Internal combustion engines - Lind

The purpose of this book is to provide a practical and up-to-date text on the subject of Internal-Combustion Engines. The endeavor has been to arrange and present the subject matter in such a manner as to bring it well within the comprehension of the average student. For more advanced students, who have a knowledge of thermodynamics, the writer has presented in Chapter III the theoretical considerations of the various cycles which are applicable to internal-combustion engines. No attempt has been made to treat the problems of actual design, these problems being fully covered in many excellent books on this particular subject.

There is but little information available as to the origin of the internal combustion engine. In 1680 Huygens, a Dutch- man, proposed to use gunpowder for the purpose of obtaining motive power. A small quantity of gunpowder, exploded in a large cylindrical vessel filled with air, expelled the air through check valves, thus leaving, after cooling, a partial vacuum. The pressure of the atmosphere then drove a piston down to the bottom of the vessel, lifting a weight or doing other work.

The Abbe Hautefeuille advanced similar ideas but does not seem to have made actual experiments. These early engines cannot be classed as gas engines. Papin, in 1688, stated that the experiments along this line were unsuccessful, so devoted his attention to steam. About one hundred years later (in 1794) Robert Street, an Englishman, patented the first real engine. It contained a motor cylinder in which worked a piston connected to a lever. This lever operated a pump. The bottom of the motor cylinder was heated by fire. A few drops of turpentine were introduced and evaporated by the heat. The piston was then drawn up, admitting a quantity of air, which mixed with the inflammable vapor. Ignition was secured by drawing in a flame through a port uncovered by the piston. The resulting explosion drove the piston up to the end of its stroke and forced the pump piston down, so performing work in raising water. The details of this engine were crude, but the main idea was correct. In 1799 Lebon, a Frenchman, patented a gas engine in which gas and air were supplied from separate compressing pumps to a combustion chamber where the mixture was detonated. The hot gases were then distributed by means of valves to a motor cylinder. Both motor and pump cylinders were double-acting. The engine resembled what became known later as a constant-pressure engine. The inventor's notions were vague, however, for he does not distinguish very clearly between explosions and constant pressure. Lebon had but little time to continue his experiments or to develop his ideas, for he was assassinated in 1804.

In 1820 the Reverend W. Cecil of Cambridge, England, described an engine which was moved by pressure of the atmosphere upon a vacuum caused by the explosion of hydrogen gas mixed with air. In his paper he described an engine which he had constructed to operate according to the explosion-vacuum method. He stated that at sixty revolutions per minute the explosions took place with perfect regularity. This paper gives an account of the first gas engine which appears to have been worked in England, and, it is believed, in the world.

In 1838 William Bamett, an Englishman, invented the compression system now so largely used in gas engines. It is true that Lebon described an engine using compression in 1799, but his cycle in no way resembles that proposed by Bamett, nor is it used in the modem engine. Barnett describes three engines, the first is single-acting, the second and third are double- acting. All these engines compress the explosive mixture before igniting it. In the first and second engines the gas and air are compressed by pumps into receivers separate from the motor cylinder, but communicating with it by a short port which is controlled by a piston valve. The piston valve also serves to open communication between the cylinder and the air when the motor piston discharges the exhaust gases. In the third engine the explosive mixture is introduced into the motor cylinder by pumps, displacing, as it enters, the exhaust gases resulting from the previous explosion. The motor piston by its ascent and descent compresses the mixture. Part of the compression is accomplished by the charging pumps, but it is always completed in the motor cylinder itself.

In all three engines the ignition takes place when the crank is crossing the dead center, so that the piston gets the impulse during the whole forward stroke. The flame method of ignition invented by Bamett was very efficient. It was widely used until about 1892. Previous to 1860 the gas engine was in the experimental stage. Many attempts were made to improve it, but none of the inventors sufficiently overcame the practical difficulties to make their engines a commercial success. Lenoir, a Frenchman, occupies the position of inventor of- the first gas engine that was actually introduced regularly to public use. This engine was first constructed in Paris in 1860. It was built along the lines of a double-acting steam engine. The ignition was obtained by means of a primary battery and coil producing a jump spark. Altogether the engine was a very decided advance over all existing forms of gas engines up to that time. The motion of the engine was as smooth and silent as that of the best steam engine. No shock whatever was heard from the explosion. The engine, however, was very uneconomical, and the great heat required that the piston be flooded with oil. For these reasons the Lenoir engine soon disappeared. The real reasons for the uneconomical working of this engine were lack of compression, incomplete expansion, and heat loss through the walls.

In the year 1862 M. Beau de Rochas, a French engineer, took out a patent setting forth, theoretically, the best working conditions for an internal-combustion engine, with a view to utilizing more completely the heat supplied. His cycle of operations was in all respects the same as that in use at the present day in the so-called Otto cycle engines. The following four propositions were embodied in his patents :

1. The largest cylinder capacity with the smallest possible cooling surface.

2. Maximum possible piston speed.

3. The greatest possible pressure at the beginning of the working stroke.

4. The greatest possible expansion.

To obtain the results which he laid down as being necessary for high efficiency, Beau de Rochas proposed to use a single cylinder and to carry out the cycle in four strokes as follows:

1. Drawing in the charge of gas and air on the first, or suction, stroke.

2. Compression during the following stroke.

3. Ignition at the dead point and expansion during the third stroke.

4. Forcing out the burned gases from the cylinder on the fourth and last, or return, stroke.

At the Paris Exposition of 1878 Otto and Langen, two Germans, brought out the celebrated Otto engine, which almost immediately superseded all other motors and created a revolution in the construction of gas engines. In this engine the whole cycle of Beau de Rochas was carried out in one cylinder. The four cycles were divided into four piston strokes covering two revolutions, thus obtaining one working stroke for each two revolutions in a single-cylinder single-acting engine. This engine has become the standard type of internal-combustion engine.

