Inlet Port Design
Page 12
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and Volumetric Efficiency
Mr. C. B. Dicksee, of A.E.C., Ltd., on an Important Aspect of OilEngine Operation THAT the velocity of the air stream at the valve opening was the chief factor governing volumetric efficiency in a compression-ignition engine, was one of the conclusions arrived at by Mr. C. B. Dicksee, M.I.Mech.E., research engineer of A.E.C., Ltd., after a series of experiments with inlet ports of different cross sections.
In giving details of these experiments in a paper which he read to the Institution of Mechanical Engineers, in London, on Tuesday, Mr. Dicksee said that, whilst it would be unwise to draw too detailed conclusions from the experiments on account of their narrow scope, there was a number which he felt could justifiably be set down.
No Remedy
He was of the opinion, for instance, that the venturi form of port did not provide a remedy for an inadequate valve diameter, and under running conditions, with equal seat diameter, it did not possess any great advantage. At small valve lifts, however, the venturi did show only a small pressure loss under a steady air flow.
Amongst other conclusions arrived at by Mr. Dicksee were:—
(I) For engines with a speed range up to 2,000 r.p.m., for a given diameter at the seat of the valve, the diameter of the port, taken at a short distancebehind the seat, might be reduced to 80 per cent, of that at the seat, without loss of volumetric efficiency.
(2) By permitting a considerable reduction in diameter to be made at a short distance behind the seat the B4 venturi throat possessed the advantage of providing more room for the injector and for making provision for its cooling.
(3) There was a definite advantage in giving a valve a lift greater than a quarter of its diameter at the seat, but the extent of the gain depended upon the adequacy of the valve diameter for the size of cylinder. Where the diameter of the valve was restricted, as when a high stroke-bore ratio was used, a high valve-lift could be of material benefit
Orderly Swirl Needed In .opening his paper, the author pointed out that whereas the .eddies produced in the induction systan of a petrol engine were not without their uses, in the compression-ignition engine indiscriminate turbulence was not required. A well-defined orderly swirling movement was needed.
A compression-ignition engine could make use of only about N per cent. of the air it received, and in some designs the. amount was even below 70 per cent. This being so, it was essential that the engine should have as high a volumetric efficiency as could possibly be attained.
This applied with particular force at the higher end of the speed range, where it was. not so easy to maintain good combustion conditions as at the lower speeds. Further, in view of the
• smallness of the combustion chamber and the need for compactness in design. overhead-valves were essential. In many designs the injector must be fitted in a central position, which meant that it encroached on space that would otherwise be available for larger valves.
One solution of this problem, said Mr. Dicksee, was to machine recesses in the cylinder walls, so as to accommodate valves of larger diameter, but this, he added, had the disadvantage that the air contained in the pockets so formed was not readily accessible to the fuel during the most critical part of the cycle. There was little purpose in passing additional air into the cylinder if no use were to be made of it.
In the case of the A.E.C. company and of a number of others, the injector was placed slightly off-centre in one, direction, and the valves off-centre in a diametrically opposite direction: the injector was inclined outwards at an angle of 20 degrees.
The engine used in the experiments previously referred to was a singlecylindered unit of 120-mm. bore and 142-mm. stroke (swept volume, 1.605 litres). For the experiments the cylinder head was modified by machining away the original .valve seat, so that inserts could be fitted to provide the different types of valve port to be tested.
Material Advantages
Mr. Dicksee dealt in detail with the characteristics -of the different port shapes, which were tested under engine conditions and tinder a steady air flow. The experiments went to show, said the author, that under a steady air flow, the venturi form of valve port offered material advantages.
For a given minimum throat diameter an increase of diameter at the valve seat resulted in a considerable reduction
in pressure loss. Under engine conditions, however, the venturi throat, for equal seat diameters, did not possess any great advantage.