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Practical Solutions of the Leaded-fuel Problem

26th November 1943
Page 28
Page 31
Page 28, 26th November 1943 — Practical Solutions of the Leaded-fuel Problem
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An American Technician Lifts the Veil as the Steps Which . Should be Taken to Prevent Burnt-out Valves and Improve Engine Performance

F all that has been written con . cerning the use of. the present leaded petrol, and the disastrous effects it is having in some engines, it must be confessed that little advance has been made towards any practical solutions of the problems presented. This is not to be wondered at betause, td offer cures for the malady considerable technical research into the real causes of the disease is necessary, and such work takes time and needs apparatus of a type found only in research laboratories.

It was for the foregoing reason that we obtained permission to hear a talk given by Mr. Errol p Gay, Technical Consultant to the U.S. Army and Navy Petroleum Board, which was given through the regis of the M.O.W.T., to a gathering of operators at the offices of the Motor Agents' Association, Great Portland Street, London, W.1, on Friday last. • . Whilst the sulSject of leaded fuel involves problems of chemistry, Mr. Gay wisely confined his remarks to the practical engineering aspect of the subject. Although, as he pointed out, his comments were not to be taken in any sense as being final, they should, at least, enable operators to obtain much better running from their vehicles and greater life for the engine valves.

It may be stated that, broadly,. the direct cause of valves burning-out prematurely is that, after a period of running, they become stuck-up in their guides. Once this happens, it is not long before the accumulative effects of flame. temperatuie and gas speed commence' their work of destruction,

Itniky be of interest to mention that the octane rating of the fuel at present being sold is 80, the quantity of lead content being from 1 c.c. to 2i c.c. per

Imperial gallon. . Two points worth bearing in-mind are that lead does not make the engine run hotter, neither does it slow up combustion. What it does do, however, is to give aform of regulated flame travel and permits a higher compression ratio being adopted. In dealing with sticking valves, Mr. Gay pointed out that most valve guides extend some way into the valve-port passage, the extended portion, or boss, reaching a temperature as high as 1,250 degrees F. In the initial stages, when the valve gels hot, lead oxide forms on that portion of the stem above the top of the guide boss and the underside of the head of the valve. In time, the. coating becomes of such a nature that the movement of the valve stern is impeded, and from that moment onward the trouble bZgins.

Considerably improved results fol.-. lowed the complete removal of the valve-guide boss, but when carrying this out certain features had to be care.

fully noted. -The most important is that the edges of the guide should be left sharp., so as to act as a scraper to the valve stern.

The speaker said that many engines would be more satisfactory were their valve guides made shorter, and it has been found that recessing of the guide; below the level of the cut-off boss, gave improved results in some units, -Bearing in mind the high temperature reached by valve-guide bosses, it was the present tendency to raise the lowe'r wall of the port passage, in order that the Cooling water could be brought as near as possible to the boss. This, of course, means a reduced area in the port passage, but this has been balanced out by making the passage an oval shape at the throat.

The next item touched upon was that of the valve-steam clearances, and the figure of .001 in, per Fin, stem diameter 1.va.l given as that generally followed in the case of Sikh Mille valves. As pointed out, however, such clearances could not be taken to unreasonable. limits, without introducing other problems.' It was important, said Mr. Gay, to see that the valve seat in the engine was concentric: with the valve guide, and this could be checked by the use of blue. Seat ''run the term applied to any lack of concentricity, contributed towards bad seating of the valve and, therefore, should be corrected.

Considerable attention was paid to the question of valve seats, the chief points mentioned being those concerning their width and angle. •The speaker pointed out that excessive width was undesirable and, as 'One example, the figures recommended for Bedford units' were mentioned: In this case, the exhaust-valve seat width was given as between 1-T6 in. and 5--64 in., and that of the inlet-valve seat as 1-32 in. to 1-16 in.

