The car engine: how it is made and how it works | Auto for Dummies

The car engine: how it is made and how it works | Auto for Dummies

In a previous Auto for Dummies article, we explained the difference between a diesel and a petrol engine. Today we focus specifically and at the same time generally on the operation of the internal combustion engine in its main parts. This type of engine, used on the first thermal machines of the late nineteenth century, allows you to apply a chain of energy transformation through various well-designed components in synergy with each other.



 

One of the visions that help to understand the functioning of a car and its beating heart, the engine, is given by imagining each component as an element capable of playing a specific role. Thanks to this premise it will be very easy to conceive how the internal combustion engine is capable of overall transform the chemical energy of the fuel into mechanical energy transmitted to the wheels.


Engine block and displacement

As anticipated in the introduction, before understanding how in practice the engine is able to transform fuel, be it diesel or petrol, into motion, let's analyze the main elements with which the internal combustion engine works. The engine block it represents the most massive and robust part of what is found under each hood. The base of this "box", made of cast iron or aluminum, has evident cylindrical holes in which the pistons are housed. The latter are free to move up and down, to be more precise than reciprocating motion, thanks to the chemistry of combustion which we will discuss later. But what regulates the movement of the pistons inside the so-called combustion chamber?



3D model of a V12 engine (six cylindrical holes on each side) credit: 3D Horse

Let's start giving some immediate definitions. When you hear about V6, V8 o V12 it refers to the number of cylinders present inside the engine block. So if we go to analyze a 4-cylinder engine, one of the most common on everyday cars, we will therefore have to deal with four holes coupled with four pistons in motion. There displacement instead it represents the size, in liters or cm3, of the cylindrical holes that contain the pistons - to be precise it would be the volume swept by the piston inside the cylinder along its stroke but we limit ourselves to analyzing the main concepts. A four-cylinder 1000 cm3, corresponding to 1 liter, will therefore be an engine in which each cylinder will be able to contain approximately ¼ of a liter, the equivalent of a glass full of water, of the explosive mixture of air and fuel. .


credit: autotecnica.org

Let's analyze a single piston for simplicity. In the upper part of the cylinder there are two holes: one where the mixture enters and one from which the burnt gases exit at the end of the thermodynamic cycle. This is where the valves, mechanical elements able to close and open these holes with a fundamental timing. Inside the combustion chamber of a four-stroke engine, the phases of intake, compression, expansion and exhaust. Exactly in the first and last of these phases the valves, respectively inlet and outlet, open and close allowing the mixture to enter and the exhaust gases to escape. Techniques such as ignition timing allow you to optimize everything.



coupling piston cylinder with crankshaft (bottom). Double camshaft and valves (top). In this case we see four valves for each cylinder, two for intake and two for exhaust

The camshaft and valves

You understand well that adjusting these different phases for all the cylinders present is not easy at all. This magic, in addition to the electronic setting, is regulated by thecamshaft: a long cylinder that runs along the entire head of the engine and mechanically “pulls” and pushes the valves in order to open and close the intake and exhaust holes at the perfect moment. The rotation of this shaft, small compared to the crankshaft which we will discuss later, is given by the belt . The camshaft is set in rotation by this component which exploits the movement of the engine itself in constant rotation.

Detail of the cams, small elements connected to the rotating camshaft (gray), able to push and open the valves (in orange) thanks to the non-alignment of the cams themselves

It sounds complicated but think of it like this: the purpose of the engine is to turn the wheels. The pistons generate an alternative movement ("up and down") which is transformed by means of appropriate kinematics (a complicated word to define mechanical elements in motion) into the rotation of a "wheel" (flywheel). This wheel is connected to the drive shaft which, by turning on itself, carries the movement to the wheels through the whole broadcast which we will not talk about in this article.




Part of the rotation is "stolen" from the movement of the wheels to make the camshaft turn right through a belt, chain or set of gears. Now you know how our motor, once the "vertical" movement has been generated, carries the rotation to thecrankshaft and at the same time opens and closes the valves that regulate the inlet and outlet of mixture and exhaust gases.

The thermal part: how movement is generated

Mechanically, the internal combustion engine must be well designed in terms of dimensions (geometry) and resistance of the components: each element must work in synergy. But how is reciprocating movement of the piston produced? The question, always partly mechanical as regards the physical displacement of this component, moves further under a energy plan. Each cylinder is able to accommodate a precise amount of air-fuel mixture and literally make it explode immediately after the compression phase. The movement of the piston is given precisely by this esplosione which by expanding the mixture generates a pressure effect on the head of the piston itself, pushing it downwards.

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In the case of a petrol engine, the explosion is generated by Candles, small and resistant components capable of producing a spark and capable of igniting the mixture. Regarding a motor a diesel, the explosion is instead given by the compression itself inside the combustion chamber. For more details on this, we refer you to a previous Auto for Dummies article. Just as gunpowder pushes a bullet into the barrel of a gun, so the explosion of fuel pushes the piston down. But how does it then get back "up"? Here a bit of mechanics and anticipation come back into play connecting rod-crank mechanism (kinematics).


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You must know that the cylinders are not all aligned; in other words, if we consider a four-cylinder engine, in the same instant two are still at the top in the compression phase while the other two have just reached the so-called bottom dead center. The inertia of these moving components (with a specific mass) and the different position of the pistons help to naturally return the pistons to their starting point. The thermodynamic cycle four times it can thus start again by aspirating, compressing, expanding and discharging everything that enters the cylinders. Additional components such as the intake and exhaust ducts, injectors, cooling system and the turbocompressione these are just some of the other synergistic components of the combustion engine.

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