How To Eliminate Knock Retard - Feedforward

Eliminating Knock Retard

Ray T. Bohacz Apr 1, 2009 0 Comment(s)

It defines a system that monitors a condition or state and then uses that information via a microprocessor to either maintain, adjust or alter an operation. The most common use of the term was attached to fuel correction and can be viewed through scan data as Block Learn Multiplier (BLM) and Intergrator (INT). The oxygen sensors are used as the auditor of the mixture strength and that data is sent to the engine controller.

A common misbelief in the enthusiast community is that the oxygen sensor is used to control the mixture. That is not its task. Its purpose is to monitor the air/fuel ratio and if something goes astray, let the ECM/PCM know--an over simplification of a complex operation. When writing a calibration, the opposite of a feedback edict is employed that can be identified as feedforward.

A feedforward logic, when it is applied to injector pulse width, means the mathematically correct air/fuel ratio is created by the opening time and no correction is required. This would be represented by a BLM and INT of 128. Since these values can be between 0 and 255 or 256 places, 128 is the mid-point. When at 128 there is no correction to the calculated fuel delivery and the proper mixture strength is being created, confirmed by the oxygen sensor. As the value goes lower, the injector pulse width is being trimmed and as it skews higher, opening time is being added. Now let us examine how this logic is applied to ignition timing control.

Knock, Knock
Most, if not all, high-performance EFI GM engines employ a system called ESC, for Electronic Spark Control, that works in conjunction with EST (Electronic Spark Timing). First seen on older engines that still employed a distributor, ESC has been a standard issue component on later applications. Evolutionary changes in the circuit components and software has been seen but the original premise has not been altered. The desired timing curve is created and the knock sensor listens for detonation. If it occurs, the engine management then responds by pulling out some ignition timing. Sounds simple enough. But there is more going on that needs to be recognized.

The ideal ignition event in a cylinder has a flame originating at the tip of the spark plug and spreading out across the bore from there. Only one flame should exist. If another rouge flame is created it is identified as abnormal combustion. Depending on where in the piston's stroke the abnormal event happens will define preignition or detonation. The enthusiast does not qualify the beginning of the abnormal combustion event, and simply identifies it as knock or ping. Thus, with an ESC system a knock sensor is employed.

The purpose of this primer is to establish methods to eliminate or minimize evoking the knock sensor circuit. Attaching the feedforward logic, if the original timing curve is correct and all is well with the engine, no knock should occur. This is the theory, but is hardly ever the case in the real world.

Anyone that has some experience with fighting knock knows that octane is only one of the areas that needs to be addressed. Often throwing octane at an engine, especially a high compression or boosted one, does little to control knock. By definition, octane is the fuel's ability to resist heat and pressure and wait for a spark to ignite. Low octane fuel is more anxious to ignite when exposed to heat or cylinder pressure. Likewise, higher octane gasoline will withstand more of these elements. Abnormal combustion is the result of extreme cylinder pressure or heat and in most instances a combination of the two.

Deposit Formation
Carbon deposits form in an engine in two distinct areas with each having its own impact on performance. Intake valve deposits (IVD) form on the backside of the valve (the part attaching to the stem) while CCD or combustion chamber deposits collect on the piston crown and the walls of the combustion chamber of the cylinder head.

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