Knock Sensing

[RABBITT_Andrew at mv8.orbeng.com.au]

> OEMs use an advance determination method called MBT (Minimum spark
> advance for Best Torque), unfortunately knock can sometimes set in 
> before best torque is realized.

just to clarify a point, MBT stands for Max. Brake Torque which is 
usually arrived at by manipulation of the ignition angle.  When you 
are increasing advance to approach MBT and run into detonation, this 
is known as the Borderline Detonation Limit (BDL).

> since detonation is caused by a too-lean condition 

not generally the case in the sense that you can knock at 12:1 as well 
as 16:1.  However for a given ignition advance, increasing the A/F 
ratio will probably induce knock.  The point at which this happens is 
a function of many variables including fuel quality, charge 
temperature and cylinder/coolant temperature.  The point is that it's 
not determinable solely on A/F ratio.


> When refering to placement, they talk of "nodes" where knock can not 
> be detected and "anti-nodes" where background noise swamps the 
> signal entirely. You wouldn't want to place a sensor at either of 
> these points.

Sensors are also 'window' sampled at certain crank angles.  Knock 
usually most intense shortly after TDC, however depending on the 
number and arrangement of the cylinders this window must avoid closing 
valves and other impact type events which can excite structural 
resonances at frequencies that can look like knock.

Andrew Rabbitt