When Knock, detonation, or pre-ignition occurs, the air/fuel mixture is ignited at an improper time in the cycle. Detonation is an uncontrolled combustion event which occurs after the spark event. Pre-ignition is an uncontrolled combustion event which occurs before the spark event.
- Unlike pre-ignition that occurs during the compression stroke, detonation occurs at the end of the power stroke. Detonation (knock) occurs when a small portion of the remaining air/fuel mixture spontaneous ignites away from the flame front started by the spark plug.
What does a pre-ignition knock sound like?
Engine spark knock, sounds like a metallic knocking, pinging or rattling noise, coming from your engine.
What are the symptoms of pre-ignition?
When pre-ignition happens, something ignites the Air/Fuel Mixture during the Compression Stroke. This creates too much pressure inside the cylinder, too soon. Common culprits are:
- Glowing hot or melted spark plugs.
- Glowing hot exhaust valve.
- Burning carbon embers.
How is pre-ignition different from knocking in SI engine?
Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires. Many engines have suffered such failure where improper fuel delivery is present.
What damage does pre-ignition cause?
Preignition creates excessive heat and substantial damage to the pistons, bearings, spark plugs and cylinders. Preignition is not easily detectable as it does not make an audible noise when it occurs. An engine experiencing Preignition will exhibit a lack of power and will run rough.
Is pre-ignition bad?
Damage: Damage from pre-ignition is much more severe and instantaneous than that from detonation. Typically, with pre-ignition, you will see holes melted in pistons, spark plugs melted away, and engine failure happens pretty much immediately.
How do you fix pre detonation?
There are several ways to cure pre-ignition:
- Run higher octane fuel. Premium gas rated at 92 or 94 octane is best for an engine with a compression ratio between 9.25 and 10.25:1.
- Run the engine on the rich side.
- Try playing with ignition timing.
Is detonation bad for an engine?
Because detonation interrupts that design, it robs the engine of power. Most engines can handle some minor detonation. Modern, fuel injected engines can sense a knock and adjust the Air/Fuel Ratio and Ignition Timing. However, if the detonation is not fixed, it will damage the engine.
Which is the best option to avoid knock in SI engine?
To prevent knock in SI engine the end gas should have low temperature, pressure and density, a long ignition delay and a non-reactive composition.
How detonation or knocking affects the performance of SI engine?
The usage of hydrogen in compression ignition engine leads to production of knocking or detonation because of its lower ignition energy, wider flammability range, and shorter quenching distance. Knocking combustion causes major engine damage, and also reduces the efficiency.
How does SI engine reduce detonation?
In spark ignition engines knocking can be reduced by:
- Increasing the cooling water temperature.
- Retarding the spark advance.
- Increasing the inlet air temperature.
Does detonation cause power loss?
In the engine tuning world, detonation is defined as one of the following: combustion that causes engine damage; combustion that causes banging or pinging noises; or combustion that causes power loss, bucking, or kicking. Detonation does not always cause damage.
Detonation vs Pre-Ignition
There are several methods to wreck a perfectly decent engine, but I’d like to focus on two of the more severe ones for the sake of this article. Even though the terms Detonation and Pre-Ignition are frequently used interchangeably and/or to describe the same event, they are actually entirely separate things that result in similar ends. Both of these conditions are referred to as aberrant combustion, and they are particularly detrimental to your engine’s performance. In order to effectively describe both Detonation and Pre-Ignition, I must first discuss regular combustion, which is necessary for both concepts.
During normal combustion, the flame front originates at the spark plug and travels outward evenly and steadily across the combustion chamber, starting at the spark plug.
Whenever you blow, the balloon grows in a very regulated and even manner away from the source of the air supply.
This transmission of heat occurs as the flame front transfers heat to the piston, which then transfers heat to the cylinder wall, which transfers heat to the coolant system, as shown in the diagram.
- This is not the case.
