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

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Discussion Overview

The discussion revolves around the mechanisms by which an explosion in an engine pushes the piston. Participants explore the role of pressure, gas laws, and chemical reactions involved in combustion, focusing on both theoretical and practical aspects of the phenomenon.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants assert that pressure is the primary factor pushing the piston, with references to molecular expansion due to exothermic reactions.
  • Others suggest that the ideal gas law (PV = nRT) provides a basic framework for understanding the relationship between pressure, volume, and temperature during combustion.
  • One participant proposes that electrical or electromagnetic forces may also play a role in pushing the piston, citing atomic interactions during the explosion.
  • There is a discussion about the change in state from liquid gasoline to gas, with some participants noting the increase in the number of gas molecules as a contributing factor to pressure changes.
  • Concerns are raised about the modest increase in the number of molecules resulting from combustion, with calculations provided to illustrate the limited volume increase from the reaction.

Areas of Agreement / Disagreement

Participants generally agree that pressure is a significant factor in pushing the piston, but there are competing views regarding the specific mechanisms and contributions of molecular changes and forces involved. The discussion remains unresolved with multiple perspectives presented.

Contextual Notes

Some participants highlight the limitations of using the ideal gas law as a crude approximation, noting that real combustion gases may not behave ideally. Additionally, there are unresolved assumptions regarding the nature of forces at play and the exact contributions of molecular changes during combustion.

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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Qaiphyx said:
... what about it pushes the piston.

An increase of pressure.
 
russ_watters said:
It's the pressure.

Caused by the molecular expansion due to the exothermic reaction?

Where is the pressure coming from?
 
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.
 
Qaiphyx said:
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.
 
oh ok I get it ha, wow, dumb question looking at it now. So basically it has to do with liquid changing into gas. the hydrocarbon gasoline reacts to create carbon dioxide and hydrogen?
 
Qaiphyx said:
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.
 
  • #10
The force is a repelling force caused by the atoms that were too close each other.
 
  • #11
Dr Lots-o'watts said:
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.
 
  • #12
Sakha said:
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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