In connection with the different ignition methods referred to in the preceding paper, it may not be amiss to mention that in the case of engines using either flame or tube igniters the consumption of gas by the igniting or heating burners is an item not always duly taken into account, and may assume appreciable proportions. Where gasoline or some other oil is used in such engines instead of gas, and where the latter is not to be had, the gas flame must, of course, except in a few engines of special design, be supplanted by an oil torch, and the same additional fuel consumption, above that taking place in the cylinder of the engine itself, is there encountered, the torch, moreover, being a rather undesirable annex to the whole outfit. These circumstances have, in a great measure, helped to stimulate the improving of electric igniting devices, and several of them are now affording very satisfactory accounts of themselves. This much may already have been gathered from what has gone before. With the electric igniter there is, of course, to be considered the expense of the battery which furnishes the electric current, but this has been claimed, and with good reason, to be a very small proportion of the whole operating expense,—a much smaller one, in fact, than that represented by the gas or oil cost in a flame or tube igniter. It would seem to be a pretty fair conclusion, under the circumstances, that electric igniters are destined to a yet wider application and a growing share of favor.
Not less important than the methods of igniting are those of governing, and considerable ingenuity has been expended for years past in developing various contrivances designed to satisfactorily solve the gas engine governor gear problem. At first thought, some sort of throttling gear by which the gas or oil vapor supply to the cylinder is gradually reduced as the speed increases, and vice versa, is apt to suggest itself as a desirable one, and, as a matter of fact, many gears of this kind have been made and are used, both Lenoir and Hugon having followed this method of governing in their early engines. Its wastefulness, however, becomes apparent on even slight consideration, so that one may well wonder that at this late day the throttling governor is still countenanced in gas engine practice. It is obvious that if, with such a governor, the volume of gas or oil vapor admitted to an engine cylinder is diminished, the volume of air admitted is correspondingly increased, and' it is well known, too, that the limits of variation permissible in explosive mixtures of gas and air are comparatively narrow, so that if, in a total volume of mixture, the volume of gas either exceeds or falls short of a certain, elastic proportion, no explosion can be produced. While, therefore, a gas throttling governor may either increase or diminish the force of the explosions in the engine cylinder by varying the strength of the gas and air mixture between these limits, yet if one of the limits be passed, and the gas volume, for instance, be too greatly diminished, ignition of the mixture will be missed and whatever gas has been taken into the cylinder will then be discharged into the exhaust pipe unburned and without having given up any of its energy. Under these circumstances the engine will work exactly as though no gas whatever had been admitted, and the speed will cease to increase, gas being simply pumped through the cylinder and wasted until its proportion is again increased, by the subsequent opening of the gas valve to that point where the mixture once more becomes explosive.
No better evidence is needed of the fact that the wastefulness of the throttling governor method has met with a fair share of recognition, than the extended application which for years has been given to what have been termed "hit - and - miss '' governors. With these the strength of the explosive gas and air mixture is never changed, but the number of explosions in the engine cylinder in a given time is varied to suit the prevailing requirements, being increased when the speed falls below, and decreased when it rises above a certain normal / or putting it in a slightly different way, when the speed falls off, the governing device hits the stem of the gas supply valve, causing the latter to open and admit gas into the cylinder, while when the speed becomes too high, the governor by changing its position, misses the gas valve stem, the valve consequently remains closed and no working charge reaches the cylinder.
One objection to this method of governing always has been that it gives the engine a jerky, unsteady motion which is fatal to success in electric light work, and in order to overcome this, some engine builders now provide the governor cam which strikes against the gas valve spindle with several steps, so that instead of being either full on or full off, the gas valve may be opened through intermediate degrees. The strength of the explosive mixture may thus be varied to some extent, but whenever the lower limit of gas percentage necessary to constitute an explosive charge is reached, the gas valve is closed entirely, and the engine then runs without explosions until the speed again drops. The stepped-cam governor, in fact, combines a throttling with a cut-off action.
The ideal method of governing, however, would seem to be one akin to that followed in modern, high-class steam engine practice,—one by which the strength of the working charge would be kept always the same and only the quantity for each working stroke would be varied. In all processes of combustion there is a certain percentage of oxygen, which, by combining with a certain percentage of combustible, produces a maximum effect, and any variation from these relative proportions will represent a loss of efficiency. This is fully as true of the combustion of gas in a gas engine cylinder as of the combustion of any other kind of fuel in any other place, and the economic bearing of maintaining a constant and certain strength of explosive mixture in gas engine work will, therefore, be at once recognized.
/fig. 115.jpg) Fig. 115— The Pittsburgh Gas Engine |
An engine in which advantage has been taken of this circumstance is that recently put on the market by the
Fuel Gas and Manufacturing Company, of Pittsburgh, Pa., and known as the Pittsburgh engine. In external appearance it strikes one much as one of the well known Westinghouse steam engines. As in these, a crank case encloses the bearings and lower end of the cylinders. This case is filled almost up to the shaft with a mixture of oil and water, into which the crank shaft and connecting rods splash at every revolution, so as to completely deluge the bearings, piston and interior of the cylinders, thereby not only affording copious self-lubrication, but also cooling the piston. Oil for the crankcase is introduced through the main bearings, which are supplied from the only two oil cups on the entire engine. A simple pipe connection with a city main supplies the necessary water. Another pipe, serving to carry off the overflow, is made, by means of a funnel head, to indicate the level of the lubricants in the crank chamber.
