Practical Hand Book of Gas, Oil and Steam Engines / Stationary, Marine, Traction; Gas Burners, Oil Burners, Etc.; Farm, Traction, Automobile, Locomotive; A simple, practical and comprehensive book on the construction, operation and repair of all kinds of engines. Dealing with the various parts in detail and the various types of engines and also the use of different kinds of fuel.

by John B. Rathbun

(3) Combustion In the Cylinder.

As the working medium in an internal combustion engine is in direct contact with the fuel it must not only be uninflammable but it must also be capable of sustaining combustion and must have a great expansion for a given temperature range. Since atmospheric air possesses all of these qualifications in addition to being present in all places in unlimited quantities it is natural that it should be used exclusively as the working medium for gas engines. Unlike the vapor working medium in a steam engine the medium in the gas engine not only acts in an expansive capacity but also as an oxydizing agent for burning the fuel, and therefore must bear a definite relation to the quantity of the fuel in the cylinder to insure complete combustion.

In the gas engine the use of gaseous fuel is imperative since there must be no solid residue existing in the cylinder after combustion and also for the reason that the fuel must be in a very finely subdivided state in order that the combustion shall proceed with the greatest possible rapidity. In addition to the above requirements the introduction of a solid fuel into the cylinder would involve almost unsurmountable mechanical problems in regard to fuel measurement for the varying loads on the engine. This limits the fuel to certain hydrocarbon or compounds of hydrogen and carbon in gaseous form of which the following are the most common examples:

(a) CARBURETED AIR consisting of a mixture of atmospheric air and the vapor of some hydrocarbon (liquid) such as gasoline, kerosene or alcohol.

(b) OIL GAS formed by the distillation of some heavy, nonvolatile oil, or the distillation of tar or paraffine.

(c) NATURAL GAS obtained from natural accumulations occurring in subterranean pockets in various parts of the country.

(d) COAL GAS, made artificially by the distillation of coal, commonly called “illuminating” gas.

(e) PRODUCER GAS, some times known as “fuel gas,” produced by the incomplete combustion of coal in a form of furnace called a “producer.”

(f) BLAST FURNACE GAS, the unconsumed gas from the furnaces used in smelting iron, somewhat similar to producer gas but lower in heat value.

It should be noted that there is no essential difference between engines using a permanent gas or an oil as in either case the fuel is sent into the cylinder in the form of a vapor. In the case of oil fuel, the vapor is formed by an appliance external to the engine proper. In this book, the heat action of an engine using one form of fuel applies equally to the engine using another. The selection of a particular fuel for use with a gas engine depends not only upon its value in producing heat, but also upon its cost, the ease with which it meets the peculiar conditions under which the engine is to work, and its accessibility.

Neglecting for the moment, all of the items that do not affect the operation of the engine from a power producing standpoint, the principal requirement of a fuel is the production of a high temperature in the cylinder since the output is directly proportional to the temperature range. Since a very considerable mass of air is to be raised to this high temperature, the heat value, or CALORIFIC VALUE of the fuel in British Thermal units is of as much importance as the temperature attained in the combustion. The calorific value of different fuels vary widely when based either on the cubic foot or pound, and a considerable variation exists even among fuels of the same class owing to the different methods of production or to the natural conditions existing at the mine or well from which they originated. The principal elements of gas engine fuels, carbon and hydrogen, exist in many different combinations and proportions, and require different quantities of air as oxygen for their combustion because of this difference in chemical structure.

Since complete combustion is never obtained under practical working conditions, the actual evolution of heat and the actual temperatures are always much lower than those indicated by the CALORIMETER or heat measuring device. Besides the loss of heat due to imperfect combustion, there are many other losses such as the loss by radiation, connection, and slow burning, the latter being the principal cause of low combustion temperatures. From the statements in the foregoing paragraphs it will be seen that the theoretical or absolute calorific value of a fuel is not always a true index to its efficiency in the engine.

