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Can Oil Engines be as Quiet as Petrol Units ?

31st October 1958
Page 56
Page 56, 31st October 1958 — Can Oil Engines be as Quiet as Petrol Units ?
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Which of the following most accurately describes the problem?

IN certain oil engines, oscillations from the induction system are the predominant source of noise. This surprising point was made in a paper entitled "Origins of Diesel Engine Noise," by A. E. W. Austen and T. Priede of C.A.V., Ltd., which was read to the institution of Mechanical Engineers in London last Friday. It was one of four papers making up a symposium on engine noise and noise suppression.

The authors felt that the oil engine was well on the way to overcoming its initial disadvantages over the petrol engine of a lower power output from a given swept volume and weight and increased initial cost. However, the greater noise emission of the oil engine was perhaps hampering its universal adoption for all forms of road transport. Thus research into the problem of noise was opportune.

Present methods of exhaust silencing were thought to be adequate so far as the occupants of the vehicle were concerned, and this was therefore discounted as a field for any great improvement. However, intake silencing had received little attention. Noise produced in the induction manifold started with oscillations set up as the inlet valve opened which were rapidly damped by the downward movement of the piston and replaced by high frequency noise developed by air flow across the valve seat.

Valve Tinting

This was followed by further oscillations in the manifold as the valve shut, lasting for some time after the completion of the intake sequence. Improvements in the degree of sound produced by the induction system could be affected by design and particularly by alterations to the valve timing. Late inlet timing, late exhaust-valve closing and a large exhaustvalve flow area all tended to reduce noise.

On test, a considerable improvement was made by fitting a Stewart low-pass intake silencer of about five times the cylinder volume lined with a soundabsorbing material.

Having effectively silenced the intake, it was possible to tackle a basic source of oil-engine noise, the characteristic "Diesel knock." This was found to be directly proportional in intensity to the rate of increase in internal cylinder pressures following combustion. It was deduced that if the rate of pressure rise could be adjusted so as not to exceed 30 p.s.i. per degree of crankshaft angle, the noise was greatly reduced. • This figure was considered to be theoretically attainable on an oil engine with a 15 : 1 compression ratio limited to a maximum cylinder pressure of 1,000 p.s.i. without losing more than 14 per cent, of the overall efficiency.

Limiting the rate of rise in cylinder pressure also reduced the vibrational noise developed by the main structure of c.20 the engine. However, there were other Means which could be used to alleviate this. Valve and timing covers were found to be a major source of noise in engines using cast aluminium for these components. Replacing these highly resonant structures with substitutes made of rubber-reinforced steel strip and asbestos-wool lagged double-skin sheets made, a satisfactory improvement. • Other vibrational areas • Were 'more difficult to locate and rectify, but it was provisionally accepted that bending moments in the crankcase, caused by transmission of force from cylinder pressure . and crankshaft vibration, were major factors in engine noise.

It was difficult to see how a nonresonant or " dead " crankshaft could be produced, but the crankcase itself could be damped by using a double-skin structure with closely spaced cross connections. The ribbed type of stigening, normally used for other reasons than noise reduction, might help in this but would most likely have little effect on the magnitude of the noise, only displacing it towards the higher frequencies.

Engine tests with the clutch housing removed showed that the flywheel too could produce noise due to endwise displacement of its centre and top edges setting up high-frequency oscillations. It was considered important, therefore, that no apertures should be left in the Clutch housing on an assembled engine-gearbox unit. The pulley on the front of the crankshaft or a counterweight, if fitted, could also set up oscillations.

Though the noise arising from injec Lion equipment was normally lost in the overall sound pattern of the present oil engine, improvements in silencing the engine structure and induction system could make this more apparent.

Noise from the fuel pump was only obvious when it was mounted directly on the engine and could transmit the small vibrations which it sets up to the engine structure, the radiation of that vibration from the larger surfaces of the engine causing it to become tiresome. Noise from the fuel pump arose from bending of the pump crankshaft because of variations of fuel pressure in the pumping elements.

Flexible Mounting

if the unit were too flexibly mounted or located on an .unstiffened structure, these vibrations could be seriously amplified. However, if the pump were flangemounted on a properly stiffened platform, the noise intensity would be negligibly small.

Injectors in which the needles always seated on fuel were found to be quieter than those which seated on a dry surface. As it was impracticable to ensure that fuel wasalways present during seating, other means were sought to effect an improvement, Reductions in the needle mass and spring force were found to improve the quietness of the injector, and this modification, in conjunction with a " dead " valve cover, produced a satisfactory reduction in noise.

It was felt that a study of fuel injection combined with improved combustion space airflow might help in achieving a desirable cylinder-pressure pattern.

The paper concluded by foreseeing improvements in oil engines which would make them at least as quiet as the equivalent petrol engine of the present day.

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