How to Reduce

, Valve-seat Width

Various ways of reducing Valve-seat width were explained, these including the use of cutters or stoQes. It did not matter whether the metal be removed from the top of the seating or the bottom, so long as care be exercised in keeping the area of contact of the valve seating as nearly as possible in the centre of its width, but, naturally. it would be necessary to use some form el pilot on the cutter, in order to maintaineconcentricity with the valve guide.

The Speaker then passed .00.19' a new aspect in the matter 'of valve Seatings,. to which he referred as " differential angle." In plain language, this means that the valve is faced to one a,ngle and the seating in the engine to another. but the difference between the two' should not exceed half 'g degree. Taking accepted angles, this would mean that with an engine seating of 30 degree-A, that of the valve lace wosuld be 29i degrees; with a 45-degree valve seating, the valve would be faced to 44ito 441 degrees. So far as Mr. Gay knew, there was no particular advantages claimed for either of the valve angles at present being generally adopted. in endeavouring to obtain this differential angle,, It was necessary to avoid producing the opposite effect, ,which would mean that the valve face would contact at the

bottom. •

Tappet adjustment; of course, has a Marked effect on unit impact force, which is initially. governed by cam con-.. tour, and although the speaker was, naturally not able to give any general ruling, the indication was 'that greater clearances-than those normally adopted

were called for. •

It was essential to renew ,any•va.1Ye spring which showed signs of fatigue, but at the same dine, it was not desirable to increase valve-spring tension merely as a Means for .givingsnappier valve a,ction,.is this might increase the load impact force beyond • the safety limit of the valve head. .The prithary purbose of the valve sprin, was to keep the tappet in contact Nyitlf the Cani and so prevent the phenomenon known as valve bounce, .

Excessive clearance, too: was undesirable for the same reakan, as it was unwise to increase the seating velocity beybrid 1.5 It, per. second.

The speaker • devoted considerable time to •yalvesand the materials of which, they are made, because on the latter depends the degree' of resistance offered to flarhe temperature and gas. speed: He Said that tests had been,. and -were .being, conducted with different types of:valve and, so far as they had gone, satisfactory . progress had been recorded, • He dealt at some length with a new type of steel 'known as ZB, of which Valves were • being made in America, and he said•it-was hoped that supplies wOuld become available next month in this country. ZE 'steel' has a content of 19 to 22 per cent. chromium, 'Lit° 3 per cent, silicon, and 1 to 2 per

cent,. nickel, but he made no reference to the percentage of carbon.

FollowingMr. Gay's practical remarks on the prevention • of burnt• valves, the speaker. was asked certain questions:, the ans:Wess to which should be Of general interest to all 'operators. ' The use of leaded fuel, he said, did not have any deleterious effect upon the lubricating oil although, from the general look of the oil, which may assume a gray or a reddish-grey appearance, it might be assumed that ._its lubricating value had been affected.

It may be of,sorne assurance to those operators Who 'hat'e felt that the bearinglife may hereduced' to know that 'no cases of 'bearing failure, as a direct result of Using leaded fuel have, so far, been recorded, No Need to Add Oil to Petrol As to the use of small quantities of hib-ricating 'oil in the petrol, Mr. Gay said •that, so far as he cc:Mid see, it

served no useful purpose. •

Dealing with the use of reclaimed oil, he was of the opinion that this should he used more in the nature ofan additive to new oil than for .crankcase replenishment atter, drainage—at least, he said, that was the general 'practice

adopted, in America. •

; Questioned as to the advisability 61 _ advancing the ignition, the speaker said it should be advanced .only to the degree which the .engine would stand, and that thee waS no object in advancing the spark without considering N-;,1ether the power unit needed it , or natl.

. It will certainly be of interest to all transport managers to know that, even if trouble is not being experienced, attention to the features touched upon should improve the performance of most engines, although, as Mr: Gay painted out'. it is not possible yet to give any hard-and-fast rules that are generally applicable to all 'engines.


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