Operating Principles Of Four-Stroke Engines
The Four Strokes In Internal Combustion Engines
The internal combustion engine in most cars works on the four-stroke engine working principle. This means that to produce one pulse of power the piston must travel up and down the cylinder four times. Each stroke of the piston performs a separate function in the cycle as follows:
INDUCTION STROKE: the stroke begins the engine combustion process. In it, the inlet valve is open and rotation of the crankshaft is moving the piston down the cylinder, sucking in a mixture of fuel and air which travels from the carburetor along the inlet manifold and past the open valve.
COMPRESSION STROKE: in this stroke which is regarded as the next stage of operation in the combustion chamber, both valves are shut and the rotating crankshaft now raises the piston, compressing the mixture above it into the combustion area.
POWER STROKE: this is the third stage in the entire stroke, in it; both valves remain shut and a spark from the plug jumping across the electrodes of the plug will set the compressed air-fuel mixture alight in the very close range between the top of the piston and the base of the spark plug.
This will lead to a spontaneous explosion due to the burning of the gaseous fuels in a highly pressurized and hot temperature condition.
The whole action will lead to the tremendous release of power. The power released will force the piston to go down immediately, with such force the piston can turn the crankshaft to its half-turn while the upward rising of the piston will prepare the combustion chamber for exhaust stroke.
EXHAUST STROKE: this is the last stroke of the four-stroke engines, in this stroke; the burnt air-fuel mixture had formed the smoke (carbon monoxide) and will be expelled from the chamber to prepare it for the next recycling of strokes.
The exhaust valve will open as the piston starts going up for the last stroke, during the period all the smoke in the chamber will be forced to enter into the exhaust channels making the engine ready for the next stage of the stroke which is sucking of fuel and air as the piston starts going done.
In that stage, the exhaust valve will close again allowing the fuel and air to move into the chamber.
THE COMPRESSION RATIO OF THE IC ENGINE
The power that an IC engine develops depends on how much energy can be released above the piston at each power stroke. This in turn depends on the n quantity of fuel/air in the cylinder and the efficiency with which it is compressed.
The amount that the mixture is squeezed up is known as the compression ratio. It is the difference between volume of the mixture in the cylinder when the piston is at the bottom of its stroke, and the volume when the piston is at its highest position.
If the upward movement of the piston reduces the mixture to one-eight of its original volume, the compression ratio is 8:1.
In theory, the more the mixture is compressed, the more energy it releases when it burns.
In practice, however, very high compression ratios result in knocking or pinking in which some of the mixture furthest away from the spark plug explodes or detonates causing uneven burning, overheating and loss of power.
For maximum efficiency, the burning of the mixture should occur rapidly but smoothly.
HOW VALVE OVERLAPPING DURING THE ENGINE STROKES CAN BE RESOLVED
So far we have assumed that the incoming mixture rushes past the inlet valve as soon as it opens.
In practice, the mixture is slow to accelerate and in order to fill the cylinder as completely as possible, the inlet valve is opened a little early, when the piston is near the end of the exhaust stroke, and while the exhaust valve is still open. This is called valve overlap.
It might seem that opening the inlet valve early would offer an alternative exit for the exhaust gas, but provided the amount of overlap is carefully chosen, the opposite happens and the last waste of exhaust leaving the cylinder will help drag the fresh mixture in past the inlet valve.
Once it is moving, the inlet mixture does not stop automatically when the piston reaches the bottom of the cylinder, and if the closing of the valve is delayed, the cylinder fills more completely even though by then the piston would have started to rise on for the compression stroke.
In practice, in order to make the most of the momentum of the fresh mixture and exhaust gas flowing in and out of the cylinder, the exhaust valve opens before the piston reaches the bottom of the cylinder and closes after it has reached the top.
Similarly, the inlet valve opens before the piston reaches the top of the cylinder and closes after the piston reaches the bottom.
CROSSFLOW CYLINDER HEAD IN THE IC ENGINES
Crossflow engines have the inlet manifold on one side of the cylinder head and the exhaust system on the opposite side, so that during the four-stroke cycle, the inlet charge and the exhaust gas flow across the combustion chamber an arrangement that gives efficient cylinder filling during the period of valve overlap.
Engines with the inlet and exhaust manifolds on the same side of the cylinder head have a reverse flow arrangement.
This is a little less efficient during the valve overlap period, but it means that the heat from the exhaust can be easily and cheaply used to warm the inlet manifold and so improve vaporization of the mixture inside.
On a crossbow engine, it is necessary to pipe water from the cooling system to heat the inlet manifold.