- In an ideal situation, after the spark plug ignites the mixture, the flame fills the cylinder in a very short period of time while remaining extremely regulated.
- Detonation always occurs after the regular combustion process has been initiated by the ignition source.
- This is most likely the result of excessive heat and pressure.
- There are a variety of components that come together to provide the optimal conditions for detonation to take place.
- An too advanced ignition timing causes the burn to end too soon, resulting in pressure building up too rapidly.
- As you can see in the illustration, the graph at the top has a smooth pressure profile and would be regarded to be a typical combustion process in most cases.
After then, you’ll notice a significant pressure surge caused by the aberrant combustion.
This resonance is what is detected by the knock sensor and sent to the ECU through the knock sensor.
Knowing what they are seeing with a monitoring device such as the Cobb Tuning AccessPort provides consumers with a window into what is going on with their engine at all times.
I get questioned a lot about the explosion that continues to occur even when the throttle is just partially opened.
Low levels of detonation occur on a regular basis and can even be sustained for extended periods of time without causing damage to the environment.
It will also present itself at shift points when the engine moves considerably between gear changes when the engine is running at full throttle, although this should not be reason for alarm.
It is recommended that you contact your tuner if you are experiencing substantial knock at Wide Open Throttle (WOT).
The pitting or abrasion on the piston crown is the less serious of the failure modes to consider.
The pitting appears to be the result of the piston being struck with bird shot from a shotgun.
You will frequently find yourself with crushed or shattered ring lands as a result of the sudden pressure surges.
When it comes to cast pistons, this is frequently the case since they were never intended to endure those kinds of cylinder pressures, let alone such abrupt and dramatic swings in pressure.
In addition, the pressure spikes cause the boundary layer of gas that quenches the flame front to break down, protecting the comparatively cold piston from being exposed to the relatively hot combustion that occurs.
This will ultimately result in the necessity to rebuild the engine.
Indicators: It is noticeable at higher degrees of explosion, and it will sound like quarters tapping on glass.
With a tuning device that detects knock retard, such as the Cobb Tuning Accesport, you are just watching the engines response to perceived noise, which is a very limited view.
Pre-Ignition: Definition: Pre-Ignition is the process of igniting the air/fuel combination before the spark plug ignites the mixture.
The fuel air mixture reaches the combustion chamber when the piston is on its downward intake stroke, allowing the combustion to take place.
Due to the fact that the more compressed a combination becomes, the more difficult it is to ignite it, while the piston is on the low side of the compression stroke, the mixture is actually simpler to ignite than when the piston is closer to the Top Dead Center position (TDC).
Now, the upward motion of the piston is in conflict with the increasing combustion force of the engine.
The pressure generated by pre-ignition is not as quick as the pressure generated by detonation.
Damage: The damage caused by pre-ignition is far more severe and immediate than the damage caused by detonation.
Because of the extended duration of the heat and pressure brought on by pre-ignition, you will observe a significant increase in the number of melted pieces, but with detonation, you will note an increase in the number of parts that are simply blasted apart.
The greatest thing you can do to avoid it is to ensure that the engine is set up as optimally as possible to reduce potential hot spots before you start.
When doing plug swaps and increasing the amount of boost in your engine, which results in greater cylinder temperatures, it is critical to ensure that you have the proper spark plugs and spacing in your ignition system.
Engine knocking – Wikipedia
It is known as knocking in spark ignition internal combustion engines. It occurs when combustion of some of the air/fuel mixture in the cylinder does not result from the propagation of the flame front ignited by the spark plug, but rather when one or more pockets of air/fuel mixture explode outside of the envelope of the normal combustion front. Knocking is also known as detonation, spark knock, pinging, or pinking in spark ignition internal combustion engines. The fuel-air charge is intended to be ignited solely by the spark plug and at a particular time in the piston’s stroke, according to the manufacturer.
- The shock wave produces the distinctive metallic “pinging” sound, and the pressure in the cylinder rises significantly as a result.
