Physics

First Law of Thermodynamics four Stroke Internal Combustion Engine



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The first law of thermodynamics, or conservation of energy principle, states that one type of energy can be transformed into another type, but that energy will never be created out of nothingness, nor can it be destroyed into nothingness.  Conservation laws also apply to mass because matter is a type of energy (e = mc^2).  Matter can be converted into energy through processes such as fission and fusion. The first law in layman’s terms is: you don’t get something from nothing, nor will something blink out of existence into nothing.  What currently exists has always been, and always will be, in one form or another.  The origins debate is pointless, there is no origin, there is no grand beginning, there was never a time of nothingness.  Conservation laws state that everything that currently exists is eternal, without beginning, and without end.  There is no such thing as ex-Nihilo creation

An open system is one which can exchange matter and energy with its environment.  By keeping track of everything that crosses the system boundaries you can keep track of what is in the system.  As an example, suppose you have an empty cardboard box.  Mary puts three apples into the box, and Tom takes two apples out.  How many apples are now in the box?  The number of apples in the box = the original number (0) + Apples in (3) – apples out (2) = 1 apple left in the box.  The box is an open system, because stuff is coming into, and out of, the box.  Conversely, a closed system would be a box which has been completely duct-taped shut so that no apples, or puppies, or any other matter, can go in and out of it.

Apples and puppies are not the only things which can jump into, and out of, an open system.  The combustion chamber in an internal combustion engine is an open system too.  In four-stroke piston engines the combustion chamber undergoes a 4 part process in which energy and mass travel into and out of a box, or open system.

1. intake stroke – fuel and air (not sand or apples) go into the box, or into the box / combustion chamber.

2. compression stroke – A piston comes into the box pressurizing the air/fuel mixture.  This is putting work and energy into the box.

3. power stroke – the spark plug ignites the compressed air-fuel mixture in the box.  The high temperature, high pressure gas expands and shoves the piston back out of the box, which in turn, makes the wheels on the bus go round and round.  Work and energy go out of the box.

4. exhaust stroke – the piston comes back into the box, pushing the burnt up fuel out.  Work in, mass and a little wasted energy out.

This four part process requires four pistons, each piston at a different stage in the cycle, each pushing the other pistons around, in a hopefully well-timed process (if it is not well-timed, then engine knocking occurs).  Basically, the combustion chamber is an open system which converts the internal energy of a fuel, into mechanical energy which can be used to do work.  An example of a typical combustion reaction is:

CH4 + 2 O2 → CO2 + 2 H2O + heat/energy

Molecules in the fuel (CH4) chemically react with air (O2) to form new compounds (CO2 and water).  The new molecules have a lower internal energy, and so heat and energy is evolved.  The chemical reaction is sort of like releasing a rock from a slingshot.  When you stretch the rubber band in a slingshot back, you are creating potential energy.  Fuel molecules have a lot of stored potential energy.  When you light the fuel on fire, it’s like letting go of a stretched out rubber band.  The atoms in the fuel molecules let go of one another, the C lets go of the H, and they go flying off like rocks released from a slingshot.  These high velocity, high temperature atoms (temperature is a measure of the velocity of atoms) go flying into the piston, and push the piston back, which is connected to gears, and the drive shaft. 

So, back to conservation of energy. We all know that our cars will not run if we do not provide them with fuel.  In order to get work and energy out, you have to first put work and energy in; that’s the first law of thermodynamics.  In steady state, (work + energy + mass)in = (work + mass + energy)out.  Again, one type of energy can be transformed into another type, the internal energy of the fuel molecules can be released through combustion and transformed into mechanical energy, but unfortunately the energy cannot be created out of nothingness. Sadly, until someone figures a way around the first law of thermo, there’s no such thing as a free ride.  Not only are there no free rides in life, we can’t even break even, but that’s getting into the second law of thermodynamics. 

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