LOW-SPEED v. HIGH-SPEED ENGINES.
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Points For and Against Each Brought Forward by Two Eminent Designers.
N INTERESTING debate, based on a comparison between the lowcompression moderate-speed engine and the high-compression high-speed engine, was held last Wednesday by the Institution of Automobile Engineers at the Institution of Mechanical Engineers, Storey's Gate, London, S.W., Major F.
Strickland, being the champion of the moderate-speed engine, and Mr. H. R. Ricardo, 1VI.I.A.E., of the high-speed type.
The discussion, athorigh alluding in particular to the power units employed on private cars, contained many points of interest to builders and users of commercial vehicles.
The average commercial vehicle engine of the heavy type is certainly in a moderate-speed category, but even for heavy vehicles there are signs that the normal engine speeds are being increased, as, for instance, in the S-type L.G.0,C. bus, in which -the normal engine speed is some 200 r.p.m. faster than in the B type. so that increased power is obtained without increasing the size or weight of the engine. With the smaller types of commercial vehicles, such as vans up to 30 cwt., car-type engines are often employed, as, for instance, in the Austin, where a similar power unit is employed in the tractor, car and lorry. In most cases the smaller engines rim at considerably higher normal speeds than those employed on heavy vehicles.
Major Strickland, the protagonist of the moderate-speed engine, pointed out that the steam engine had an advantage over the four-stroke petrol unit in that the reciprocating parts could be brought to rest by a steam cushion so that, other things being equal, the power would increase directly as the number of revelations. In the case of the petrol eugine, however, the reciprocating parts could only be cushioned once in every four revolutions, so that the inertia forces became serious at high speed. To illustrate this point he showed mil-yes taken from an engine of 80 mm. bore and 150 mm. stroke, in which, at 3,400 r.p.m., the bending stress on the shaft, due to inertia, was double that due to the explosion pressure. After pointing out that the actual size of parts often depended inure upon the bearing surfaces than upon considerations of strength, he showed that the bearing pressures which, in the case of the steam engine, remained practically constant, rose very seriously in the petrol engine with increases in speed, so that the dimensions of the parts, and consequently their weight, would have to be increased, which appeared to indicate that there would be some point at which the weight of the engine would increase more rapidly than the horsepower. Following this up, he came to the conclusion that the lightest engine was one with a normal speed of 1,200 r.p.m. to 1,500 and with a cylinder of from 92 mm. to 103 mm. diameter.
He showed by curves that the conclusion as to the limiting speed applied in the same way to engines having a stroke equal to the bore, and followed this by dealing with the advantages of the moderate-speed engine, apart from the question of weight, but chiefly in connection with piston and cylnder wear, simplicity of lubricating parts and noise. In conclusion, he pointed out that, although the modern high-speed engine gave a very high power for the size of its cylinder, it did not, give a high proportionate _e power per unit of weight, and, in addition, required more attention.
Mr. Ricardo, the advocate of the highspeed engine, pointed out that, the ultimate objectives in any design were efficiency, durability, the best possible use of the materials employed in relation to weight and cost, silence and balance.
In order to show the limiting compression ratio, he described tests made on a variable-c.ompression engine which resulted in the conclusionthat from 60 lb. per sq. in. compression pressure up to 95 lb. per sq. in. the power rose and the fuel censumption fell, whilst at 97 lb. per sq. in. the engine began to run harshly with no increase of power or economy. At 100 lb. per sq. in.
detonation occurred; this became very definite at 102 lb. per sq. in.,. accompanied by a drop in power and economy.
In giving this warning against excessive compression pressures, he pointed ont that the smaller the cylinders the higher the speed and the compression pressure which could be adopted, thus leading to better economy than with the ,slow-speed• engine with large cylinders. By increasing the compression pressure from 75 lb. to 110 lb., the.-e would.he a 20 per cent, gain in economy, which wonld increase to 35 per cent, at the average load factor of , a motor vehicle engine.
So far as the heat flow was concerned, this increased very rapidly as the compression pressure approached 100 lb. per sq. in., thus leading to trouble with the sparking plugs, valves, piston rings, etc., and by detonation, and when the lastnamed once, appeared the high-compression engine lost all its advantages.
In dealing with the question of high speed, he made the assertion that such an engine must last as ;ong as the'lowspeed engine or show some compensating advantage. The higher mean pressure would allow the use of smaller cylinders, and therefore smaller and lighter pistons, which, owing to the reduced heat flow, would run cooler, and would allow more efficient lubrication and consequently reduced wear, and thus, for equal ratios of wear, would allow a higher road factor, whilst the smaller and thinner pistons would reduce the inertia forces down to little more than those of the low-speed engine of the same horse-power.
Ile made the remarkable statement that if the parts were carefully designed, and if the gases, after passing the valves, had free entry into the cylinder, large and heavy valves were unnecessary.
Summing up, he stated that development always tended towards the attain, ment of the maximum amount of energy from the minimum both of fuel and material. He gave as an example the fact that the speed of steam engines had greatly increased from the Winans to the Daiss type.