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Spark ignited engines with direct fuel injection into the combustion chamber can be started by injecting fuel into the combustion chamber of the stopped engine and igniting it afterwards with the spark plug. The explosion of the air fuel mixture initially rotates the crankshaft. The engine is started without motoring the crankshaft with a starter device. The described start procedure is called “Direct Start”. For a successful Direct Start a maximum oxygen concentration in the “Direct Start Cylinders” (the cylinders utilized initially for the Direct Start) is required, because the oxygen concentration in the start cylinders determines the maximum amount of energy, which can be exploited to move the crankshaft over the following “TDC” (Top Dead Center). After the engine stop the Direct Start Cylinder valves are closed. Therefore the oxygen concentration has to be maximized during the preceding engine shut down.

The ever increasing demands on combustion engines with regard to exhaust emissions and fuel consumption require the use of modern analytical methods. Especially optical measurement techniques have contributed a lot to the realization of complicated engine concepts. The disadvantages of complex measurement techniques are the expenditure of time and the necessary changes of the engine. This paper introduces a new practice oriented optical measurement technique with the aid of an example. With this technique it is possible to record the combustion process with a sampling rate of up to 200 kHz through a small drilling. Even quick events like knocking can be recorded. The combustion luminosity is watched through a special UV-capable borescope, transmitted to an array of photoelectric transformers and finally recorded as a flame figure. We will show an SI-engine which is limited by knocking.

Optical measurement technique became more and more common for the last few years. Especially optical fibre technique is often used to detect flame propagation. With optical sensors the ignition process can be investigated with high temporal and spatial resolution. An in-cylinder optical sensor has been developed and tested to analyze the ignition of mixture and luminous emission of burning gas. The sensor consists of eight optical probes fitted in a conventional spark plug. The results show good correlation between measured luminosity and combustion parameters such as load, engine speed, ignition timing and air-fuel mixture ratio. A correlation between development of light intensity and pressure was found. For evaluation of light signals different analysis methods are presented. Furthermore it is shown that the luminosity of the flame can be used to control the combustion process.