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The paper describes investigation in origins of automotive gasoline noise with a wide range of combustion characteristics from normal to severe knock. WITH NORMAL COMBUSTION the noise is due to impulsive oil pressures in the main bearings and a peak cylinder pressures control the emitted noise. The onset of knock produces almost instantaneous pressure rise from 0.5 up to 30 bar, which not only excites the engine structure at natural frequencies between 800 to 2000 Hz, but also excites the natural modes of the combustion chamber cavity resulting in large amplitude cylinder pressure oscillation and emitted noise in the frequency range from 6500 to 20000 Hz. THE CLEAR DEFINITION of knock induced noise of the engine offers a simple method which can be employed for knock detection and control in a car, particularly that of high speed knock.

The paper quantifies the forces applied to the main bearings of three six-cylinder turbocharged diesel engines and reviews their exciting properties in both time and frequency domains. The engine structure response at the bearing supports and the outer engine surfaces are correlated. It is shown that the engine structure response is a transient phenomenon and is a maximum in the vicinity of the applied force. By representing the engine response in terms of displacement it is possible to recognise the applied force time history and thus the identification of the specific parts of the engine structure primarily excited by moments and by direct force. The displacement technique for quantifying engine response provides detailed information of the distortion of the running engine enabling the prediction of mechanical inputs which can control the turbocharged engine noise.

The paper exemplifies the necessity to quantify the various noise sources present in modern automotive engines. Results demonstrate the non-linearity problems associated with many of the non-running engine rig tests currently in use. A new technique is described for combustion induced noise simulation using hydraulic excite ion. Results obtained using this experimental method are compared with those measured on the running engine.

In this paper the noise of engines of different size, duty and combustion system are compared. Measured differences, not consistent with combustion charges, are analysed in detail and these differences are shown to be the result of mechanical impacts. Results suggest that the kinetic energy of impact is not the only significant parameter influencing mechanical noise, and in the general case the magnitude of the force accelerating a component across a clearance must also be considered. The analysis shows that the dominant parameters affecting the magnitude of the impact change with speed and size of engine and offers an explanation for the apparent discrepancies observed in some previously recorded data.

The paper traces from the introduction of the internal combustion engine the numerous attempts that have been made to understand the fundamental origins of noise. It relates the progress made from very early subjective assessments to the present day sophisticated experimental and theoretical analyses.

The paper critically reviews the requirements for internal combustion reciprocating engines to meet present and future noise legislations according to their specific road vehicle applications. It reviews the significance of the vibration characteristics of structure elements in relation to the combustion systems employed and attempts to identify the importance of the balance between mechanical and combustion induced noise in engines falling into specific categories.

The paper classifies all current automotive engines into four distinct groups relative to engine usage and vehicle noise legislation. The noise characteristics of the engines in each group are widely different except that at their maximum rated speeds the overall noise levels approach the same value The sources of noise rank in different importance according to the size and type of engine. Therefore quiet engine design must also vary. The importance of mechanical noise is emphasised and it is considered that this source could be a limiting factor to future low noise engines.

The paper discusses the fundamental origins of truck noise and shows the rate at which the noise of each individual source increases with speed. Various means of controlling noise from each component are considered. A method of predicting engine noise, and hence vehicle noise, from basic engine speed and piston diameter data is given and the significance of this information to the engine designer is emphasized.

This paper discusses the sources of noise pollution in the civilized human environment and reviews the numerous criteria which have evolved for their assessment and description. It is stressed that criteria should be established in simple basic units and dBA values are recommended. By far the most common single source of annoyance is that of road transportation noise and this aspect has been covered in specific detail. Methods of predicting vehicle noise from the engine design and operating parameters are shown and some measures for noise reduction are discussed.

This paper describes the relation between the diesel engine structure design, its characteristics of vibration, and emitted noise. Special techniques developed for the investigation of the origins of noise and modes of vibration are discussed. On in-line engines the effect on noise and vibration of various design alternatives, such as wet and dry-liner, underslung crankshaft and skirted crankcases, intermains and without intermains crankshaft, designs, engine size, and number of cylinders are explored. On vee engines the origins of noise and the basic modes of vibration are established and the significance of the contribution of noise by covers is shown. A new form of in-line engine structure design has been investigated giving 10 dbA overall reduction of noise.

A twelve year investigation of diesel engine noise due to combustion, particularly in the high-frequency range, has indicated that changes in engine structure can achieve substantial reductions in noise. Test results obtained at the Research Laboratory of CAV Ltd. showed that the predominant noise in diesel engines of automotive size is produced by the rapid pressure rise resulting from combustion. Work directed toward “smoothing” the cylinder pressure is still in progress. At the same time, it has been determined that two forms of engine structure could improve structural attenuation. They consist of a separate load-carrying structure with attached highly damped outer walls and a construction of great bending stiffness cast in low density material such as magnesium.

The effect of engine speed load and size (swept volume) on noise has been evaluated on a limited range of engines. Total sound intensity (but omitting low frequency components due to air intake noise) varies as (speed)3, as (swept volume) and very little with load. The noise from a petrol engine was of the same order of magnitude as that of the diesel engines at full load but considerably less at low load. The following sources of noise have been identified and means of reducing them adequately established in principle. Narrow band frequency analysis has been shown to be a convenient criterion of the “noisiness” of the form of the cylinder pressure and in conjunction with noise measurement makes it possible to evaluate separately the noisiness of the engine structure. In the small diesel engines tested in this respect, it should be possible to quieten some (by about 5 db in the critical frequency range) by smoothing the cylinder-pressure-rise.