Hocking, C., Antonov, S., and Shahlari, A., "Instrumentation, Acquisition and Data Processing Requirements for Accurate Combustion Noise Measurements," SAE Int. J. Passeng. Cars – Electron. Electr. Syst. 8(2):470-476, 2015, doi:10.4271/2015-01-2284.
The higher cylinder peak pressure and pressure rise rate of modern diesel and gasoline fueled engines tend to increase combustion noise while customers demand lower noise. The multiple degrees of freedom in engine control and calibration mean there is more scope to influence combustion noise but this must first be measured before it can be balanced with other attributes. An efficient means to realize this is to calculate combustion noise from the in-cylinder pressure measurements that are routinely acquired as part of the engine development process. This publication reviews the techniques required to ensure accurate and precise combustion noise measurements. First, the dynamic range must be maximized by using an analogue to digital converter with sufficient number of bits and selecting an appropriate range in the test equipment. Second, the encoder resolution (sampling rate) must be chosen in order to achieve the required frequency range and to avoid aliasing that is typical in event-based acquisition systems. Third, to calculate combustion noise the resulting cylinder pressure must be suitably chosen or windowed before applying an FFT and processed on a cycle-by-cycle basis. Each of these aspects is discussed including the errors that can be incurred with each step and how to minimize them. Recommendations are provided with an emphasis on a single setting that minimizes the errors across the wide range of speeds and loads encountered in engine dynamometer testing.