OPINIONS FROM OTHERS.
Page 27
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The Eiitor invites correspondence on all subjects connected with the use of commercial motors. Letters shOulg be on one side of the paper only and typewritten by preference, The right of abbreviation is ueserved, and no responsibility for views expressed is accepted, America and the Crude Oil Engine.
The Editor, TEE COMMERCIAL MOTOR.
L19241 Sir,—From Mr. Sturmey's letter in year issue of March 14th, relative to the Dodge-lividDiesel-Hornsby-Ackroyd engine, it would appear that he has slightly misconstrued the points we raised in connection with his statement, that, the engine concerned would utilize suecessfully " crude oil or any other hydrocarbon except petroleum spirit." In the circumstances, therefore, may we,, without the slightest intention of seeming hypercritical, offer a further commentary? At this stage, too, we should like •clearly to indicate that in joining a discussion of this character we do so with the abject of openly ventilating, from a disinterested standpoint (which, unfortunately, technical controversies of the kind are not always devoid of), a reasoned consideration of the subject with a jealous proclivity for the-dissemination of accurate technical information.
What caused us to offer friendly criticism in the first instance was the use of the term ." petrol " or " petroleum spirit" in a connotative sense, and, secondly, Mr. Sturmey's omission to give an explanation of the somewhat bald statement. The term " motor spirit", connotes all those fuels suitable for ordinary motorcar engines, and it denotes the attributes of volatility and low flash-point. The word "petrol " has reference to a part4eular fuel— distinct from benzele, for instance, and was, therefore, hardly correctly used.
Mr. Sturiney next says that the ignition pressure of the Dodge engine is nothing like so high as the explosion pressure of an ordinary engine. We think, however, that Mr. Sturiney will find that he is mistaken on this point. The explosion pressure of an ordinarypetrol engine does not usually exceed 250-350 lb. per sq. in., hut, in order to compress the air to a temperatUre sufficiently high to ignite the fuel in a Diesel type of engine, a compression pressure of seme 500 to 600 lb. per sq. in. is unavoidable. Further, in order to reduce this pressure during the working stroke to a reasonably, low value at release, expansion must commence very early, so that the constant-pressure portion of the diagram will only be maintained for about one-twelth of the stroke— which is very different from the state of affairs in a single-cylinder steam-engine, as referred to in the original article.
Again, Mr. Sturrney surmises that " the power exerted on the (following working) stroke would be proportionate to the initial pressure as with an ordinary petrol engine." Eut this scarcely is the case, for it is well known that the power is .proportionate to the mean_ effective pressure of the cycle, and the value of the latter has but little relation to the, initial value, being, furthermore, quite insignificant compared with it.
High initial tempetature and pressure mean thermal efficiency, but a narrow indicator diagram.; that is why the Diesel engine is so efficient, but it also means very strong construction of the cylinder. And this indicates the reason why engineers are striving to produce the successful compound Diesel curgine— because then the high initial pressure can be readily provided for in a small high-pressure cylinder, The pressure during the remainder of the expansion —in the low-pressure cylinder—is, very moderate, so that light low-pressure ylinder construction is permissible, with a corresponding increase in power-weight ratio.
It is a pity that Mr. Sturmey descends to unParliamentary language in the last paragraph of his letter ! We had no intention whatever of belittling
the efforts and achievements of our American friends; We simply said that we were unable to see any essential difference between the Dodge cycle of operations's and thatof . the well-known HornsbyAckreyd engifie ' a this Country (A.D. 1890).
We note the more complete statement that our distinguished friend now gives concerning the question of the introduction of the fuel (by gravity) and we trust he realizes, and also does not mind, that we were gently pulling his leg on this point!—Yours faithfully, THE AHTOCRAFT Boelin,
Lendon. J. MORSE SCOTT, Controller: Thermo-siphonic Water Circulation.
The Editor, TEE COMMERCIAL MOTOR.
[1925] have followed with much interest the correspondence on your page "Opinions from Others" relating to pump .v. thermo-siphonic cooling water circulation.
I freely admit that there is a lot to be said for both types, but, on the whole, I think that pump circulation is the more suitable to meet general requirements. Nowadays, one must think imperially, and not so far, as this country alone is concorned. Vehicles should be designed. to be efficient, whether employed here or in any part of ,the Dominions, or, or, in fact, of the world, and whilst then-nosiphonic circulation may do very well in temperate climates, I have grave doubts as to the advisability of employing this means of water circulation in places where the temperature is high.
There is something positive about the pump which appeals to the majority of engineers and users. Thermo-siphonie circulation often acts in a very satisfactory manner until the interior of the radiator and of the water piping becomes furred with deposits fm urn the water. When this happens, the fur has a retarding effect on the flow, and a certain amount of cooling efficiency is lost, but in such cases the pump, if properly designed, can attend to the slight extra friction, and there is practically no difference in the circulation. The pump may cause a little more trcuble in upkeep, but the extra amount of attention required is really very slight.—Yours faithfully,
Railways and Road Transport.
The Editor, THE COMMERCIAL MOTOR.
[1926] Sir,—When railway companies attempt to pose as public benefactors the position is just a little absurd. The bulk of the traffic Met by them is due to sheer neglect on their part, and not so much to increased rates. They have no qualifications for under-. taking road transport. They ro not pretend to have and their only defence is that the present road haulier is insufficiently. taxed. They obviously would not labour that point if they sincerely thought of permanently operating on the roads! I think of all the most nonsensical arguments put forward by the railway companies the silliest is that if they could e,c,nvershort-distance traffic by road that would release rolling stock for long-distance traffic. The fact is they are at a loss to know what to do with their rolling stock. Road hauliers have only seriously taken to the longdistance huSiness since the war, vet I Maintain that in this short space of time they have .achieved astonishing results, and given the public greater cause for satisfaction than the railway companies have in 50 years, hence the jealousy of the latter.— Yours faithfully, . WALTER GAMMONS. London, E.(). I.