Optical Piston Temperature in an internal combustion engine

In the last ten year the mechanical power output of car engine increased significantly. This result has been possible especially through new injection systems that brought to an optimization of the combustion (direct injection, common rail) and to an improvement of the turbo charging. Moreover, these technical devices brought to a reduction of the exhaust emissions and an increasing of the engine efficiency. In particular, the specific power is increased from 34kW/litre of the 1992 to the 63kW/litre of the 2010. Furthermore, the pressure peaks into the combustion chamber and the fuel injection pressure have been increased to the aim of emission reduction and higher engine efficiency. In this scenario, car manufacturers are following the direction of the engine downsizing that means to have the same engine power by a lower engine displacement. The downsizing has as main consequence a thermal risks specially for the hot parts of the combustion chamber (exhaust valves, spark plug body and piston) because these parts have an increased high heat load and thermal stress. These temperatures could have as main consequence to the risk for engine damage by irregular combustion due to "surface ignition" by hot spot or by deposit of the combustion chamber. In this paper a method for optical measurement of piston temperature during the fired and unfired operation of an internal combustion engine has been developed. An optical access to the combustion chamber is achieved by an optical element made from sapphire lens that can be located in a special spark plug or other engine parts (injectors, cylinder head). The thermal radiation of the piston observed with this optical element is guided to the detector with optical fibers. As a detector InGaAs-photodiodes are used, which allow a detection of the infrared radiation up to 2.6 μm. The relation between the signal intensity and piston temperature was determined outside the engine in a calibration unit: the piston was heated up to 400°C. In the calibration unit the sensor and the piston are arranged in the same geometry as in the combustion chamber. During cool down the thermal radiation is simultaneously measured with the temperature of the piston surface, obtained from a thermocouple. AVL uses optical sensors in spark plugs to investigate combustion phenomena in spark ignited internal combustion engines. In a recent application the measurement of valve temperatures has been demonstrated using these sensors. In this paper the possibility to measure piston temperatures with this measurement system is investigated. For this purpose following tasks have been performed: The measurement range was extended from 400° down to 200° by selecting appropriate optical materials and detectors. A calibration unit was developed to determine the relation between radiation signal and piston temperature. The piston temperature was measured for different pistons in a single cylinder research engine.