In 1879 a modification of this engine was produced by Dugald Clerk, an English engineer. In the Clerk engine the charge was compressed and exploded once every revolution, as against one explosion every two revolutions in the engines of the Otto type. The Clerk engine is known as the two-cycle type.

In 1873, before Otto took out his patents, George B. Brayton, an American, took out patents for a gas and an oil engine. In both these engines combustion took place at constant pressure. The gas engine was never successful, so oil was resorted to as fuel. This engine was mechanically better than any previous design of internal-combustion engines, but its economy was insufficient to enable it to compete with other types. In 1893 a new form of internal-combustion engine was described by Rudolph Diesel, a German scientist and inventor. This engine does away with many of the difficulties of the gas and oil engines, and at the same time gives a much higher efficiency. The essential feature of his engine consists in the compression of atmospheric air to a sufficient temperature to ignite the fuel, which is injected at a predetermined rate during a part of the expansion, or working stroke. The oil used as fuel is injected in the form of a spray by air that is compressed separately in a compressor under a pressure 300 or 400 pounds above that in the main cylinder. The engines have this advantage: the work can be regulated by the amount of fuel supplied. This amount is not controlled, as in explosive engines, by the necessity of forming an explosive mixture. The cycle has a resemblance to that of the Otto engine, but differs from it in that the air only is compressed in the main cylinder, and the combustion is not accompanied by an explosion. This type of engine, or modifications of it, is used in practically all submarines.


CHAPTER II - ELEMENTARY CONSIDERATIONS

There are three important sources of energy available for industrial uses. The first of these is the muscular power of man and animals. The second of these sources of energy is that due to the position or motion of a body whereby it possesses potential or kinetic energy. The third is the energy which is manifested by the chemical reactions that occur in combustion or oxidation. The most important manifestation of this third group is heat, and it is in the conversion of this heat into mechanical energy that we are now interested.

The heat energy released by the combustion of fuel is converted into mechanical energy by means of an engine. If a steam engine is used, an intermediate member is needed, this member being the boiler. The chemical energy of the fuel is changed into heat energy in the furnace, and this heat energy is transferred to the water in the boiler. The water in this case acts as the carrier of the heat. Other heat carriers could be used in place of water, but water is plentiful and possesses characteristics which make its use for this purpose most desirable. The water being changed into steam in the boiler, the heat in this steam is converted into mechanical energy by forcing a piston back and forth in a steam-engine cylinder.

In the internal-combustion engine, combustion takes place in the cylinder itself, which thus acts as the furnace of the boiler. The heat carrier, or medium, in this case is the air required for the combustion of the fuel. The mechanical action of the internal-combustion engine is similar to that of the steam engine, differing principally in that most engines of the former, type are single acting and hence require no piston rod or crosshead and the cylinder need be closed at one end only.


Parts of the Internal-Combustion Engine and their functions

The function of the mechanism of any engine is to provide a means whereby the heat energy of the fuel can be efficiently converted into useful mechanical work. A study of its construction and parts is needed in order to become familiar with the engine as a whole and with the terms used in describing it. Some of the principal parts are described below.

The cylinder is usually made of hard, close-grained cast iron and may be arranged either horizontally, vertically, or at an angle to the vertical, according to the type of engine. In most aircraft engines the cylinders are made of steel.

For reasons already given, the cylinders must be provided with some means of cooling the walls. In small engines this may be accomplished by placing fins, or ribs, on the outer surface of the cylinder to expose a large cooling area to the outside air. The largest engines and the majority of smaller ones are water cooled, the cylinders being cast with outer walls, or jackets, inside of which water is circulated to cool the inner wall.

Pistons are usually made of a good grade of close-grained cast iron or aluminum alloy. In larger engines, cast steel is sometimes used. The pistons are similar to the steam-engine pistons except that they are longer. For single-acting engines the trunk piston is the type generally employed. Usually no special provision is made for the cooling of this type of piston, as the circulation of air within it is sufficient. All pistons are provided with rings to prevent leakage between the piston and the cylinder walls.

The valves of an internal-combustion engine serve the same purpose as the valves of a steam engine. They admit the fresh mixture of air and fuel into the cylinder at the proper time and permit the exhaust gases to escape. In some engines slide valves are used to perform these functions, but poppet, or lift, valves are most commonly used. Valves for small cylinders are made of alloy steel. The valves are worked from an auxiliary, or cam, shaft. In most engines the valves are actuated by cams, while in large engines eccentrics are generally employed.

The force on the piston of the internal-combustion engine is transmitted to the revolving crankshaft through a connecting rod and crank in the same manner as in the steam engine. In single-acting engines there is no piston rod, but the connecting rod is fastened directly to the trunk piston, which also acts as a crosshead. In double-acting engines the arrangement is the same as that of the steam engine, there being a piston rod, cross-head, and connecting rod. These parts are usually made of open-hearth steel.

To reduce cyclic variations in speed a flywheel is used. This is commonly made of cast iron. For high-speed automobiles and motor boats a steel disk is generally used.


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Internal combustion engines