Walter C. Runciman

“ If we take an extreme case as an example, where, to get any gas to speak of into the cylinder the air-supply would have to be cut down or throttled to an abnormal extent, we will realise at once that such a small quantity of both air and gas would have been drawn in, and consequently the mixture would be so rarefied that on the compression stroke the pressure would possibly be extremely low and totally inadequate to produce efficient working. ”

“ With the gas, as with any other kind of engine, the valve settings are of primary importance. On very small engines it is often the case that only the exhaust valve is operated mechanically.
Again, there are several well-known makes which operate the gas and exhaust mechanically while the air valve is opened by suction alone. Though opinions differ as to which is the best course to take, there can be little doubt that, with all three valves mechanically operated, a greater nicety of adjustment is obtainable than would be otherwise possible. ”

“ The small oil engine is practically the same as the gas engine, with the addition of a vaporiser for converting the oil into gas, or vapour, to be exploded in the cylinder; consequently the one may be converted into the other in many cases without much trouble. The difficulty of producing an efficient oil engine lies principally in devising a satisfactory and reliable vapouriser—one which will work equally well under all loads. The heat supplied to the chamber must be sufficient to vaporise the oil, but not great enough to decompose it. ”

“ The adjustment of the ignition tube, although one of the most important and necessary to be made on the whole engine, is in itself a perfectly simple matter. It must be understood that the ignition tube cannot, with the ordinary means at our disposal, be kept at too high a temperature; but it must not be assumed that either the size of the flame, or the time the flame has been alight, is conclusive evidence that the tube is, or ought to be, sufficiently hot to fire the charge successfully. ”

“ In place of giving a mere list of common engine troubles and their remedies, I have thought it better to endeavour to explain thoroughly the fundamental principles and essentials of good running, so that should any difficulty arise, the engine attendant will be able to reason out for himself the cause of the trouble, and will thus know the proper remedy to apply. ”

“ The settings of the valve being of primary importance, no matter what size engine we are dealing with, and being also the most confusing matter for anyone unacquainted with gas engines to grasp, it will not be out of place to suggest a simple method of checking these settings.
Let us begin by pulling the fly-wheel round backwards until we feel the piston is on the compression stroke, then from this point—the crank being about 45° above the front centre—pull the wheel round until the crank is in the position for the exhaust opening (see fig. 28) . ”

“ A common fault is that there is too much gas allowed to flow through the nipple, compared with the amount of air being drawn in at the air aperture, fig. 13. The result is, we get a flame of great length, but one which is not at all suited to our requirements; and instead of giving up its heat to the tube and the asbestos lining of the chimney, a large amount of gas we are presumably burning in the chimney is not being burnt there at all, for, on applying a light just above the chimney top, a quantity of this gas we are wasting will be seen to burn with a flickering blue flame. ”

“ An asbestos washer is interposed between the tube at each end and the metal it bears against, thus making a more or less flexible joint. A thumb screw is arranged at the outside end of the tube, by means of which pressure can be applied to clamp it up between the washers to the desired extent. Some care has to be exercised in adjusting this form of tube for running. When heated to the working temperature it, of course, expands, so that, if tightened up too much when cold, it is under a fairly high compression ”

“ Hence, in the first case, when a further forward movement is given to L by the cam, the pecker P is clear of B, and omits to open the gas valve V; in the second case, P engages with B, and the gas valve is held open during the time the portion of cam Y to Z is passing over the roller R on arm L.
The great drawback to some forms of governors is not that they fail to govern well when new, but that no provision is made to ensure them working steadily when a bit worn. The shape of the cam has everything to do with the regular working of this form of governor. ”

“ Fig. 40a
Having once thoroughly grasped the important part the cams play in the working of the engine, it will be an easy matter to adjust the valve settings, and to keep them adjusted correctly. The effect of a wrong setting will then be strikingly apparent. On small engines a separate cam to operate the gas valve is not a necessity; and the practice of fitting [Pg 73] the gas valve spindle (or the pecker, the effect would be the same) with a device for increasing or diminishing its length, is also unnecessary and unsound. ”

“ There are innumerable designs of each one of these parts, and no two makes are precisely alike in detail, as every maker employs his own method of achieving the same end, namely, the production of an engine which comprises maximum efficiency with a minimum of wear and tear and attention. ”

“ An end view of the three cams keyed up on the side shaft is given in fig. 40A. In small engines it is convenient to have the air and exhaust cams made in one casting, when one key only will be required. On some engines, instead of employing a movable roller or valve lever, the exhaust cam is fitted on side shaft with a "feather"—i.e., a headless key—and the cam being capable of longitudinal movement, such movement being controlled by a small lever or handle, called the half-compression lever. ”

“ The gas engine of the present day, although from a structural point of view is very different to the early engine, or even that of fifteen years ago, is, in respect to the principle upon which it works, very similar. The greater number of smaller power engines in use in this country work on what is known as the Otto or four-cycle principle ”

“ When dealing with engines which have no separate gas valve—the gas being admitted with the air, which is sometimes the case with very small engines—the above notes referring to the gas setting independently, will, of course, not hold good. ”

