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A microcomputer-based analyser is being developed to estimate subjective reaction to noise from diesel-powered road vehicles. An interactive technique is envolving to match certain combinations of objective measurements to the subjective preferences of individuals, using a control signal from the microcomputer. For a pilot experiment on diesel engine noise at cold idle conditions, the sound stimuli were obtained from engine surface vibration signals. The pilot experiment results suggest that the impulsive characteristic of the sound could be more important than the overall level.

As new designs of diesel engine, which emit less noise, are evolved to meet increasingly stringent noise legislation, noise from diesel fuel injection equipment could become a significant proportion of the total noise from the engine. Quieter fuel injection pumps are being produced for such engines by Lucas CAV, however their potential will not be realised unless special care is taken in the design of pump drive and mounting brackets. Impacts from the sudden take-up of backlash in the drive to the pump, and vibration of the engine surfaces to which the pump is secured, can increase pump-radiated noise considerably.

The contribution to the external noise from the combustion process has been considered in relation to the gaseous emissions, smoke and specific fuel consumption. To satisfy the requirements of proposed legislation in different markets, whilst maintaining economical operation, the fuel injection equipment for a diesel engine must compromise between several conflicting requirements. The conflicts are presented in the form of “trade-off” curves over a range of dynamic injection timings. Four means to achieve better “trade-off” curves have been investigated:- higher rate of injection, fumigation, turbocharging, and turbocharging combined with a higher rate of injection.

The contribution to the noise of automotive diesel engines from the DPA rotary fuel injection pump has been assessed by comparing its overall noise with that of a number of engines, to which the pump is, or may be fitted. From this data, a ‘worst case’ has been selected for a more detailed assessment. The noise generating mechanisms of the pump are examined. Two improvements are described which are suitable for mass production with existing facilities, and which reduce the overall noise by 3 to 5 dB (A). An outline of a modular mathematical model is presented which will guide future efforts towards further practical cost-effective noise control.

If the noise generating mechanisms inherent in a diesel engine design are analysed thoroughly, several alternatives for controlling the noise at source become apparent. Research has shown that there are several ways of modifying an engine structure to reduce the noise radiated by the external surfaces of the engine in response to combustion and mechanical forces developed within the engine. A combination of structural improvements and fairly modest reductions in the excitation, by incorporating a smoother combustion process and by minimising mechanical impacts between the working parts of the engine, could result in reductions in truck engine noise of up to 10 dB(A).

A simple technique has been developed for measuring the noise radiated by diesel engine surfaces in response to combustion excitation. Results using this technique correlate well with the established computer-based analysis technique. Combustion noise measurements from a direct injection engine show increases of 3dB(A) as fuel cetane number is reduced from 50 to 40CN. Combustion noise in high speed direct injection engines is much greater than in comparable indirect injection engines, and a fundamental study is described which indicates the rate of heat release diagrams required to minimise and optimise combustion noise. This work is supported by engine results with pilot injection which gave reductions of 5 to 8dB(A).

There is now a growing interest in every means of reducing the noise of current diesel engines and designing even quieter engines for the future, perhaps as awareness of noise as a social problem becomes widespread. This paper discusses the production of engine noise, its transmission through the structure, and its subsequent radiation as a sequence of events. Diesel engine noise can originate from several mechanical sources such as piston slap, timing gear rattle, etc. as well as from the sharp rise in cylinder pressure which attends combustion. The relative importance of these sources is affected by clearances between the working parts which are dictated by design and manufacturing constraints. If reduction of noise at source is contemplated, it is necessary to diagnose which of these sources are primarily responsible for the noise, bearing in mind that the relative importance of the sources varies with engine load and speed.

Emphasis is placed on noise control at source in the hearing conservation programmes in the U.K. factories of Lucas companies, especially when new plant is ordered. This paper briefly describes the overall plan and Company standards with five case studies to illustrate how they work in practice. It concludes with a summary of the key steps in the Company strategy.