Theoretical cycle of a four-stroke engine

In this internal combustion engine whose combustion cycle requires four phases. The first to let the air or mixture enter, the second to compress it, the third to detonate it and obtain energy with the expansion of gases and, finally, a fourth to expel the exhaust gases. The names of these phases or times are: admission, compression, expansion y Escape.

It differs from the two-stroke engine in which the former groups the intake and compression in the first phase and the expansion and exhaust in the second. Below we detail the parts of a four-stroke engine, how it works, what is the mixture of air and fuel that it needs and why its use in cars has become widespread.

Parts of a four-stroke engine

A four-stroke piston engine has many more basic components than a two-stroke engine:

• Engine block: where the holes or cylinders that make up the combustion chamber walls.
• Head: part that closes the combustion chambers from above and where the valves and camshafts are housed.
• Piston: mobile part that closes the combustion chamber below and that is attached to the crankshaft by connecting rods.
• Crankshaft: axis that transform linear motion rise and fall of the pistons in circular motion.
• Cranks: moving parts that attach the pistons to the crankshaft.
• Valves: Are the gates that open and close at the right time to let the air in or not and let the exhaust gases out or not. Two-stroke engines do not have
• Camshafts: they are the axes with protuberances called cams that they push the valves open when appropriate. Two-stroke engines have neither valves nor camshafts.
• Intake manifold: ducts through which air enters in the combustion chambers when the valves allow it
• Manifold: ducts through which the exhaust gases exit when the valves allow it
• Plugs: in four-stroke gasoline engines, some spark plugs that cause a spark to detonate the mixture. In diesel engines they are not necessary, because the reaction is achieved by pressure. Only a few heaters are necessary to facilitate the cycle at startup
• injectors: In direct injection engines, only air is allowed to enter the combustion chamber and fuel is supplied to them already inside through the injectors. In indirect injection engines, the injectors are outside the combustion chamber.
• Carter: It is the part that closes the engine from below and contains the lubricating oil when the engine is at rest. This piece is not the same in two-stroke engines, because the oil is mixed with the air and the fuel.
• Oil pump: It is the in charge of taking the oil to all the places that is required for engine operation.

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Four-stroke engine operation

In a engine four-stroke, there is a theoretical cycle that is the basis for understanding the operation of the real cycle, but as its name indicates, it does not describe exactly what happens in reality.

1. El first time of the cycle is the aspiration of the fresh load, composed of air (oxidising or also called oxidant) and gasoline (fuel). At this time, the piston descends from the top dead center to the bottom, while the crankshaft rotates 180 degrees.  The intake valves open to let in the mixture.
2. El second time is the compression. The intake valves close leaving the inside of the cylinder sealed. The piston rises and raises the pressure of the mixture reducing the volume that contains it, while the crankshaft completes its first lap turning 360º.
3. El third time starts at the moment when the piston is in the top dead center, and the spark plug emits a spark that fuel combustion starts. At this time, a large amount of heat is generated and the mixture expands, causing the piston goes down violently, producing the work. The crankshaft starts its second turn with 540º.
4. In the fourth time we have the piston at the bottom and the combustion gases filling the cylinder. The exhaust valves open and the piston rises again expelling them. At the end of this phase, the crankshaft finishes its second revolution because it has rotated 720º. The combustion chamber is ready to restart the cycle with the first time.

What mix does it work with?

As we have indicated before, a two-stroke engine burns a mixture of air, fuel and oil. However, in four-stroke engines this mixture is Composed only of air and fuel. The correct ratio under ideal conditions for a gasoline is 14,7 grams of air for every gram of fuel (14,7/1). These quantities are known as the stoichiometric or ideal mixture.

In the diesel engines this ratio is slightly different, with a slightly lower amount of air: 14,5 grams of air for every gram of diesel. These mixtures seek to achieve the amount of oxygen necessary to burn all the fuel that enters the combustion chamber. If the proportion is incorrect, the combustion will not be of good quality and more smoke and emissions will be produced.

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Why is a car engine four-stroke?

The choice of four-stroke engines for cars is because consume less gasoline and oil than the two strokes. That is, they emit less gases and therefore they pollute less those. Also, since they only produce one explosion for every two revolutions of the crankshaft, they are quieterHave less vibrations and are easier to refrigerate. Hence they are also more reliable and durable.

Images 1 and 3 – Robyn Gallant, Tim Dobbelaere