When researching engines, you may come across the term "knocking."
Knocking is a type of abnormal combustion that occurs in the combustion chamber of a gasoline engine. It is commonly described as a metallic sound, such as "clicking," "pinging," or "rattling."
However, not every such sound from an engine is knocking. Moreover, modern engines are equipped with knock sensors and electronic controls that detect knocking and adjust the ignition timing accordingly.
This article provides an easy-to-understand explanation of what knocking is, why it occurs, its relationship with octane rating, what knock sensors do, what happens if it's left unaddressed, and what to check if you experience knocking.
What is knocking?
In short, knocking is an abnormal combustion in the gasoline engine's combustion chamber where a portion of the air-fuel mixture spontaneously ignites, separate from the normal flame propagation, causing rapid pressure fluctuations and vibrations.
In a typical gasoline engine, the combustion process follows these steps:
- Air and gasoline are drawn into the cylinder.
- The air-fuel mixture is compressed.
- A spark plug ignites the mixture.
- The flame spreads within the combustion chamber.
- The resulting combustion pressure pushes the piston.
However, if the temperature or pressure in the combustion chamber becomes too high, the air-fuel mixture that has not yet been reached by the flame may spontaneously ignite. This leads to rapid pressure fluctuations within the combustion chamber, which are transmitted as vibrations to the engine structure.
Difference between normal combustion and knocking
In normal combustion, the flame initiated by the spark plug spreads relatively orderly within the combustion chamber.
In contrast, knocking occurs when the unburnt air-fuel mixture spontaneously ignites due to high temperature and pressure while the normal flame is still propagating.
The rapid pressure wave generated at this time propagates within the combustion chamber and is transmitted to the engine as vibrations. Under certain conditions, these vibrations can be heard as "clicking" or "pinging" sounds.
Therefore, knocking is not the sound of "the engine physically hitting something," but rather caused by pressure vibrations resulting from abnormal combustion.
Why does knocking occur?
Knocking does not occur due to a single cause. The primary contributing factors include:
- Temperature inside the combustion chamber.
- Pressure inside the combustion chamber.
- Ignition timing.
- Compression ratio.
- Boost pressure.
- Fuel octane rating.
- Intake air temperature.
- Condition of the combustion chamber.
Simply put, the more conditions that favor premature autoignition of the air-fuel mixture, the higher the likelihood of knocking.
Cause 1: Octane rating does not match the engine
The octane rating is extremely important when considering knocking. The octane rating is an indicator of a gasoline's resistance to knocking. A higher number indicates higher knock resistance.
Next Science's "What is Octane Rating? Differences between Premium and Regular Gasoline, and Relationship with Knocking" also explains octane rating as one of the main differences between regular and premium gasoline.
In high-compression ratio engines and high-performance turbo engines, where combustion chamber temperatures and pressures tend to be high, the engine may be designed to require fuel with high knock resistance.
Therefore, if a vehicle designated for premium gasoline uses fuel with a lower octane rating than specified by the manufacturer, the likelihood of knocking may increase.
The basic rule is to always use the fuel specified by the vehicle manufacturer.
Cause 2: High compression ratio
When the compression ratio is high, the air-fuel mixture is compressed more intensely. As a result, pressure and temperature tend to increase.
A high compression ratio is advantageous for improving thermal efficiency, but it also makes anti-knock measures important.
In other words, a high compression ratio is not inherently bad. To effectively utilize a high compression ratio, combinations of combustion chamber design, exhaust system, fuel injection, ignition control, and knock control are employed.
Cause 3: High intake air temperature
When the temperature of the air entering the engine increases, the temperature inside the combustion chamber also tends to rise.
Especially in summer, during traffic jams, under high load, or in turbocharged cars, intake air temperature and engine ambient temperature can increase.
Honda also explains that in high-temperature environments, increased intake air temperature affects engine performance and reliability, and knocking can occur depending on combustion conditions.
For this reason, in turbocharged vehicles, intercoolers, intake air temperature management, ignition control, and fuel injection are also crucial.
