MOTOR CAR ENGINES: THEIR OPERATION AND CARE. An internal combustion engine is one in which the fuel is burned directly inside the machine which converts the heat energy into work. Practically all motor cars use this kind of engine, the fuel being petrol (gasoline). The engine consists of several cylinders in which pistons are fitted. The top of each cylinder is covered over with a head which contains a hol lowed-out space opening directly from the cylinder. This is called the combustion chamber. A rod connects each piston with a shaft which rotates, called the crankshaft. The burning of fuel and air in the combustion chamber produces heat energy which causes expansion of the gases formed by the process of combus tion. The gases push the pistons down the cylinder and thus cause the crankshaft to rotate. A flywheel on the end of the crankshaft keeps the latter rotating smoothly between the power impulses from each cylinder; it also transmits the power developed by the engine to the clutch.
The easiest way to understand how an engine works is to keep in mind only one cylinder. For the engine to run and do work, certain things must take place over and over again in a regular order. The fuel must be sucked in, it must be compressed and burned ; the resulting gases expand, and thus do work, and after expansion they must be removed. These steps make up what is known as a cycle; in most motor car engines it takes four strokes of the piston to make up one cycle. During this time the crank shaft will have made two complete revolutions. This type of engine is said to operate on a four-stroke cycle. The four strokes of the piston are named according to what they accomplish. The first is the suction, the second the compression, the third the power, and the fourth the exhaust stroke.
At the start of the suction stroke, the piston is at the top of the cylinder. It moves downward and the inlet valve opens. This sucks in the charge of gases from the carburettor. The compres sion stroke starts when the inlet valve closes and the piston is moving upward. The mixture is ignited while under pressure by a high tension electric spark, which is produced across the points of the sparking plug, either by means of a battery and coil (see INDUCTION COIL), as in American cars, or by a high-tension magneto (see MAGNETO, HIGH-TENSION), as in British cars. The power stroke begins with a downward thrust of the piston. The burning of the gases gives off heat, which causes expansion. Pres sure is thus exerted on the piston, forcing it downward as far as it will go. This completes the power stroke. The last movement
of the piston is the exhaust stroke. The exhaust valve opens toward the end of the power stroke and most of the burned gases rush out. The piston moves upward, pushing the remaining gases out of the cylinder. The four-stroke cycle is now complete. (See INTERNAL COMBUSTION ENGINES for more extended treatment.) Fuel and Carburetion.—In order to be utilized by the en gine, the gasoline must first be mixed with air in the proper proportions. This is accomplished in the carburettor (q.v.), which meters the correct amount of fuel as a fine spray into the air being sucked into the intake manifold. The intake manifold distributes the air containing atomized fuel to the several cyl inders. During the atomizing of the fuel into the air stream, some of the small gasoline particles are evaporated. This evapora tion of gasoline causes a drop in temperature of the fuel charge which is counteracted by the manifold heater. Too little heat in the manifold will cause unequal distribution of fuel charge to the various cylinders, too much heat will cause excess evapora tion of the fuel particles. Since gasoline in atomized liquid form occupies much less space than it does when vaporized, excess evap oration of the fuel cuts down the total amount of air-fuel mix ture which can enter the cylinder during the suction stroke, thus decreasing the power output of the engine. It is also the duty of the carburettor to admit the correct amount of air needed to burn the fuel. This is important, for if too much air is pres ent the mixture will be too lean, resulting in a lack of power. The most common fault, however, is the use of a mixture that is too rich, i.e., one containing so much fuel that there is not enough air present to burn it completely. This happens when the choke valve (which cuts off the supply of air to the carburettor) is used too freely. Maximum power is developed when the 12.5 parts by weight of air are mixed with one part of fuel; maximum economy is obtained when the mixture is 14.o parts of air to one of fuel. At the latter mixture ratio, all the fuel is burned to car bon dioxide and water; very little carbon monoxide is present in the exhaust gas. However, exhaust gases are dangerous, as they may contain up to 7% of carbon monoxide with rich mixtures. As very small amounts of carbon monoxide may cause death, the engine should not run in an enclosed space.