Complete combustion results in the carbon of the fuel being reduced to carbon dioxide (CO₂) and the hydrogen to water (H₂O), with the liberation of atmospheric nitrogen that was previously combined with the fuel, and some oxygen. The reduction of the fuel to carbon dioxide and water produces every heat unit available since the latter compounds represent the lowest state to which the fuel can be burned. Carbon however may be burned to an intermediate state without the production of its entire calorific contents when there is not sufficient oxygen present to thoroughly consume the fuel. Incompletely consumed carbon produces a gas, carbon monoxide, as a product of combustion, and a quantity of solid carbon in a finely subdivided state known as “soot.” Unlike the products of complete combustion, both the carbon monoxide and soot may be burned to a lower state with a production of additional heat when furnished with sufficient oxygen, both the soot and the monoxide being reduced to carbon dioxide during the process.

Fig. F-2. Sunbeam Engine with Six Cylinders Cast “En Bloc” (in one piece). At the Right and Under the Exhaust Pipe is the Compressed Air Starting Motor that Starts the Motor Through the Gear Teeth Shown on Flywheel. From “Internal Combustion.”

As the soot and monoxide have a calorific value it is evident that much of the heat of the fuel is wasted if they are exhausted from the cylinder without further burning at the end of the stroke. To gain every possible heat unit it is necessary to furnish sufficient oxygen or air to reduce the fuel to its lowest state. As the free oxygen and nitrogen contained in the fuel are without fuel value, their rejection from the cylinder occasions no loss except for that heat which they take from the cylinder by virtue of their high temperature.

With complete combustion the TEMPERATURE attained increases with the rate of burning, while the number of heat units developed remain the same with any rate of combustion. Because of the conditions under which the fuel is burned in the gas engine the fuel is burned almost instantaneously with the result that high temperatures are reached with fuels of comparatively low calorific value. With a given gas the rate of combustion is increased with an increase in the temperature of the gas before ignition and remains constant for all mixtures of this gas in the same proportion when the initial temperature is the same. The rate of combustion also varies with the composition of the gas, hydrogen burning more rapidly than methane. As a rule it might be stated that the rate of burning decreases with the specific gravity of the gas, the light gases such as hydrogen burn with almost explosive rapidity, while the heavier gases such as carbon dioxide are incombustible or have a zero rate of combustion. In practice an increased rate of burning is obtained by heating the charge before ignition by a process that will be explained later.

Another factor governing the output of an engine with a given size cylinder is the amount of air required to burn the fuel. The quantity of air necessary for the combustion of the fuel determines the amount of fuel that can be drawn into a given cylinder volume, and as we are dependent upon the fuel for the expansion it is evident that with two fuels of the same calorific value, the one requiring the least air will develop the most power. Since the air required to burn hydrogen gas is only one fourth of that required to burn the same amount of methane it is clear that more hydrogen can be burned in the cylinder than methane. This great increase in output due to the hydrogen charge is however, considerably offset by the greater calorific value of the methane.

Should the air be in excess of that required for complete combustion, or should a great quantity of incombustible gas, such as nitrogen be present in the mixture, the fuel will be completely burned, but the speed of burning will be reduced owing to the dilution. As the air is increased beyond the proper proportions the explosions become weaker and weaker as the gas becomes leaner until the engine stops entirely. Because of the fact that it is impossible in practice to so thoroughly mix the gas and air that each particle of gas is in contact with a particle of air, the volume of air used for the combustion is much greater than that theoretically required. A SLIGHT excess of air, making a lean mixture, increases the efficiency of combustion although it reduces the temperature and pressure attained in the cylinder. This is due to the fact that while the temperature of the mixture is lower than with the theoretical mixture the temperature of the burning gas itself is much higher. A mixture that is too lean to burn at ordinary temperatures will respond readily to the ignition spark if the temperature or pressure is raised.