Cause 4: High boost pressure
Engines using turbochargers or superchargers compress air and send it into the engine.
Compressing air causes it to become denser, hotter, and increases combustion pressure. Therefore, knock prevention is crucial in high-boost engines.
That is why in turbocharged cars, fuel, intercoolers, ignition timing, and boost pressure control are designed as a set.
Cause 5: Inappropriate ignition timing
In gasoline engines, when the spark plug ignites is extremely important. Advancing the ignition is called ignition advance.
Generally, advancing the ignition timing within an appropriate range can improve efficiency and output. However, if the ignition is too early for the conditions, combustion pressure can become too high, increasing the likelihood of knocking.
For this reason, modern engines use knock sensors to precisely control ignition timing.
Cause 6: Combustion chamber temperature becomes too high
If the engine internal temperature becomes too high, the air-fuel mixture is more likely to self-ignite. Therefore, the state of the cooling system, engine water temperature, intake air temperature, and high-load operation are all related to knocking.
However, high water temperature does not necessarily mean knocking. Modern cars manage various conditions, including temperature, with the ECU.
Cause 7: Deposits in the combustion chamber
Depending on mileage and usage, deposits may accumulate in the combustion chamber.
The presence of deposits can affect the effective combustion chamber shape, local temperature, and heat retention. As a result, knocking may become more likely under certain conditions.
However, it cannot be definitively stated that "carbon deposits always cause knocking." Knocking occurs due to a combination of multiple factors.
What is a knock sensor?
Modern gasoline cars use knock sensors. A knock sensor is a sensor that detects vibrations transmitted to the engine block and other parts.
When knocking occurs, specific vibration components are transmitted to the engine. The ECU uses this vibration information to:
- Determine the possibility of knocking.
- Retard ignition timing.
- Adjust control as necessary.
These actions are then taken.
Why does retarding ignition timing occur when knocking happens?
Retarding ignition timing is called ignition retard.
When knocking is likely to occur, retarding the ignition timing can suppress the peak combustion pressure, making it easier to avoid knocking.
However, retarding ignition timing may lead to a decrease in engine efficiency, torque, and output.
In other words, modern engine control constantly balances "wanting to use the most efficient ignition timing while avoiding knocking."
Therefore, if fuel with a lower octane rating than specified is used, the ECU may adjust the ignition timing to avoid knocking, making it difficult for the engine to deliver its full performance.
Does knocking reduce power?
If control intervenes to avoid knocking itself, it may result in a decrease in output.
Honda also explains that if knocking occurs under high-temperature conditions, performance may temporarily need to be reduced to protect the engine.
In other words, knocking does not directly increase power.
Rather, in modern vehicles, control may detect knocking, retard ignition, and if necessary, limit torque.
What happens if knocking is left unaddressed?
A brief, light knock may not immediately damage the engine. Modern cars also have knock control systems.
However, if strong knocking occurs for a prolonged period or repeatedly, it places significant pressure and heat stress on the combustion chamber.
Honda also explains that knocking can damage the engine and adversely affect both performance and reliability.
Therefore, it is not recommended to ignore it for a long time, thinking "it's just a rattling sound, but it's still running, so it's fine."
What does knocking sound like?
It is generally described as a clicking, pinging, rattling, or a light metallic tapping sound.
It may be heard particularly on inclines, at low RPMs under high load, during sudden acceleration, or at high temperatures.
However, a sound alone cannot definitively determine knocking. Similar sounds can come from other sources such as valve train, timing chain, injectors, auxiliary components, exhaust system, or heat shields.
If abnormal noises persist, it is safer to have your vehicle inspected at a repair shop.
Is knocking more likely when "pressing the accelerator hard at low RPMs"?
It may become more likely under certain circumstances.
For example, if you press the accelerator hard at low RPMs while in a high gear, the engine speed may be low, but the cylinders may be under significant load. This can create severe conditions within the combustion chamber.
However, modern ATs and CVTs will shift gears as needed or control ignition and throttle.
In other words, low RPMs do not necessarily mean knocking. The combination of engine load, temperature, fuel, and control is what's important.
