This book is not intended as a complete treatise on the subject. The object of the author is to present the principles governing the salient features of gas engine construction and operation in as simple a manner as possible, and to that end academic discussions of the characteristics of gases and of hypothetical heat-energy cycles, of the character commonly found in text books, have been avoided. Since the pressures, temperatures, and energy transformations which occur in a gas-engine cylinder cannot be adequately explained without the use of algebraic equations which appear complex to a beginner, such equations have been employed in that connection, but their use has been restricted to a single chapter. This chapter may be omitted without sacrifice by readers who wish merely general, rudimentary information, but not by real students of the subject.

Gas and oil engines differ from other forms of heat engine chiefly in that the pressure which gives the engine its power is produced within the cylinder by the combustion of the gas or oil. For the operation of all other heat engines the working medium (steam, hot air, etc.) is raised to a pressure much higher than that of the atmosphere before it is delivered into the cylinder; after entering the cylinder, the working medium is expanded to a low pressure, the expansion driving the piston forward. The working medium of the gas or oil engine is delivered to the cylinder at about atmospheric pressure and there compressed and ignited; the rise of temperature produced by the combustion causes a corresponding rise of pressure and the high-pressure gases are then expanded behind the piston in the same manner that steam expands in a steam-engine cylinder, and with similar results.

In order to burn anything, no matter how inflammable it may be, it is necessary for oxygen to be brought into contact with the substance to be burned, because combustion is nothing more than the union of oxygen particles with the combustible particles of the substance "burned," under the influence of heat. Air, which consists of oxygen and nitrogen, is the only free source of oxygen and is therefore universally used to supply the oxygen required for combustion of any kind. Hence, the gas or oil used as fuel in an engine is always mixed with air either immediately before it enters the engine cylinder or immediately afterwards. The proportion of air to gas or to oil is of great practical importance; if too little air is supplied, combustion is slow and incomplete, and if the proportion of air is too large, the inflammability of the mixture is reduced and combustion is retarded. Again, it is important that the mixing of the air and fuel should be thorough; otherwise some of the gas or oil either will not be burned at all or will burn too late to do much good in the way of producing pressure behind the piston.

Gas and oil burn quietly in the open air chiefly because the gases produced by the combustion can expand as rapidly as they are formed, having the whole universe into which to expand and being restrained only by the moderate pressure of the atmosphere. Moreover, the atoms of gas or oil are not mixed intimately with the atoms of oxygen in the air before burning begins, so that the combustion is very gradual. When gas and air or oil vapor and air are thoroughly mixed and highly compressed in a closed vessel, however, igniting the mixture will produce almost instantaneous combustion of the whole mass, resulting in a sudden rise of temperature and pressure amounting practically to an explosion. This is what happens in the cylinder of a gas or oil engine when conditions are right. The explosion of the mixture occurs when the piston is at one end of its travel, the mixture being compressed in the clearance space between the piston and the near cylinder head; and as soon as the crank passes the dead center, the burned and burning gases expand, forcing the piston away from the end of the cylinder. At the end of that stroke, the spent gases are exhausted into the atmosphere, just like the expanded steam in a non condensing steam-engine cylinder.

There are two general classes of gas and oil engines; one works on the four-stroke cycle and the other on the two-stroke cycle. All small engines of both classes are single-acting : the piston is commonly of the trunk type, as illustrated in Fig. 1, and the combustion of fuel occurs in one end only of the cylinder.

The four-stroke cycle is more widely used than the two-stroke, for reasons which will be explained later on. The cycle comprises live events, namely: Admission, compression, combustion or explosion, expansion and exhaust. In this type of engine the charge is taken into the cylinder at atmospheric pressure, and is therefore necessarily drawn or "sucked" in by the piston of the engine, acting for the time as a pump piston. This occurs during one out-stroke of the piston (from position A to position B, Fig. 2), during which the inlet valve is held open either by the valve gear or by the atmospheric pressure. At the end of this stroke, commonly called the "suction" stroke, the inlet valve is closed, and when the piston comes back on the return stroke (5 to C, Fig. 3) it compresses the cylinderful of mixed air and gas (or air and oil vapor) into the clearance space, which is relatively much larger than in a steam-engine cylinder.

