BURNING FUEL IN THE ENGINE CYLINDER
ONE OF the handicaps of steam power is that the heat produced by the combustion of fuel is not used directly to drive the piston. A large part of the heat energy in coal goes up the chimney or is wasted by radiation from the furnace walls. With anthracite coal in the furnace the loss may be as low as 22 per cent in the very best types of boilers. In the engine other serious losses occur, so that in the best condensing reciprocating steam engines the power delivered is only 10 to 16 per cent of that stored in the fuel. In locomotives the efficiency is as low as 4 to 6 per cent. In turbines the steam is used to better advantage and the actual power delivered may run up to 20 per cent of that in the fuel.In internal-combustion engines the furnace and boiler losses are largely overcome by burning the fuel right in the cylinder, where the heat energy of the combustion may be utilized directly upon the piston. There are other losses, however, so that the most efficient gasoline engines deliver only 28 per cent of the energy in the fuel, and coal-gas engines may run up to 31 per cent.
A wide range of fuels may be used in an internal-combustion engine. They may be either gaseous, liquid, or even solid. In stationary engines hydrogen, coal gas, natural gas, blast-furnace gas, and producer gas are employed to advantage. Volatile fuels, such as alcohol, bensol, gasoline, and kerosene, are turned into a mist or vapor and then burned as a gas. In engines of the Diesel type any liquid fuel ranging down to thick crude oils may be employed, and in one type of engine, which, however, has not proved commercially practicable, coal dust is burned in the cylinder.
BURNING AIR IN THE AUTOMOBILE ENGINEOwing to the widespread use of automobiles, the general public is better acquainted with internal-combustion engines than with steam engines. Nevertheless, there are many elementary facts in connection with gasoline engines that are not generally known. The average motorist probably does not realize that he burns far more air in his motor than gasoline, and he probably does not understand why it is that the mixture of gasoline vapor and air must be compressed before it is ignited, or why only one out of four strokes of the piston is a power stroke.
If a volume of gasoline vapor be thoroughly mixed with an equal volume of air, the mixture will not explode. Only when there is an excess of air will combustion take place, and the most intensive explosion takes place when there are nine parts of air to one part of gasoline vapor. Where gasoline is vaporized in a carburetor the best mixture is one part of gasoline-saturated air to eight parts of pure air. Hence it is mainly air that is burned in an automobile engine.
The energy of combustion is much greater when the mixture is compressed. The particles of gasoline and air are forced into more intimate contact by the compression. The same thing is true of gunpowder. If gunpowder is ignited in the open air, it will burn quickly, but not with explosive violence. If, on the other hand, the powder is compressed in a cartridge and is then ignited, an explosion takes place. Nearly twice as much gas was required in the early noncompression-type motors as is required to-day in the compression motors of the same power.
In an internal-combustion engine the cylinder serves as a furnace. This “furnace” must be charged with fuel, and after the fuel has been burned the “ashes”—i. e., the products of combustion—must be removed. The piston serves as the furnace stoker.
In the ordinary four-cycle engine the action of the piston is as follows: On the first or down stroke of the piston a mixture of air and gasoline vapor is drawn into the cylinder; on the next or rising stroke the charge is compressed. Then the charge is ignited by an electric spark, and the rapid combustion of the charge produces gases which drive the piston down. On the fourth stroke the piston rises again and pushes the burnt gases out of the cylinder. The piston receives energy intermittently, or only once out of four strokes, and a flywheel has to keep it going the rest of the time. It is just like propelling a bicycle with a single pedal and pushing the pedal every other time it comes out. It can be done as long as the wheel is moving fast enough to carry itself along between power strokes. Naturally a single-cylinder motor cannot be slowed down very much without stalling, and it will not start of itself because it needs outside help in stoking its furnace before it acquires the power to do this job alone.
There are no bothersome poppet valves in this engine. The piston itself acts as a valve, opening and closing the inlet and exhaust ports as it slides by them. Every other stroke of the piston is a power stroke, so that the engine acts like a one-pedal bicycle, which receives a push every time the pedal comes around. Unfortunately, the burnt gases are never completely scavenged, and the fresh charge of fuel is always more or less diluted by the product of combustion remaining from the previous charge. This represents just so much loss of power, and may interfere with the ignition of the charge. For this reason the four-cycle engine is generally considered more reliable and efficient, and is far more generally used, particularly on motor vehicles.
To overcome the intermittent character of the internal combustion engine a number of cylinders are used, which come into play successively. In the four-cylinder motor one piston is always on the power stroke. In recent years the number of cylinders has progressively increased from four to six, eight, and twelve cylinders, while in racing power boats the number of cylinders has gone up as high as twenty-four.
