MODERN METHOD OF ECONOMIC TUNING
Page 24
Page 25
If you've noticed an error in this article please click here to report it so we can fix it.
By L. Mantell
FUEL economy, always desirable, now becomes necessary. The greater the proportion of the yearly fuel bill to the gross annual standing and running costs, the greater is the need for locating and eliminating all sources of curable fuel extravagance.
Whilst many of these may be due to fundamental shortcomings in design, modern research has brought to light numerous hitherto unsuspected " mickles " which can, in the aggregate, total up to a " muckle " well worth having. It is on these that I would now offer some advice.
Compression and Efficiency
• To impress upon readers of The Commercial Motor the need for strict attentioa to such primitive necessities as the maintenance of good compression, corrs_t tappet clearances, requisite exhaust valve spring strengths, etc., would certainly be advice of the " You're telling isle " order; but I must risk flying missiles by discoursing, to commence with, on the modern relationship between high compression and efficiency—in respect of both power and economy.
We all know, of course, that these two qualities go hand in hand up to a point, and that this point is the detonation critical,which constitutes always the" thus far and no farther' to any advantages accruable from increased „ ratios. It is now recognized, however, that there is much more here than meets the eye.
Nearly all observant charge hands have already noted that the most economic state of adjustment, whether by ignition advance or by raising the compression to the detonation threshold in whatever way it can be done, represents always the maximum, both of power and economy; but a great many may not be aware how closely these two move in parallel and that every measure which can be applied to raise the useful spark: advance critical (i.e., without encroaching on the knock critical) will duly bring as"reward a proportionate efficiency improvement.
In technical research circles, in fact, It is now more or less customary to refer to the successful application of one or another experiment in terms of the number of additional degrees of non-pinking advancement it has conferred upon the engine.
Obviously, therefore, one of the first and most important lines, along which to carry out an economy-improvement campaign, is to explore all those avenues which confer this most desirable capacity for taking a greater useful spark advance. You will, of course, know many of them already; but here are a few of recent discovery that can be applied to existing designs without fundamental alterations. First comes super-high-tension ignition. This is employed, I think, snore with a view to straightenigg out bottom-end inflexibilities in the power curve, especially in small high-compressicin high-revving engines, than through any particular bearing it may have on the fuel-economy question, which is of minor importance to thc type. The ordinary coil produces somewhere around 4,000 or 5,000 volts secondary pressure, and takes a plug gap of about 0.02 in., but the super-high-tension coil will increase this output three or fourloll and enable plug gaps of about 0.045 in, • to be used.
Whilst realizing that this can readily flatten out irregular bottom ends of power curves,, one may wonder, perhaps, how it can possibly have any action on the knock critical. Here, therefore, is how it works.
The prime incentive to knocking is local super-hot spots in the cylinder head. The plug is One of them, but by far the greatest aggressor is the exhaust valve, and the cooling of this member is a problem of the highest importance. I am presently going into this matter from other points of view, but, meantime, let us see how spark strengths affect it.
All mixtures, if one can imagine a graphic slow-up • section of them, are mottled. The public and most of the trade love to think of the cylinderhead contents as homogeneous "gas," but, alack, it is not so; on the contrary its section is quite coarsely mottled with rich and lean areas.
Unstable Ignition This is the reason why—especially in small, high-revving engines with big choke tubes and low charge velocities— the ignition is apt to be so unstable at low speeds. Sometimes the spark will pass in a normally ignitable area of mixture, but at others it will occur in air-petrol ratios differing widely from the normally inflammable ranges and either will produce a considerable lag in the flame initiation or perhaps will not fire at all.
At higher speeds the general turbulence tends greatly to improve this state, but there is always a delay period in the initiation, and investigation shows that this delay is roughly an inverse measure of the spark dimensions; that is, the bigger and longer the spark the quicker the initiation.
The. succeeding chain of events is easy to follow, because the more rapidly the early development takes place, the sooner is the primary phase over, the cooler is the tail flame, and, the lower is the exhaust-valve temperature. Furthermore, the position of the pressure peak is more constant in relation to the crank angle, which is also known now to be of importance. This brings me to another and very recent discovery.
It has long been known that for certain and often undiScovered reasons engines wh.ch are apparently identical in every way will produce markedly different results. It has also been noted that, for reasons difficult to trace, inflammations develop at different rates, even with consecutive inspirations in the same cylinder. There is often, in fact, an unaccountable variability in the position of peak development spread quite indiscriminately over the whole of the cylinders.
Investigation now showsthat this is largely due, if ordinary reasons be absent, to lack of uniformity in the precise crank angle at which the spark passes and it has been found that many engines have an ignition-point tolerance of as much as 5 degrees. This can be variously due to slack in the drive or dissimilar gaps in the distributor, which, with plugs of varying electrode clearances, and perhaps designs, will unstabilize the duration of the potential " pile" lag and, therefore, in effect, the timing.
An interesting experiment is to take a plug lead from the distributor, pass it via any convenient arrangement of ring collector and brush to a specific and insulated point on the rim of the flywheel, whence it can visibly jump to a stationary and insulated metallic strip graduated in degrees, bent to suit the rim curvature, and joined up. thence, to its plug. This fitment will show, by unaided optical observation of the passing spark en route to its plug, exactly the amount of timing variation.
Precise Timing Angle It is now found that, by correcting a 5-degree tolerance, which is by no means unusual, to I degree, an improvement in effective advancement of 5 to 8 degrees can be made.
Not a lot, of course, but it all counts and is of further value in that from it follows the apparefit corollary that irregularities in explosive-pressure-peak positions are always inimical to the attainment of a maximum effective spark advance. A most useful find, in my opinion, for it opens a considerable vista of possible avenues of research to inquiring minds.
