This is a new technical paper regarding Engine Knock; What it is, the physics, how to measure and monitor for engine knock and how to use the information for tuning. This article was written by Brian Barnhill, Technical Director of Tuner Tools LLC and published in Issue #4 of Juiced Magazine.
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Knock Detection and Prevention
Much like that annoying neighbor or mooching family member, your engine is something you would rather not come knocking. Knock is a warning and indication of potential and pending trouble. Thankfully there are some simple tools to monitor and determine knock and help tune your engine to prevent the conditions which lead to knock and pre-ignition.
Knock and pre-ignition are phenomena that describe improper or uncontrollable combustion, and both lead to reduced performance and drivability and can even cause catastrophic engine damage. Understanding what these terms describe, the physics behind the phenomenon, why they occur and how to prevent them is the most critical aspect of tuning your engine, as a mistake here will essentially guarantee eventual engine failure.
To fully understand the phenomena we must first discuss and understand the basic ignition process itself. Recall that an engine is essentially an air pump and relies on changes on pressure to move air in and out of the engine. Ignition of the air/fuel mixture is initiated by the spark plug when either the ECU or distributor signals the ignition coil to fire. This causes expansion of the previously compressed gas mixture.
Proper timing to ensure maximum power requires ignition of the mixture at the peak pressure in the cylinder. However, since ignition is not an instantaneous event and the flame front propagates through the combustion chamber. This requires the initial spark event to occur Before Top Dead Center (BTDC) and is always measured in degrees of rotation in reference to top dead center. When tuning ignition timing in an ECU map it is these values that are being modified. Typically positive values indicate degrees BTDC while negative values are ATDC.
Timing the spark event to ensure full ignition of the mixture always occurs at peak cylinder pressure will create the most torque from the engine being tuned. The key is to have as little timing advance as possible, while still maintaining this pressure – this is a torque limited ignition mapping method and the value may be referred to as the Mean Best Torque Spark Advance and should always be the target when not limited by engine behavior such as knock or pre-ignition. This is precisely why monitoring and controlling knock is so important.
Knock, pinging and detonation describe improper propagation of the flame front – in other words, the air/fuel mixture is burning in an inconsistent and unpredictable rate. This causes the final ignition event to occur at a time other than peak cylinder pressure and a decrease in pressure. This can be caused by many issues, including incorrect spark timing, failure of the fuel to atomize, or local hotspots in the combustion chamber. This will cause multiple areas of ignition not in the same location as the flame front. This can be basically described as the ignition force “jumping” around the combustion chamber, rather than coming from one location.
The result is each of these ignition events producing its own pressure or shockwave. These waves propagate from the local ignition point, and run into each other within the cylinder. The result is a combined shockwave resonating at a different frequency than normal combustion and even creates an audible metallic knock or pinging noise, hence the name of this phenomenon. This can case holes in engine components and small indents on the piston face as well as very rapid wear and stress on all engine components.
If knock occurs at an ignition timing value less advanced then that for best torque, the engine is said to be knock limited, as ignition timing is tuned to prevent knock at this map location rather than for maximum torque. This is often the case in higher compression and force induction engines, where pressure and temperature in the cylinder can be very high. The key is to request as much spark advance as possible while avoiding knock during any engine operations and parameters.
Pre-Ignition is a very separate and differing phenomenon than knock. Pre-Ignition occurs when the mixture is caused by a means other than the timed spark event, often significantly before the intended timing. This causes massive stresses on the rotational components of the engine, as full ignition occurs before peak cylinder pressure and far before TDC. The premature ignition attempts to push the piston backwards against the rotational inertia and can often lead to broken connecting rods, wrist pins and other component failure.
Pre-ignition is often caused by excessive temperatures in the combustion chamber from high intake temperature, high compressions, high levels of boost (which cause high intake air temperatures), physical hot spots due to piston imperfections or damage, or low-grade gasoline (lower octane gasoline is easier to ignite.) Prevention of pre-ignition is achieved largely through proper selection of parts, operating parameters (boost, fuel used, etc) and engine building. Pre-ignition is usually an indication of a larger scale problem with the engine of vehicle. Pre-ignition can also be avoided by using items such as water/methanol injection or an intercooler to lower intake temperatures or methods to increase the effective octane rating of the air/fuel mixture.
Tools of the Trade
Thankfully there are many tools for detecting, predicting and controlling knock. The cost of these tools range from under $25 to a $1000 or more – The simpler tools will allow the user to determine when the engine is knocking, while the more sophisticated tools will use assumptions and prior data to predict and prevent knock before it becomes severe. Understanding how each tool works, what the information means and how they are applied is critical to deciding which tools are required for your tuning strategy.
