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Knock Control

Knock Control

There are several ways to approach Knock control. Some manufacturers use more of a noise control than a knock control system. These systems use the knock sensor with a non specific band pass filter to filter out some back ground frequencies. Then a user defined table is used to limit the amount of “noise” before it is considered to be knock. Once this threshold is reached action is taken by pulling timing and adding fuel in attempt to limit the noise. There are several problems with these types of systems. As the engine is producing more power, it also produces more noise, so if the noise isn’t really knock, then power is greatly reduced for no reason. Using a broad band pass filter, you can’t determine the frequency that knock actually occurs in the engine being monitored. Also by looking at the “noise” level, you don’t know which cylinder is actually responsible for know. It then gets even more complicated because you can’t determine the window in which knock will occur in each cylinder. The systems that are monitoring noise rather than knock also lack the processing speed to properly determine what knock really is.BACKGROUNDThe term “knock” refers to the spontaneous combustion of in-cylinder end gases (un-burnt premixed fuel and air in front of the flame front) that occurs independent of the combustion associated with the spark initiated flame front traveling through the cylinder.  This spontaneous combustion is a function of temperature, pressure, and time – that is, if the temperature and pressure of the end gases reach the self-ignition threshold before the flame front has had time to propagate to the cylinder walls, knock will occur.  The effect of this spontaneous combustion creates shock waves and thermal explosions throughout the cylinder, attributes that both audibly and physically affect the consumer and engine, respectively.

Various steps can be taken to decrease an engines tendency to knock.  Increased burn rates will allow less time for the end gases to heat up and ignite before combustion is completed.  Therefore any agent that improves mixing, such as swirl and tumble, will also improve the knock characteristics.  Colder charge and block temperatures will guarantee more time for the end gases to reach there self-ignition temperature threshold, thus giving the flame front more time to reach the end gases before knock occurs.  Higher octane fuels have a similar effect on the end gases, by raising the activation energy required to self-ignite.  Rich mixtures burn faster than lean, and heat the cylinder up less for each consecutive combustion event.  Spark retard shifts peak pressure of the combustion event away from TDC, thus avoiding the dramatic rise in cylinder pressure (and temperature) when the two are in close proximity relative to crank angle.  (It should be noted here that fuel is typically used to cool exhaust gas temperatures (EGTs) when significant spark retard is used to for knock control, and does not have a substantial affect on knock control.)  Some of the steps listed above must be addressed in the base engine design, but some can be utilized in an active cylinder-by-cylinder, cycle-by-cycle knock controller to effectively reduce knock only in operating conditions where knock is present, thus retaining the majority of the engines potential power whenever possible.

The calibration process for the knock controller involves four basic steps.  These steps are sequential, and described in Figure 2.0.  Location refers to determining the location of the knock accelerometers on the engine that captures the largest knocking energy relative to non-knocking energy.  Once chosen, a band pass Filter must be selected that isolates the knock frequencies from the full spectrum to maximize signal to noise ratio of the knock signal.  Next Windows are selected relative to crank angle to direct the controller which portion of the knock signal to use for calculating cycle-by-cycle knock intensity.  Finally, a Threshold is chosen to define a knock intensity level above which the engine is considered to be knocking.

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