xorl %eax, %eax

The Basics of 4-stroke Internal Combustion Engines

with 63 comments

So, first of all this post is extremely simple and I’m fairly sure that most people are already aware of these. I’m posting this for future reference on more advanced concepts and redesigns of core components used in modern cars. Here, I’ll try to demonstrate the simplest possible 4-stroke internal combustion engine in order to be understandable even by people with no prior knowledge.
Warning: As you’ll see in future posts, the engine I’m discussing here has very little in common with the engines found in modern vehicles apart from the general operation principles.

That said, let’s start this… As a general unofficial rule, in internal combustion engines air (more specifically oxygen) + fuel = power. With this in mind let’s have a look at the image below.

Cylinder’s Displacement
What you can see at a glance is that this is a 4 cylinder engine. The cylinder’s displacement is what you commonly hear as a 3.2L, 3.200cc, 195.3ci in liters, cubic centimeters and cubic inches. In fact, this is the swept volume of a cylinder multiplied by the total amount of cylinders.

Intake/Exhaust Valve(s)
Now locate at the left of the image the intake valve. As you can see, this is controlled by intake camshaft which in real life looks more like this.

As it rotates it forces some valves to open and others to remain closed. For better understanding here is a figure that demonstrates this.

Of course, valves have springs to return to their original position when not pushed down. If you have a look at the end of the camshaft you will notice that it has a gear. This is driven with a belt (known as cambelt) using the rotating crankshaft at the lower part of the engine. Here is a photograph where hopefully, you will be able to identify the discussed parts.

Similarly to the intake camshaft and valves you can locate at the opposite side of the cylinder the exhaust camshaft and valves. As you have probably already guessed, the intake opens to fill the combustion chambers of the cylinders with the mixture (fuel + air) while the exhaust opens after the combustion to release the exhaust gas from the chambers. This means that valve timing is crucial since valves should be consistent.

In the previous paragraph I talked about the rotating crankshaft. Check out the first image again to locate the crank at the lower part of the engine that holds the pistons with the connecting rods and has a flywheel at one of its end. Assuming that two of the four pistons are compressing the mixture inside the chamber (such as the first image’s chambers 1 and 4), when the spark plug will ignite the mixture the pressure from this “explosion” will force those two pistons move downwards while the middle two will have an opposing movement since they were already down. This is what makes the engine running and the crankshaft rotating giving us the ability to connect it to wheels and roll. In most vehicles I have seen, what you see as the RPM indicator (revolutions per minute) on your dashboard, is the rotation of the above mentioned crankshaft.

Clutch (Transmission)
With the above knowledge you should be frustrated by now. If the engine’s crankshaft doesn’t stop moving as long as the engine is running then why isn’t the car moving all the time? The answer is because at the the end of the flywheel which is there to transfer the power from the crankshaft to the wheels is another part named clutch (and gearbox that we’ll discuss next).
There are numerous different types of clutches for both manual and automatic gearboxes. The most simple is the one you see here:

As you can see from the figure, it is attached to the end of the flywheel which is constantly rotating. When the clutch is pressed, the pressure plate pushes the clutch disc and thus moves it away from the flywheel so that the power isn’t transfered. Of course, after some time (or truly bad use of it), the clutch will be damaged by the friction during this connection/disconnection and it’ll need to be replaced. The aim of the clutch is to allow us disconnect the crankshaft, select a gear appropriate to produce the RPMs and torque we need and then reconnect it. This moves us the the next part…

Gearbox (Transmission)
By now, you should know that clutch is between the engine’s flywheel and gearbox to separate them when necessary. A gearbox is nothing more than what it’s name suggests. A box of gears which could be either automatic (using various technologies such as hydraulic systems to select the right gear and push/pull the clutch), or manual. Here is a simple gearbox to get you started.

When the selector is set to neutral the layshaft will rotate (at the same RPMs as the crankshaft) but there will be no gear connected to it to transfer the movement to the wheels (ignore that differential mentioned in the figure for the moment). Hopefully, you’ll know from school that when you connect a smaller gear to a larger one it will rotate at less RPM and of course the larger transferring power to a smaller will have the opposite result. That’s the whole concept behind transmission here. So, depending on the driver’s selection the transmission can provide the required RPMs and torque. The reverse gear is nothing really special. If a gear is rotating clockwise and you connect another gear on it, the latter will rotate counter-clockwise. Consequently, to have reverse the only thing you have to do is insert another gear between layshaft’s gear and the 1st gear (although the diagram shows otherwise this is how it’s done in most vehicles).