When the piston has reached the end of the compression stroke, and while the particles of mixed air and fuel are compressed into intimate contact, the mixture is ignited and burns explosively, as already explained, producing a, sudden rise of pressure behind the piston. This enhanced pressure drives the piston forward on its power stroke {C to D, Fig. 2a), during which the pressure is gradually reduced by the expansion of the hot gases, the valves remaining closed, of course, until the stroke is almost completed; then the exhaust valve is opened by the valve gear and the burned gases allowed to expand through the exhaust port down to atmospheric pressure. On the return stroke {D to E, Fig. 2a), the piston drives the remaining , hot gases out of the cylinder, except what remain in the clearance space at the completion of the stroke (position E).

It is evident from the foregoing that the five events in the engine cylinder occur during four strokes of the piston: the suction stroke, compression stroke, expansion or power stroke, and the exhaust or expulsion stroke; hence the term " four-stroke cycle." Fig. 3 is an indicator diagram made by a gas engine working on the four-stroke cycle. The line s from po to Pa was traced by the indicator during the admission or suction stroke of the piston, and lies a trifle below the line e, which is at practically atmospheric pressure. The reason for this is that the piston, moving away from the cylinder head, forms a slight vacuum behind it before the fresh mixture begins to enter the cylinder, and this partial vacuum is maintained throughout the remainder of the suction stroke. Consequently the entering charge is at a slightly lower pressure than that of the outside atmosphere, as shown by the admission line s on the diagram. This is necessary, of course, because if the pressure within the cylinder were not lower than that of the atmosphere the mixture of air and gas would not enter, unless previously compressed to a pressure higher than the atmospheric pressure.

The degree of vacuum required to draw in the charge depends on the resistance offered to the charge by the passages and inlet-valve port through which it reaches the cylinder, and, in order to keep down the work done in "sucking" the charge through these passages and port, their area is made as large as practicable and all bends are of as large radius as constructional considerations will permit. On the diagram here reproduced the admission pressure was 13| lbs. (inaccuracy in redrawing it has made it appear higher); the degree of vacuum, therefore, was 1.2 lbs. per square inch or 2.43 ins. of mercury.

The curve marked "Compression" shows the rise of pressure produced by compressing the mixture in the cylinder during the return piston stroke. If the mixture had not been ignited until the compression stroke was completed, the curve would have been continued to the point pc , as indicated by the dotted extension of the curve, and if ignition had then occurred, the combustion line would have started abruptly upward, as indicated by the vertical dotted line. But it has been found advisable in practice to ignite the mixture Just before the end of the compression stroke, and that is what was done in this case, producing an upward change in the compression curve at the point marked "Ignition occurs." The reasons for igniting the mixture before, the piston completes the compression stroke are fully explained in the chapter on Ignition.

It will be noticed that the lower part of the combustion line is strictly vertical and that the line leans over slightly toward its upper end. That is due to the fact that combustion was not instantaneous, but continued after the piston had started^ forward on its power stroke. Absolutely instantaneous combustion is hot obtained in an actual engine because of the impossibility of getting a perfect mixture and inflaming the whole of it at the same instant.

The expansion curve of the diagram will be recognized as very similar to the expansion curve of a steam-engine indicator diagram. It drops more rapidly, however, than the curve of steam expansion, and at the point pe its direction changes rather abruptly; this is due to the fact that the exhaust valve opens before the piston completes the expansion stroke, which is necessary in order to give the hot gases time to expand to atmospheric pressure before the piston starts back on the return (expulsion) stroke, and thereby avoid excessive backpressure during the first part of that stroke. The expansion of the burned gases to atmospheric pressure is represented by the reverse curve from the point pe to the extreme "toe" of the diagram, and although the exhaust valve is open while the pressure is falling from the release pressure pe to atmospheric pressure, the expanding gases do some work on the piston. If it were practicable to have an exhaust-valve port large enough to let the gases drop instantaneously to atmospheric pressure when it was opened, the expansion curve could be continued to the end of the stroke, as indicated by the dotted extension, but this is, of course, impossible. The line e, at practically atmospheric pressure, is traced during the exhaust or expulsion stroke of the piston. The actual pressure is very slightly above the atmosphere, of course, owing to the resistance to the flow of the burned gases presented by the exhaust port and channel and the piping leading away from the engine, but the difference is not measurable on an indicator diagram.


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