Monday, June 28, 2010

How an Internal Combustion Engine Works

Combustion Engine Principle

Here is one approach how to describe an internal combustion engine and link the energy conversion theory with the conception of novel ideas that resulted in the engine design as it is today, starting from the crank to turn the shaft to statistical thermodynamics at the molecular level with macroscopic thermodynamics laws inside the cylinder with piston, valves, connection rod. Complex formulae are avoided, but physical principles that led to winning initial and boundary conditions for PDEs are outlined. Note that physical feel and intuition how the engine should work are leading drives for successful design.

Let’s say we have started with the question how to turn the wheel of a car or of a coach.

First thought is to turn it by hand. But, obviously it is not practical. We don’t want to sit in the coach and turn the shaft all the time while we travel. Is there anything else that we can use to turn the shaft around it’s axis and transfer power to the wheels?

What if you imagine a cylinder and a piston, and inside the cylinder a lot of small particles that push the piston up? Then the particles wait until the piston is down, then push again up. Sure, that looks like a nice idea. You can easily connect the piston via some kind of connection rod to the shaft and with one or two gears you can translate vertical motion of the piston to the rotational motion of the shaft. But, the major point is how the particles get energy to push the piston up. Why particles? Why not something else? What are these particles anyway? I will answer these questions in this article.

Let’s start by answering what would be the major invention here. It is a chemical reaction between the fuel molecules and oxygen. Don’t forget, a chemical reaction, called combustion, is only process of breaking bonds between certain atoms and creating new bonds between the same atoms. Bond creation is done by sharing electrons, in most cases. So, here we have only electric forces in game, no others. Atoms are clusters of protons and neutrons held together by nuclear forces, with electrons orbiting around. So, in chemical reactions, the atom core remains intact, only electrons got shared or exchanged between these clusters of protons and neutrons! Hence no nuclear forces are acting in the chemical reactions, only electrical. Hence, chemical reaction can be called an electrical reaction. Note here the significance of bonds breaking and bonds forming. While combustion is usually associated with flames, smoke, etc. that is not the major characteristic important to us. Combustion is like any other chemical reaction, and, moreover, as we have seen, each chemical reaction is actually electrical reaction. Hence, combustion is an electrical reaction, where energy after atoms and electrons reorganization is responsible to move the piston.

During this reorganization of atoms, the stored energy in the bonds got released and newly formed molecules with newly formed bonds accelerate, away from each other, with high speed from the point of initial impact. Note that impact is not the source of the increased speed of the new molecules. It is the bonds formed that supply that energy. This is the most important moment in internal combustion engine theory. These newly formed molecules (usually CO2 and H2O) are the ones that will hit the wall of the piston and push the piston up. Of course the molecules will hit the other sides of the cylinder but that work is lost.

Now, given that you have these particles that accelerate away from each other once the bonds are broken and new bonds are created, and initial molecules are broken and new molecules are created, you want to make these particles to work for you. How would you do that? You will confine them in a closed space, some kind of enclosure, and a metal cylinder will serve that purpose nicely. So, let’s put molecules of oxygen and fuel in a container, a cylinder. Then, we will make one side of the cylinder movable, top lid say. That will be our piston. You can easily attach a connection rod to the piston and use this rod to turn the shaft connected to the wheel. At this point we have fully working internal combustion engine!

What happens when all molecules are reorganized, i.e. when there are no molecules to push the piston up, or, in popular language, when all fuel is burnt, in one "cycle"? You have to somehow empty the container cylinder from the combustion products, i.e. molecules of CO2 and water, H2O, that did their role in pushing the piston up. Once the container is empty, you can add new mixture of fuel and oxygen, ignite it and get new molecules to push your cylinder up again. How is this emptying and filling done? Well, you can manually open the lid, i.e. remove the piston from the container, empty the content, then fill the container with the new amount of mixture of fuel and oxygen. But, again, that’s not practical and even might not let the engine work continuously. What’s the solution? The solution is to create two openings in the cylinder, and two valves to open and close each opening. They will be referred to as intake opening and exhaust opening. These valves can easily be connected to the camshafts, intake camshaft and exhaust camshaft. Both camshaft can be again easily connected to the main crankshaft which is powered with our accelerating molecules from the cylinder. So, once the intake and exhaust camshafts are synchronized, they will do the required tasks, namely allowing fuel and oxygen to enter combustion chamber, and, after burning, let them out. When we need a new amount of mixture of fuel and oxygen in the cylinder, intake camshaft will open intake valve and the mixture will go in. When the burning process is finished, the exhaust shaft will open the exhaust valve and the piston will push the combustion products out of the cylinder, through the exhaust opening. Again, we have used the energy from accelerating molecules which are the result of the combustion (combustion is, in it’s essence, an electrical reaction between the fuel and oxygen molecules) to turn the main crankshaft, and, by attaching convenient configuration of gears to the main crankshaft, we control opening and closing of the intake and exhaust valves. At this point we have, essentially, designed the 2 tact motor, internal combustion engine. Here is the illustration of the real internal combustion engine with its essential parts:

Elements of an Internal Combustion Engine

Here is a real world illustration of the inside of an automobile engine:

And, let’s repeat again how it works.
1. Vacuum will suck in the mixture of fuel and oxygen.
2. Spark will ignite and initiate the electrical reaction, also called chemical reaction, between the molecules of fuel and oxygen. The accelerating molecules will push the piston down and do the work.
3. Once the combustion is completed, the flywheel will push the piston up and empty the combustion chamber, i.e. our cylinder, from the products of combustion.
4. Flywheel will close the exhaust valve, pull the piston down, and open the intake valve. Vacuum will suck in the new mixture of fuel and oxygen and the process will start again.

But, the major thing is that the energy to push the piston to do the work, to rotate the crankshaft, is obtained from the bonds breaking and bonds forming between the molecules of fuel and oxygen, during the combustion process. During these bonds breaking and formation, molecules of CO2 and H2O get accelerated and push the piston thus transferring energy to shaft. It is these accelerated molecules that moves the crankshaft, and hence, car.

As a side note, although we talk here about energy and work, they are the same physical concept. Moreover, although we talk about giving energy, transferring energy over distance, etc. energy is not an object. Energy is a calculated value, obtained by measuring mass, time, distance, keep that in mind. The motion is what we are after.

[ car engine, chemistry, combustion engine, fuel, Internal Combustion Engine, physics, Thermodynamics ]

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