With two sensors, each listens to the cylinders nearest to it.
When it detects knock in a cylinder, it takes a few degrees of advance away at once and then gives them back little by little. If the knock keeps happening, it learns to run less advance in that operating zone. That learning is what some cars show as retard per cylinder.
Pre-ignition in modern turbo engines
In small direct injection turbo petrol engines, at low revs and high load (flooring it in a high gear at 1,500 rpm), a very violent pre-ignition can happen. It has been linked to oil droplets and deposits getting into the chamber. The newest oil standards include specific tests to curb it, which is why using exactly the oil the manufacturer specifies matters so much on these engines. And driving them without lugging in high gears.
What damage it does
Light, occasional knock is corrected by the control unit and nothing happens. Heavy, continuous knock, for example with a carelessly modified engine map or a sensor that does not work, erodes the piston crown, breaks the lands between the rings and can blow the head gasket. And sustained pre-ignition can melt a piston in minutes. So if you hear a rattle when accelerating, it is not the engine being fussy: it is a warning.
All sections of the mechanics guide
See it in your own car
Almost everything in this guide can be seen with Final OBD and an adapter plugged in: faults explained, live data and the details of each system.
