L. 'THE MIXING' OF PUPS. AND. AID by means oi Carburetors and Atomisers.
These problems are of a chemico-physico mechanical nature, and are intimately inter woven. Eac.liA Abe rqqiiirqmots cannot, how ever, be realized" 'to its hitheit 'degree. For instance, wiile'rOtattl tOr'SertiSt-'tontlftfons, it is very desirable to keep the cooling medium at the lowest possible temperature; yet the low L Cyclic Functions.--In order to satisfy the above five basic requirements, the internal com bustion engine has to pass through a definite of Operation)); that is, its piston has to make a number of single strokes to gain one 'Tower° or ((Impulse))stroke, known as the stroke) To-day the four-stroke cycle and two-stroke cycle are most commonly used, and are briefly called four and two cycle, respectively.
(a) Four-Stroke Cycle.— In the diagrams, est fuel consumption is attained only at a fairly high temperature of the cooling water. Thus in all practical cases there can be only an ap proximate solution, which must necessarily be a happy balance between contradicting condi tions. Furthermore, the location of the engine as well as the kind of service causes still more difficulties, and indeed difficulties of a special nature. 'The problems of the air-craft engine, for instance, are decidedly different from those of the automobile enne, while the latter re quires features of which are seldom embodied in the stationary type of engine.
Figs. 1 to 4, a four-stroke cycle single acting gas engine is briefly illustrated.
Fig. 1 represents the engine with the piston on the suction stroke, or its first out-stroke. During this stroke a charge of gas and air, previously mixed in the mixing chamber, is taken in through the inlet valve. In Fig. 2 the piston is starting on its first in-stroke or com pression stroke. The inlet valve is closed and the charge compressed. At or near the end of the stroke, ignition occurs. Fig. 3 shows the piston, just after the ignition of the gases, at the beginning of the working stroke, which is now the second out-stroke. Due to the com bustion of the mixture within the cylinder, both the temperature and the pressure of the gases increase suddenly. The high-tension gases thus produced drive the piston out, and while doing this useful work gradually expand to nearly atmospheric pressure. At the end of this stroke the exhaust valve is opened and the inert gases are forced out of the cylinder during the fourth stroke, Fig. 4, known as the exhaust stroke, at or near the end of which the mixture inlet valve is opened, starting the cycle over again. The changes of pressure within the cylinder during the different strokes are represented in the pressure-stroke or pressure-volume dia grams, Figs. la, ?a, 3a, 4a; in Fig. 5 these pressure-volume diagrams are developed. Dur
ing the first, second and fourth strokes the engine performs the functions of a pump, de riving its power from the excess energy stored up in the flywheel during the power stroke. The power is automatically adapted to the load demand by a governor acting on the gas inlet throttle value. During the cycle, a cooling medium, usually water, flows through the jacket surrounding the cylinder to carry away the heat transmitted through the cylinder walls, thus preventing overheating of the latter.
(b) Two-stroke Cycle.—To illustrate the two-stroke cycle principle, a two-stroke cycle single acting oil engine has been selected. See Figs. 6 to 10.
In Fig. 6 the piston is in its outer dead centre position. The ports for both the scav enging air and the exhaust are uncovered by the piston, while the air inlet valve in the crankcase is closed. The cylinder as well as the crankcase is filled with air at atmospheric pres sure. The piston on its in-stroke (Fig. 7) slides over the air inlet port, cutting off the cylinder from the crankcase. This causes the pressure in the latter to fall below that of the atmosphere, thereby opening the automatic air inlet valve and admitting fresh air to the crank case. Further sliding of the piston then cuts off the exhaust port from the cylinder and com pression starts. Before the piston reaches the inner dead centre (Fig. 8) the fuel is sprayed into the combustion space, which in this case is the hot bulb, where it is vaporized, mixed with air and ignited near dead centre; the first operation in the two-stroke cycle — the compression stroke — is now completed. The temperature and the pressure of the gases within the cylinder increase quickly, and the high tension gases, expanding gradually, force the piston out (Fig. 9), thus doing useful work. At a point where the piston has yet to travel about 15 per cent of its stroke, the exhaust port is uncovered by it and the exhaust gases are discharged. Further sliding uncovers the inlet port (Fig. 10), and the air in the crank case, being compressed slightly above atmos pheric pressure, rushes into the cylinder and sweeps out or scavenges the inert gases. This completes the cycle. Figs. 8a, 9a and 10a illus trate again the changes of pressure within the cylinder during the cyclic events. One further feature to be noted is the necessity of a fuel pump for the two-stroke cycle engine. During the compression stroke, the excess of energy which was stored up in the flywheel during the power stroke, is consumed. The power is adapted to the load by the action of the gov ernor on the fuel pump. To assure continuous operation some heat is transmitted through the walls and carried away by the cooling medium passing through the water jacket.