2  The Process of Designing Low-Noise Engines

The function of the engine is to convert fuel into the mechanical forces which power the vehicle. The vibration of the engine structure arises from the mechanical events within the engine and the direct excitation from the fuel combustion process. The noise observed by the occupant or bystander is the perceived acoustic response of the air to the various radiating surfaces. These surfaces may be those of the powertrain itself or of the vehicle structure. The vehicle structure may be excited either from the air (in turn excited by the powertrain surfaces), or directly, for example via the engine mounts. Background work on this can be found, for example in Grover and Lalor (1973), Challen (1982), Morrison (1985).
Radiated noise levels must be considered at every stage in the development of the design of modern engines. At concept design only broad guides to the overall engine structure will be given. During the initial design phase, however, accurate predictions of engine vibrations may be made, these may form the basis of noise prediction systems. Such analysis work may be used to help generate improved designs, prior to manufacture of prototypes.
The overall design process may be divided into the pre-prototype and post-prototype phases. Each of these phases is likely to be partially iterative in nature.
At any given point during the post-prototype phase the engineer may be satisfied that the noise is sufficiently low and that no further deliberate action is necessary. On the other hand, the engineer may be already planning further measures to reduce the noise. One such measure is that of partial enclosure of the engine block using noise shields.
In this section a simplified and schematic description of each design stage is given. It is shown how computational techniques may be usefully employed in the design process.

2.1  The Pre-Prototype Phase

In the process of designing an engine, methods for predicting the vibratory and acoustic properties from a draft of the design would be useful. Such methods would show whether or not the engine would be too noisy when built, and also give indications as to how the design may be improved. Once the design of the engine is such that the predicted noise is satisfactory, a prototype of the engine may then be built.
Figure 1 illustrates this part of the design process. The reason for carrying out this pre-prototype phase, rather than directly building a prototype, is that it makes the overall design process more economical. The efficiency and even the success of this process is dependent on the accuracy of the prediction of the vibratory and acoustic properties, the interpretation of these properties and the ability to make successful modifications to the design of the engine based on this interpretation.

Figure 1. The Pre-prototype design phase.

2.2  The Post-Prototype Phase

Once the prototype is built, the vibratory and acoustic properties can be measured. However, some of the desired acoustic information may be difficult or costly to measure and it may therefore be more feasible to predict the acoustic properties from the surface vibration. This part of the design process is illustrated in figure 2.

Figure 2. The Post-prototype design phase.

2.3  Noise Shields

On fitting one or more noise shields around the engine block, the change in noise will depend on the shape of the engine block and the nature of its vibration, the size, shape and position of each shield and the vibratory properties of the shield itself. Diagnostic information about the vibratory and acoustic properties of the isolated engine block will help the design engineer decide where the shields should be placed. Without the aid of computational tools for predicting the effect of the shield on the noise, the shield may only be developed through generating a physical model and measuring the effects. The overall process of designing an engine-shield system is illustrated in figure 3.

Figure 3. Noise shield design process.

2.4  Discussion

Noise has been an important factor in the design of engines for several decades. Over this period, engineers have become more experienced in designing low-noise engines, using simple relationships between design parameters and noise (see, for example, Grover and Lalor (1973)). However, this approach is severely limited since the relationship between the design parameters and the noise produced by the engine is not trivial. With the increasing capabilities of computational tools, the more valid approach of basing the noise prediction methods on the principles that govern the properties of the engine structure and the air which surrounds it has been considered over recent years (see, for example, Dowling (1991)).
In the design process, illustrated in figures 1-3, the design engineer has to be able to interpret the vibratory and acoustic information and judge whether corrective treatment in the design is required. Clearly, the more reliable and comprehensive this information is, the better placed the engineer is to make these judgements. Hence the aim of developers of both computer software and measuring equipment is to provide the engineer with as much reliable information as he can usefully handle.