A detonation, or “det” can is essentially a stethoscope for an engine and allows the tuner to physically listen to the internal heart beat of the combustion process. It is the simplest and cheapest tool for detecting knock yet is still a very reliable means of detecting knock – and the good news is you can build one yourself for $25 with parts from your local hardware store. The det can is basically an amplifier for the vibrations of your engine. It attaches a cylinder to the block that will vibrate and create a pressure wave inside the cylinder, and then connects this cylinder via an air hose to a headset or another can to allow the user to listen to the noise. This works on the same principle as homemade “two cans on a string” trick.
During normal operation the noise generated in the det can will resemble normal engine noise and have the same frequency. If knock were to occur, however, there would be a different and very distinct noise heard. By listening for these events you can determine when your engine is knocking, and tune accordingly.
There are 2 common types of sensors that may be used to detect knock. The first and more common type - which will typically referred to as a knock sensor, is the microphone type. The second is an in-cylinder pressure sensor. Both are discussed and have distinct applications and advantages.
- Microphone Knock Sensor
The microphone style sensor is the most common knock sensor used, and is even commonly found on most OEM vehicles with a closed loop knock system. Many aftermarket knock lights, knock monitoring systems, and standalone engine management systems also heavily rely on this style sensor as the backbone of their system. This common sensor is really just a microphone in the most basic sense. The sensor is attached solidly to the engine block, and detects engine noise the using the method any common microphone uses. This sensor, however, is calibrated to listen and detect very distinct frequency ranges.
As discussed earlier, knock will create a pressure wave in the combustion chamber that will vibrate at a frequency easily differentiable from background engine noise. The microphone knock sensor has to be calibrated to determine the normal frequency range of the engine, and will filter this noise from the output. When any noise outside of this frequency is detected, the knock system will pass this information to the ECU or to the indication system being used (I.e. knock light). By tying this information with other data in the ECU, such as AFR, RPM, Load, Spark Advance, etc the tuner can determine the location in which knock is occurring and change the tune accordingly.
There are a few drawbacks of this sensor, however. The largest is it susceptibility to noise. While most engine noise can be filtered, there are cases were excessive background and engine noise will prevent the sensor from providing a reliable and accurate signal. The opposed Subaru engine is a common culprit - the OEM knock detection system will actually operate in open loop and ignore the sensor in the upper rpm ranges due to this issue. More sophisticated systems may provide better filtering and sensitivity to prevent this issue, but care should always be taken when determining a mounting location for a knock sensor of this type.
Since this sensor works by detecting vibrations transmitted through the block from the combustion chamber it is really detecting the symptoms of knock, and not knock itself. Due to this, it is especially critical that the sensor be bolted to the block itself, and as close to the combustion chamber as possible. Problems arise if there are differing materials or gaskets/seals between the material the sensor is bolted to and the cylinder. This method of measuring also means that knock must first occur before the system can detect it, making prediction of the onset of knock difficult and based on assumptions and historic data rather than real time measurements.
- Pressure Sensor
Utilizing a pressure transducer the actual pressure in the combustion chamber can be measured. Since knock creates a pressure spike and fluctuations within the combustion chamber, this is a very accurate way of detecting and measuring the severity of knock. The sensor can also measure real time propagation of the flame front as pressure will rise as the ignition event occurs.
Since the pressure sensor will detect very small changes in cylinder pressure the tuner can see fluctuations or inconsistencies which may indicate the onset of knock. This additional resolution and foresight allows the tune to be more aggressive with less risk of potential performance degrading situations or harmful engine conditions. Additionally, this sensor allows the tuner to accurately determine the point of maximum cylinder pressure and provide another useful tool for determining optimal ignition timing.
This method is not without drawback however. The sensor is typically more expensive than other methods of knock detection. Mounting requirements can also increase the cost of the engine if installed in an OEM system. Additionally, mounting of such a sensor, if not originally equipped, can be extremely difficult, if not nearly impossible without modification to the engine block itself. One solution to such a problem is incorporating the sensor into the spark plug. A piezoelectric pressure transducer is fitted onto the spark plug, and then the spark plug is installed as normal (albeit with an additional wire lead for the new sensor.) This is a very simple and elegant solution to the problem – however it does significantly increase the price of the spark plug as well as the consequences for damaging or fouling plugs.
The application of these types of spark plugs is still fairly narrow, and it may not be possible to purchase such an off the shelf solution for many cars at the present time. It is however possible for the electronic savvy individuals to fabricate their own plugs with such a solution, though care should be taken not to disrupt the behavior of the spark plug or influence the combustion process with placement of this sensor. Also note that many sensors may be negatively influenced by the electrical activity during a spark event or the heat of the combustion process. These areas will require special attention if constructing a homemade sensor. Only the most experienced and knowledgeable tuners and engine specialists should attempt such a sensor, as the consequences of a mistake and failure during operation can and will cause dangerous conditions and can lead to catastrophic engine damage.