An Oil-engine Designer Upholds Two-stroke Merits
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The Relative Merits of the Two-stroke and Fourstroke Systems are Exercising the Minds of Many Designers Throughout the World
APARTICULARLY interesting controversy, is arising as a result of the publication of the article
" Two Stroke or Four Stroke?" in our issue dated March W. In this contribution, Mr. C. W. Chapman, M.Eng., gave reasons why he believes that the apparant advant age of the two-stroke cycle for vehicle oil engines is unreal. One reply is given as a letter elsewhere in this issue. Another is contained in the following article by Mr. C. B. M. Dale, chief designer, Diesel Department, Brush Electrical Engineering Co., Ltd., in which he says that he might indeed question some of the assertions made by Mr. Chapman, and he continues (1) Referring to "Two-stroke valve systems" your contributor implies that there is some novelty in the introduction of an engine having exhaust valves in the cylinder head and inlet ports at the bottom of the working cylinder.
" Uniflow " scavenge with this arrangement of port and valve gear, has been known for a long time. To cite a few examples, Burmeister and Wain and their licensees,Harland and Wolff, have been building this type of engine for many years. The " Petter Superscavenge" engine is amongst the recent designs, being introduced into successful service just over a year ago. The Winton engine is another of this type.
There are also further examples of " Unifiow " scavenge such as the " Doxford " and " junker " engines, which have been in production for a number of years. These latter have opposed pistons to control the ports, instead of exhaust valves, as in the case of the former types mentioned.
(2) Under the heading of" Heat-dissipation difficulty" your contributor does not appear to have appreciated the latest developments in two-cycle engines, particularly as exemplified in those of the " Superscavenge " type.
• Two-stroke Gains by Superseavenging • When the principle of " Superscavenging "is adopted, use is made of the "self-induction" effect, whereby the discharge of the exhaust gases from the working cylinder assists in bringing in a fresh charge. Thus the blower power required is very much less than in engines built under previous accepted practice. For instance, on a certain ported (loop-scavenge) type of two-stroke engine, which had blower scavenge, the blower pressure was approximately 4-3 lb. per sq. in., the best b.m.e.p. obtained with a clear exhaust being of the order of 50-55 lb. per sq. in.
When the engine was converted to the " Superscavenge " principle with exhaust valves in the head and uni-directional flow, the blower pressure was reduced to only li•lb. per sq. in., with consequent direct gain in mechanical efficiency. Further, the b.m.e.p. was increased to 70 lb. per sq. in. for normal working, with a peak output of PO lb. per sq. in..
Incidentally, this engine will develop some 35-40 lb. per sq. in. b.m.e.p. when the blower is disconnected, " clearly showing the assistance obtained from the " self-induction " effect. The " Petter Superscavenge" engine is, of course, manufactured under licence of the Kadenacy patents issued by Armstrong Whitworth Securities, Ltd.
Further, under all conditions, the exhaust was perfectly smokeless and fuel consumptions of 0.369 lb. per b.h.p.,/hr. are usual at the normal rating of 70 lb. per sq. in. b.m.e.p.
It is true that these results are obtained on a fairly large engine of 8i-in. bore by 13-in, stroke of medium speed (500-600 r.p.m.) but our development work has shown that similar results can be obtained equally well on smaller engines of 41-4i-in. stroke by 6-in. bore.
Of course, on the larger engines oil-cooled pistons are a necessity for the high-duty two-stroke-cycle types, but provided suitable precautions be taken in the design, this does not present any difficulty.
Owing to the fact that the " Superscavenge " type of engine is working much more efficiently, a supply of excess air can be obtained without doing an excess amount of negative work. Further, due also to the efficient discharge of the gases from the cylinder, We are able to ensure that the exhaust valves are considerably cooled by the excess air.
In the engines above referred to, the exhaust tempera.. ture, measured close to the exhaust valve, does not exceed 730 degrees F. at the .normal industrial working load of 70 lb. per, sq. in. b.m.e.p.
• What is Wanted for Traction Purposes • Due to the presence of an appreciable quantity of excess air, the engine has a very high overload capacity (about 33? per cent) with a smokeless exhaust, which is just what is wanted for traction purposes.
(3) No trouble has ever been experienced with piston rings sticking. Much of this is undoubtedly due to the use of oil cooling. A special design of piston ring has been developed to avoid ring flutter and blow-by.
(4) In the conclusions given by your contributor in comparing the latest British four-stroke and the American two-stroke oil engines, I do •not think the comparison has been correctly stated.
In the published data on its new two-cycle engine, issued by the General Motors Corporation, whilst the normal b.m.e.p. is given as 70 lb. per sq. in, the maximum b.m.e.p. is actually 94 lb. per sq. in. ; thus, taking the four-stroke figures as being correct, the twostroke performance is just double that of the fourstroke engine.
Again, I do not think that the comparison stated is a fair one, as the two-stroke engine is doing only 1,800 r.p.m., as against the 2,600 r.p.m. of the four stroke.
If the latter were limited to 1,800 r.p.m. there would be a very different story to tell in respect of performance. There can be no doubt as to which engine will be in a better mechanical condition, after long service, if the four-stroke engine has to run at 2,600 r.p.m. to give equal power.