- There should be no confusion between knocking and pre-ignition Because these are two very different phenomena.
- Lodge Brothers (spark plug producers and sons of Sir Oliver Lodge) published a letter in November 1914 describing the occurrence of detonation as a result of a disagreement about the source of “Knocking” or “Pinging” in motorbikes.
- As a result of studies conducted out between 1916 and 1919 to determine the cause of failures in aviation engines, Harry Ricardo was able to further examine and characterize the phenomenon.
Within the confines of an ideal environment, an internal combustion engine burns the fuel/air combination contained within the cylinder in an ordered and regulated manner. According to a variety of conditions, including engine speed and load, the ignition is initiated by the spark plug at a position between 10 and 40 crankshaft degrees before top dead center (TDC). This advance in ignition provides enough time for the combustion process to reach its maximum pressure at the optimal moment for the most efficient recovery of work from the expanding gases.
As it grows in size, the amount of heat it produces rises, allowing it to grow at a faster pace and spread more quickly within the combustion chamber as it expands.
In normal combustion, the flame front travels across the fuel/air combination at a velocity that is distinctive to the particular mixture under consideration.
In order for the force applied to the piston (resulting from the increasing pressure applied to the top surface of the piston) to give its hardest push precisely when the piston’s speed and mechanical advantage on the crank shaft gives the best recovery of force from the expanding gases, thus maximizing torque transferred to the crankshaft, the maximum cylinder pressure is achieved a few crankshaft degrees after the piston passes TDC.
Detonation may occur when an unburned fuel/air mixture outside the boundary of the flame front is exposed to a combination of heat and pressure for an extended length of time (that is, longer than the delay period of the fuel utilized), as described above. It is distinguished by the rapid and explosive lighting of at least one pocket of fuel/air combination outside of the flame front, which takes place in a matter of seconds. A local shockwave is generated around each pocket, causing the cylinder pressure to increase abruptly – and perhaps beyond its design limitations – and resulting in damage to the cylinder walls.
It is possible for engine parts to be damaged or destroyed if detonation is permitted to continue under high conditions or for a long period of time.
The most straightforward negative impacts are typically generated by particle wear.
During severe knocking, physical holes are melted and pushed through the piston orcylinder head (i.e., rupture of the combustion chamber), either of which causes depressurization in the affected cylinder and the introduction of large metal fragments, fuel, and combustion products into the oil system, resulting in a catastrophic failure.
Detonation can be avoided by employing any or all of the strategies listed below:
- It is possible to lower detonation risk by delaying ignition time, using a high-octane gasoline with increased combustion temperature, or by using a high-octane fuel with reduced detonation risk. The air–fuel ratio is being increased, which modifies the chemical processes that take place during combustion, lowers the combustion temperature, and raises the margin for detonation. lowering the peak cylinder pressure
- Lowering the manifold pressure by lowering the throttle opening or boost pressure
- Lowering the strain placed on the engine
In addition to reducing peak combustion chamber temperatures through compression ratio reduction, exhaust gas recirculation, appropriate calibration of the engine’s ignition timing schedule, and careful design and construction of the engine’s combustion chambers and cooling system, knock can also be reduced by controlling the initial air intake temperature. When certain types of fuels are utilized, the inclusion of specific elements such as lead and thallium will significantly reduce the likelihood of explosion.
- The addition of lead dust to the intake charge will also help to lessen knock while running on various hydrocarbon fuels.
- Knock occurs less frequently in colder areas.
- The presence of steam (water vapor) will decrease knock even if no additional cooling is provided.
- Branched chain paraffins are more resistant to knocking than straight chain paraffins, which are more susceptible to knocking.
- As previously indicated, turbulence has a significant influence on knock.
- It is not only during the engine’s inhalation that turbulence occurs, but it is also during the compression and burning of the mixture.
- For example, all current side valve or flathead engines are examples of this.
