What is it exactly about an explosion that pushes the piston in an engine?

  • Thread starter Qaiphyx
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What are all the given aspects of this?

I understand how the reaction happens, but what about it pushes the piston.
 
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russ_watters

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It's the pressure.
 
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If you need an equation, the ideal gas law should be a first crude approximation : PV = nRT. The temperature in the cylinder clearly increases as the fuel/air mixture burns, and whatever gas is present has to increase either P, V or both to satisfy the gas law. Since the cylinder is made of solid steel, the only way V is going to increase is by having the piston head move away, bringing along a crankshaft rotation.
 
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Caused by the molecular expansion due to the exothermic reaction?

Where is the pressure coming from?
A mole of an ideal gas occupies 22.4 liters at standard temperature and pressure. Gasoline combustion gasses are not ideal but you get the idea.
 
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oh ok I get it ha, wow, dumb question looking at it now. So basically it has to do with liquid changing in to gas. the hydrocarbon gasoline reacts to create carbon dioxide and hydrogen?
 
What are all the given aspects of this?

I understand how the reaction happens, but what about it pushes the piston.
Actually, in my opinion there is the electrical force (or electro-magnetical force) which pushes the piston. The fact that in the explosion the atoms are generating smaller molecules and reorganize, make increase drastically the volume.
 
The force is a repelling force caused by the atoms that were too close each other.
 
295
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If you need an equation, the ideal gas law should be a first crude approximation : PV = nRT. The temperature in the cylinder clearly increases as the fuel/air mixture burns, and whatever gas is present has to increase either P, V or both to satisfy the gas law. Since the cylinder is made of solid steel, the only way V is going to increase is by having the piston head move away, bringing along a crankshaft rotation.
Besides the Temperature increase, there is a very high increase in the 'n' of the equation -number of moles-. The gasoline enters as liquid and changes into gas.
 
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Besides the Temperature increase, there is a very high increase in the 'n' of the equation -number of moles-. The gasoline enters as liquid and changes into gas.
The increase is very modest. you get something like

C5H12 + 8 O2 -> 5 CO2 + 8 H20

only 13/8 as much molecules. But air contains only ~20% O2 so the
reaction should be

40 N2 + C5H12 + 8 O2 -> 40 N2 + 5 CO2 + 8 H20

so you get only 53/48 as much molecules, so only an 10% volume increase from the
increased number of gas molecules
 

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