All the Pittsburgh gas engines are built with two cylinders on a single shaft, and, as usual, abnormal heating is obviated by the employment of water jackets. Each revolution made by the engine operates valves admitting the gaseous fuel alternately to the one or the other cylinder. As the period of admission is controlled by a positive action, the crankshaft receives an impulse once each revolution, no matter what the load, but the energy of that impulse is predetermined by an independent piston valve. In order that the maximum amount of energy may be developed by the explosion of the gaseous fuel, there is, as already explained previously, but one value that the relative amounts of gas and air can bear to each other, and the company design their measuring piston valve so that, it is claimed, it always admits gas and air in their correct proportions for producing the desired maximum result, but at the same time varies the total amount of mixture directly as the work of the individual piston stroke. The governor is mounted upon the shaft, between the cranks, and, by direct connection between the eccentric rod and valve stem, insures an accurate and positive travel to the measuring slide valve. The igniter employed is of the electric spark type. Unfortunately, more complete particulars of this engine are not available at the present time.
/fig. 116.jpg) Fig. 116— 25 H. P. Union Marine Gas Engine |
In connection with the Union engine, described in the preceding paper, and built, as there stated, by both the
Union and the
Globe Gas Engine Company, of San Francisco and Philadelphia, respectively, the illustration on the opposite page will prove interesting, representing, as it does, the latest type of double cylinder, marine engine of Union make, rated at twenty-five horse-power. This engine, according to advice’s received within the past few weeks from the builders, was completed only a short time ago, and is now in a schooner in San Francisco Bay giving highly satisfactory results. It is the first one of the kind that they have built, and is probably the largest marine oil engine now in use, except a four-cylinder engine of forty-five horse-power of somewhat different form which was turned out by the same builders a number of years ago. The main features of the engine are essentially the same as those of the horizontal Union stationary engine, shown in the September number; the valves and igniting devices are similarly operated, and the same form of vaporizer as that then illustrated is used.
The propeller reversing gear shown tends to give the impression that there is considerable complication about the engine, but this will disappear upon closer study of the details. A muffler around the exhaust pipe is used to deaden the noise of the exhaust. The schooner, in which the engine is placed, is 59½ feet long with 14 feet beam, and carries freight between San Francisco and Bodega Bay. On her trial trip over the Government course in San Francisco Bay she developed a speed of over 8 miles an hour. This was regarded as a very good showing, as the engine was new and stiff, and the boat was not built for speed. The owners of the vessel also have a 10 horsepower engine in one of their other boats.
/fig. 117.jpg) Fig. 117— Charter Gas Engine |
As the last of the series of illustrations are presented those of the Charter gas and gasoline engine, which is made by the
Charter Gas Engine Company of Sterling, Ill., and which in some respects is similar to the Caldwell-Charter engine described in one of the earlier papers. When using gasoline no carburetting device is used between the oil tank and the cylinder, but the oil is delivered directly into the suction pipe by a pump controlled by the governor, a few drops only being admitted at a time. There are three cut-offs between the tank and engine cylinder, viz., a cock at the tank, a throttle valve to regulate the amount of gasoline delivered, and the plunger of the pump just mentioned.
/fig. 118.jpg) Fig. 118— Details of the Charter Gas Engine |
The engine works according to the Otto cycle, the exhaust valve being pushed open at every other revolution by a rod worked through reducing gearing from the crank shaft. The arrangement of the gasoline pump and the manner in which it is controlled by the governor will be easily understood both from the general view of the engine and from the details shown in Fig. 118, the latter representing an elevation and a plan of the governor and its connections. The governor, it will be observed, is mounted on a sleeve on the main shaft, and when the governor balls, under the influence of unduly high speed, move outward, the sleeve is carried along the shaft and moves with it a cam roller, which is mounted loosely on the upper end of the rocker arm A. When so displaced, this cam roller is missed by the cam on the larger of the two gear wheels shown, and the rocker arm, which is connected with the injector rod operating the gasoline pump, remains undisturbed, and no gasoline is permitted to enter the suction pipe leading to the cylinder. When, on the other hand, the governor balls are in the position shown in the engraving, the cam on the larger gear wheel will, in the course of its revolution, come in contact with the cam roller on the arm A, force it over to the right, and cause a stroke to be made by the gasoline pump through the intervention of the injector rod. The displacement of the pump plunger then admits the proper gasoline supply to the suction pipe, and the entering air carries the oil along into the cylinder in which the mixture, after compression, is fired by a tube igniter. For engines above six horse-power a slightly different form of governor connection is employed, the working principle, however, being the same. The engine is turned out in sizes of from one and one-quarter to thirty-five actual horsepower.
Before finally leaving the subject, the writer would attempt to forestall criticism on the score of incompleteness of the series of articles presented by stating that no attempt could be well made to embrace in them all the engines of the class considered which are now in use and built in different countries. It was deemed advisable, in fact, at the outset to try to present only engines of English and American make, and such foreign designs as were represented in English and American markets, and even this undertaking was found beset with many difficulties. Aside from the fact that the addresses of some makers of engines could not be ascertained, there were a number of builders who simply ignored requests for information, and others again who flatly declined to furnish particulars of any kind. That the list of engines considered in these articles is by no means comprehensive is, therefore, natural.
Information Sources
- Cassier's Magazine, V4, Oct 1893 pgs. 437-443