This was not done in the early days of side valve heads, which resulted in a far lower compression ratio being required for any given fuel.
Indiesel engines, in which fuel is injected into highly compressed air at the end of the compression stroke, are prone to knocking, and it is almost impossible to avoid.
By this point, there is already a significant amount of fuel in the combustion chamber, which will ignite first in places with higher oxygen densities before the entire charge is burned.
Knocking may be significantly reduced by careful design of the injector pump, fuel injector, combustion chamber, piston crown, and cylinder head, and newer engines that use electronic common rail injection have extremely low levels of the phenomenon.
Diesel engines do not suffer from the same “knock” as gasoline engines since the cause is only known to be the extremely rapid rate of pressure rise, not unstable combustion, as the source of the knock in gasoline engines.
During the compression stage of a gasoline engine, the fuel is slowly oxidizing all the while as it is compressed before the spark is ignited.
Because of this, changes in the structure and make-up of the molecules might take place before the extremely important period of high temperature and pressure.
Because of the wide range of variations in fuel quality, air pressure, and ambient temperature, as well as the likelihood of a malfunction, every contemporary internal combustion engine is equipped with devices to detect and prevent knocking from occurring. A control loop is constantly checking the signal from one or more knock Sensors to ensure that they are functioning properly (commonlypiezoelectric sensorwhich are able to translate vibrations into an electric signal). Upon detection of the typical pressure peak of a knocking combustion, the ignition time is postponed by a few degrees in increments of a few degrees.
Today’s knock control loop systems have the capability of adjusting the ignition timings for each cylinder on an individual basis.
In this manner, performance is maintained at its peak while the danger of engine damage caused by knock is reduced to a large extent, for example, while using low-octane gasoline.
Given how critical it is for development engineers to avoid knocking combustion, a range of simulation systems have been created to help them determine if an engine design or operating state is likely to result in knocking combustion. Using this information, engineers can devise strategies for mitigating knocking combustion while yet maintaining good thermal efficiency. Because the onset of knock is sensitive to the in-cylinder pressure, temperature, and autoignition chemistry associated with the local mixture compositions within the combustion chamber, simulations that take into account all of these aspects have proven to be the most effective in determining knock operating limits and enabling engineers to determine the most appropriate operating strategy for their particular engine.
In order to achieve the best possible trade-off between protecting the engine from harmful knock occurrences and optimizing the engine’s output torque, knock control systems must strive to maximize this trade-off. Knock events are a completely random process that occur on their own. It is hard to create knock controllers on a deterministic platform because of the nature of the platform. Because knock events arrive at random times in a time history simulation or experiment, a single time history simulation or experiment using knock control systems will not be able to offer a repeatable evaluation of the controller’s performance.
So the desired trade-off must take place within the context of a stochastic framework that can provide an appropriate setting for constructing and assessing alternative knock control techniques’ performances in terms of their strict statistical features.
- On page 528 of The Motor Cycle, on November 12, 1914, a letter from Lodge BrothersCo Ltd is found
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- WirthHorn-Informationssysteme GmbH-“Modern Automotive Technology – Fundamentals, service and diagnostics – Europa-Lehrmittel”
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Detonation is the result of spontaneous combustion occurring inside the cylinder AFTER the spark plug is ignited. Though similar to Pre-Ignition, it differs in several ways. Normal ignition occurs when the spark plug fires just before the piston hits top dead center (TDC). During its travels through the combustion chamber, the flame ignites the Air/Fuel Mixture. This results in a gradual increase in cylinder pressure, which drives the piston down on the Power Stroke engine. When detonation occurs, a portion of the air/fuel ignites before the regular burn has a chance to reach the rest.
Because of the sound it creates, detonation is often referred to as a “Engine Knock,” “Knocking,” or “Pinging.”
How is it indicated?
- Exhaust gas temperature (EGT) dropping
- Broken piston rings and/or spark plugs
- Knocking or pinging noise
- The piston and/or valves have been damaged.
