THE FOUR-STROKE-CYCLE ENGINE

Prime Movers [43]. We have what are called prime movers doing a lot of useful work for us. For instance, there is the windmill which takes advantage of the energy of moving air to pump water. There is the hydraulic turbine which uses the velocity [44] and consequent energy of moving water to create power. The water wheel uses the energy of falling water; the steam engine, the energy of expanding steam. These are examples of physical changes adding to man’s source of power.

External Combustion Engine. Another important example of a prime mover is the heat engine. Certain organic compounds which contain carbon are caused to burn and give off heat. This heat furnishes power in one of two ways. In the external type of heat engine, the fuel is burned in a chamber or firebox; the heat is applied to some liquid such as mercury [45] or water which, in turn, is piped to the engine as it changes from a liquid to a gas. The heat has caused the molecules of the liquid to separate so far from each other that they form a vapor which may be considered as a gas. The enormous energy of the moving molecules of the gas creates a tremendous pressure against a reciprocating [46] piston or against the blades of a turbine. However, it must be noted that the fuel is burned on the outside of the engine itself; it is, therefore, of the external combustion type. The steam engine and steam turbine are good examples. The steam engine has been inefficient in the conversion of heat into the energy of movement. The average steam engine of the reciprocating type has only been able to convert about 5 per cent of the energy of the fuel into, useful work. That is one of the reasons why the diesel engine with a comparable efficiency of approximately 30 per cent has been displacing the steam locomotive engine.

The Internal Combustion Engine. The most important type of prime mover is the internal combustion engine. Most of the engines of this type use liquids as their fuel, although some burn gas from manufactured or natural sources. The two most common fuels used are gasoline and a less volatile [47] fuel called diesel or fuel oil.

Engine Principles. In the internal combustion piston engine, the fuel is forced into the space between the cylinder head and the piston which is usually called the combustion chamber or space. The size of this space is variable, that is it depends upon the position of the piston. If the piston is at the head end (HEDC [48]), the space is small. If the fuel is in this space and is ignited, it will tend, to expand. As the head is securely fastened to the block and as the cylinder walls are either in the block or are a part of the block, the only object that can move is the piston. This it does, going as far as it can toward the crankcase end (CEDC [49]).Thus the internally burning fuel causes expansion, and movement in the internal combustion engine.

Engine Types. There are several types of such engines, depending upon cycle, location of parts, and other items. A partial designation of engine types could be as follows:

1. Type of fuel burned - gasoline, kerosene, fuel oils, and gases.

2. Method of cooling - liquid or gas (air).

3. Arrangement of cylinders.

4. Valve location.

5. Cycle - four-stroke and two-stroke.

6. Type of valves.

7. Ignition - spark or compression.

8. Moving parts - piston or blade (turbine).

9. Compression - low, medium, high.

One of the most important engine designations is by the cycle used. The usual outboard boat engine [50] is of the two-stroke cycle. The usual tractor engine and all known automobile engines are of the four-stroke type.

The four-stroke-cycle engine is sometimes improperly called the four-cycle engine. This is not a correct technical designation. It is simply an abbreviation of the longer descriptive term; it does not indicate that there are four piston strokes per cycle.

Four-stroke-cycle Engine. In order to simplify the explanation of the operation of the four-stroke engine, one of the gasoline-burning type will be used. This is the usual type found on garden tractors, lawn mowers, and most wheeled farm tractors.

Cycle Processes. By necessity we must start with one of the four events. The most logical one for beginning is the intake of the fuel. As the piston moves down (in the vertical engine), there is a tendency for a vacuum to form in the cylinder space. This is caused by the expansion of the gas (air only, for our beginning stroke). For instance, if one volume of air occupies six times as much space, say in an engine with a compression ratio of 6: 1, there should be about one-sixth the absolute atmospheric pressure, according to Boyle’s gas law. This means that the pressure on the outside is much greater than on the inside. If now the intake valve is opened, there will be a rush of air into the cylinder space. If by carburetion some fuel is mixed with air, we have a combustible mixture in the engine. If the intake valve now closes, the mixture is trapped. If this mixture is ignited at this point, no work wi11 be done as the piston is down as far as it can go. Also the fuel would not have much force as the molecules of fuel and oxygen (in the air) are quite widely separated. Therefore the piston is moved to the top of the cylinder by moving the flywheel [51] or hand crank [52]. This will compress the fuel-air mixture into approximately one-sixth of its former volume. The mixture will now be under six times the pressure it was formerly, more if the temperature of the mixture rose during compression. If both valves are left closed and the mixture is ignited, the trapped and compressed gases will exert a tremendous force while burning. This force will cause the piston to move to the crank end of the cylinder, thus causing motion and doing work with heat energy.

To complete the cycle, the exhaust valve is opened when the power of the rapid combustion is mostly spent, and, as the piston moves toward the head, the exhaust gases are pushed from the combustion chamber. Energy stored in the flywheel as momentum provides the power to exhaust the products of combustion, to provide the engine with another charge of fuel and air, and to compress the mixture ready for ignition and another power stroke.

Briefly the events are as follows: intake, one valve open; com­pression, both valves closed; burning or combustion, both valves closed; exhaust, one valve open. Thus one cycle of four events with four strokes of the piston has been completed (ignition could be a fifth event).

Вариант №5