Note: The following texts are for the master students who are preparing for the English state exam. The texts will be followed by reading comprehension and vocabulary exercises. Also will be added speaking and grammar tasks in the exam.
School of Mechanical Engineering Text 1
Lubrication of Motor Vehicle.
Friction causes heat and wear. In an engine, oil lubricates the moving parts and reduces the heat and the wear. The oil also collects any small particles of dirt or metal and carries them to the oil filter.
Some of the oil will leak out of the engine when it is used. The amount of oil in the engine will need checking regularly. The dipstick is used for checking the amount of oil. If there isn’t enough oil in the engine, friction between the moving parts will increase and the engine will quickly become damaged.
The oil in the engine will need changing about once every 5000 km. If it is not changed, it will become thin and full of impurities and it will not lubricate efficiently. The oil filter will also need changing regularly. If it is not changed, it will become blocked by particles of dirt and metal. If the filter becomes blocked, the oil will not flow around the engine and heat and water will increase very rapidly.
Some parts of a car need greasing-usually about once every six months. Fifty years ago cars needed greasing every week. Modern vehicles need much less greasing. They only need greasing about twice a year. Cars in the future will probably need no greasing.
In gas welding, it is necessary to use a mixture of two gases. To create a hot enough flame, a combustible gas must be mixed with oxygen. Although acetylene (C2H2) is normally used, the combustible gas need not be acetylene. Hydrogen or petroleum gases can also be used.
Oxygen can be stored at very high pressure. It is dangerous to compress gaseous acetylene in the same way and so it is dissolved under pressure in liquid acetone but at a much lower pressure than oxygen. To create a suitable flame, the gases must be supplied to the welding torch at low pressure. Pressure regulators are therefore used to regulate the gas flow from the cylinders. They are screwed into the top of each cylinder.
Gas welding is normally used to join steel to steel. To make a very strong joint, the work pieces must be composed of the same metal. Welding rods are used to provide filler metal. In gas welding, these rods are generally composed of steel. Bronze or brass rods may sometimes be used. When bronze and brass filler metal is used the process is called brazing.
To light the welding torch, the combustible gas must be turned on first. The oxygen must not be turned on before the flame is lit. The oxygen supply must be adjusted to give the correct flame.
The Instruments in a Car
All vehicles require certain instruments to provide information for the driver. For instance, every car has a speedometer to indicate its speed. It also has a fuel gauge to indicate the amount of fuel in the petrol tank. Many cars also have a tachometer to indicate the engine speed. They may also have an ammeter to indicate if the battery is charging or discharging.
The speedometer is indicating zero kph. The car is not moving. The engine is turning at minimum speed (approximately 750 rpm). As the engine is only turning slowly, the alternator is also turning slowly. It is not producing enough current for the engine. Therefore, the battery must supply some of the necessary current. The battery discharging and so the ammeter is indicating about -5A.
If the car is moving at 60kph, the engine is turning at 2500 rpm and so the alternator is turning quite fast. It is producing a strong current for the engine and so the battery is no longer needed to supply current. The battery is now recharging from the alternator and so the ammeter is indicating +10A. after a short time, the battery will be fully charged again.
If the car is moving at 90kph, the engine is turning at a speed of 4500 rpm. However, the alternator is not producing any current. The ammeter is indicating -20A. In other words, the battery is discharging rapidly although the engine is turning at high speed. Therefore, the alternator is not producing any power and the battery is discharging at 20A. So, unless the fault is put right or the engine stopped, the battery will soon become completely discharged. The electrical items, such as the headlights, should be switched off as soon as possible. When they are switched off and the engine is stopped the ammeter will read zero and the needle will point vertically.
A car battery can easily become discharged if there is an electrical fault in the car. If the fan belt is broken, for instance, the battery may become discharged in quite a short time. If the lights are left on while the car is not in use, the battery will also become discharged.
A battery (d.c.) cannot be recharged directly from the mains (a.c.). A battery charger is needed to rectify the a.c. to d.c. and to reduce the voltage to 12V. Before charging the battery, remove all the filler plugs. While battery is charging, hydrogen will be produced. This gas cannot escape easily from the battery if the filler plugs are not removed.
When connecting the crocodile clips to the battery, check the connections. The positive clip must be connected to the positive terminal and the negative clip to the negative terminal. Make sure the clips are connected before switching on the charger. After charging, switch off the charger before disconnecting the clips.
Charging started eight hours ago. During the first hour, the ammeter needle was indicating 5A. (the battery was being charged at the maximum rate). During the second and third hours, the ammeter was indicating about 4.5A. During the next two hours, the charging rate was decreasing more rapidly. After five hours, the rate was only 2A. After eight hours, the ammeter is now indicating 0.5A. The battery is almost fully charged. It will be fully charged in about an hour from now.
Energy Conversion Process
Fuel and oxygen are converted to heat energy by combustion. The heat created during the combustion process causes the gases trapped inside the cylinder to expand. This expansion causes a pressure build-up, which is then converted to mechanical energy as the expanding gases force a piston down the cylinder.
