Working Principle of an Engine
When a suitable fuel in gas or vapour form is mixed with air in a proportion, a combustible mixture and is formed and is then ignited in a confined space. The heat generated by combustion will cause the gases to expand. If the combustion takes place in a straight cylinder containing a well-fitted piston then the piston is forced down the cylinder. By connecting the piston to a crankshaft, the up and down (reciprocating) motion of the piston is changed into rotary motion. To enable the engine to operate continuously, provision must be made to ensure that the following operation continues satisfactorily as long as the engine is required to run:
(a) Air and fuel must be supplied in the correct proportion to the cylinder and at correct time in relation to the piston position.
(b) Combustion must occur at the correct time so that maximum advantage can be taken of the expanding gases, i.e., when the piston is moving down. Otherwise effort would be wasted in acting against the piston on the up stroke.
(c) The hot gases must be expelled from the cylinder immediately after their usefulness is ended.
It is the four-stroke cycle of operation, which produces the power within the cylinder of an internal combustion engine. It is so called after Dr. Nicolas Otto (1833-1891), a German Engineer, who introduced it in his silent gas engine in 1876. Beaude Rochas, a French scientist in 1862, however, first suggested this four-stroke cycle.
Four Stroke Cycle. In modern passenger cars, the cycle is extended through four stroke of the piston or two revolution of the crankshaft. In the four-stroke cycle engine, the four strokes are named – Induction, Compression, Power and Exhaust in accordance with the operations of the cycle which occur during each particular stroke. In every cylinder there is a piston secured in the crankshaft by a connecting rod. As the crankshaft rotates the piston is moved up and down in the cylinder. In each cylinder two valves are arranged one as inlet and the other exhaust valve and they are geared to the crankshaft through a camshaft. The cams on the camshaft are so arranged that the valves open and close at certain intervals. Each cylinder is fitted with sparking plugs and are separately connected by rubber-covered copper wire cables to the magneto or coil and opens in combustion chamber. Other ports or opening is controlled by the exhaust valve; lead the exhaust manifold to the silencer and exhaust pipe. The ignition system may be either high-tension (High voltage) magneto or coil but whichever system is employed it is driven by the camshaft at half of crankshaft speed.
Four Stroke Cycle Operation
(a) Suction Stroke. During suction stroke, the piston is moved downward by the crankshaft, which is revolved either by the momentum of the flywheel or by the power generated by the electric starting motor. The inlet valve remains open and the exhaust valve is closed during this stroke. The downward movement of the piston sucks air-fuel mixture in the cylinder from the carburetor through the open inlet valve. Here the fuel petrol is mixed with air, broken up into a mist, and partially vaporized in the carburetor.
(b) Compression Stroke. During compression stroke, the piston moves upward thus compressing the charge. The heat produced by the compression makes more homogeneous mixture of air and petrol inside the cylinder. The heat makes the petrol easier to burn, while the compression forces it into closer combination with the air. The mixture under compression is ignited by the spark produced by a spark plug. Both the inlet and exhaust valves remain closed during the compression stroke.
(c) Power Stroke. Sparking takes place from the spark plug which ignites the compressed charge. The expansion of the gases due to the heat of combustion exerts a pressure on the cylinder and piston. Under this impulse the piston moves downward thus doing useful work. Both the valves remain closed during this stroke.
(d) Exhaust Stroke. During this stroke, the inlet valve remains closed and the exhaust opens. The greater part of burnt gases escapes because of their own expansion. The piston moves upward and pushes the remaining gases out of the open exhaust valve. Only the small quantity of exhaust gases remains in the clearance space, which will dilute the fresh incoming charge. Thus, in this type of engine four strokes of the piston are required to complete the cycle, and the four strokes make two revolution of the crankshaft. The operations are repeated over and over again in running the engine. Each alternative revolution of the crankshaft has one power stroke.
Working Principle Two Stroke
(a) The two-stroke engine is much simpler in operation than the four stroke engine. The two-stroke engine of the present day was first developed by Sir Dugaid Clerk In 1881. The two-stroke is so named because it has a power stroke to every two strokes of piston or every one revolution of the crankshaft. It is remarkably simple in action and is sometimes referred to as valve less engine, because it has no valves. The piston carries out the function of inlet and exhaust valves. The essential parts are an intake port or opening connected to the carburetor, an exhaust port immediately above it and a transfer port or passage connected with the crankcase.
(b) In two stroke cycle engines, the suction and exhaust strokes are eliminated. There are only two remaining strokes, the compression and power strokes and these are usually called upwards stroke and downward stroke respectively. As a power stroke occurs every time the piston reaches TDC, the magneto must be driven at the same speed as the crankshaft.
(c) The compression charge is ignited and at the same time a fresh charge from the carburetor enters the crankcase. Again the piston descends and when reaching the bottom of its stroke the burnt gases is expelled and a fresh charge from the crankcase is transferred. It will be noted that the transfer and exhaust ports are both open at the same time and in order to prevent the fresh charge rushing out through the exhaust port, the top of the piston is specially shaped to deflect and separate the gas. Each downward stroke, of the piston is both a power and an exhaust stroke, and each upward stroke is a compression and induction stroke.
Operation of Upward Stroke
During upward stroke the piston moves upwards from bottom dead centre (BDC) to top dead centre (TDC), compressing the charge (air/petrol mixture) in combustion chamber of the cylinder. Due to upward movement of the piston, a partial vacuum is created in the crankcase and a new charge is drawn into the crankcase through the uncovered inlet port. The exhaust port and the transferred port are covered when the piston is at TDC position. The compressed charge is ignited in the combustion chamber by a spark given by the spark plug.
Operation of Downward Stroke
As soon as the charge is ignited the hot gases compress the piston, which moves downward, rotating the crankshaft thus doing the useful work. During this stroke the inlet port is covered by the piston and the new charge is compressed in the crankcase further downward movement of the piston uncovers first the exhaust port and then the transfer port and hence the exhaust starts first through the exhaust port. As soon as the transfer port opens, the charge through it is forced into the cylinder. The charge strikes the deflector on the piston crown, rises to the top of the cylinder and pushes out most of the exhaust gases. The piston is now at BDC position. The cylinder is completely filled with the fresh charge, although it is somewhat diluted with the exhaust gases. The cycle of events is then repeated, the piston making two strokes for each revolution of the crankshaft that theoretically the inlet valve opens exactly at the beginning of suction stroke and closer at the end of the stroke. Both valves remain closed during the compression and power strokes. The exhaust valve opens exactly at the beginning of exhaust stroke and closes at the end of stroke.
Modified otto-cycle is the modification of Otto-cycle of Four Stroke engines to ensure that the maximum possible charge can be induced during induction stroke and maximum scavenging of burnt gases can be obtained during the exhaust stroke. By this arrangement the maximum possible output can be obtained from an engine.