What causes it?
Detonation can be induced by a number of different circumstances. Here are a few examples of common causes: Ignition Timing that is too advanced It is possible that the ignition timing is excessively advanced, and the spark plug will ignite too soon. As a result, the flame burns out too soon. The leftover gasoline has the potential to explode. Mixture of Lean Air and Fuel A rich air/fuel combination operates at a lower temperature than a lean one. A lean combination has the potential to become too hot and explode.
Heat is produced via compression.
Overheating of the engine Overheating can be caused by a lack of coolant or a malfunctioning water pump.
Low-Octane FuelThe “knock resistance” of a fuel is measured by its Octane Rating.
How does it affect performance?
It is the design of an engine that it operates in a specified manner. Because detonation disrupts that design, it deprives the engine of its ability to produce power. Most engines are capable of withstanding a little amount of detonation. Modern fuel-injected engines are capable of detecting a knock and adjusting the Air/Fuel Ratio and Ignition Timing accordingly. However, if the explosion is not corrected, it will cause significant damage to the engine. Even a single large explosion event might inflict considerable destruction.
Pre-ignition, Detonation & Knock
The culprit was apprehended before any damage to the engine could be done. It is possible to use a cooler spark plug when racing in order to assist defend against pre-ignition because cold-start characteristics, plug fouling, and exhaust emissions are not usually a concern when racing. If your engine is running too rich, however, the buildup of carbon deposits in the cylinder might result in pre-ignition. Keep an eye on your spark plugs and exhaust-gas temperatures to identify whether or not pre-ignition is taking place in your vehicle.
- In terms of temperature, the ideal temperature at the firing end of the spark plug ranges between about 930°F and 1470°F (500°C and 800°C), according to the manufacturer.
- By starting with a reasonable heat range (often three steps cooler than what you would use on the street), you may run your engine for at least 30 seconds under racing conditions before checking the plugs and adjusting your heat settings.
- A temperature shift of 160°F to 210°F (70°C to 100°C) is typical when moving up or down one heat range, so you should have an idea of how far to travel in one way or the other based on this.
- This will help you to detect any indicators of an overheated electrode before you notice any substantial damage to the piston (this is not always the case, as severe pre-ignition can kill an engine quickly, but such a severe case is not typical).
In fact, many drag racers inspect their spark plugs after each and every lap for precisely this reason: Using it to locate problems is represented by the spark plug on the right in this illustration: This damage occurred as a result of a lean state, which resulted in the overheating of the electrode, which might have resulted in pre-ignition in the process.
- Detonation Detonation is defined as the uncontrolled burning of the end-gasses in the cylinder and occurs, by definition, immediately following the spark-ignition event in the engine (as opposed to before spark-ignition, as is the case with pre-ignition).
- However, it may also be produced by a fuel combination that is too lean.
- The effects of detonation on an engine are tolerable if they are not severe in comparison to the engine’s design, but they may be severely devastating if they are severe enough.
- Neither, however, is ideal since neither contributes to the stability and control of the combustion process.
Assuming you’ve tuned your engine and have corrected the airflow modeling characteristics by properly adjusting the MAF curve and/or mapping the VE table to achieve the desired fuel mixture, you should set the power enrichment values at WOT to the value that will result in the greatest possible power output (MBT) while not compromising the engine longevity that you desire.
- Assuming you value power output over all else, you may run as light as 13.2:1 (if dyno testing show a power boost) without damaging the engine; but, you will put considerably more strain on the engine than you would with an AFR of 12.8:1.
- You may test leaning again with lower rear axle ratios (3.55:1 and numerically higher) and you may find that detonation does not occur as a result.
- When a detonation happens, you have a number of alternatives (assuming you’re staying with the particular fuel in question): Increasing the amount of fuel in the mixture and testing it to ensure that detonation is prevented without a reduction in power is Option 1.
- Option 3 is to put up with the explosion if it is only a little one.
