• Why Do Leaded Fuels Cause Excessive Engine Trouble?
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THE harmful effects following the use of leaded fuels in some engines have led to a series of divergent views as to the real cause. In the following article the writer gives his reasons for holding diametrically opposite views from those expressed by Mr. Anthony High in his article in our issue dated October 15 last. By Walter Roper Lindsay, M.S.A.E.
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. HE article in your issue dated OCtober 15 entitled " Keep the Engine Hof to Keep the Valves Cool," indicates conclusions not altogether logically deducible either from existing theory or established practical experience.
Much has been written in the past few years about the so-called " corrosion theory," that is, the effect of certain constituents in the products of combustion in causing excessive cylinder-bore, ring,. and piston wear. In fact, sometime around 1936, the Research Committee of the I.A.E. made a special investigation of the subject. The broad conclusions then reached indicated that, under certain conditions, such as low jacket-temperatures that occur when starting from cold, coupled with the high petrol-air mixture ratio that is inevitable when using the choke, microscopic, but dangerous detrimental quantities, of 1-1550 are formed, although the sulphur content of both petrol and oil may be practically. negligible.
I believe that investigations also proved conclusively *e theory—long held but not hitherto verified undet laboratory conditions—that 95'per cent. of normal cylinder-bore and ring wear occurred during the starting-andwarming-up period.
I do not think, however, that it was found possible to apportion the blame equally betweenthe two causes:—(a) the acid-corrogive content of the burnt gases, and (b) dilution, or adulteration, of the lubricating oil caused by unburnt fuel breaking down the film strength of the lubricant coating the cylinder walls and rings. However, it was made clear that these two causes mainly affected the bores, pistons and rings, but not the valves or valve seats.
Water Not a Contributory Factor Mr. Anthony High describes, in detail, the processes of combustion and apparently deduces that, as water, in the form 'of steam, is one of the products of combustion, this steam, on condensing, must neaessarily be responsible for a considerable amount of damage to the valves. He mentions condensation "on the cold parts of the engine," although he had previously emphasized the temperature of the exhaust gases at the exhatist ports as bring in the region of 700 degrees C.
Actually, even with very low jacket-temperatures there is little or no opportunity for the steam content of the exhaust gases to condense, either on the cylinder walls or exhaust valves. The working temperature of exhaustvalves often exceeds 450 degrees C. Expansion of the exhaust gases in the silencer and subsequent cooling in a long exhaust pipe will, of course, greatly assist condensation, and there may be .a steady dribble of water from the exhaust pipe untilthat assembly gets thoroughly warmed up. With short, stubby silencers and pipes, as fitted to most trucks—especially those of ' early Vintage—no appreciable visible evidence of water may be discerned, even on a cold morning. The water is there, nevertheless, but in the form of superheated steam.
Assuming that some condensation did occur in the exhaust manifold, of even in the combustion chamber, then the deleterious effect would be confined to that of emulsification of the lubricant on the bores; and rings. Unless water be formed in excessive quantities, •there would be no " washing " effect on the oil, owing to the latter's greatly superior surface tension'. Were it not for this, we would look for, and probably find, altogether excessive piston, ring, and bore wear on steam engines, and these units operate with exhaust tealperatures but a fraction of the 700 degrees C. mentioned by Mr. High.
In tracing the cause of excessive valve burning when leaded fuels are used, we should look for particular conditions that are not present when normal, unleaded fuels are burnt. Certainly, the water content of the exhaust gases is not .greater with doped fuel.
A Misconception on Leaded Fuels Nowadays, the terms " high-octane" and " leaded " are freely used when describing certain.grades of petrol and, unfortunately, the implication has crept in that such fuels, when used in any engine, will inevitably increase the power output. It is forgotten that standard petrol is " doped " mainly with the object of utilizing .a higher compression ratio and .thereby improving
thermal efficiency. • Thg use of a low-octane petrol, say, 67 rating, in an engine having, say, a 6.5 to 1:compression ratio, would result in pinking, knocking or detonation, and consequent heavy loss of power. This detonation, of course, is due to the mixture burning on compression some time before the piston has reached' top-dead-centre.
