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.
The physics
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.
Ignition
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
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
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.
“Det” Can
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.
Knock Sensor
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.