- If you’re like drag racing, this could be something to consider.
You may want to consider creating a separate engine calibration specifically for these tracks to run a little bit richer of a fuel mixture at WOT and, if necessary, a couple of degrees less of spark advance to ensure that the engine survives happily at WOT during these heavy-throttle endurance races, such as those at Daytona, Sebring, and Road America.
Re-check the spark plugs after you’ve made the necessary adjustments to your ECU calibration to ensure that they are operating within a suitable temperature range for the newly configured vehicle.
When detonation occurs, engine knock is more usually linked with it because it is more frequently heard as a result of large-amplitude pressure waves that bounce off the engine block walls and cylinder head.
Knocking and pinging, on the other hand, can be caused by any combustion event that results in a pressure rise in the cylinder that exceeds a certain pressure-rise threshold for the engine’s specific design and configuration.
The accuracy and sensitivity of modern knock sensors are fairly good when it comes to monitoring engine knock, however it should be recalled that like with any sensor, the knock sensor’s accuracy and sensitivity cannot be totally depended upon to keep an engine safe in the event of detonation.
- Engines with big, aggressive cam lobes can generate enough mechanical noise to induce the knock sensor to register false knock readings, which in turn causes the ECU to delay the engine’s ignition timing.
- It is recommended that those of you who are running big bore engines use fuel with the fastest burn rate possible to limit the chance of detonation to an absolute bare minimum.
- ), you’ll be able to race and have the most fun possible while spending the least amount of money possible.
- Pre-Ignition When the fuel mixture in the cylinder burns before the spark-ignition event at the spark plug, this is referred to as pre-ignition (or self-ignition).
- A considerable reduction in power production and efficiency occurs as a result of pre-ignition since it deprives the crankshaft of power that must now be utilized to propel the piston upward through the forces of early fuel mixture combustion and expansion.
- Engines built for street usage are typically fitted with “hot” spark plugs, which provide superior cold-start characteristics as well as decreased fouling and corrosion.
As a result, when the engine is kept running at full throttle for extended periods of time, the OE manufacturers run excessively rich mixtures at full throttle in order to keep the spark plugs cool and prevent them from causing pre-ignition events that are severe or continuous enough to destroy the engine.
- It is improbable that a carbon build-up will form within the cylinder if an abnormally rich fuel mixture is only present during WOT, but it is possible.
- On the other hand, if the engine is run excessively lean when at WOT, cylinder temperatures can rise dramatically, increasing the risk of pre-ignition.
- During a race, if you see what appears to be a considerable decline in power output from the engine, you should immediately stop the engine when it is safe to do so and examine for symptoms of pre-ignition.
- Consequently, because the cylinder heads and engine block have a higher thickness and thermal inertia than the pistons, they have a better tolerance to high temperatures and high pressures than do the pistons and rings, which are typically the first to fail when pre-ignition occurs.
- What is the difference between pre-ignition, detonation, knocking, and pinging?
- Without a thorough understanding of each of these occurrences and how they manifest themselves, you will be unable to prevent them from occurring or even recognize them when they do occur.
When this happens in your engine, it’s not quite as dramatic as we depicted in the photo (laugh, chuckle), but the effects might be irreversible engine damage if it happens again. So, what exactly is the distinction? Let’s have a look.
Fuel pre-ignition or auto-ignition in an engine’s combustion chamber is referred to as detonation. It is most commonly produced by low-octane gasoline. The early ignition of fuel (before the spark plug ignites) causes a shock wave to propagate throughout the cylinder when the burning and expanding fuel-air combination collides with the piston, which is still heading towards top-dead-center. The sound of the pistons hitting against the cylinder walls causes the knocking or pinging that occurs as a result.
Knocking that is prolonged and powerful can cause the piston or the engine to fail, yet it is possible to drive thousands of miles with this little problem.
