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

(4) Compression.

In the practical gas engine the gas is not ignited at the beginning of the suction stroke by which it is drawn into the cylinder, but is compressed in the front end of the cylinder by the return stroke of the piston, and then ignited. The process of compression adds greatly to the power output of a given sized cylinder and increases the efficiency of the fuel and expansion. In order to understand the relation that the compression bears to the expansion let us refer to Fig. 2 in which C is the working cylinder, P the piston and G the crank. While the piston is moving towards the crank in the direction of the arrow A it draws the mixture, indicated by the marks x x x x x, into the cylinder, the quantity being proportional to the position of the piston. In this particular case let us assume that the area of the piston is 50 square inches and that the entire stroke (B) of the piston is 12 inches. To prevent confusion due to considerations of heat loss we will further assume that the cylinder is constructed of non-conducting material.

With the piston at the position H, midway between J and I, the volume D is filled with the explosive mixture at atmospheric pressure and a temperature of 500° absolute. Since D = 6 inches and the area of the piston is 50 square inches, the volume D is equal to 6 × 50 = 300 cubic inches, and the entire volume is 2 × 300 = 600 cubic inches. On igniting this mixture (at atmospheric pressure) the temperature will rise immediately, say to 1000°F with the piston at H. According to a law governing the expansion of gases, known as Gay-Lussac’s Law, the expansion v × T
t = V where v = the initial volume of the gas before ignition = 300 cubic inches; t = the temperature before ignition 500° absolute; V = the volume of the gas after expansion; and T = temperature after ignition = 1000° absolute. Inserting the values in numerical form we have as the final volume:—300 × 1000
500 = 600 cubic inches = the volume after expansion, or twice the original volume of gas. This means that the expansion is capable of driving the piston from H to I before the pressure is reduced again to atmospheric pressure. As the volume is expanded to twice that of the original volume at atmospheric pressure (14.7 pounds per square inch), the pressure against the piston before it starts moving will be 2 × 14.7 = 29.4 pounds per square inch.

Let us now consider the case in which the charge is compressed before ignition occurs and compare the expansion and pressure established with that produced by ignition at atmospheric pressure. To produce the compression the piston will travel through the entire stroke to the position I on the suction stroke filling the entire cylinder volumes of 600 cubic inches with the mixture. On the return stroke the piston stops at H, reducing the original volume of 600 cubic inches to 300 cubic inches, doubling the pressure of the gas. The initial and final temperatures will be considered as being the same as those in the first example, 500° and 1000°. From Gay-Lussac’s Law—v × T
t = V and substituting the numerical values 600 × 1000
500 = 1200 cubic inches, or the expanded volume will be four times the compressed volume, or four times the initial volume of the first case where the gas was ignited at atmospheric pressure.

It should be noted however, that while the expansion has been greatly increased by the compression, that this is not all gain, as equivalent work has been expended in compressing the charge. With the exception of doubling the fuel taken into the cylinder, and consequently doubling the output for a certain cylinder capacity, there has been no increase in fuel efficiency except that due to conditions other than the mere reduction in volume. In the second case the volume was increased four fold which resulted in a piston pressure of 4 × 14.7 = 58.8 pounds per square inch before the piston increased the volume by moving from H to I.

The work done by the engine on the charge in compressing is converted into heat energy causing a rise in the temperature of the gas. This would not be a loss as it would reappear as mechanical energy on the return stroke of the piston through its expanding effect on the gas. This heat would, in effect, be added to the temperature due to ignition, and the sum would produce its equivalent expansion. The temperature due to the combustion may be determined by reversing Gay-Lussac’s Law—

t		I		Pt
	=		or T =
p		P		p

Where t = initial temperature; T = temperature combustion; P = pressure after combustion; p = pressure before combustion.

Because of the fact that the act of compressing the charge in the cylinder before ignition increases the temperature of the working medium, the compression will increase the speed of combustion and efficiency of the fuel as the rate of combustion increases with the initial temperature. This increased temperature due to initial compression of course results in a greater temperature range and output due to the increased rate of burning, and this rate of combustion may be varied for different fuels by changing the compression pressure. In a previous paragraph it was explained that the fuel efficiency was increased by a slight dilution or excess of air, and that while the temperature and pressure of the mixture were reduced by the dilution the temperature of the fuel was increased, provided that the inflammability was not decreased.

Compression affords a means of using dilute mixtures without loss of inflammability, as the heat gained by the compression restores the inflammability lost by the effects of dilution. Increased compression pressures increases the possible range of dilution, so that extremely lean gases and mixtures may be used with success with appropriately high compression. As an example of this fact we can refer to the engine using blast furnace gas, a fuel that is so lean that it cannot be ignited under atmospheric pressure. By increasing the piston speed, the heat of the compression can be made more effective as the gas lies in contact with the cylinder walls for a shorter time which of course reduces the heat to the jacket water.