Now that you hopefully have a very general idea of how an engine works let’s have a look at the most important part for most modern vehicles. Their engine operation is named 4-stroke because the engine does a cycle of tasks which is separated in 4 stages.
1: Intake Stroke
The piston moves downwards and the intake valve(s) opens up to fill the chamber with the mixture (fuel + air) while the exhaust valve(s) are closed. Here is a figure for better understanding.

When the piston reaches the lowest position the intake valve(s) will start closing (because of the camshaft’s rotation and the springs shown above).
2: Compression Stroke
During this stroke, the cylinder looks like this:

Both intake and exhaust valves are closed and the piston compresses the mixture. It’s very common among performance tuning communities to hear about the ideal compression ratio to have the best possible ignition. Compression ratio is the the difference between:
a) The volume of the cylinder (and combustion chamber) when the piston is at the bottom
b) The volume of the cylinder (and combustion chamber) when the piston is at the top
Here is a nice diagram to understand this:

The wikipedia article on compression ratio (you can find it here) has a nice and simple example which I’ll paste here:

Picture a cylinder and its combustion chamber with the piston at the bottom of its stroke containing 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.

So, assuming that we have reached the top of the combustion chapter we can move to the next stroke.
3: Power Stroke
Once again, both intake and exhaust valves are closed and the piston is at the top of the combustion chamber. The third stroke is when the spark plug ignites the mixture.

Of course, here is important to have a homogeneous mixture of fuel/air to have the best ignition. The pressure from the released exhaust gases of this ignition will force the piston move downwards. Earlier in this post, I said “explosion”. I put this in quotes because I wanted to explain this here. In fact, this is a fast, uniform burn of the mixture. Not an actual explosion. As you probably already know, in order to have ignition you need to have oxygen. This is why the air/fuel ratio is so important (and many of the most common performance tunings (superchargers, turbo, nitrous oxide, aftermarket air filters, etc.) have to do with providing higher quality mixture). The most common representation of this ratio is using lambda (λ) representation but I’ll talk about this in detail in future blog posts. If you want to learn more, you can always read wikipedia.
4: Exhaust Stroke
If you have a look at the first engine’s diagram you’ll notice that two of the four cylinders are down while the others are on top. This is done to provide a consistent operation since the intake (stroke one) and the exhaust (stroke four) strokes move the pistons with the power of the crankshaft which is moved by the other two cylinders that are giving power to it.

Now, as you’ve been expecting. With the power of its neighbor cylinder(s) that moves the crankshaft, the piston that just passed the third stroke will start moving upwards and the exhaust valve (using the camshaft’s timing) will open to release the exhaust gases from the previous ignition.

This 4-stroke cycle continues as long as air/fuel mixture is provided to the engine. Here is an animated image that will hopefully be understandable by now.

These are the principles of 4-stroke engine’s operation which are the most widely used in cars and modern bikes.
Again, I have intentionally omitted many, many, many details since this is supposed to be a simplistic introduction that I will be using for future reference in other blog posts.

The pictures and photos are taken from Google Image search, sorry for not mentioning every single site.


Written by xorl

March 5, 2011 at 21:32

Posted in motorcycles & cars

63 Responses

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  1. yaar its really good


    October 5, 2011 at 17:39

  2. so knowledgeable this thing I.C. Engine


    December 9, 2011 at 17:41

  3. Cool…


    February 17, 2012 at 05:32

  4. this realy owsome very educated, i cant believe it


    March 23, 2012 at 00:46

  5. Again, awesome!


    March 26, 2012 at 03:48

  6. nice


    May 6, 2012 at 12:01

  7. it’s realy awesome n it,s included all things of ic eng


    May 18, 2012 at 21:50

  8. i read it for first time but i have understood about the gear shift pattern and how it works.


    May 21, 2012 at 17:37

  9. wao its is one of the best av ever read

    charles omojola

    May 24, 2012 at 17:32

  10. Very helpful diagram. Thank you!

    Tech Exec

    June 16, 2012 at 18:55

  11. its so easy to anderstand


    June 20, 2012 at 00:15

  12. Thank you for explaining in detail, it is very informative and easy to understand, splendid work! Great!


    June 27, 2012 at 13:50

  13. it is very easy to anderstand for that type of explanation thank’s

    sachin yadav

    July 12, 2012 at 22:13

  14. Your explanation is good


    July 16, 2012 at 17:01

  15. so nice sir……………….

    rahul kashyap

    August 5, 2012 at 22:50

  16. this stuff about engines and more, cool i want to learn more abt this things!!! luv mechanical engineering

    Renang Tohlang Isaac

    August 27, 2012 at 17:39

  17. your english is simple. it’s easy to under stand.i like it…..thank u…


    September 5, 2012 at 19:38


    Ayo Godswill

    September 8, 2012 at 18:00

  19. the engine is coming up soon

    Ayo Godswill

    September 8, 2012 at 18:07

  20. superb! my query is over now .thank u sir

    ujjwal kumar

    October 1, 2012 at 23:07

  21. very helpful thanks!

