In its element: the new cylinder pressure sensor for large engines pushes the limits

Winterthur, December 2019 – The innovative 6635A1 cylinder pressure sensor from Kistler marks a decisive step forward in improving the efficiency and life expectancy of large engines. This exceptionally rugged and accurate piezoelectric sensor with an integrated charge amplifier is the key to optimized engine operation with closed loop control.

Closed Loop Combustion Control (CLCC) is firmly established as a suitable method for increasing the efficiency and life expectancy of large engines such as those used to power ships. The sensors required for continuous measurements of cylinder pressure are exposed not only to high pressures and temperatures but also to severe mechanical shocks. In view of the trend towards higher mean effective pressures (MEP) in engines, the ruggedness of pressure sensors is becoming a critical factor. But there is a drawback here: making the sensor more rugged almost inevitably means that it will also be less accurate.

Developed in close collaboration with engine manufacturers, the new cylinder pressure sensor 6635A1 was designed for use in engines running at medium speeds. The new sensor from Kistler optimally combines maximized operational lifetime with the greatest possible accuracy thanks to a host of innovative features:

• PiezoStar crystal: Kistler grows its own piezoelectric crystals – up to five times more sensitive than quartz, they deliver excellent measurement stability and have minimal temperature dependance.
• Optimized diaphragm design: the new, patented shape of the sensor diaphragm minimizes the thermal shock error, ensuring very good measurement characteristics.
• Robust signal transmission: instead of a plug-in connector, a welded connection ensures reliable wear-free signal transmission – even in case of severe stresses induced by shocks or vibrations.

Simulation, validation and intensive performance tests

Complex FEM (Finite Element Method) simulations played a key part in the development of the new sensor. For this purpose, the prototype based on the first simulations was compared to the model across several cycles to ensure that the results could be reliably reproduced.

The completed sensor underwent comprehensive load and lifetime testing. This presented considerable challenges, including replication of the harsh environment of a combustion engine as well as long-term tests over several thousand operating hours. With the help of suitable hydraulic test stands developed by Kistler, it was nevertheless possible to expose the sensor simultaneously to pressure and temperature changes as well as mechanical stresses with high repetition rates.

Close collaboration with engine manufacturers

One factor that has proven essential for successful product development is direct collaboration with engine manufacturers. It is impossible to provide detailed predictions or simulations of boundary conditions inside the engine – regarding optimal positioning of the sensor in the cylinder head, for example. However, accurate knowledge of the specific engine allows better adaptation to the sensor's environment.

The new 6635A1 cylinder pressure sensor from Kistler is used in medium-speed four-stroke engines to perform functions such as reliable knock detection, cylinder balancing and calculation of mean effective pressure. It features a pressure range of up to 350 bar. Signal transmission and power supply are galvanically isolated, and the sensor is also certified as per EU RO Mutual Recognition (MR), ATEX and IECEx.