be less, but it is a fact that such plants as the 85-hp Westinghouse engine running at the Erie terminal, Jersey City, has operated for three years on 22-hour service and has only required a new exhaust valve and one governor spring during that period. The Wood's producer of this plant has run for six years and ten months without drawing the fires. It may be interesting to state at this point that the rating of a gas engine is determined by the ability of the crank shaft to carry the load. The fact that one gas engine has bigger cylinder dimensions than another, both having the same sized crank shaft, simply indicates that the engine having the larger cylinders is designed to consume more B. t. u. per horse-power than the other. It must be borne in mind that gas-engine cylinders are always designed to carry the maximum loads with such margins for overload as the builder may guarantee to provide for. While on the subject of the combustion chamber, it is interesting to note that the theoretically perfect combustion chamber would be a sphere, because that contains the greatest quantity of mixture with the minimum radiating surface. This conformation is modified to suit practical conditions, for if the piston had a concave surface the edges of its outer diameter would offer sharp corners difficult to reach with water or air for cooling, the small cross-section of which would rapidly heat, so the piston head is made flat. The same thing applies to the cylinder openings. for the valves and igniters. Their edges are tapered off so as to avoid corners resulting in hot spots. In short, the desideratum in` gas-engine design is to burn the greatest amount of gas with the smallest possible amount of radiating surface under the highest possible compression without prematurely firing the gas. Practically every point of design centres around this principle. One engine builder found it necessary to completely change the design of the valves because the exhaust outlet was so close to the inlet that the incoming gases passed over the hot exhaust parts, were heated thereby, resulting in the rarefaction of the incoming gases, and thus made it impossible to get as much of the mixture in the combustion chamber as would be possible with the cold gas. It is with reference to these features that the various types of engine will differ. The limitation of compression is not determined by the strains that the cylinder can stand, but rather by the ignition point of the gas that may be reached by the heat resulting from compression. The designer that can get the greatest volume of gas within a given combustion chamber, without firing it until he is ready, is the one who will work his engine at the highest efficiency, the crank shaft being the determining feature as concerns capacity. It is plain to see from this wherein troubles have occurred in the past. Gasengine designers have had to learn just how far they could go in any one direction before the limitations of good service were reached, and, furthermore, they have had to develop their skilfulness in the arrangement of the inside of the combustion chamber so as to thoroughly cool every section and prevent any one particular part reaching a point where it would raise the temperature of the charge sufficiently to fire it prematurely. It is interesting to note that the gas-engine indicator card has the same characteristics as a diagram made by superimposing the cards of a quadruple-expansion steam engine. Passing on to the devices that control the gas admitted to the cylinder, we are all familiar with the cut-off valve with its inlet valve, the cut-off varying throughout the stroke or else acting as a straight throttling valve; the exhaust valve with its watercooled jacket; the gas and air cocks, which determine the proportions of gas and air that are to enter into the mixing chamber; and the igniters, which may be of either the make-and-break or the jump-spark type. Igniters should be equipped with a relay system of current supply, and generally two ignition plugs entering each combustion chamber. All of these parts are operated by a lay shaft and have reached such a state of development in the well-designed engines now offered that the earlier troubles have disappeared and are now only interesting as matters of history. As concerns governing, gas engines run on better than 2 per cent variation from the mean, their heavy flywheels helping them out in this respect. As concerns variation in the angular velocity, they do not do as well as steam engines. However, we have before us cases of 3-cylinder single-acting engines, operating 60-cycle, 3-phase generators in parallel; in fact, it might be stated that from a 3-cylinder single-acting engine to a 2-cylinder double-acting engine 60-cycle apparatus can be operated in parallel, provided belts or flexible couplings can be interposed. With two power strokes per revolution and on a good gas, such flexible connections may not be necessary for operating 60-cycle apparatus in parallel. Twenty-five-cycle apparatus can be operated in parallel without such flexible connections on 3-cylinder FIG. I-DIAGRAM OF EXPLOSIONS PER REVOLUTION IN SINGLE- FIG. 2-DIAGRAM OF EXPLOSIONS PER REVOLUTION IN DOUBLE ACTING ENGINES single-acting engines, provided the gas is of a constant quality. The characteristic of the alternating-current generators is an important element in this matter of paralleling. I submit diagrams (Figures 1 and 2) showing the various explosions per revolution in the different types of gas engines, each circle representing a revolution and each perpendicular line a stroke of the piston. The quality of the gas has a great deal to do with the operation of the engines when running in parallel, for a premature explosion, a failure to explode or a weak mixture will cause the engine to lose headway. TYPES OF GAS The majority of the engines of this country operate on the following gases, the characteristics of which are given : The consumption is based on an engine requiring 12,000 B. t. u. per brake hp-hour. Now, of course, natural gas is the most desirable form for engine purposes. It is cheap as to first cost, eliminates the gas-generating plant and contains a large percentage of marsh gas, which is the best ingredient in a gas used for engine purposes. Illuminating gas works well in an engine, despite the high percentage of hydrogen that it contains. But illuminating gas, because of its enrichment, is too expensive. The question has often arisen as to why engines that have operated successfully on natural gas and illuminating gas have failed on producer gas, and the writer has it on the authority of a skilful gas-engine designer that it is because the producer gas has not the marsh and olefiant gases to tone off the snappy sensitive and quickfiring properties of hydrogen. Coke-oven gas can be used in gas engines, although, of course, it is not available in all localities; furthermore, because of the large and varying quantities of hydrogen, it is a treacherous gas to handle in an engine. Happy is the engine designer who can adjust his engine to a constant quality of gas. This quality corresponds to a constant boiler pressure in a steam engine. We next come to producer gas, which has the advantage of being available in all localities and for most fuels. It is extremely cheap and is a satisfactory gas to use. Blast-furnace gas is practically producer gas, only it is leaner in hydrogen, and because of this it is, when well cleaned, one of the most desirable forms of gas. It might be said that it comes of poor but honest parentage. I have named these gases in the order of their calorific value and should like to return to producer gas for a moment to explain in a few words how it is made, at the risk of going over ground already familiar to most of you. The chemistry of producer gas is exceedingly simple. The carbon of the coal is combined by heat with the oxygen of the incoming air, forming a carbon monoxide gas (CO), which, mixed with the inert nitrogen (N) of the air and such hydrogen (H) as may be liberated by disassociation of the water in the coal or introduced as steam, constitute practically all of the component parts of the gas in the ratio given in the table found elsewhere. It will be seen that the carbon monoxide is the constituent of the gas which makes the principal fuel element. It is not uninteresting to note the exact action that takes place in the producer. The air first coming in contact with the carbon and subjected to combustion burns to carbonicacid gas (CO2). If this CO2 were to pass off into the atmosphere, carrying with it the sensible heat resulting from that combustion, no further chemical reaction would take place, |
