Sound Quality Evaluation Method for Engine Combustion Noise in an Engine Acoustic Test Cell

In order to efficiently enhance engine sound quality under acceleration, the authors have developed an evaluation method for primary judgment of the sound quality of engine combustion noise at the stage of advanced engine development before the prototype vehicle is built. This method is an application of an existing method for evaluating the sound quality of engine combustion noise in vehicle interiors to the evaluation of noise and vibration at an engine acoustic test bench. In this method, it is necessary to consider the air-borne and the structure-borne components separately. The analysis procedure for the air-borne component is as follows. First, the sound pressure at a point 1 m away from the engine and the in-cylinder pressure of each cylinder are measured simultaneously in a semi-anechoic engine dynamometer test chamber. Next, the signal correlated with engine combustion is extracted from the measured sound pressure using the time domain combustion noise separation method. Then the obtained signal is converted into the equivalent signal of combustion noise in the vehicle interior using an acoustic filter that simulates the standard vehicle response characteristic. Finally, its sound quality is quantified using the existing sound quality index for combustion noise in the vehicle interior. The analysis procedure for the structure-borne component is the same as that for the air-borne component, except that vibro-acceleration at the engine mount is measured instead of measuring the sound pressure of the engine-radiated noise. This paper presents an overview of this method, introduces an example of application of the method to advanced engine development, and discusses the effectiveness of the method in automotive development.