Common Causes of Detonation
Detonation is most commonly caused by the use of low-grade engine gasoline and the degradation of your engine’s components as a result of this use. The design of the engine’s chamber, on the other hand, is critical in deciding when and whether it may detonate unexpectedly. The form, size, position of the spark, and geometry of the design all contribute to determining where these detonations are most likely to take place. Pre-ignition can be caused by an overheated spark plug tip as well. It is possible that this could make a pinging sound in your vehicle when going down the interstate, but it is possible that this will last in the engine for thousands of miles.
Detonation can result in three forms of engine failure, depending on the source and degree of the detonation: abrasion, mechanical damage, and overheating, among others. Mechanical damage occurs because the increased impact of nature might cause elements of the internal combustion engine to fracture as a result of the increased impact of nature. This can have a particularly negative impact on the top or second piston ring land, as well as the exhaust and intake valves. Abrasion causes the piston head to be gradually eroded, resulting in the formation of a tiny Swiss cheese effect on its surface, which reduces efficiency and eventually causes the piston head to fail.
This overheating of the engine is caused by the boundary gas layer becoming interrupted against the cylinder head and heat moving to the coolant via the cylinder head.
Fortunately, there are a variety of options for preventing pre-ignition. The most effective approach is, of course, to consult with your technician about the problem. However, if you have prior engine repair knowledge, you may want to consider the following measures to lessen the likelihood of engine detonation. The most effective method of preventing erroneous firing is to use higher octane gasoline in order to lessen the heat generated in the firing chamber and burn fuel at a slower rate. Similarly, lowering the temperature of the engine’s intake air will significantly lessen the likelihood of pre-ignition and detonation.
Adjusting the engine timing may also be beneficial in resolving this issue. If your engine is firing at low engine speeds when the throttle is depressed, you may need to alter the timing by two to three degrees.
Knock Knock, Detonation Time
In my experience, the most often requested question by my clients who have forced induction engines is: “Why can’t we create more power?” Why aren’t you able to just turn the boost up a notch more? Why aren’t you able to just add additional time? This is the answer to the issue of whether or not we are knock-limited. These people aren’t going to let me off the hook. Unfortunately, it is not an acceptable response. They are curious as to why this knock is occurring and what they can do to stop it.
- It’s a little more involved than simply purchasing some parts, to be honest.
- It is only after that that we can investigate what causes knock and what may be done to mitigate it.
- That way, new clients should be more appreciative of a tune’s safe limit and understand why you can’t just “add more boost, dude” to a tuned vehicle.
- When pockets of mixed fuel and air inside the combustion chamber burn independently of the original flame front that is ignited by the spark plug, this is referred to as detonation.
- Because the pressure wave from the explosion reverberates through the engine block, these explosive detonations can cause damage to an engine, which can be heard as an audible pinging sound.
- In this comparison, the cylinder pressure trace from a knock event (blue) is shown against the regular clean burn (red).
- High Power Media is credited with the photo.
Heavy detonation over an extended period of time may and can damage pistons, head gaskets, spark plugs, and cylinder heads.
Consider the alternative: if you drop a spark plug on the ground, you will almost certainly break off a portion of the plug.
Unknown photographer captured this image.
Under normal conditions, detonation under mild load does not produce a big enough pressure to cause harm to the engine.
What Is the Meaning of Pre-Ignition?
It occurs when the engine is sufficiently dissatisfied with its existing condition of operation that a portion of the fuel mixture ignites before the spark plug ignites the first flame.
Detonation can cause internal engine parts to glow or become extremely hot due to the heat transfer caused by the explosion.
Once pre-ignition has begun, it is reasonable to expect that it will be a very harmful event, since no alteration in ignition timing will make a substantial difference in preventing it.
Detonation occurs when pockets of air and fuel are subjected to sufficient heat and pressure to ignite on their own.
Fuel: The use of fuel is the most widely recognized method of preventing detonation.