Adding an inhibitor, such as tetraethyl lead, to the fuel, or an inert gas such as carbon dioxide to the manifold mixture, has the effect of preventing, not just delaying, spontaneous detonation either before or after ignition. (Note that detonation can still take place after combustion has commenced., that is, one portion of the charge may detonate whilst, on the other side of the combustion chamber, the remainder may burn through the normal and correct process of intensely speedy flame propagation.)
Now, in a low-compression engine (say, 4.2 to 1), the conditions favouring detonation may not be present, unless, in an exceptional circumstance, heavy carbon deposits have appreciably increased the original ratio. Hence, leaded or doped fuels are" not called for and, if used, could not be expectednoticeably to increase
the poVver output. ,•Al!
the above facts are already well known. They are recounted here primarily with the object of emphasizing the point that decreased, father than increased, engine . temperatures result from the use of leaded fuels.
In referring to the benefits accruing from the use of high-compression ratios,. Mr. .High has tended to make the implication that leaded fuels, used in such engines, would result in increased exhaust-port temperatures.' As thermal efficiency may roughly be described as the ratio of the heat made use 'of in the engine to the total heat supplied by complete combustion of the mixture, then the greater the initial compression the lower will be the exhaust temperature.
Practically expressed, an engine with a compression ratio of 6.5 to 1 directly converts into mechanical work 25 per cent, more of the latent heat (or energy) in an equivalent mixture volume than an engine with a compression ratio of 4.0 to 1•. Comparison of the manifold teffiperatures of a 1928 two-litre unit with those of a 1938 model would convincingly bear this out. More convincing still is the low manifold temperatures of oil engines.
We must, therefore, rule out the passibility of leaded fuels being directly responsible for• valve burning on the theory that such fuels increase engine operating temperatures.
More Failures in Low-compression Units In the writer's experience with the operation of over 90 tippers of 4-, 5and 6-cubic-yd. capacities, all engaged on sand, shingle and ballast haulage, with 50 per cent, loaded running mileage and average " lead " miles from 20 to 25, valve failure has been most severe,. on 1934-36 models with 5.0 to 1 compression ratios. With 16 1941 models of a popular4make, operating on 6.3 to I compression ratios, valve failure has occurred but infrequently, and in only two cases in under 10,000 miles service, whereas with the low-compression engines valves fail cdnsistently after 6,000 miles.
As for sparking-plug failure, this occurs more frequently-, usually as the result of excessive erosion of the central electrode coupled with cracking of the insulator; but, again, failures have been much more frequent with the low-compression engines.
The only explanation the writer can suggest—and this is based on pure speculation and with no practical grounds of proof whatsoever—is that this red " petrol, apparently, contains a much higher lead content than the pre-war etlaylized fuels, thereby greatly increasing its octane rating—actually far above that called for by the highest compression ratio used in any engine yet built for commercial use. The resulting excessive quantity of lead bromide formed after combustion, under conditions of comparatively low compression and, therefore, poor turbulence, is deposited accumulatively on the valves where, acting in the form of a catalyst, erosion and corrosion of the silicon-chrome steels take place.
Deposits Which May Act as Insulators
Possibly, these deposits act as insulators, preventing the dissipation of heat from the valve heads to the incoming fresh mixture charge on the induction stroke. Similarly, deposits on the underside of the valve heads, particularily on the seats, would prevent the escape ot heat to the cylinder block.
Incidentally, it is remarkable to note 'that although the valve may be •found completely corroded the seat or insert in the block is often barely marked.
If this theory be correct, we should expect to find corroboration by tracing the results of using similar, or still higher, octane rated fuels in aero engines operating on compression ratios of 8 to 1, or higher.
Another interesting point, noted by the writer, is the apparent connection between the general condition of the engine and the frequency with which this valve failure occurs. Where big-ends, bores, pistons and rings are in obviously poor condition, showing poor compres sion and, .particularly, heavy oil consumption, then -the valves have apparently but half the life of similar valves on engines in Al condition.
It is feasible that the carbon formed from the burning lubricating oil may have some affinity for the leadbromide precipitate, the accumulation of deposits on the valves being thereby encouraged. If such be the case, then it would not appear to be advisable to recommend the addition of oil oto the petrol.