Every day we take the car to go to work, run errands or just enjoy driving, but we may not know how does a gasoline engine work and what is it that produces its movement, whatever your type.
A four-stroke gasoline engine has, as its name suggests, four phases of operation. These are: intake, compression, power and exhaust. Each of them corresponds to a stroke of the piston or, what is the same, half a turn of the crankshaft and are carried out inside the engine block.
Operating phases
Intake phase: air and fuel enter
At the beginning of the phase of admission, the piston is at the top of the cylinder, called TDC (Top Dead Center). At this time, the intake valves are open and the piston descends to the bottom of the cylinder, bdc (bottom dead center). During the descent of the piston, a depression is created inside the cylinder that draws in the air-fuel mixture. That is to say, it "sucks the air" as if it were a syringe.
At pmi, the intake valves close and the air-fuel mixture remains inside the cylinder. Incidentally, it is a mixture of these two components because the fuel would not burn if it did not have the oxygen in the air. In scientific terms it is called fuel to gasoline and oxidising to the oxygen that enters with it.
Compression phase: air and fuel are compressed
The second phase, compression, the piston rises again to TDC, compressing the whole mixture inside the cylinder, which also causes it to heat up. The pressure inside the cylinder can reach about 15 bars with a temperature close to 450º C, achieving a more homogeneous mix. At this stage, the crankshaft has turned another half turn.
An interesting fact is that the more the mixture can be compressed, the more the explosion that will occur immediately will be used. this is called engine compression ratio, which is usually measured in proportions as 10 to 1, 11 to 1, 12 to 1, 14 to 1…. Which means nothing more than the volume that the mix occupies before compressing and the volume that it occupies at the end of the compression phase. We recommend the article compression ratio of an engine, to better understand this factor.
However, one must be careful with not cause uncontrolled self-ignition, because that would damage the engine. What happens when the mixture is compressed too much, when there is too much temperature in the combustion chambers or a mixture of both circumstances. If you want to understand the latter, take a look at the post about Self-ignition, detonation and knocking of connecting rod.
Explosion phase: we start working…
In this phase, the piston is at TDC with all the mixture compressed. It is here when a spark jumps from the spark plug (gasoline engines) igniting the mixture of air and fuel. As a result, an explosion occurs. quickly expands the burned gases and pushes the piston up to the pmi. This is the work phase of the engine, since the piston receives a great impulse with the explosion, to transmit it to the crankshaft. During this phase the crankshaft rotates another half turn.
It should be noted a very important issue in combustion engines. This description of the phases is simplified to understand how the gasoline engine works, but in the explosion phase we are going to skip this simplification for a moment. As we have told you, the explosion expands rapidly to push the piston.
However, this speed is not so much for the speed at which the engine sometimes moves. Hence there is the ignition advance, which is about starting the explosion while the mixture is still being compressed. In this way it burns sooner and the most profitable part of the explosion coincides with the descent of the piston. That is to say, that the explosion phase and the compression phase "coexist for a moment" in favor of making use of the fuel. If you want a more detailed explanation of this process, we recommend the article What is ignition advance?
Exhaust phase: gases leave the combustion chamber
Finally, we come to the stage of Escape. On this occasion, the piston is at PMI and the exhaust valves open, where exhaust gases are evacuated quickly. This occurs because the piston moves back up to tdc pushing them out of the inside of the cylinder. When the piston is at TDC, the valves close. During the exhaust phase, the crankshaft rotates another half turn, completing the 4 phases of operation of the gasoline engine.
As you can see, the piston is again at TDC, ready to start the admission phase again. In such a way that the engine will repeat this entire process as long as we keep it on.
More questions to know how a gasoline engine works
crankshaft
It has already been mentioned several times in the phases of the four-stroke engine and you have been able to see it in action in the video at the beginning. Its function is that of convert linear motion of the pistons in the circle that we are going to use to move the car. The best analogy is that of a cyclist, who pedals to get around on a bicycle. If you look closely at the movement of the engine, that is what each of the pistons does when turning the crankshaft: «pedal«.
If you want to go deeper into this part of the engine, we recommend the articles crankshaft operation y The piston: what is it, what are its parts, price... You will see that the operation of an engine is simpler than it seems.
Distribution of an engine
As you may have already noticed, in the engine there is elements that require perfect coordination. For example: that the valves open to let in the mixture when the piston goes down (intake phase) or that they close when the piston goes up (compression phase). The distribution system of an engine is responsible for this coordination, which are a series of elements that connect each component through wheels, belts, camshafts and others.
Specifically, the fundamental part of the distribution system in today's most common engines is the timing belt or chain. She is in charge of transmitting the turn of the crankshaft Camshafts that open and close the valves. That way they do it at the right time and not another. A defect that would be fatal to the engine.
If you don't know these components, you can see what they look like and how they work in the articles: camshaft y timing belt. Also, if you want a little more depth on the subject, we also recommend the article on the types of distribution in current engines. It will help you to better understand how a gasoline engine works. Not all motors are the same, even if they are governed by the same basic principles of operation.
Images – Marco Bernardini, Matt
When the engine has good compression but it smokes and they waste oil, it is surely a seal problem (cap or rubber), we have a product called YELLOW SEAL, which corrects any wear on valves and guides, prolonging the useful life of these pieces.
I find your video very interesting, I wanted to ask if I can copy it to use it later.