Determining locations at which knocking events appear frequently is a very important tool for optimizing the efficiency of S.I.-Engines.
The knock behavior of two Direct-Injection Engines (one with wall guided fuel delivery - Mitsubishi GDI-engine - and a single-cylinder test engine with narrow arrangement of spark plug and injector) was tested and the location of appearing knocking was detected by the use of optical fiber technique.
The optical investigations were conducted using a spark plug equipped with 8 optical sensors evenly distributed in a ring on the ground electrode of the standard spark plug. The intensity of flame radiation in the observing area of the optical sensors were measured.
As knocking combustion produces pressure waves exciting gas oscillations, each compression of residual exhaust gas causes a steep increase of the measured flame intensity curve.
Knowing the precise angular position of the first steep increase of flame intensity caused by pressure waves in each cycle and at all of the optical probes the determination of pressure wave propagation was made possible as well as the detection of knocking location.
To achieve knocking combustion in the driving mode without interfering with the engine control system, the ignition point of a single cylinder of the Mitsubishi-GDI-engine was set by an external engine control system. Additionally, the standard knock sensor was disconnected. Thus, independent control of ignition timing was achieved.