2020-07-12
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The internal combustion engine in which the combustion (or radip oxidation) of gas and air occurs in a confined space called a combustion chamber. This exothermic reaction of a fuel with an oxidizer creates gases of high temperature and pressure, which are permitted to expand.
The first internal combustion engines did not have compression, but ran on air/fuel mixture sucked or blown in. The most significant distinction between modem internal combustion engines and the early designs is the use of compression and in particular of in-cylinder compression. The term Internal Combustion Engine (ICE) is almost always used to refer specifically to reciprocating engines and similar designs in which combustion is intermittent. However, continuous combustion engines, such as jet engines, most rockets and many gas turbines are also internal combustion engines.
For a typical four-stroke engine, key parts of the engine include the combustion chamber, one or more camshafts, cams and intake and exhaust valves. There are one or more cylinders and for each cylinder there is a spark plug, a piston and a crankshaft. The defining feature of an ICE is that useful work is performed by the expanding hot gases acting directly to cause pressure, further causing movement of the piston inside the cylinder. A single sweep of the cylinder by the piston in an upward or downward motion is known as a stroke. If there are four movements, or strokes, of the piston before the entire engine firing sequence is repeated, we have a typical four-stroke cycle, or Otto cycle.
The cycle begins with the intake stroke as the piston is pulled downward towards the crankshaft. The intake valve is open, and fuel and air are drawn past the valve and into the combustion chamber and cylinder from the intake manifold. At the end of the intake stroke, the piston begins to move back (upward). The cylinder and combustion chamber are full of the low pressure fuel-air mixture and, as the piston begins to move, the intake valve closes. Cams and rocker arms provide control and timing of the valves’ opening and closing.
As the piston is pushed downward again, the volume is reduced and the fuel-air mixture is compressed during the compression stroke. During the compression, no heat is transferred to the fuel-air mixture. As the v olume is decreased because of the piston’s motion, the pressure in the gas is increased. When the volume is the smallest, and the pressure the highest, the contact is closed, and a current of electricity flows through the plug.
At the beginning of the power stroke the spark plug produces a spark in the combustion chamber which ignites the fuel-air mixture. Rapid combustion of the fuel releases heat and produces exhaust gases. Because the intake and exhaust valves are closed, the combustion of the fuel takes place in a totally enclosed and nearly constant volume vessel. The combustion increases the temperature of the exhaust gases, any residual air in the combustion chamber, and the combustion chamber itself.
The high pressure of the gases acting on the face of the piston cause the piston to move downward which initiates the power stroke. Unlike the compression stroke, the hot gas does work on the piston during the power stroke. The force on the piston is transmitted by the piston rod to the crankshaft, where the linear motion of the piston is converted to angular motion of the crankshaft. During the power stroke, the volume occupied by the gases is increased because of the piston’s motion and no heat is transferred to the fuel-air mixture. As the volume is increased, the pressure and temperature of the gas are decreased. Heat that is now transferred to the water in the water jacket until the pressure approaches atmospheric pressure. The exhaust valve is then opened by the cam pushing on the rocker arm to begin the exhaust stroke.
The purpose of the exhaust stroke is to clear the cylinder of the spent exhaust in preparation for another ignition cycle. As the exhaust stroke begins, the cylinder and combustion chamber are full of exhaust products at low pressure. As the piston moves upward, the exhaust gas is pushed past the open exhaust valve and exits the engine. At the end of the exhaust stroke, the exhaust valve is closed and the engine begins another intake stroke.
Internal combustion engines can contain any number of cylinders, with numbers between one and twelve being common. Most car engines have four to eight cylinders, with some high performance cars having ten, twelve, or even sixteen, and some very small cars and trucks having two or three. Having more cylinders in an engine yields two potential benefits. First, the engine can have a larger displacement with smaller individual reciprocating masses (that is, the mass of each piston can be less) thus making a smoother running engine since the latter tends to vibrate as a result of the pistons moving up and down. Second, with a greater displacement and more pistons, more fuel can be combusted and there can be more power strokes in a given period of time, meaning that such an engine can generate more torque than a similar engine with fewer cylinders.
The down side to having more pistons is that the engine will tend to weigh more and tend to generate more internal friction as the greater number of pistons rub against the inside of their cylinders. This tends to decrease fuel efficiency and deprive the engine of some of its power. For high performance engines using modern materials and technology there seems to be a break point around 10 or 12 cylinders, after which addition of cylinders becomes an overall detriment to performance and efficiency, although exceptions such as the W16 engine from Volkswagen exist.
