Internal Combustion Engine
Structure Of The Internal Combustion Engine
Internal combustion engines generally employ reciprocating motion, although gas turbine, rocket, and rotary engines are examples of other types of internal combustion engines. Reciprocating internal combustion engines are the most common, however, and are found in most cars, trucks, motorcycles, and other engine-driven machines.
The most basic components of the internal combustion engine are the cylinder, the piston, and the crankshaft. To these are attached other components that increase the efficiency of the reciprocating motion and convert that motion to the rotary motion of the crankshaft. Fuel must be provided into the cylinder, and the exhaust, formed by the explosion of the fuel, must be provided a way out of the cylinder. The ignition, or lighting, of the fuel must also be produced. In the reciprocating internal combustion engine, this is done in one of two ways.
Diesel engines are also called compression engines because they use compression to cause the fuel to self-ignite. Air is compressed, that is, pushed into a small space, in the cylinder. Compression causes the air to heat up; when fuel is introduced to the hot, compressed air, the fuel explodes. The pressure created by compression requires diesel engines to be more strongly constructed, and thus, heavier than gasoline engines, but they are more powerful, and require a less costly fuel. Diesel engines are generally found in large vehicles, such as trucks and heavy construction equipment, or in stationary machines.
Gasoline engines are also called spark ignition engines because they depend on a spark of electricity to cause the explosion of fuel within the cylinder. Lighter than a diesel engine, the gas engine requires a more highly refined fuel.
In an engine, the cylinder is housed inside a engine block strong enough to contain the explosions of fuel. Inside the cylinder is a piston which fits the cylinder precisely. Pistons generally are dome-shaped on top, and hollow at the bottom. The piston is attached, via a connecting rod fitted in the hollow bottom, to a crankshaft, which converts the up and down movement of the piston to a circular motion. This is possible because the crankshaft is not straight, but has a bent section (one for every cylinder) called a crank.
A similar structure propels a bicycle. When bicycling, the upper part of a person's leg is akin to the piston. From the knee to the foot, the leg acts as a connecting rod, which is attached to the crankshaft by the crank, or the bicycle's pedal assembly. When power is applied with the upper leg, these parts are made to move. The reciprocating motion of the lower leg is converted to the rotary, or spinning, motion of the crankshaft.
Notice that when bicycling, the leg makes two movements, one down and one up, to complete the pedaling cycle. These are called strokes. Because an engine also needs to draw fuel in and expel the fuel out again, most engines employ four strokes for each cycle the piston makes. The first stroke begins when the piston is at the top of the cylinder, called the cylinder head. As it is drawn down, it creates a vacuum in the cylinder. This is because the piston and the cylinder form an airtight space. When the piston is pulled down, it causes the space between it and the cylinder head to become larger, while the amount of air remains the same. This vacuum helps to take the fuel into the cylinder, much like the action of the lungs. This stroke is therefore called the intake stroke.
The next stroke, called the compression stroke, occurs when the piston is pushed up again inside the cylinder, squeezing, or compressing the fuel into a tighter and tighter space. The compression of the fuel against the top of the cylinder causes the air to heat up, which also heats the fuel. Compressing the fuel also makes it easier to ignite, and makes the resulting explosion more powerful. There is less space for the expanding gases of the explosion to flow, which means they will push harder against the piston in order to escape.
At the top of the compression stroke, the fuel is ignited, causing an explosion that pushes the piston down. This stroke is called the power stroke, and this is the stroke that turns the crankshaft. The final stroke, the exhaust stroke, takes the piston upward again, which expels the exhaust gases created by the explosion from the cylinder through an exhaust valve. These four strokes are also commonly called "suck, squeeze, bang, and blow." Two-stroke engines eliminate the intake and exhaust strokes, combining them with the compression and power strokes. This allows for a lighter, more powerful engine—relative to the engine's size—requiring a less complex design. But the two-stroke cycle is a less efficient method of burning fuel. A residue of unburned fuel remains inside the cylinder, which impedes combustion. The two-stroke engine also ignites its fuel twice as often as a four-stroke engine, which increases the wear on the engine's parts. Two-stroke engines are therefore used mostly where a smaller engine is required, such as on some motorcycles, and with small tools.
Combustion requires the presence of oxygen, so fuel must be mixed with air in order for it to ignite. Diesel engines introduce the fuel directly to react with the hot air inside the cylinder. Spark-ignition engines, however, first mix the fuel with air outside the cylinder. This is done either through a carburetor or through a fuel-injection system. Both devices vaporize the gasoline and mix it with air at a ratio of around 14 parts of air to every one part of gasoline. A choke valve in the carburetor controls the amount of air to be mixed with the fuel; at the other end, a throttle valve controls how much of the fuel mixture will be sent to the cylinder.
The vacuum created as the piston moves down through the cylinder pulls the fuel into the cylinder. The piston must fit precisely inside the cylinder in order to create this vacuum. Rubber compression rings fitted into grooves in the piston make certain of an airtight fit. The gasoline enters the cylinder through an intake valve. The gasoline is then compressed up into the cylinder by the next movement of the piston, awaiting ignition.
An internal combustion engine can have anywhere from one to twelve or more cylinders, all acting together in a precisely timed sequence to drive the crankshaft. The bicyclist on a bicycle can be described as a two-cylinder engine, each leg assisting the other in creating the power to drive the bicycle, and in pulling each other through the cycle of strokes. Automobiles generally have four-, six-, or eight-cylinder engines, although two-cylinder and twelve-cylinder engines are also available. The number of cylinders affects the engine's displacement, that is, the total volume of fuel passed through the cylinders. A larger displacement allows more fuel to be burned, creating more energy to drive the crankshaft.
