Are the Days of the Four-stroke Numbered ?
Page 36
If you've noticed an error in this article please click here to report it so we can fix it.
By A.Q.M.S. E. G. FROGLEY,
I T was with' interest that I read the contribution by " Northern Engineer '' in your issue dated January 12, and whilst the explanation of thermal efficiency was made clear, together with several • other facts regarding fuels,,etc, I feel that, on the whole, it would appear that the writer of the article gives the impression that the design of compression-ignition and petrol engines is likely to remain unaltered for ever, . Surely, this can he very far from the
truth. Obviously,. he cannot have weighed the pros and cons of some of the facts which I should like to outline, and which will, perhaps, prove to the lay-man and others who may be labouring under the same delusion, that the working principles have reached such a stage of perfection that they cannot-be
improved upon.
In the first. place, as the Editor pointed out in the article in question, there is that excellent and well-tried unit, the G.M. two-stroke oil engine. I have had some considerable practical experience withthis unit, and a more efficient engine it would be difficult to produce. It is simple, economical and capable of operating under extreme conditions of heat and cold, and, from the servicing point of :v-iew, compares more than favourably with its four
stroke counterpart, _ .
. Perfect scavenging is performed, bymeans of the blower, consequently,
thermal efficiency is _very, high. The fuel-injection equipment, although a little unorthodox as compared with the normal fuel pump and injectors, presents no difficulties Whalsoever, and no special skill is required; this, in itself, is a great point in its favour, ' The injector-turn-pump units are easily removed for servicing, and lave been found to be trouble-free. The
compression ratio, is in the. region of 16:1yery high for a two-stroke—and by virtue of the. annular air-admission ports a:round the-Cylinder, a-high degree
of turbulence exists. ,
The power-weight ratio is in the region of 12 lb. per hp., which, I Should think, compares more than favourably -with. the majority of four.. strokes. The enginp has a '11.m.ep, of 821b,
A Two-Stroke Prophecy A further example, and the're are several, is found in the lunkers jumo, which is an opposed-piston two-stroke compression-ignition unit. Whilst I have no • intimate knowledge of this engine, I have examined one very thoroughly, and, in my opinion, we shall see. ranch more of this design in the post-war years.
It lends itself admirably to use in aircraft, owing to its extremely small frontal area. Bearing this iii mind, it should -.prove ideal for rear7engined chassis; which are certain to come at
no distant date. .
For the benefit of readers who may not be technically informed, this engine is of the opposed-piston type, with geared crankshafts. Its working principle is similar to that of the G.M., in that it i,s supercharged and scavenged in exactly the same way. The coinpression -ratio is 14:1 and b.m.e.p. 90 lb.
It is my opinion that there is ahnost unlimited scope for the two-stroke engine., The old bugbear of incomplete scavenging has 'been entirely elimin ated, and the employment of high . •
compression ratios boosted by means of a blower will enhance the thermalefficiency to a large degree, The principle need not be confined to compression-ignition engines; 1 can visualise a petrol engine employing a fairly low compression ratio, with a simple variable-boost blower, in which the rbutter and induction-manifold system is dispensed with. Instead, there will he direct petrol injection.
I am convinced that this will see the ..end of " the, well-tried veteran cycle and proved -principles that Took like being with us until the day of the internal-combustion engine is over," to quote "Northern Engineer."
In this type, pre-ignition would be impossible, because there would be nothing to ignite. Fuel distribution to the cylinders would be.of the same high. order as in the compression-ignition engine, and the old inherent -faults, attendant upon the use of tortuous manifold passages and inefficient carburetters, to say nothing of abnormal "cylinder weal-, would be gone for ever.
Initial cost should not be excessive and, once established, such a unit should compare on favourable terms, so far as Production is concerned, with the higher-class of four-stroke engine. The great strides which have been made during recent years in the field of metallurgy will put at the disposal of designers. alloys of. such strength and lightness, that compreSSion-ignition engines will be able to be built much lighter, with an irnprovenaent in their power-weight ratio.
With such unlimited scope, I think our own designers should, for once, set' a, precedent, and disabuse the minds of the vast number of people who believe that the four-stroke engine is " the be all and end all " n internalcombustion engine design.