Measuring Cylinder Distortion With Maximum Precision

Measures, such as downsizing, highpressure supercharging and weight reduction, constantly increase the stress and strain on all engine components. This is moving the piston-cylinder system closer into the developers’ focus. It has the greatest impact on oil consumption and blow-by, and it is also responsible for most of an engine’s mechanical losses. IAV has developed a patented method that is capable of measuring cylinder distortion with maximum precision – as a key to designing and configuring optimized piston-cylinder systems.

The piston group is responsible for the lion’s share of oil loss and for around 40 percent of an engine’s mechanical losses. Blow-by – the quantity of gas entering the crankcase from the combustion chamber – also reduces the engine’s overall efficiency. As such, there are many good reasons for optimizing the pistoncylinder system. Cylinder distortion plays a key part, mainly being influenced by the tensioning between the engine block and cylinder head as well as by a thermal stress and dynamic strain on the engine block while the engine is running.

The aim is to design the engine block as well as cylinder head and gasket in a way that only slight and non-critical cylinder distortions occur in the tensioned operating state and while driving. “This leads to a direct reduction in piston and piston ring friction”, explains Dr. Hubert Schultheiß, Technical Consultant for Powertrain Tribology at IAV. “This would also lower piston ring pre-tensioning, further reducing friction.”

Today, developers use tools for configuring, designing and computing the base engine. “Yet the programs are only as good as the computation models they are based on”, Schultheiß says. “Over recent years, this has prompted us to develop a patented method that lets us measure cylinder distortion with maximum precision while the engine is operating. The data obtained are used for optimizing the computation models and, ultimately, designing better engines. This makes cylinder distortion measurement an important building block in the process of cylinder optimization.”

In assessing cylinder distortion, developers distinguish between different orders, corresponding to a Fourier development of geometrical variations from the ideal. The zero and first order describe the variations from the prescribed cylinder inner diameters or from the ideal center position. The second, third and fourth orders indicate the extent to which the piston resembles an oval as well as a three or four-leaf clover. These three variables are crucial in optimizing the piston-cylinder system.

Measuring them demands the utmost precision: “This is about just a few micrometers – and under extremely difficult conditions”, Schultheiß says. “Although there are other measurement techniques, a thorough analysis shows their measurement errors to be so large that they fail to provide reliable information on cylinder distortion.” One problem lies in the fact that the errors from all sensors go into the overall shape deviation error, making it impossible to determine deviations in the shape of the piston-cylinder system with adequate precision.

IAV’s measuring method uses eight eddycurrent displacement sensors which, each offset by 45 degrees, are arranged in an annular sensor carrier made of invar. It is integrated into a special measuring piston and supported in bearings between piston top and bottom. A measuring lever arm is responsible for outward signal transmission.

The basic idea behind IAV’s measurement method: the cylinder contour is measured seven times between which the sensor carrier is turned by 11.25 degrees each time. This gives the measurement engineers readings for the sensor carrier positioned at 0 degrees, 11.25 degrees, 22.5 degrees, 37.75 degrees, 45 degrees, 67.5 degrees and 90 degrees. This means that for the measurements at 0, 45 and 90 degrees, different sensors measure the same variables under identical boundary conditions.

For each contour level, these three measurements produce a total of 24 measured values for an equation system with 22 unknown variables (eight each for shape deviations and sensor errors, six for the different measurement piston eccentricity levels measured in each case in x and y direction). By means of iteration, IAV’s “IAV Engine Analyzer” uses this overdetermined equation system to compute the information being sought on cylinder distortion.

“Our unique approach lets us define the relative sensor error and make allowance for it at the evaluation stage”, Schultheiß says. “This improves precision in the all-important second, third and fourth distortion order. And higher distortion orders can also be reliably evaluated.” IAV cylinder distortion measurement with the engine operating delivers measured values that are accurate to within one to two micrometers – good enough to further improve base engine development tools. “Our customers never cease to be impressed that we can measure cylinder distortion with this level of accuracy”, Schultheiß reports. “And it really is quite fascinating to witness how sensor accuracy increases to produce this level of precision measurements.”