Combustion engine power - Principle of function

 To understand how recent cars work, you have to understand how an engine work.

What do engine need to work?

To be more technical, I will employ technical words from mecanical engineering that is primary to understand with explanations in order to be understandable. Also, I will explain graphics linked with engines.

Basically, an engine need air and fuel in the cylinder chamber to produce a mecanical energy to rotate the crankshaft helped by the pistons strokes and this movement makes engines work. The mix with fuel and air is called a chemical energy and it is transformed to become a mecanical energy by rotating the crankshaft. Then, this mecanical energy is distributed on the flywheel engine which spins. Then, the clutch makes the transition between the engine and the gearbox. Finally, the gearbox transmits the rotation to the differential gear thanks to different height of gears and the wheels can spin in last.
 

This image is taken from a website and it is a free published image for free use. Introduction to the Second Law of Thermodynamics: Heat Engines and Their Efficiency · Physics (philschatz.com)

In an automobile, a four stroke engine has two parts:

- The cylinder head cover. (on the top part)

-  The bottom motor part (I cannot give a technical word here...)

Cars work with a cycle which is called "Otto cycle" that represent the function of an engine. Some says that it is called "Otto Cycle" and other says that it is called "Beau de Rochas' Cycle". Whatever the origin of where and who it came from, because it is the same schedule. This cycle is a theoric of an ideal combustion engine. In fact Beau de Rochas is the first one that demonstrates how the four stoke combustion engine works in a theorical way. Otto applied what Beau de Rochas did to confirm this thermodynamic theory, with also lot of friends who helped him, such as Gottlieb Daimler who is one of the founder of a famous automotive brand: Mercedes-Benz.  

You have four cycles in a combustion engine:

                                            1- Intake

The inlet valve opens to let the air (at atmospheric pression) in the cylinder chamber. The exhaust valve is shut. An injection of fuel is controlled electronically to inject the right amount of fuel. The air and the fuel take place in the cylinder chamber, the piston is in a first position that is called "TDC" (Top Dead Center) and it transfers down to be at the "BDC" (Bottom Dead Center).

                                            2- Compression

The inlet valve shuts to not let air go into the cylinder chamber, the exhaust valve is still closed. The piston transfers up to compress the amount of fuel and air into the cylinder chamber and it is placed at the "TDC".

                                            3- Power

The inlet valve and the exhaust valve are shut. Then the piston stays at the "TDC" point. The spark plug lit with an electric impulsion commanded electronically. As soon as the spark ignites, it makes an explosion of the mixture to create the working part of the engine. This is the most important part where the engine works and creates power in order to move the car forward.

                                            4- Exhaust 

The inlet valve is shut and the exhaust valve is opened to let go all residues that did not combust with air and fuel. The piston is at "BDC" to let maximum residues and these are pushed away thanks to the piston which goes up at "TDC". 

All these cycles are represented on a timing diagram which looks like a circular shape. However you need to know how to read this diagram. Otherwise you will be lost. By the way, all cars have not got the same pattern because it depends of the fuel injected in the engine. Here is just a picture representing how petrol engines work.

                                                        Otto cycle diagram

This image is taken from a website and it is a free published image for free use. Image Viewer - HomeworkLib

This image is taken from Wikimedia and was modified by myself. File:Valve timing 4T.png - Wikimedia Commons

The way of read the circular diagram is up to down and right to left from n°1 "Intake". 

As you can see, the first timing cycle "Intake" does not start on the top where it is wrote "TDC" (Top Dead Center). In fact, the inlet valve opens 15° before the piston reach the "TDC" to let air goes into the cylinder chamber. 

Then, the second timing cycle is not correct because the exhaust valve does not open when it came to this cycle. As you can see, it shows that the exhaust valve opens but it must not open, in that case the exhaust valve is closed and the inlet valve closes 80° after the second cycle began, in order to compress the mixture, the piston reach the "BDC"(Bottom Dead Center) before the inlet valve closes and the "compression" cycle starts before the piston stroke. The compression continues with both valves closed to arrive at around a 360° point and where the piston goes up again to reach "TDC". The crankshaft made a "half" cycle (360°) and also the camshaft made a "half" cycle (180°- is not represented on the diagram in general).

Now, we look at the third timing cycle "Power". Both valves are still closed and the ignition begins while the compression is made. Thanks to the spark plugs, they ignite the mixture and creates power in the cylinder chamber. The piston moves down to reach "BDC" point and it new position is 540°  .

Finally, we arrive to the final cycle "Exhaust". To explain that, the cycle begin before the piston arrives at "BDC" and the exhaust valve opens to let all residues which have not combust when ignition lit the mixture of fuel and air. Consequently, the piston goes up to reach "TDC" with the exhaust valve opened
with letting all these residues go to the exhaust part. Now, the crankshaft have made 720° and this is called a full cycle of a four stroke engine. However the exhaust valve closes 15° after the piston reaches "TDC" and the inlet valve is opened 15° before "TDC" to begin a next cycle.

I think I lost all my readers at this point, but REMEMBER THIS:

720° crankschaft rotation = a full cycle of the crankshaft = 360° camshaft rotation = a full cycle of the camshaft.

Videos:

                             go to 2:45 directly