OPINIONS FROM OTHERS.
Page 20
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The Editor invites correspondence on all subjects connected with the use of commercial motors. Letters should be on enie side of the paper only and typewritten by preference. The right of abbreviation is .reserved, and no responsibility for views expressed is accepted.
The Tyre Mileage Guarantee.
The Editor, THE COMMERCIAL MOTOR.
[1677] Sir,—All readers"of The Commercial Motor who are also users of heavy motor vehicles are indebted to you for the publication of the British Rubber Tyre Manufacturers' Association's explanation of the abandonment of the 10,000 miles guarantee. I gather, from the wording of your brief article, that you are not entirely in sympathy with the tyre ft manufacturers, and venture to respond to your request for opinions of users on this important question. The explanation—or apology—and the announcement which was the first public intimation of the alteration in the policy of the tyre manufacturers had surely better have been withheld. Far better ,would it have been had the makers merely stated their plain intention to drop the guarantee, and have "left it at that..
The makers, having arrived at the conclusion either that owing to the very bad condition of our roads, the guarantee will for some time prove an expensive one, or that the user has to have tyres anyway, and therefore will have to submit to whatever conditions • the makers imPose, decided to eliminate the guarantee from the condit;ons of sale. Why trouble to explain a-matter so crystal clear l In any event such an explanation as that which appeared on page 33 of your issue of March 6th is merely adding insult to injury. I need only refer to one paragraph as indicating its hollowness. I mean that one which runs as follows :—".They (the .makers) will for the future he content to allow their products to he judged by the service given (as distinct from merely guaranteed) and are making it their business to see that their tyres "give the best mileage, the best service, and fullest satisfaction to 'their customers." Either the manufacturers did not essay such high-class service in pre-war days—which is surely a confession—or, if, as I anticipate, they insist that this was also their pre-war policy, then we users are clearly the losers by whatever value there was in the guarantee, and as the majority of tyreS, notwithstanding what is said to the contrary, did not, and do not outrun the 10,000 miles, the inference is plain.—Yours faithfully.
OWNER OE FOUR.
T. he Waste of Power.
The Editor, THE COMMERCIAL TVIOTOR. ' [1678] the last issue of The Commercial•Motor you treated of the extent to which internal
combustion engines converted into useful work the power put into them in the form of the calorific value of their fuel. You then showed that even the most efficient prime movers wasted about 67 per cent. of this calorific, value, these lost heat units being dissipated principally in the cooling water and in the exhaust gases.
Attempts have been made in the past -to utilize the heat of exhaust gases for warming the feed water of steam engines. Shortly ihefore the outbreak of war, a vessel was built for the Royal Navy, in which were installed a Diesel engine and two turbines : the exhaust heat from the Diesel engine was made to effect the preliminary heating of the feed water for tterirtilkrs that supplied the steam to the turbines.
• At first sight it would seem that this arrangement ought really to effect substantial economy of ealorifir, value ; actually this is not found to be so, and the system has long since been abandoned. Remembering that steam engines in. themselves are far more wasteful than internal combustion engines, it is evident that it will not in the long run he profitable to save a small proportion of the calorific value of the fuel in the latter only at the cost of maintaining
c15 mite or more extremely wasteful steam engines. The worst of it is that, clue, to what is known as the latent heat of steam, so many heat units are required to effect the evaporation of water, that the amount of heat being carried away in the exhaust is found to be inadequate to the needs of a steam engine. This is best expressed in figures, although all users of commercial vehicles have formed some idea of the amount of heat necessary to vaporize a liquid : on cold days petrol engines manifest a reluctLance to start, because the heat required for converting liquid petrol into a gas is taken, in the ordinary way, from the material of which the carburetter and induction pipe are made ; and on a cold morning there is not an adequate supply of this heat available. Hence the necessity for warming carburetters, a necessity which serves to illustrate in a simple manner the "problem presented by the latent heat of steam.
To consider the heat wasted_ in exhaust gases alone, it is desirable to consider an efficient Diesel engine working under normal conditions. The data may then be formulated as follow:—
Temperature of exhaust, 800° F.
(Rise in temperature, say, 750°F.)
Fuel consumption-0.33 lb. per 1 h.p. hour.
Sp. Ht. of gases-0.3.
Proportion by weight of air to fuel, 14 to 1. Then weight of mixture per 1 h.p. hour = 4.95 lb. And loss of heat to exhaust : 4.95 x 750 x 0.3 =1,110 'B.Th.U.
Havingestablished -that-in the exhaust we have 1,110 B.Th.U. per indicated h.p. hour, let us examine into the possibilities of producing steam. with this amount of heat. The number of units required to raise a pound of water at 32° F. (freezing point) to,
say, t° F. is •
H = 1,082 -I.305 x t.
Supposing that the steam has to be produced at a temperature of 360° F. (corresponding to an initial steam pressure of about 160 lb.), it is found that 1,192 I3.Th.U. are required to effect the evaporation of one pound of water. It will thus be ,seen that the heat of exhaust gases in a Diesel engine is sufficient to evaporate one pound of water per indicated h.p. hour, converting it Into steam at a temperature of 360° F. and at a pressure of 160 lb. But a noncondensing, simple, super-heated steam engine having this initial pressure would require about 16 lb. of steam per indicated h.p. hour.
It is certainly impossible, then, to hope to drive a steam engine by means of steam derived entirely from the heat of an internal-combustion engine's exhaust.—Yours faithfully, • PEP.TINAX.