COOLING THE CYLINDERSThe temperature of the combustion in a gasoline engine may be over 3,000 degrees Fahrenheit. Evidently it would melt the cylinder walls were not special provisions taken to keep them cool. The usual method is to surround the cylinders with water which absorbs the heat, and then cool the water by passing it through a radiator. Air driven by a fan through the radiator carries off the heat. Of course this represents just so much wasted energy and lowers the efficiency of the motor, but it is the most convenient way of getting rid of the intense heat of combustion. Smaller motors, such as are used on motorcycles, have their cylinders cooled by direct action of air on the outer surface of the cylinder. The heat radiated depends upon the surface exposed and the velocity of the air current. The radiating surface of the cylinders is increased by forming them with external ribs or flanges, so that the air that flows over them, while the machine is in motion, carries off enough heat to keep the temperature of the cylinders within safe limits. In some systems, particularly in the case of stationary engines, to insure a good circulation, the air is positively driven against the flanges by means of a fan.
There are serious disadvantages in using water to cool an engine. Besides the bother of keeping the water system supplied with water there is the danger of clogging the radiator with lime deposits, and in extremely cold weather the water is liable to freeze and burst the water jackets or the radiator; for this reason, air cooling is recommended by some for automobile engines. In certain air-cooled automobile engines the cylinders are air-jacketed; that is, they are surrounded with casings through which the air is sucked by a powerful fan driven by the engine shaft, so that the air is bound to flow if the engine is turning. The cylinders are ribbed, so that they present a large radiating surface.
Air-cooled engines have hotter cylinders than water-cooled engines and hence in cylinders of the same size less fuel is drawn into the air-cooled cylinder. But the fuel is used more efficiently because less of its heat energy is wasted; so that air-cooled engines show a slight fuel economy over water-cooled engines. On the other hand, when the air-cooled engine is overloaded or is run at very high speeds it is liable to become overheated and may ignite the incoming charge of gas prematurely.
THE SELF IGNITING DIESEL ENGINEAs has already been pointed out, compression of gas generates heat, and one of the reasons for cooling the cylinders of an internal combustion engine is to prevent the gas from exploding prematurely on the compression stroke of the piston. In the Diesel engine, instead of avoiding such a critical temperature it is deliberately sought, because it is the heat of compression, instead of an electric spark, which ignites the charge. Pure air is drawn into the cylinder on the suction stroke. This air is under ordinary atmospheric pressure when it enters, but on the return stroke of the piston it is compressed to about 500 pounds per square inch. This raises the temperature up to the neighborhood of 1000 degrees Fahrenheit. Into this highly heated air a spray of oil is injected by air at still higher pressure and immediately the oil flashes into flame and the gases resulting from the combustion drive the piston down.
This is a very economical type of motor. Any liquid fuel may be used, from light gasoline to heavy crude oils, or the oils that remain after the more volatile fuels have been distilled from them.The heavy oils have more heat value in them than is to be found in light volatile fuels, such as alcohol, gasoline, and kerosene, and because the oils do not throw off any inflammable vapors unless highly heated, they can be stored more safely. This is of highest importance on submarines and Diesel engines are commonly used for propulsion on the surface or to drive the generators which charge the storage batteries for submerged travel.
In order to inject the fuel into the cylinder against the pressure of air therein, a powerful air pump is required. This does not need to be of very large capacity, but it must compress the air to from 700 to 1,000 pounds per square inch. It may seem at first as if the work done by the engine in compressing the air in the cylinder and in operating the injector pump represents so much loss, but a moment’s consideration will show that it is all recovered. The air in the cylinder acts like a spring, rebounding when the piston starts down on its power stroke and adding its energy to that of the burning gases, while the air from the injector pump also enters the cylinder and helps to push the piston down.
Diesel engines are made to operate on the two-cycle as well as the four-cycle principle.
In one type of engine, known as the semi-Diesel, a lower compression is used in the cylinder. This is not high enough to ignite the oil that is sprayed in, and so a hot tube or bulb is employed at the top of the cylinder against which the fuel jet is directed. This bulb is heated to a dark cherry red by means of a torch until the engine has developed enough heat to keep the bulb at the requisite temperature.
THE DOUBLE-ACTING JUNKER ENGINE
The efficiency of Diesel engines, although greater than that of the gas and gasoline engines, is still very low. The semi-Diesel will yield about 30 per cent of the energy in the fuel, the Junker engine about 34 per cent, and the best Diesel, four-cycle engine in large units, about 36 per cent. This is a wonderfully high efficiency compared with that of a locomotive, and yet it seems pitiably low when we consider that nearly two-thirds of the energy stored in the fuel is thrown away.
