The Internal Combustion Engine - TM 10-570 War Department Technical Manual

Fundamental principles Four stroke Otto cycle.

2. General. — a. An engine may be described as a machine that uses power to do work, particularly one that converts some form of energy, especially heat, into mechanical power. An internal combustion engine is one which obtains its power by the combustion or burning of fuel in a confining space — its cylinders.

&. In gasoline engines of the type commonly used for automotive purposes, mechanical power is generated from the potential chemical energy of the fuel used. The engine, acting as a pump, draws the fuel mixture (air and gasoline) from a device known as a carburetor into the cylinder. The gasoline in the ideal mixture is highly vaporized or atomized and is in the proportion of about 1 part of gasoline to 15 parts of air by weight. After this vaporized mixture is drawn into the cylinder it is compressed to a fractional part of its original volume and then ignited. The mixture burns and forms water vapor and carbon dioxide. This burning raises the gases to a high temperature, and since heating a confined gas increases its pressure, the gases of combustion exert extremely high pressures on the piston of the engine, giving to it a rectilinear motion and allowing the gases to expand. This motion is converted into rotary motion by means of a connecting rod and a crankshaft.

3. Work. — a. Any applied effort which overcomes a resistance or results in objects being lifted or moved is work. Thus, if you turn a crank, lift a chair, or slide a table across the floor you are doing work. The time it takes to move an object is also involved in work. A man could unload a ton of bricks in an hour by handling them one at a time whereas a power crane could be used to lift the whole load at one time.

&. (1) In each instance the same amount of work was accomplished but the crane did it in a fraction of the time. This indicates a greater ability to do work. Work is done when the man, figure 1 0, exerts force through distance in turning the wrench to tighten the nut on a bolt.

(2) In figure 1 (D the bicycle rider exerts a force downward against the pedal, making the sprocket turn, which moves the chain and re- Figure 1. — Principles of force, torque, and power.

volves the rear wheel. The work done by the rider is used or consumed in overcoming the resistance of the bicycle to motion, in keeping it moving, or in overcoming gravity when going up hill.

c. It is an accepted fact that the total energy of the universe remains constant. Its form may change and its distribution may change, but it cannot be destroyed or created. F or example, work is done in raising a book to a table top, but, by virtue of its position, the energy that has been stored in the book then enables it to do work. Likewise, heat energy stored in gasoline is liberated to do work when it is burned with oxygen.

d. An engine cannot create energy but it can transform the heat energy in a fuel into mechanical energy. In this process all the potential energy of a fuel is not used because no machine is 100 percent efficient. Some of the heat energy is lost in the exhaust gas, some is lost by radiation to the surrounding air, and some by conduction to the cooling system of the engine through cylinder walls, piston, etc. Some of the heat energy transformed into mechanical energy is used in overcoming the friction between the various moving parts of the engine. By this it is seen that the usable mechanical power delivered by the engine is only a part of the total heat energy supplied by the fuel.

e.

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