Improving efficiency and reducing emissions are the principal challenges in developing new generations of internal combustion engines. Different strategies are pursued like downsizing or sophisticated after-treatment of exhaust gases. Another approach gears forward to optimize the parameterization of the engine. Correct adjustments of ignition timings, waste gate position et cetera have significant influence on the combustion process. A multitude of application data is generated during the development process to predefine appropriate settings for most situations. Improvements regarding the application effort and the quality of the settings can be achieved by measuring the combustion process and optimizing the parametrization in a closed loop. However, cylinder pressure sensors that are used during the development process are too expensive for series applications. This paper focuses on an affordable combustion sensor based on the measurement of the electromagnetic radiation of the combustion flame. The intensity of electromagnetic radiation by chemiluminescence is a good indicator for the status of the combustion as it is primarily dependent on the number of excited molecules. A characteristic spectral band is emitted by the OH radical, which is created in the flame front and corresponds to the number of reactants that are burned. A research engine with pressure indication was equipped with an access for an optical measurement system for chemiluminescence. To achieve sufficient amplification and temporal bandwidth for the weak radiation signal, a special detector circuitry was designed. The measurement data was used to derive and verify a model that calculates the heat release from the signal and thus reveals live information about the combustion process for closed loop combustion control.