Topics:
Knocking:
Knocking only occurs in spark-ignition engines (petrol / gas). Engine knocking (also called detonation or engine knocking) is the self-ignition in a petrol or gas engine. When the engine knocks, the fuel is not ignited by the spark plug, but ignites earlier by itself. In that case, the mixture ignites while the piston is still compressing it. The piston is moving up and the explosion has already taken place. The piston force is now greatly opposed, which can cause severe engine damage (such as a hole in the piston). Knocking only occurs in the low engine speed range.
Possible causes of knocking:
- The engine cooling is not in order: Because the engine gets extremely hot, the temperature in the combustion chamber also increases more than normal. During compression of the mixture, heat is also generated, which may cause it to ignite prematurely.
- Wrong type of petrol: When a car manufacturer specifies that petrol with octane rating RON 98 must be used, RON 95 must never be filled up. RON 98 has a lower tendency to ignite than RON 95. That means that 98 ignites less quickly than 95.
If 95 is used anyway, there is a high chance that the engine will start knocking, because the temperatures in the combustion chambers rise higher than in other engines. The fuel then ignites earlier than it should. It may even ignite while the piston is still compressing and the spark plug has not even produced a spark yet.
That is why 98 is suitable for higher temperatures and pressures in the combustion chamber. - Incorrect driving style: When climbing a steep hill at too low an engine speed, e.g. with a caravan or other heavy load, the chance of knocking is high. Cooling is then minimal and combustion pressures and temperatures are maximal. It is better to shift down a gear (from 5th to 4th), so that the engine runs at a higher speed. Often less throttle is needed and the engine may even use less fuel.
- Contamination: An incorrect driving style can also cause a lot of contamination in the engine. By always driving short distances and/or always driving at low revs (shifting up too early so that the engine never really gets going), a lot of engine contamination occurs. Due to the low revs, carbon deposits stick to the valves and the piston crown. When the engine is then subjected to a heavier load, these carbon deposits start to glow (just like on the barbecue) and take over the task of the spark plug. The fuel then ignites due to the glowing carbon deposits before the spark plug fires, which greatly increases the chance of knocking.
A turbo can also fail in this way, because the glowing carbon deposits can attack the material.
These kinds of cases occur a lot with “the new driving style”, where it is advised to shift up as early as possible. Driving in the city at a low engine speed certainly saves fuel, but as in example 1, when driving in the mountains with a caravan it can have fatal consequences for the engine and shifting down is actually more economical. Always driving gently always causes contamination problems. Especially with the newer generation of engines with EGR and variable valve timing etc. It is therefore good to rev the engine higher from time to time with a warm engine (e.g. up to three-quarters of the maximum engine speed).
Knock sensor:
In modern engines, knock sensors are being used more and more often. These sensors are mounted on the engine block. Often there are several mounted, one sensor between every two cylinders. During knocking, a specific vibration frequency occurs in the engine, which is recognized by the knock sensor. In that case, the engine control unit retards the ignition timing in steps of 3 degrees. The knock sensor basically ensures that the engine just does not knock and operates right at the limit. The engine control unit will constantly advance the ignition timing of all cylinders in steps of 0.5 degrees up to the normal value.
The image below shows a V8 engine block with 4 knock sensors mounted. The tips of the arrows indicate the knock sensors. They are a bit hard to see. The sensors are attached to the engine block with a single bolt. You can see the wiring of these sensors running towards the front.
Water injection:
To prevent knocking, water injection can also be used. The injected water vapor absorbs the heat in the combustion chamber. The temperature, and thus also the chance of knocking, decreases. This is particularly applicable to turbo engines that deliver a lot of power and therefore need to be well cooled.