The up-and-down movement of the piston is converted to rotary motion by a connecting rod and crank. This is like the rider’s legs and the chain wheel of a bicycle.
The energy of motion and of vehicle movement is called kinetic energy. To stop a vehicle this energy must be converted to another form. The brakes do this by converting kinetic energy into heat. When all the kinetic energy has been converted the vehicle is stationary.
The internal combustion engine is not a very efficient energy converter, as it can not turn all the fuel into mechanical energy. Surprisingly, much of the fuel is wasted in the form of heat, either down the exhaust pipe with the waste gases, or absorbed by the cooling system and radiated to the atmosphere. Only some 25 percent of the chemical energy fed into the engine is used to drive the vehicle.
As the piston moves up and down the cylinder it must stop at the top and bottom every time it changes directions. These points are known as top dead-centre and bottom dead-centre. The distance travelled by the piston between top dead-centre and bottom dead-centre. The distance travelled by the piston between top dead-centre and bottom dead-centre is called the stroke. The diameter of the cylinder is called the bore.
Text 6 Crankshaft
The crankshaft of an engine is similar to the cranks on a bicycle. The rider’s feet push on the pedals, which turn the cranks mounted on the centre spindle, converting the up-and-down movement of the legs into rotary motion. In that way a pushing effort is converted into a turning force called torque.
The piston in an engine is connected to the crankshaft by the connecting rod. In much the same way as the bicycle, the up-and-down movement of the piston is converted into the rotary torque as the crankshaft. Because far greater forces are being converted, all the components are much stronger, and the crankshaft is supported in more bearings.
The crankshaft is normally a robust, one-piece alloy steel forging machined to very fine tolerances. Some manufacturers use steel alloys or cast iron containing copper, chromium and nickel. Cast iron crankshafts have proved to be very durable; they have good wearing properties and are less prone to fatigue than forged steel shafts. The journals may be hardened by processes such as nitriding or induction hardening.
The crankshaft has a number of identifiable parts:
Main bearing journal. Any part of the shaft that rotates in a bearing is called a journal. The main bearing journals support the shaft in the cylinder block.
Crank pin journal.This is the part of the crankshaft to which the connecting rod is attached, and is often called the big-end journal.
Crank radius. This is the term used to describe the offset from the main journals to the crank pin; it is like the length of the pedal crank on a cycle. In the same way as the cycle rider’s foot moves two crank lengths from highest to lowest position, so the piston moves an amount that is twice the crank offset. This is the stroke of an engine.
The webs. The big-end journals and the main bearing journals are held together by the webs, which may also incorporate counterbalance weights.
Fillet radius. A sharp corner in this position would create a weak spot, so a radius is provided to avoid any problems.
Crank throw.A single-cylinder engine has a single-throw crankshaft, while a four-cylinder engine would have a four-throw crankshaft. However, engines of ‘vee’ configuration often share big-end journals between two opposing cylinders.
Crankshaft throw. This describes how far the centre of the big-end journal is offset from the centre of the crankshaft main journal: the larger this measurement, the greater the turning force applied to the crankshaft while increasing the piston‘s effective stroke.
Internal oilways. To supply oil to the big-end journals the crankshaft has internal oilways drilled from the adjacent main bearing journal. Oil flows into main bearings from the oil gallery, and from there it is fed along the crankshaft oilways to each of the big-end bearings.
Other requirements. One end of the crankshaft forms a boss to which the flywheel is attacked. The other end usually has some form of key way machined into it to provide a positive drive for the timing gears, sprockets or pulleys. Pulleys for auxiliary drives may also need to be mounted there.
At both ends of the shaft some form of oil sealing must be used to prevent leakage from the revolving journals. The scroll or quick thread ‘screws’ the oil back towards the inside of the engine. At the same time, the centrifugal force of the spinning crankshaft forces oil to climb the thrower. When it reaches the edge it is thrown off into a channel and returns to the sump.
The cover at the timing gear end usually has a lipped seal, made from synthetic rubber stiffened by a metal shell. It has an inner lip that rubs on the crankshaft to stop oil leakage. Light contact is maintained by the use of a steel garter spring.
Text 7 Historic-atmospheric Engines.
Most of the very earliest internal combustion engines of the 17th and 18th centuries can be classified as atmospheric engines. These were large engines with a single piston and cylinder, the cylinder being open on the end. Combustion was initiated in the open cylinder using any of the various fuels which were available. Gunpowder was often used as the fuel. Immediately after combustion, the cylinder would be full of hot exhaust gas at atmospheric pressure. At this time, the cylinder end was closed and the trapped gas was allowed to cool. As the gas cooled, it created a vacuum within the cylinder. This caused a pressure differential across the piston, atmospheric pressure on one side and a vacuum on the other. As the piston moved because of this pressure differential, it would do work by being connected to an external system, such as raising a weight.