Are knocking and pre-ignition the same?
They are not the same. Knocking is an abnormal combustion where unburnt air-fuel mixture spontaneously ignites after normal ignition.
On the other hand, pre-ignition is a phenomenon where the air-fuel mixture ignites before the spark plug fires.
Both are abnormal combustions, but their timing and mechanisms of occurrence differ.
What is LSPI?
LSPI (Low Speed Pre-Ignition) has become a topic of discussion recently, particularly in small-displacement direct-injection turbo engines. In Japanese, it's called "low-speed pre-ignition" (low-speed early ignition).
This is a phenomenon where abnormal ignition occurs before the spark plug fires, under low-speed, high-load conditions. It is a different phenomenon from general knocking.
Therefore, it is more appropriate not to assume that "anything that causes abnormal combustion at low speeds is knocking."
Is it the same as diesel knock?
Diesel engines also have a term called "diesel knock." However, the combustion method differs from knocking in gasoline engines.
Gasoline engines primarily use spark plugs for ignition. Diesel engines, on the other hand, inject light oil into hot compressed air, causing it to self-ignite.
Therefore, the concept of abnormal combustion and the important fuel indices also differ. Octane rating is important for gasoline, while cetane rating is important for diesel.
This difference will be explained in detail in a future article, "What is Cetane Rating?"
Will using premium gasoline always eliminate knocking?
Not necessarily.
If the cause is fuel with an octane rating lower than the manufacturer's specification, then switching to the appropriate fuel is crucial.
However, if the cause lies in cooling system abnormalities, ignition system, sensors, combustion chamber condition, supercharging system, engine control, or other issues, simply switching to premium gasoline may not solve the problem.
Using premium gasoline in a regular gasoline vehicle does not resolve all knocking issues.
The basic rule is to always use the manufacturer-specified fuel, and if abnormalities still occur, investigate the cause.
Can fuel additives fix knocking?
This cannot be stated uniformly.
Fuel additives come in various types with different purposes, such as cleaning, octane adjustment, fuel stability, rust prevention, and improving combustion.
Next Science also explains that not all fuel additives are the same, and each has a different purpose.
Therefore, fuel additives are not products that "fix knocking."
Furthermore, fuel additives cannot repair knock sensor malfunctions, cooling system abnormalities, ignition system abnormalities, or mechanical problems within the engine.
If abnormalities persist, prioritize diagnostic testing.
Does old gasoline cause knocking?
Gasoline stored for a long time can change in condition due to oxidation and evaporation. However, it cannot be simply concluded that old gasoline always causes knocking.
Changes in fuel condition can affect starting performance, combustion state, engine feel, and other factors.
For long-term stored fuel, Next Science's article "Does gasoline degrade? Changes in old gasoline and precautions for long-term storage" provides a detailed explanation.
Is poor acceleration also knocking?
This is also not necessarily the case.
Symptoms like a heavy-feeling engine or poor acceleration can have many causes, including ignition, intake, fuel, exhaust, oil, sensors, friction, and transmission.
Next Science's "What causes an engine to rev poorly? Explaining reasons for a heavy-feeling engine" also explains the causes of poor acceleration from multiple perspectives.
It is possible that knocking causes ignition timing to be retarded, resulting in reduced power. However, poor acceleration does not necessarily mean knocking.
What to check when you feel knocking
First, you should check:
- Are you using the fuel specified for your vehicle?
- Did the symptoms appear after refueling?
- Does it only happen at high temperatures?
- Does it only happen on inclines or during sudden acceleration?
- Is the engine warning light on?
- Is there any abnormality in the water temperature?
- Is the abnormal noise continuous?
It is.
Especially if you put regular gas in a car that requires premium, if the warning light is on, if a water temperature warning appears, if a strong metallic sound persists, or if output drops sharply, it is best not to continue driving under high load.
Fundamentals of preventing knocking
The most basic measures to prevent knocking are:
- Use the manufacturer-specified fuel.
- Maintain the cooling system in good working order.
