FOUR-STROKE ENGINE
The four-stroke engine has four basic phases that are repeated with every two revolutions of the engine.
1. Intake Stroke: The first stroke of the internal combustion engine is also known as the suction stroke because the piston moves downwards in the cylinder to the maximum volume position or bottom dead centre (BDC). The timed inlet valve opens as a result of piston movement, and the vaporized fuel mixture enters the combustion chamber. The inlet valve closes at the end of this stroke.
2. Compression Stroke: Through this stroke, both valves are closed and the piston starts its movement to the minimum volume position in an upward direction in the cylinder compressing the fuel/air mixture. During the compression stroke the pressure, temperature and the density of the fuel/air mixture increases drastically.
3. Power Stroke: When the piston reaches the minimum volume position or top dead centre (TDC), the spark plug ignites and in turn the fuel mixture and explodes transferring this energy into the power stroke which forces the piston down which is in turn transmitted to the crank shaft.
4. Exhaust Stroke: At the end of the power stroke the exhaust valve opens. During this stroke, the piston starts its movement in the minimum volume position from TDC. The open exhaust valve allows the exhaust gases to escape the cylinder and by the end of this stroke, the exhaust valve closes and the inlet valve is opened. The above sequence repeats itself in the next cycle. All four-stroke engines require two revolutions to complete the process.
All internal combustion engines depend on the combustion of a combustible fuel type ignited by either spark ignition or compression ignition and this is only made possible with oxygen from the air. The combustion process typically results in the production of a great quantity of heat and energy, as well as chemicals such as carbon dioxide at very high temperatures determined by compression ratio and the charge which is ignited that in turn creates combustible energy.
The most common modern fuels are made up of hydrocarbons and are derived mostly from fossil fuels. Fossil fuels include diesel, petrol and petroleum gas, and propane. Except for the fuel delivery components, most internal combustion engines that are designed for petrol use can run on natural gas or liquefied petroleum gases without major modifications to the engine type.
Liquid and gaseous biofuels, such as ethanol and biodiesel which is a form of diesel fuel produced from crops such as soybean oil that yield triglycerides can also be used. Engines with appropriate modifications can also run on hydrogen gas.
Almost all diesel engines use a four-stroke cycle, Intake, compression, ignition and exhaust. These engines rely on a compression heating ignition system. This variation is called the diesel cycle whereby diesel fuel is injected directly into the cylinders combustion formed chamber so that combustion occurs at constant compression pressure of a ratio of roughly 20:1, (as the piston moves upwards towards the cylinder head creating pressure resulting in combustion).
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