Every time you fill up your car with gas, one of the numbers you’ll notice at the pump is the octane number of the gasoline — this is a measure of the fuel’s resistance to knocking.
In order for the crankcase gasses to be re-circulated back into the intake tract, it is critical to have a very effective crankcase air-oil separator installed.
Race fuel has a substantially higher octane rating than regular gasoline because of the chemical makeup of the fuel.
There have been several studies conducted, as well as a large number of dyno graphs that compare identical setups using one type of gasoline to another.
Mechanical changes are difficult to assess for a variety of reasons.
Mechanical Characteristics: However, while the way gasoline influences detonation is pretty straightforward, the mechanical factors that influence the likelihood of knock (mechanical octane) are significantly more intricate, and many of them are being investigated by OEMs using cutting-edge engine modeling software.
In a combustion chamber, the sooner the fuel/air combination is ignited, the greater the amount of pressure that will be created.
The term “knock limited” refers to an engine that experiences knock before it can be operated at its optimal ignition timing for peak torque production.
The rate of power loss increases as we get further away from MBT, finally reaching a point where the engine begins to miss and run badly as a result of the engine’s excessively delayed ignition timing.
Using the left graph, you can observe what happens to cylinder pressure when the ignition timing is advanced — it increases significantly in amount.
It is also worth remembering that cylinder pressure will continue to rise with ignition advance even after the engine has reached its maximum power output.
Photo courtesy of an unidentified text book.
The temperature of the water and the temperature of the charge are important factors.
Engine Load– As the engine load increases, you get closer to the knock threshold.
Knock may be reduced by providing efficient airflow through proper valve timing, adequate head flow, and a low exhaust gas back-pressure.
Even the original equipment manufacturers (OEMs) suffer with knock.
Mazda Hong Kong provided the image (Skyactiv) A substantial part of the mechanical octane rating is determined by the engine’s capacity to swirl produce and maintain turbulence in the combustion chamber.
This results in a reduction in engine ignition timing and an increase in mechanical octane.
The design of cylinder heads and pistons is time-consuming for OEMs since they must consider coolant flow and cylinder quench in order to enhance thermal efficiency while also reducing detonation, as previously stated (to allow higher compression ratios).
End gas quench zones, also known as “squish” zones, offer substantial cooling (quenching) to end gases, causing these end gases to move closer to the spark plug as the piston approaches top dead center (TDC).
Featured Image Credit: Unknown Fuel Mixture– The chance of knock increases with the leaner the mixture is.
As previously stated, oil decreases the effective octane of the fuel; thus, if the blow-by fumes are unable to exhaust out of the crankcase, they will re-enter the combustion chamber and lower the effective octane of the fuel even further.
Carbon buildup, spark plugs that are too hot for the application, and sharp edges on the piston or heads are all examples of things that might cause heat to accumulate.
Even throughout the engine building process, there is very little you can do to alter the cooling qualities of your engine, or the design of the cylinder head and pistons.
Finally, a word on conclusion: The most straightforward technique to get more power out of your engine is to use higher-octane fuel and make certain that your engine’s cooling system is adequate to the task.
Finally, ensure that you have a proper air/oil separator in place and that no oil is getting absorbed into the combustion chamber throughout the process.
I’d love to hear the results of testing that try with coatings, porting, squish/quench, and other techniques to increase mechanical octane, so please share your findings if you have any.
So, now that you’ve gained a basic understanding of knock, you should be able to recognize when your engine has reached its safe operating limit.
Providing you with a tune that your tuner feels will be safe for the engine under all predicted operating circumstances is his/her responsibility.
That being said, engines with properly calibrated knock control systems can afford to run more aggressive ignition timing because they have a system that will only allow one or two knock events before removing ignition timing and adding fuel to prevent the engine from being damaged by the knocking events.
Engines that do not have knock control must be tuned with a larger buffer since knock tends to cause additional knock, which creates a dangerous snowball effect that can cause serious damage to your engine in a short period of time.