Finding the Culprit: A Detailed Analysis of the Causes of Engine Detonation in Cars
As cars become increasingly integrated into daily life, more and more vehicle usage issues are coming to light. One common issue that has recently surfaced is engine detonation (knocking). Many people who aren't familiar with this phenomenon feel confused when they hear about it. Some even give up on certain models they like due to this concern. For example, a friend really liked the Focus but asked me if it was as prone to knocking as rumored online.
In fact, engine knocking isn't complicated; once you understand its causes, you can take targeted measures to solve the problem. Below, we'll delve into the "culprit" behind this issue together with Car Exploration.
What is Detonation?
In gasoline engines, when the air-fuel mixture enters the combustion chamber during the intake stroke, it is compressed by the piston during the compression stroke. The spark plug ignites the high-pressure mixture, and the pressure generated by combustion is converted into the power that drives the engine.
Although the process of engine combustion can be described simply in a few words, research on internal combustion alone has produced countless doctoral and master's theses. Many scholars and engineers have spent their entire lives studying the science of combustion. Therefore, truly understanding an engine requires a great deal of effort.
Due to the complexity of engine combustion, precise design and control are essential. Even slight control errors or abnormalities can lead to abnormal combustion, and "detonation" is one form of abnormal combustion. Simply put, detonation refers to abnormal pressure within the combustion chamber caused by irregular combustion.
Causes of Detonation:
Before discussing the causes of detonation, we need to understand two key points:
1. When the air-fuel mixture burns inside the combustion chamber, the flame spreads outward from the ignition point in a "wave." Thus, there is a brief time lag between ignition and complete combustion.
2. Although the air-fuel mixture needs to be ignited by the spark plug, overly high temperatures and pressures in the environment can also cause spontaneous combustion.
Typical detonation occurs when, after ignition in the combustion chamber, the flame has not yet fully spread, and the remote unburned air-fuel mixture spontaneously combusts due to high temperature or pressure. The collision of this flame with the regular combustion flame generates immense pressure, causing abnormal knocking in the engine.
Main Causes of Detonation:
1. Excessive ignition timing advance:
To ensure the piston immediately gains power at the start of the power stroke after reaching the top dead center (TDC), ignition typically occurs before the piston reaches TDC (since there is a time lag between ignition and full combustion). If ignition is too early, most of the air-fuel mixture will already be burned while the piston is still in the compression stroke, causing the unburned mixture to self-ignite under extreme pressure, leading to detonation.
2. Excessive carbon buildup in the engine:
Excessive carbon deposits in the combustion chamber not only increase the compression ratio (creating high pressure) but also generate hot spots on the carbon surface, which can cause engine detonation.
3. Overheated engine:
An engine operating in excessively hot conditions or poor coolant circulation can result in high engine temperatures, leading to detonation.
4. Incorrect air-fuel ratio:
A lean air-fuel mixture increases the combustion temperature, which raises the engine temperature and makes it more prone to detonation.
5. Low octane fuel:
The octane rating indicates a fuel's resistance to detonation. Higher octane ratings mean better resistance to detonation. High-compression engines operate under higher combustion chamber pressures, so using low-octane fuel can easily lead to detonation.
Detecting Detonation and Its Effects:
Detonation's English term is "Knocking," meaning knocking sounds. During detonation, the engine produces knocking sounds. Mild, intermittent detonation sounds clear, somewhat like lightly tapping a triangle. Severe and continuous detonation produces a "lililili" sound, and the engine will noticeably lose power.
Many car manufacturers set the ignition timing for the commonly used RPM range relatively advanced to maximize performance and reduce fuel consumption. Therefore, some engines may experience mild detonation between 2000 and 3000 RPM under heavy load. However, mild detonation does not significantly affect the engine, and owners don't need to worry too much.
However, severe detonation caused by engine problems such as excessive carbon buildup or poor cooling is very serious. Continuous and severe detonation at high RPM and high load can damage the spark plugs and pistons within minutes, or even crack the cylinders and destroy the engine itself.
Knock Sensor:
The fastest and most effective way to suppress detonation is to delay the ignition timing, reducing combustion pressure. The principle of operation for the knock sensor is to delay the ignition timing to a non-detonating point when it detects engine knocking. Once the knocking stops, the ignition timing is gradually restored.
The knock sensor uses an accelerometer to measure changes in the engine's acceleration, i.e., vibrations. Engineers program the vibration pattern of detonation into the ECU during calibration. Once the knock sensor detects this vibration pattern, the ECU determines that the engine is knocking and immediately delays the ignition timing. More advanced knock sensors can even identify which cylinder is knocking and individually delay the ignition timing for that cylinder.
Regarding detonation, what everyone cares about most is the type of gasoline to use. Actually, 93, 97, or 98 refers to the anti-knocking properties of gasoline, i.e., its "octane rating." What is "octane rating"? In researching the relationship between fuel and knocking, researchers found that "isooctane" resists knocking the best, while "normal heptane" is highly prone to knocking. Therefore, the anti-knocking ability of isooctane is rated at 100, and normal heptane at 0. Gasoline with an octane rating of 97 means its anti-knocking ability is equivalent to a mixture of 97% isooctane and 3% normal heptane.
So, this is purely an issue of anti-knocking capability. Adding higher-octane gasoline doesn't necessarily make the engine more powerful. Of course, if adding low-octane gasoline leads to knocking, or if the engine retards ignition during knocking, the car will indeed feel less powerful. In other words, as long as the engine doesn't knock, increasing the fuel's octane rating won't make the engine more powerful or fuel-efficient; it will just shrink your wallet.
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