SILENCING THE NOISY EXHAUSTThe waste of energy is clearly evident in the exhaust pipe of an internal combustion engine. The gases after doing their work on the piston rush out with such velocity as to produce a sharp explosive sound. The noise of the exhaust is highly objectionable in automobiles and must be overcome, but instead of utilizing the boisterous energy of the escaping gases and getting a little more useful work out of them, means are provided for hushing their noise. This is done by passing them through a series of baffles which reduces the pressure of the gases before they are discharged into the atmosphere. By letting the gases expand gradually instead of bursting suddenly into the atmosphere the noise of the discharge is reduced. However, this must not be done at the expense of the engine power. Unless the gases pass quickly through the muffler they will choke and retard the exhaust and make the engine do useless work in driving them through. Even the best of mufflers will use up nearly 5 per cent of the engine power. The energy that escapes at the exhaust is not the only loss. All the heat that radiates from the engine represents just so much wasted energy. In our water and air-cooled systems we deliberately abstract heat from the burning gases and throw it away. If we had materials that would stand the intense heat of burning gases and enable us to conserve all the heat developed in the cylinder we could use our fuels much more economically.
Many attempts have been made to utilize the wasted heat of internal combustion engines, but they have not met with any considerable degree of success, with the exception of the invention of William Joseph Still. This engine, which was the result of many years of patient work, was first made public in a paper read before the Royal Society of Arts in London, May 26, 1919, and when it was shown that the new engine had developed an efficiency of 38 to 41 per cent it was realized that here was a remarkable advance over any other machine for turning heat into power.
COMBINED STEAM AND OIL ENGINEThe Still engine is a combined steam and gas or oil engine. The heat that is ordinarily thrown away in the water jacket and in the exhaust is utilized to produce steam, and this steam is employed to return the piston to the top of the cylinder after the gases generated by the combustion of fuel have driven it down. While in previous experiments some steam had been developed from the exhaust gases it had not been found possible to generate steam in the water jacket because of the low temperature that had to be maintained. In the standard internal combustion engine the cylinder walls are made of cast iron, thick enough to withstand the heavy pressures to which they are subjected should the charge be ignited prematurely. Around them the water circulates under practically no pressure and so the wall of the water jacket is made comparatively thin. Because of the thickness of the cylinder wall the water has to be maintained at a comparatively low temperature so as to keep the interior of the cylinder from growing too hot. In the Still engine the cylinder wall is from one-third to one-fourth the thickness of the standard cylinder wall, while the water-jacket wall is of thick steel. The cylinder wall is formed with ribs which extend to the water-jacket wall, so that the latter will take care of any excessive loads due to premature ignition. Figure 53 shows how the cylinder and jacket of the Still engine compare with those of the ordinary motor. The thin cylinder wall and the ribs furnish a far better conduction of heat to the water which circulates between the ribs. The necessary cooling of the cylinder can be maintained with water at a much higher temperature than in the ordinary engine.
LATENT HEATWhen water turns into steam a certain amount of heat is absorbed, which does not show in the thermometer. This is known as latent heat. If cold water is placed in a kettle and is then heated the thermometer will gradually rise until the water reaches the boiling point. Then there will be no further rise of temperature, although heat is still applied to the kettle, until all the water is turned into steam, after which the thermometer will begin to show a rise of temperature, showing that the steam is beginning to grow sensibly hotter. In the engine, shown in Figure 54, a boiler pressure of 120 pounds gauge pressure is maintained, and therefore water will not boil until it reaches 350 degrees Fahrenheit.The water in the water jacket registers 350 degrees and is therefore far hotter than it could possibly be in the open atmosphere. However, the heat from the combustion does not raise the temperature of the water, but expends its energy in converting the water into steam. The cylinder wall is kept at a temperature of at least 350 degrees all the time, so that the air that enters the cylinder gathers heat from the cylinder as well as from its own compression, and its temperature at the end of the compression stroke is higher than it would be in a cold cylinder, thereby insuring the ignition of the fuel when it is sprayed in.
The Still engine can also be used as a common gasoline or gas engine of either two or four cycle and the efficiency in such types is from 31 to 33 per cent. This is much better than the best airplane engines, which show an efficiency under 27 per cent. Although 41 per cent for the best Still heavy-oil engine is a remarkable accomplishment, yet it does not begin to compare with the efficiency of the best water turbines and Pelton wheels, which turn into useful power from 75 to 87 per cent of the kinetic energy in the water that drives them.
by A. Russell Bond