利用小波变换分析燃烧噪声

SAE TECHNICAL PAPER SERIES

2001-01-1545

Wavelet Transform Applied to Combustion Noise

Analysis in High-Speed DI Diesel Engines

José M. Desantes, Antonio J. Torregrosa and Alberto Broatch

Universidad Politécnica de Valencia

Noise and Vibration Conference & Exposition

Traverse City, Michigan April 30 - May 3, 2001

Reprinted From: Proceedings of the 2001 Noise and Vibration Conference

(NOISE2001CD)

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2001-01-1545 Wavelet Transform applied to Combustion Noise Analysis in

High-speed DI Diesel Engines

José M. Desantes, Antonio J. Torregrosa and Alberto Broatch

Universidad Politécnica de Valencia Copyright © 2001 Society of Automotive Engineers, Inc.

ABSTRACT

Traditionally, combustion noise in Diesel engines has been quantified by means of a global noise level determined in many cases through the estimation of the attenuation curve of the block using the traditional discrete Fourier transform technique. In this work, the wavelet transform is used to establish a more reliable correlation between in-cylinder pressure (sources) and noise (effect) during the combustion of a new generation 2 liter DI Diesel engine. Then, in a qualitative sense, the contribution of each source intrinsic to the combustion process is determined for four engine operating conditions and two injection laws. The results have shown high variations in both the in-cylinder pressure and noise power harmonics along the time, which indicates the non-stationary character of this process. INTRODUCTION

In Diesel engines, when the initial fuel and air mixture ignites spontaneously, a sudden pressure rise takes place producing the well-known Diesel knock. This abrupt pressure change forces the gas in the combustion chamber to oscillate. This oscillation causes then the vibration of the engine block, which in turn radiates the aerial noise. Thus, the vibration of the block is originated by two main sources associated with the combustion process: the pressure forces of the gas, and mechanical forces like piston slap, friction, etc., which are powered during the combustion [1]. This noise radiation process during the combustion is schematically represented in figure 1. In this figure, it can be observed that in-cylinder pressure characterizes the excitation source of the system (pressure and mechanical forces), while its response is associated with the vibration of the block wall. The final effect of this vibration is noise radiation, which obviously can be coupled with other noise sources. Moreover, the pressure gradients occurring during the combustion process also originate a high amplitude oscillation of the gas, which is associated with the resonant frequency of the combustion chamber. In DI engines, this resonance frequency depends mainly on the geometry of the piston bowl and the temperature of the gas inside it. Since both the combustion chamber geometry and the gas temperature change during the combustion process, the resonant oscillation in this

system turns into a non-stationary mechanism.

Figure 1. Combustion noise process.

Due to the increasingly restrictive noise limits for commercial vehicles in the last years [2], combustion noise of Diesel engines has been widely studied. These studies have been focused either on the vibration of the block structure [3], or on the use of the block attenuation as a transfer function between in-cylinder pressure and radiated noise [4,5]. Actually, with state-of-the-art injection systems, i.e. common rail, electronic unit injector, etc., more degrees of freedom are available for low-noise engine development. However, this factor should be balanced with other engine characteristics, such as exhaust emissions, smoke and performance. In this sense, the optimization of the engine requires not