Some early steam engines also were atmospheric engines. Instead of combustion, the open cylinder was filled with hot steam. The end was then closed and the steam was allowed to cool and condense. This created the necessary vacuum.
In addition to a great amount of experimentation and development in Europe and the US during the middle and latter half of the 1800s, two other technological occurrences during this time stimulated the emergence of the internal combustion engine. In 1859, the discovery of crude oil in Pennsylvania finally made available the development of reliable fuels which could be used in these newly developed engines. Up to this time, the lack of good, consistent fuels was a major drawback in engine development. Fuels like whale oil, coal gas, mineral oils, coal, and gun powder which were available before this time were less than ideal for engine use and development. It still took many years before products of the petroleum industry evolved from the first crude oil to gasoline, the automobile fuel of the 20th century. However improved hydrocarbon products began to appear as early as the 1860s and gasoline, lubricating oils, and the internal combustion engine evolved together.
The second technological invention that stimulated the development of the internal combustion engine was the pneumatic rubber tire, which was first marketed by John B. Dunlop in 1888. This invention made the automobile much more practical and desirable and thus generated a large market for propulsion systems, including the internal combustion engine.
During the early years of the automobile, the internal combustion engine competed with electricity and steam engines as the basic means of propulsion. Early in the 20th century is the period of the internal combustion engine and the automobile powered by the internal combustion engine. Now, at the end of the century, the internal combustion engine is again being challenged by electricity and other forms of propulsion systems for automobiles and other applications. What goes around comes around.
During the second half of the 19th century, many different styles of internal combustion engines were built and tested. Engines operated with variable success and dependability using many different mechanical systems and engine cycles.
The first fairly practical engine was invented by J.J.E. Lenoir (1822-1900) and appeared on the scene about 1860. During the next decade, several hundred of these engines were built with power up to about 4.5 kW (6 hp) and mechanical efficiency up to 5%. In 1867 the Otto-Langen engine, with efficiency improved to about 11%, was first introduced, and several thousand of these were produced during the next decade. This was a type of atmospheric engine with the power stroke propelled by atmospheric pressure acting against a vacuum. Nicolaus A. Otto (1832-1891) and Eugen Langen (1833-1895) were two of many engine inventors of this period.
During this time, engines operating on the same basic four-stroke cycle as the modern automobile engine began to evolve as the best design. Although many people were working on four-stroke cycle design. Otto was given credit when his prototype engine was built in 1876.
In the 1880s the internal combustion engine first appeared in automobiles. Also, in this decade the two-stroke cycle engine became practical and was manufactured in large numbers.
By 1892, Rudolf Diesel (1858-1913) had perfected his compression ignition engine into basically the same diesel engine known today. This was after years of development work which included the use of solid fuel in his early experimental engines. Early compression ignition engines were noisy, large, slow, single-cylinder engines. They were, however, generally more efficient than spark ignition engines. It wasn’t until the 1920s that multicylinder compression ignition engines were made small enough to be used with automobiles and trucks.
Fundamental principles of generators
A generator is a machine that converts mechanical energy into electrical energy by the process of electromagnetic induction. In both d.c. and a.c. types of generator, the voltage induced is alternating; the major difference between them being in the method by which the electrical energy is collected and applied to the circuit externally connected to the generator. Plotting of the induced voltage throughout the full cycle produces the alternating or sine curve shown.
Generators are classified according to the method by which their magnetic circuits are energized, and the following three classes are normally recognized:
Permanent magnet generators.
Separately-excited generators, in which electromagnets are excited by current obtained from a separate source of d.c.
Self-excited generators, in which electromagnets are excited by current produced by the machines themselves. These generators are further classified by the manner in which the fixed windings, the electromagnetic field and armature windings, are interconnected.
In aircraft d.c. power supply systems, self-excited shunt-wound generators are employed and the following details are therefore related only to this type.
The Random House College Dictionary defines propulsion as “the act of propelling, the state of being propelled, a propelling force or impulse” and defines the verb propel as “to derive, or cause to move, forward or onward.” From these definitions, we can conclude that the study of propulsion includes the study of the propelling force, the motion caused, and the bodies involved. Propulsion involves an object to be propelled plus one or more additional bodies, called propellant.
The study of propulsion is concerned with vehicles such as automobiles, trains, ships, aircraft and spacecraft. Methods devised to produce a thrust force for the propulsion of a vehicle in flight are based on the principles of jet propulsion. The fluid may be the gas used by the engine itself (turbojet), it may be fluid available in the surrounding environment (air used by a propeller), or it may be stored in the vehicle and carried by it during the flight. (rocket).
Jet propulsion system can be subdivided into two broad categories: air-breathing and non-air-breathing. Air-breathing propulsion systems include the reciprocating, turbojet, turbofan, ramjet, turboprop, and turboshaft engines. Non-air-breathing engines include rocket motors, nuclear propulsion systems, and electric propulsion systems.