    Rahul Dhangar

    October 12, 2012 at 14:11

  22. this is so help full to understand 4-stroke engine super


    November 1, 2012 at 17:22

  23. nice explanation

    prince amaechi

    November 12, 2012 at 01:01

  24. thank you. you just saved me from confusion. i wish you can explain more thing for us the ladies who do not have a clue with engines


    November 12, 2012 at 17:26

  25. thanks!

    rajesh gulati

    December 2, 2012 at 19:54

  26. very useful for mechanical mighties


    December 9, 2012 at 11:21

  27. thnx

    Prasad Shinde

    January 12, 2013 at 20:02

  28. i love what u guys are posting. It will make people to be automotive litrate.

    joseph ekundayo

    February 7, 2013 at 17:44

  29. it is really interesting not only to learners even to tutors


    February 19, 2013 at 13:30

  30. nice…………………………….


    March 6, 2013 at 06:06

  31. very good diagram to understand

    sanjee kr machuwa

    March 24, 2013 at 07:50

  32. Small correction, just *one* guy posting. :)


    May 18, 2013 at 12:46

  33. It is good and easy to understand ,thank you sir


    June 21, 2013 at 17:49

  34. awesome data


    October 27, 2013 at 05:51

  35. if all the stock is power stock in ic enginire ,flywheel is required or not

    ajay vallabh

    November 20, 2013 at 19:57

  36. Thanks very much said i wish i could do my research with you .


    February 13, 2014 at 19:42

  37. thank you..very much sir………….


    March 13, 2014 at 14:09

  38. nice information but it is quite difficult to understood by beginners.

    noyal thomas

    March 23, 2014 at 06:37

  39. nice animation


    March 29, 2014 at 10:06

  40. Good automobile education material,I commend you

    Arinze Emmanuel

    May 22, 2014 at 17:09

  41. very intresting as it help me to refresh my maind again .thanks

    Edibo A.R

    May 29, 2014 at 19:45

  42. could you slow it down a bit more would really help.


    July 21, 2014 at 07:57

  43. It is quite an elaborating fenominon, keep it up.

    George Akech

    July 27, 2014 at 17:28

  44. Ashish dev

    Ashish kumar

    September 19, 2014 at 23:27

  45. very good diagrammmmmmmmmmmmmm thankssssssssssssssssss

    rakesh mandal

    September 29, 2014 at 09:15

  46. very good…..Really helpful


    April 5, 2015 at 15:50

  47. I like your knowledge distribute & I love your system.


    April 24, 2015 at 18:32

  48. Nice


    July 27, 2015 at 15:26

  49. 951


    September 1, 2015 at 23:29

  50. Thanks for your illustrations on how both four stroke s.I and c.I engines works.

    Clement Mawa

    November 18, 2015 at 20:10

  51. Very nice. us full information thank you

    Suresh kumar S N

    January 18, 2016 at 16:23

  52. its too good to understand


    April 11, 2016 at 16:06

  53. I have read first time. But I have understood to gear pattern or assembly to ic engine.

    Kiran Awantkar

    April 25, 2016 at 10:10

  54. Very great – genuine effort – simple & professional

    Vijitha Fernando

    April 29, 2016 at 17:47

  55. its a good one I have try to understand but I find it difficult before but now I understand it very well but pls keep us in touch and upload more thanks

    onazi james

    August 2, 2016 at 16:17

  56. its a explanation


    August 18, 2016 at 08:21

  57. I.c.engine ke part kon kon se h


    September 15, 2016 at 17:44

  58. your information is excellent.thankyou


    February 5, 2017 at 21:04

  59. makes sence

    Brian moompo

    February 27, 2017 at 12:35

  60. Nice one, I like that because I learned something from it

    K C

    August 24, 2017 at 22:57

  61. Awesome nicely i understand thank you


    November 25, 2017 at 20:38

  62. Used this blog in teaching high school physics this week. Thank you!

    Mike Christenson

    March 8, 2018 at 23:10

  63. Well explained thenks..


    March 9, 2018 at 15:13

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