- Properly maintain the ignition and fuel systems.
- Use the manufacturer-specified engine oil.
- For modified vehicles, ensure appropriate fuel mixture, ignition, and boost pressure settings.
It is important.
Specifically, it is not recommended to excessively use fuel additives based on self-diagnosis, switch to unapproved high-viscosity oils, or disable sensors with the aim of avoiding knocking.
For additive handling, please also refer to "Should you add fuel additive every time? Frequency, timing, and precautions for overuse" and "Are there disadvantages to fuel additives? Precautions to know before use."
Relationship between knocking and fuel economy
When knocking is about to occur, modern engines employ control measures such as retarding the ignition timing. As a result, torque, combustion efficiency, and output may deviate from their normal states.
Honda also explains cases where performance temporarily needs to be reduced to avoid knocking.
However, poor fuel economy does not necessarily mean knocking is occurring.
Fuel economy is influenced by many factors, including tires, driving style, oil, vehicle weight, ambient temperature, friction, and combustion.
For a comprehensive look at poor fuel economy, please refer to "10 Causes of Poor Car Fuel Economy | From Tires, Engine, Friction, and Combustion."
Next Science's view on "combustion state"
At Next Science, we emphasize not only the type of fuel but also how that fuel combusts inside the engine.
Knocking is also an important phenomenon for understanding the combustion state. However, knocking is not solely a fuel issue.
Intake, temperature, ignition, fuel injection, compression, electronic control, and the condition of the combustion chamber all combine to determine the final combustion.
Therefore, when assessing engine condition, it is important to consider not just one value or component, but the entire set of conditions leading up to combustion.
Our technological approach is introduced in Technology, and our testing, measurement, and comparison methods are described in Research & Development.
Frequently Asked Questions
What is knocking in simple terms?
It is an abnormal combustion in the gasoline engine's combustion chamber where unburnt air-fuel mixture spontaneously ignites, separate from the normal flame propagation, causing rapid pressure vibrations.
What does knocking sound like?
It is generally described as "clicking," "pinging," or "rattling." However, similar abnormal noises exist, so it cannot be definitively identified by sound alone.
Will using premium gasoline stop knocking?
If the cause is the fuel's octane rating, it may improve, but it may not be resolved if the cause is elsewhere, such as the cooling system, ignition system, sensors, supercharging system, etc. Always use the manufacturer-specified fuel first.
Does knocking damage the engine?
If severe knocking occurs for a prolonged period or repeatedly, it can place significant heat and pressure loads on the engine, potentially leading to damage.
What does a knock sensor do?
It detects engine vibrations and transmits signs of knocking to the ECU. The ECU then uses this information to adjust ignition timing and other parameters.
What is the relationship between octane rating and knocking?
Octane rating indicates a gasoline's resistance to knocking. A higher number means the fuel is less prone to autoignition and has higher knock resistance.
Summary: Knocking is "abnormal combustion" in engines
Knocking is an abnormal combustion in the gasoline engine's combustion chamber where unburnt air-fuel mixture spontaneously ignites, causing rapid pressure vibrations.
Key contributing factors include octane rating, compression ratio, intake air temperature, ignition timing, boost pressure, combustion chamber temperature, and the condition of the combustion chamber.
Modern vehicles use knock sensors and ECUs to detect knocking and adjust ignition timing and other parameters.
While control mechanisms may activate for temporary knocking, if strong abnormal noises persist, a warning light illuminates, the temperature rises, or output significantly drops, it's crucial to stop driving under high load and have the vehicle inspected.
The most fundamental countermeasure is to use the fuel specified by the vehicle manufacturer.
Understanding knocking makes it easier to comprehend why some vehicles require premium gasoline, why octane rating is important, and why combustion control is crucial for high-compression ratio and turbo engines.
Knocking is not just a "sound" issue; it's a critical phenomenon for understanding the engine's combustion state.
References (Official Manufacturer Information)
Official documents discussing high compression ratio, knock detection, supercharging and intake cooling, and combustion chamber temperature management mentioned in the main text.