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Doped Fuel Trouble Is Just Straight Burning

24th December 1943
Page 31
Page 31, 24th December 1943 — Doped Fuel Trouble Is Just Straight Burning
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Which of the following most accurately describes the problem?

'By Capt. H. R. Morgan, R.A.S.C.

The Destructive Effects on Exhaust Valves, Following the Use of Doped Fuel,' are Caused by a Slowburning Mixture, Which is Still Flaming When the Exhaust Valve Opens. Our Contributor Repudiates Suggestions that any My-, sterious or Unknown Combination of Factors is Responsible for the Trouble

WITHIN the past few months some really wonderful theories have been propounded in your columni in attenipts to account for the harmful effects of ' doped " fuels upon exhaust valves. The two that I have in mind at the moment are those from the pens of Mr. Anthony High and Mr, Roper Lindsay, both of whom claim to write from the engineer's point of view, yet they each manage to wrap the thing up in such a. mist of quasi-scientific dogma that it is difficult to appreciate either their line of reasoning or their eventual conclusions upon the root of the trouble.

For instance, Mr. High spends some time in setting out the explanation of an indicator diagram from which, eventually, he appears to draw a rather remote and ambiguous conclusion that water on the valves, while starting and warming up, is a cause.After refreshing our jaded memories upon what happens when combustion takes place, he goes on to tell us why the inhibitor was *introduced and its constitution, and then states that •" the present troubles which are being experienced by some operators started only when additions were made to the Pool spirit, and are concerned with burnt valves." Note that he says burnt valves.

' Half-way Towards The Answer

Actually, having made this statement that I -have quoted, he is practically half-way towards the answer but, without applying it, With logic, to his diagnosis, he proceeds again to wrap the whole matter up, because one operator that he knows had .not experi

enced any trouble. It so happened that this operator had blanked off hit radiators and he offers us the conclusion that the remedy is to keep the engine warm.

Whilst I must, perforce, agree that an internal-combustion engine is more efficient, thermally, when, within limits, it is hot, I cannot follow the reasoning that the hotter we keep ,the engine the cooler' the valves will be. I have alWays been under the impression that exhaust valves lose their heat to thecomparatively cool main bulk of the engine and to the air around the engine, and it does seem to follow that the hotter the main bulk of the engine

the less heat the valves can lose to it.•, Mr. Roper Lindsay, in his article,

also tells us why the inhibitor was introduced into the spirit. In effect, he says that it was because, with the high-compression ratios, the highly volatile spirit was prone to ignite by compression temperature before the spark took place. These are my own words, but I fancy that their meaning is not materially different from those of Mr. Lindsay. • We find ourselves, again; practically half-way to a reasonably logical diagnosis, and then, apparently without appreciating that •this -is so, Mr. Lindsay provides the material for the other half. He points out that the trouble is mdch more prevalent in worn engines and those of low compression. (For our purposes, here, these are practically one and the same thing—a worn engine is an engine of low corn-..• pression.) Having got that far, how

• ever, Mr. Lindsay, in his turn, ties -everything in a knot again and proceeds to offer us a theory which he says is "based _upon pure Speculation and with no practical grounds of proof whatsoever." `I don't understan& it,but that, in all probability, is my fault.

Facts That Must Not Be Forgotten

If we strip these two articles of conjecture and eimentary technical• recapitulation, we find ourselves with three substantial facts: (1) That the inhibitor was introduced to restrain combustion; (2) That the trouble is more prevalent in engines that have low compression; (3) That the valves are burnt,

These three facts seem to be fairly generally accepted and agreed upon— not only tly Mr. High and Mr. Lindsay —but by other engineers who I have talked to, upon the question. This being so, it seems to ,me logical that we should attempt to build our -diagnosis upcsh them.

Fact No. 1.—The danger of the preignition of the charge by the temperature of compression, although very real with pre-war spirit, is certainly not so real to-day. I think it can' quite reasonably be stated that the volatility of the spirit now supplied to us for use at home is, by comparison, quite low, and it rathe'r follows that, in doping it with a re-strainer, we are making a slow. burning spirit still slower in burning.

Fact No. 2.—The trouble is more prevalent in engines that have low compression because low compression means low-compression temperature, .poor turbulence and, in consequence, longer flame travel—slower burning still.

• Fact No. 3.—Certainly the valves are " burnt " because the combustion, by virtue of Facts Nos. 1 and 2, is so slow that it is still going• on when the exhaust valve opens, and the tremendously high-velocity gas impinges upon, the heads and must burn them, We have to remember that this is not taking place just every now and then but, if conditions are right for it, hundreds of times per minute, and the poor exhaust valve cannot get rid of this accumulated heat, so it becomes hater and hotter until eventually it burns.

Immediately below the article by Mr. Lindsay, which ,appeared in your • issue dated November 19, you set out some recommendations by Vaaxhall Motors, Ltd., to combat the trouble on its Bedford 28 h.p.„ engines,

This concern recommends, ip effect, that the ignition timing be advanced 5 degrees, that the tappet clearances be substantially greater (the exhaust .005' in.), :that. the felt Washer be removed from the exhaust-valve stem, and that we do not allow the eichaustvalve heady to become too thin. Curt. cerns such as this do not go off halfcocked, so let us see .how its recommendations fit M with the diagnosis that the trii-trthrefois too slow in burning,

An Analysis of . Practical Advice The Vauxhall company recommends that the ignition timing be advanced— I dc, not think that we shall be judged gical if we say that this is to ignite tne mixture earlier and so give more time for burning. It is also suggested that tappet clearances be increased— the exhaust valve will then open later and give a little more time for the mixture to burn. The thicker the valve head, the more heat it. can absorb before burning, and the removal of the felt washer will allow better heat dissipation. [A thin valve head ‘is inclined to " curl " under constant hammering.—ED.] If the valves that I have seen, taken from many engines during the past few months, suffer from anything else but just plain access of heat—then, not only am I wrong in this diagnosis, but I have been wrong for years, and I have done Many a young Artificer a grave injustice when I accused him of burning metal in the forge. (It could • have been erosion—corrosion—deposition of heat insulators, etc.) The remedy is, of course, patent from the cause, and all that we as engineers can do is . to emulate the principle of Vauxhall Motors, Ltd„ which is, apparently, to do the best we can to reduce the effects of the slow burning until the time comes when something can be done about the spirit itself. But for pity's sake don't let us tie ourselves up in knots and talk about " keeping. the engine hot 't.) keep the valves 'cool," or get all scientific and vapour about " catalysts " and " bromide blight," etc. It does hurt.


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