Spark is introduced through a spark plug placed in the cylinder head. The spark causes the gasoline to explode. Spark plugs contain two metal ends, called electrodes, which extend down into the cylinder. Each cylinder has its own spark plug. When electric current is passed through the spark plug, the current jumps from one electrode to the other, creating the spark.
This electric current originates in a battery. The battery's current is not, however, strong enough to create the spark needed to ignite the fuel. It is therefore passed through a transformer, which greatly amplifies its voltage, or strength. The current can then be sent to the spark plug.
In the case of an engine with two or more cylinders, however, the spark must be directed to each cylinder in turn. The sequence of firing the cylinders must be timed so that while one piston is in its power stroke, another piston is in its compression stroke. In this way, the force exerted on the crankshaft can be kept constant, allowing the engine to run smoothly. The number of cylinders affects the smoothness of the engine's operation; the more cylinders, the more constant the force on the crankshaft, and the more smoothly the engine will run.
The timing of the firing of the cylinders is controlled by the distributor. When the current enters the distributor, it is sent through to the spark plugs through leads, one for each spark plug. Mechanical distributors are essentially spinning rotors that send current into each lead in turn. Electronic ignition systems utilize computer components to perform this task.
The smallest engines use a battery, which, when drained, is simply replaced. Most engines, however, have provisions for recharging the battery, utilizing the motion of the spinning crankshaft to generate current back to the battery.
The piston or pistons push down and pull up on the crankshaft, causing it to spin. This conversion from the reciprocating motion of the piston to the rotary motion of the crankshaft is possible because for each piston the crankshaft has a crank, that is, a section set at an angle to the up-and-down movement of the position. On a crankshaft with two or more cylinders, these cranks are set at angles to each other as well, allowing them to act in concert. When one piston is pushing its crank down, a second crank is pushing its piston up.
A large metal wheel-like device called a flywheel is attached to one end of the crankshaft. It functions to keep the movement of the crankshaft constant. This is necessary on a four-stroke engine because the pistons perform a power stroke only once for every four strokes. A flywheel provides the momentum to carry the crankshaft through its movement until it receives the next power stroke. It does this by using inertia, that is, the principle that an object in motion will tend to stay in motion. Once the flywheel is set in motion by the turning of the crankshaft, it will continue to move, and turn the crankshaft. The more cylinders an engine has, however, the less it will need to rely on the movement of a fly-wheel, because the greater number of pistons will keep the crankshaft spinning.
Once the crankshaft is spinning, its movement can be adapted to a great variety of uses, by attaching gears, belts, or other devices. Wheels can be made to turn, propellers can be made to spin, or the engine can be used simply to generate electricity. Also geared to the crankshaft is an additional shaft, called the camshaft, which operates to open and close the intake and exhaust valves of each cylinder in sequence with the four-stroke cycle of the pistons. A cam is a wheel that is more or less shaped like an egg, with a long end and a short end. Several cams are fastened to the camshaft, depending on the number of cylinders the engine has. Set on top of the cams are push-rods, two for each cylinder, which open and close the valves. As the camshaft spins, the short ends allow the push-rods to draw back from the valve, causing the valve to open; the long ends of the cams push the rods back toward the valve, closing it again. In some engines, called overhead cam engines, the camshaft rests directly on the valves, eliminating the need for the push-rod assembly. Two-stroke engines, because the intake and exhaust is achieved by the movement of the piston over ports, or holes, in the cylinder wall, do not require the camshaft.
Two more components may be operated by the crankshaft: the cooling and lubrication systems. The explosion of fuel creates intense heat that would quickly cause the engine to overheat and even melt if not properly dissipated, or drawn away. Cooling is achieved in two ways, through a cooling system and, to a lesser extent, through the lubrication system.
There are two types of cooling systems. A liquid-cooling system uses water, which is often mixed with an antifreeze to prevent freezing. Antifreeze lowers the freezing point and also raises the boiling point of water. The water, which is very good at gathering heat, is pumped around the engine through a series of passageways contained in a jacket. The water then circulates into a radiator, which contains many tubes and thin metal plates that increase the water's surface area. A fan attached the radiator passes air over the tubing, further reducing the water temperature. Both the pump and the fan are operated by the crankshaft's movement.
Air-cooled systems use air, rather than water, to draw heat from the engine. Most motorcycles, many small airplanes, and other machines where a great deal of wind is produced by their movement, use air-cooled systems. In these, metal fins are attached to the outside of the cylinders, creating a large surface area; as air passes over the fins, the heat conducted to the metal fins from the cylinder is swept away by the air.
The lubrication of an engine is vital to its operation. The movement of parts against each other cause a great deal of friction, which raises heat and causes the parts to wear. Lubricants, such as oil, provide a thin layer between the moving parts. The passage of oil through the engine also helps to carry away some of the heat produced.
The crankshaft at the bottom of the engine rests in a crankcase. This may be filled with oil, or a separate oil pan beneath the crankcase serves as a reservoir for the oil. A pump carries the oil through passageways and holes to the different parts of the engine. The piston is also fitted with rubber oil rings, in addition to the compression rings, to carry oil up and down the inside of the cylinder. Two-stroke engines use oil as part of their fuel mixture, providing the lubrication for the engine and eliminating the need for a separate system.
Schuster, William A. Small Engine Technology. Delmar Publishers, Inc., 1993.
Stone, Richard. Introduction to Internal Combustion Engines. Society of Automotive Engineers, Inc., 1994.
M. L. Cohen