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With the introduction of lean burn and multivalve technology to produce gasoline engines with power outputs above 50 kW/litre there has also been an increasing interest in baseline engine noise. One component of this is due to the operation of valve train mechanism. The work described in this paper concentrates on this aspect of engine noise and uses a motoring rig to allow a full assessment of the problem. Usually the noise of the valve train mechanism is measured outside the valve cover. Here both noise inside and outside the valve cover is assessed using an extension of the acoustic substitution technique and the individual contribution of both airborne and structure borne noise assessed. It is found that airborne noise generated by the valve mechanism inside the valve cover and transmitted through the cover can be a major noise source at low frequencies (200 - 1200 Hz) and higher engine speeds.

The paper describes an investigation to identify the sources of forces which cause the vibration of different parts of the engine structure in a turbocharged heavy duty diesel engine of 2 litres/cylinder capacity. The differences in vibration response at the main bearings and on the engine outer surfaces is shown. Results of overall dynamic stiffness measurements at the main bearings indicate that the oil film has a negligible effect on the behaviour of the major vibration response. A model is put forward for an absolute prediction of the engine outer surface vibration. The model can be seen as an alternative or complement to current F.E. techniques. A comparison between predicted and measured vibration on the crankcase is presented. Predicted vibration response spectra are used to show the relative contribution of liner and bearing forces to the overall crank-case and cylinder block vibration of the engine.

There is concern in Europe and elsewhere that present fuel quality for diesel engined road vehicles cannot be maintained with changing patterns of demand and utilisation of crude oil. In particular, the oil industry (in the UK) has asked for a reduction of the present British Standard of 50 cetane number minimum for such fuels. Eleven diesel fuels with cetane number ranging from 26 to 58 and made from blends of gas oils refined from crudes from the Middle East, North Sea and West Africa were selected for test. The effect of these on noise and performance has been determined for three representative engines, IDI, DI and turbocharged DI. The test results show that the relation ships between the various engine parameters measured and the basic fuel properties such as cetane number and specific gravity are not simple. Cetane number appears only to be valid when comparing fuels of similar chemical composition or at the same air temperature.

This paper describes the use of digital measurements to assess acoustic intensity close to the various surfaces of a two litre gasoline engine. Using phase-locked data acquisition an acoustic power balance is constructed for the engine which is shown to correlate well with three other methods - lead covering, average surface vibration level and close microphone. However, the prediction of sound pressure level from these power balances is shown to be difficult, at least at 1 m from the surface. A simple relationship between acoustic intensity and sound pressure level in the frequency domain has been found.

The general problem of estimating the noise which vibrating surfaces will produce is discussed with particular application to diesel engines and their component parts. The measurement and prediction of engine noise balances using the surface vibration technique is described and the variation of engine block vibration with engine type and speed illustrated. Some measured radiation ratio’s for engine surfaces and components are given. Finally a method for measuring surface vibration on diesel engines using a laser doppler velocimeter is described.

From measured overall I. C. engine noise levels and corresponding measured combustion (cylinder pressure) levels the paper illustrates the major influence that the combustion system has on the engine radiated noise. The basic ‘noisiness’ of normally aspirated and turbocharged two- and four-stroke D.I. systems, normally aspirated I.D.I. (swirl chamber) diesel and gasoline systems are compared by a normalised frequency spectrum method. Using a simple linear model for calculating the direct combustion noise level of each combustion system the relative levels of noise are calculated. The results show good agreement with measured noise levels for normally aspirated two-stroke and four-stroke D.I. engines and indicate that combustion noise is low in turbocharged diesels, I.D.I. diesels and particularly gasoline engines.

The paper describes the assessment and testing of two high speed diesel engine structure modifications designed to reduce noise. The modifications tested are a five main bearing version of a standard three main bearing engine and the addition of a sump plate. The results indicate that it is difficult to assess the running engine vibration and noise characteristics using modal analysis techniques but that this is possible when using the banger test techniques. It is shown that noise reductions up to some 4.0 - 5.0 dBA can be achieved both on the test bed and in the vehicle application.

The overall effect of two methods of turbocharging a direct injection four stroke diesel engine in terms of performance, smoke, noise and gaseous emissions is described. It is shown that the effect of turbocharging is to produce a more socially acceptable engine. Matched turbocharging involving reduced compression ratios produces substantial emissions and combustion noise reductions (up to 16dB) at full load but can increase combustion noise levels considerably (8dB) at light load. Before full advantage can be taken, methods of controlling the characteristics of turbochargers to maintain short ignition delays at all conditions are required, together with reductions in engine mechanical noise levels.

The fundamental effect that cycle difference of a reciprocating internal combustion engine has on noise and performance, together with noise and vibration characteristics of 2- and 4-cycle diesel engines, is described. Some of the conclusions drawn from extensive noise and vibration measurements include: 1. Variation in engine surface vibration and noise radiated is linearly related to a force input applied to the structure. 2. A method of predicting combustion noise levels of current automotive diesel engines. 3. A combination of engine performance calculations with relations for predicting overall engine noise in initial design stages.