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Fundamental Element of Mechanics

The motion of a body occurs in space, therefore space is one of the fundamental elements in mechanics. Time is likewise such an element in mechanics.

Every point of a moving body describes a path of definite form relative to a basic system, this path is called a trajectory. A trajectory may be either of straight or curved lines, in accordance with which the motion of a point is then described either as rectilinear or curvilinear, A moving point traverses a definite distance, the length of which as covered in a definite interval of time will depend upon the speed of the moving point. If the point travels equal distances in equal intervals of time, its speed will be constant and its motion is then said to be uniform. In other cases the motion is said to be non-uniform, or variable

If speed changes at an equal rate in equal intervals of time, the motion is said to be either uniformly accelerated or uniformly retarded, a change in speed is called acceleration.

In investigating mechanical motion of bodies and their state of rest (as a particular case of motion, another quantity is met with which determines the action of one body upon another; that quantity is called force.

All these elements will be dealt with in detail along with others pertaining to mechanics, as we proceed.

 

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Mechanical Motion

There are a great many forms of motion. An incalculable number of bodies are in motion on the earth, which in its turn is rotating about its axis and also travelling around the sun, while the sun itself and all its planets are in movement relative to the stars, which in their turn are also moving through space. But in all these instances we have to do with only one form of motion - the motion of bodies themselves. Science has established that heat light, electricity, and chemical and many other phenomena are also forms of motion. Furthermore life itself in all its manifestations is form of motion.

The perpetual movement of matter causes all natural phenomena about us.

Motion occurs in space and in time, therefore space and time are inseparable from matter in motion. When a body changes its position in respect to other bodies, we say it is in motion. Thus relative change in position of a body is called "mechanical motion".

The science dealing with the laws of mechanical motion is called "mechanics".

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Carburettor Engine-

1 fan belt ; 2 generator ; 3 fan ; 4 piston pin ; 5 small end of connecting rod ; 6 water jacket ; 7 spark plug ; 8 cylinder ; 9 piston ; 10 poppet-valve ; 11 exhaust manifold ; 12 inlet manifold ; 13 flywheel ; 14 main bearing ; 15 oil pan ; 16 crankshaft ; 17 camshaft ; 18 cam ; 19 tappet ; 20 big end of connecting rod ; 21 main bearing ; 22 journal ()

The small end of the connecting rod is rotatably connected to the piston by the piston pin. The big end of the connecting rod is also rotatably connected with the crankshaft. The flywheel is located on the rear end of the crankshaft. It is equipped with a toothed rim for meshing with the starter pinion. Piston with piston pin, connecting rod and crankshaft with flywheel are called crank-drive assembly.

Inlet of the fuel-air mixture into the cylinder and outlet of the burned gases are controlled by valves. The valves are actuated by the camshaft generally supported in the crankcase. Valves, camshaft and camshaft drive are forming the control mechanism of the engine.

The crankcase bottom is closed by the oil pan holding the oil required for lubrication. The oil pan is connected to the crankcase.

The liquid fuel is mixed with air and vaporized in the carburettor. This fuel-air mixture enters cylinder and combustion chamber. Inside the combustion chamber, the fuel-air mixture is ignited and burned by a spark jumping at the electrodes of a spark plug.

The generator is located at and driven by the engine. When the engine is running at sufficient speed, the generator supplies the ignition current and charges the storage battery.

The starting motor is a small electric motor. When it is actuated, a driving pinion is shifted and meshes with the toothed rim of the flywheel. Thus the crank-drive assembly begins cranking (working).

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Fundamental Principles

All theorems and equations in statics are deduced from a few fundamental principles, which, are accepted without mathematical roof, and are known as the principles, or axioms of statics. The principles of statics represent general formulations obtained as a result of a vast number of experiments with and observations of the equilibrium and motion of bodies and which, furthermore, have been consistently confirmed by actual experience. Some of these principles are corollaries of the fundamental laws of mechanics, which will be examined in the course of dynamics. 1st Principle. A free rigid body subjected to the action of two forces can be in equilibrium if, and only if, the two are equal in magnitude collinear, and opposite in direction.

The first principle defines the simplest balanced force system, since we know from experience that a free body subjected to the action of a single force cannot be in equilibrium.

2nd Principle. The action of a given force system on a rigid body remains unchanged if another balanced force system is added to, or subtracted from, the original system.

This principle establishes that two force systems differing from each other by a balanced system are equivalent. Corollary of the 1st and 2nd Principles. The point of application of a force acting on a rigid body can be transferred to any other point on line of action of the force without altering its effect. This is known as the principle of transmissibility.

3rd Principle (the Parallelogram law). Two forces applied at one point of a body have as their resultant a force applied at the same point and represented by the diagonal of a parallelogram constructed with the two given forces as its sides.

4th Principle. To any action of one material body on another there is always an equal and opposite reaction, or, reaction is always equal to action.

5th Principle. (Principle of Solidification). If a freely deformable body subjected to the action of a force system is in equilibrium, the state of equilibrium will not be disturbed if the body solidifies (becomes rigid).

 

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A Diesel Engine

A Diesel engine is a machine which produces power by burning oil in air which has been quickly compressed to a high pressure by a piston moving in a closed cylinder. The piston is at its lowest position, about to start upwards on its compression stroke. As it moves upwards it compresses the air above it into a smaller and smaller volume. If the final volume of the air is one twenty-fifth of its original volume, the engine is said to have a compression ratio of 25.

Now the piston is in its highest position. Here the air in the cylinder is highly compressed and is also very hot. (Have you not noticed how your bicycle-pump gets hot when you pump up your tyres?) At this stage of the cycle of operation oil is injected into the hot air. The oil ignites instantaneously...The heat of its combustion raises the temperature of the air still higher and so the pressure of the enclosed air is very much increased also.

The piston is therefore forced down again violently. This second stroke of the piston is called the working-stroke.

After reaching the bottom of its working-stroke the piston begins to rise again. Its purpose now is to sweep the products of combustion from the cylinder. So a valve is opened to let the spent gases escape to the exhaust pipe as the piston rises to the top of its third stroke.

The fourth stroke of the piston is downwards again. The exhaust valve is closed and an inlet valve is opened to allow fresh air to refill the cylinder. After four strokes, two up and two down, the cylinder is again in the condition we begin with. We say that it has completed its four-stroke cycle.

A Diesel engine usually has four cylinders which fire in turn. The four pistons are connected to a rotating crankshaft so that the working stroke of one piston is always available to drive the three simultaneously non-working-strokes of the other pistons. The remaining energy of the working-stroke is available for the external work the Diesel engine has to do.

 

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