“ There is no need to use anything beyond a touch of oil when putting in a new tube, in order to make a perfectly tight joint; white or red lead are quite unnecessary, and are liable to make it a troublesome matter to remove the tube on future occasions. Neither should undue force be applied when putting in new tubes; it is liable to wear the thread in the firing block, which results in a partial stoppage of the ignition hole, as indicated in fig. ”

“ Iron ignition tubes may be used, and one heating lamp serves a double purpose in keeping the tube and vapouriser hot at the same time. This lamp is fed by means of a pump actuated from the side shaft. The plunger of the pump is loaded with a spiral spring, which may be adjusted to give any desired pressure, and is kept constant and steady by means of an air vessel. This pump is shown in fig. 48. It is actuated by means of a rod and lever from the side shaft of engine. ”

“ The object, then, is to do as little cooling as possible, and to apply the cooling effect at the right parts; hence the passages and chambers through which the cooling water circulates should be so arranged that those which require to be kept at a low temperature are in close proximity to the cooling water. On some of the engines of days gone by, the exhaust valve was carried in a large iron casting, this in turn being bolted to the cylinder casting and communicating with the combustion chamber by means of a port. ”

“ The device consists primarily of three parts—the body or chimney B, the cover C, and the tube itself T. The body is a light iron casting, carried by a couple of studs SS, which are either screwed into the firing block F, or direct into the metal of the cylinder casting if no firing-block is used; the latter may very well be dispensed with in the smaller-sized engines.
The tube is made of thin porcelain, slightly bell-mouthed at its open end, and is mounted in a thick metal washer W, as shown in fig. 18 in section, the joint being made with a little asbestos paper, moistened. ”

“ The gas valve opens just after the crank is above the back centre and closes just before the front centre is reached, that is, opening a little after the air valve and closing a shade before it, thus every particle of gas is used in the cylinder, due to a draught of air being drawn in after the gas valve has been closed. ”

“ The asbestos lining, shown in fig. 13, may be of various thicknesses, according to the size of the chimney and the tube; the reason for this will be apparent to many; but being a most important factor in the heating of the tube, and consequently the working of the engine, it will be advisable to deal with this point more fully.
Due mainly to the peculiar behaviour of iron tubes under heat and internal pressure, it is always advisable to look to them first of all when the engine shows signs of missing fire ”

“ Moreover, working at such a high vacuum as this would not only prevent us obtaining a normal explosion in the cylinder, but would upset the working of the exhaust valve. The latter being held down on its seat during the suction stroke by means of a spiral spring would be lifted off its seat by suction (the partial vacuum in the cylinder) , and any burnt gases which happened to be hanging about in the exhaust port or pipe would be drawn into the [Pg 75] cylinder again, and tend to damp the ensuing explosion. Too early closing of the exhaust should be avoided almost as rigorously as too late. ”

“ To put matters right, it is necessary to do one of two things—either cut down the supply of gas or increase the air-supply. Providing the air aperture is normal, i.e., the same size as it was originally, it is better to adjust the gas, which may be done by tapping up the nipple N, as indicated in the enlarged sketch, fig. ”

“ In the reservoir R is fitted an overflow pipe, so that the oil cannot rise beyond a certain level; hence the head of oil in the smaller one M is always constant. On the suction stroke a partial vacuum is formed in the engine cylinder, consequently the pressure in the vapouriser drops somewhat below that of the atmosphere, [Pg 88] and this small difference in pressure is enough to cause the oil to rise in the small passage X, fig. 45, beyond its normal level, and overflow into the vapourising chamber, as previously described. The valve or nipper N is shown open in the diagram, fig. ”

“ As we do not propose to enter into more than a brief explanation of why and how this apparatus generates current to produce the required spark, perhaps a simple analogy will make matters most intelligible to any reader not well acquainted with electrical phenomena. We know that when a current of electricity is flowing in a wire, and the wire be [Pg 47] suddenly broken, a spark will occur at the point of breakage. ”

“ Moreover, if the engine were running at even a slow speed, the motion imparted to lever L would be indefinite; and this, especially if the governor is fitted to the air valve lever, as in fig. 25, is very undesirable. Therefore, to obtain a definite opening we must set out the cam, as shown in fig. 31. In this diagram the roller is shown standing clear of the back of cam by about 1⁄16 in. A line MN is then drawn, forming a tangent to both roller R and circle GHJ at points F and O respectively. ”

“ A minimum amount of play must always be allowed, however. When two sets of marks are obtained, the mean must be taken and the keyway cut as shown by the thick lines in fig. [Pg 71] 39. The exhaust cam in larger engines is usually made with a swelling on the opening portion, as shown in fig. 40, so that the valve is very slightly opened some time before the crank has reached the position shown in fig. 41. Fig. 42 shows position of crank at the close of exhaust valve, and the two last-mentioned diagrams correspond with the two positions in which the exhaust cam is shown in fig. ”

“ Further reference will be made to this in another chapter, and, although this is not a necessity on a small engine, it is always employed on engines over 2 B.H.P. In fig. 1, HW is the cooling water outlet and CW the inlet. A small drain cock is shown at DC, through which the water in the cylinder water-jacket may be drawn off when required. ”

“ It has been found that better results are obtained by causing the magnetic field to move [Pg 48] relative to the armature winding than to move the latter through a stationary field. Reference to the diagrams, figs. ”