Now we will revert to the suppression of local hot areas and see what we can do with our old enemy the hot exhaust valve. It is fundamentally a difficult problem to know how to cool this. Radiation, conduction, and convection are the three methods of moving heat about; as all schoolboy physicists .know, Radiation can be written off immediately as hopeless. • Conduction was, until recently, regarded as the only remaining means, but are all exhaust valves provided with 'a maximum freedom of heat conduction? Except in very recent designs I think not, but let us consider them in detail.
Most of the heat escapes to earth via the seating so we will take this part fut. Do all modern designers insure here a •maximum amount of cooling, i.e., the coldest water, led by the coolest and most-direct route, to :the most-carefully designed water corings with a forced and definitely unidirectional current to dislodge bubble clusters? Again I think not, but, forgive the digression for this is not a discourse upon fixed fundamentals.
Turn now to the exterior of the seating where room for improvement can often be found as regards the width. Here one has to steer a course between two contradictory requirements. On the one hand, adequate width is essential to get a maximum crosssection of heat transfer path, but, on the other hand, widening it too much reduces the effective outward-flow value. Later on I Will venture a few remarks on the latter, but for the moment I have to add' only a warning against unduly narrow. seatings.
Wide Valve Seatings
have seen more than one inveterate pinker cured by having its exhaust seatings widened. This trouble of narrow seatings, if present, is, of course, one that automatically cures itself with age. It behoves one, therefore, to be careful in dealing with it and to remember that an originally toonarrow seating, with spark-advance critical set to suit it, can, by gradual widening, improve automatically. It is often noticeable that engines which started life as pickers improve. with age, and this is a frequent cause.
As to the valve itself, if one prefers to retain it, a good plan is to have the head sprayed with aluminium (the British Oxygen Co., Ltd., Thames
House, Millbank, London, S.W.1, does this operation) for purposes of aiding the heat conduction to the, seating. That is about as far as one can go with the original valve, but great opportunities immediately present themselves if one be prepared to change the valve and do a little local and minor redesigning, because we can then increase greatly our cooling 'by& conduction and call in convection—a form of heat carriage which would have been deemed impossible a, few years ago where valve cooling was concerned.
Until now the seating has been regarded as the principal path of heat dissipation, but if we obtain coppercored valves, not only do we speed up the radially outward heat flow to the seatings, but by boosting greatly the conductivity of the stem we can employ the guide also as an escape.
'This part has in the past been regarded as of little help in the above respect, but the error in that impression was brought home by an investigator who made comparative valveheat tests with a dry versus a well-oiled guide, and finding a marked improvenient in the latter case proceeded to go farther by using thicker stems and providing them with extra oiling and cool in g arrangements.
Salt-filled Valves
So promising were these tests, especially with copper-cored valves, that the idea of a loose filling with certain dehydrated metallic salts was evolved therefrom in order that the oscillations of the valve should shake about the contents add convectionally carry the heat to the escape surfaces. Again .this showed itself to be a step in the right direction, because, with each of these progressive changes, the -knock critical went up until finally we reached the sodium-filled valve.
To give some idea of the improvement, I would say that comparative tests were recently carried out by responsible observers between standard and sodium-filled valves and the following conditions were imposed to get a correct comparison. The valves were externally of identical dimensions, in order that cxact comparisons of only the valves could be made. The engine was the same, its mechanical state as constant as possible, and the test conditions identical, except that the ignition timing was set forward, in each case, to the optimum advance for each set of valves.
It was found, that the sodium-filled valves gave no less than 11 degrees greater useful advance and, if one were to include the additional cooling incentives—suitably thicker stems, better guide oiling, etc.—it is obvious how much there would still be to come from these local attentions in the way of improved knock criticals. Whilst it is true that these investigations were mainly directed at the improvement of very-high-output engines—aircraft units and suchlike— it is to be emphasized that the same knock-critical laws apply to all heavyduty engines. Every addition that can be made to the highest useful compression ratio, if I may use an old, but still convenient, expression, means economic improvement pro rata.
The mere fact of adding these improvements constitutes in itself only -a potentiality. To convert it to profitable kinetics the ignition advance, car, buretter setting and, above a certain point, the actual compression ratio itself.must he varied, each to its respective optimum, under the new basic conditions.
Combustion-head Hot-spots There is no doubt that compression ratios are still far below the thermodynamic optimum, taking all the facts into account, and there is equally no question that, among the retarding agencies to the attainment of more ideal conditions in this respedt, local combustion-head hot-spots, headed by over-hot poppet exhausts, are the chief culprits; hence my concentration in this discourse on ways of suppressing them without major alterations, So far we have dealt with only the dissipation of unwanted 'heat, but there is another thermal aspect also to be considered, namely, conservation of wanted heat.. Where the combustion head is of cast iron, an appreciable improvement in economy, and frequently in power also, can generally be obtained by having the surfaces exposed to the flame copper plated. It is easier, as a rule, to have the whole. head plated, although it is only the interiorthat matters. Ordinary plating will not do because the Contact is not sufficiently intimate.
Plating must be done by a special procesA to insure perfect " interparticulate " union and, if properly carried out, it raises the knock critical considerably, permitting, in fact, the actual compression ratio to be raised by about 10 per cent, with the same spark timing, and leaner mixtures to be usable. The action of the copper is not conductive, but apparently only reflective, and although its details are still open to debate it appears to reradiate hack into the burning charge, during primary combustion, heat rays which would otherwise penetrate the head surface and automatically become waste. I have seen various engines treated as above and, although results vary, they have practically always been worth while if the original plating were properly carried out. The Barrel and .C.lerlcenwell Plating Co., Ltd., Rosebery . Avenue, London, B.C. 1, undertakes plating of this kind,