Flash point temperature of petrol is lesser than that of diesel at the same pressure

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  • #1
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Main Question or Discussion Point

Flash point temperature of petrol is lesser than that of diesel at the same pressure.
Does this mean, if I take petrol and diesel as liquids in seperate containers and heat them, rapid vapourisation (rapid enough to sustain a combustion) of petrol occurs at a lower temperature(fire point) than that of diesel?
But if I take petrol and diesel as vapours and heat them ,will diesel self ignite at a lower change in temperature(lower increase in temperature) than petrol?

Are boiling point temperature,fir point and flash point temperature same at constant pressure(because,once vapourisation starts at constant pressure,the temperature doesnot increase till all the liquid vapourises)?
At higher pressures,a higher temperature is needed to vapourise the liquid.In engines, during compression, pressure increases.Does this mean the flash point at atmospheric pressure is lower than the flash point at the presaures inside an engine?

Does self ignition temperature depend on pressure?

In wikipedia it is mentioned that self ignition temperature decreases with pressure or oxygen concentration.Does this mean if we compress a fuel at constant temperature,it can self ignite at room temperature?
 
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Answers and Replies

  • #2
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If you guys are hesitating to answer this question because of its length( six questions), it would help me a lot if you answer at least the following question.

Are boiling point temperature,fir point and flash point temperature same at constant pressure(because,once vapourisation starts at constant pressure,the temperature doesnot increase till all the liquid vapourises)?
Example:Heating diesel in an open container(at constant atmospheric pressure).
 
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  • #3
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least the following question. Are boiling point temperature,fir point and flash point temperature same at constant pressure
No.
 
  • #4
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No.
But,how?During vaporisation,if the temperature of the liquid remains constant(constant pressure line in the saturation region), how will the three temperatures be different.
 
  • #5
berkeman
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Thread closed for Moderation...

Thread re-opened. @Mohankpvk -- please do not cross-post you question in multiple PF forums. Thank you.
 
  • #6
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But,how?During vaporisation,if the temperature of the liquid remains constant(constant pressure line in the saturation region), how will the three temperatures be different.
Try googling "petrol, diesel, and flash point." The first two are "mixtures," many components, and the third is an awkward means/method of "quantifying" fire hazard(s).
 
  • #7
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The first two are "mixtures," many components.
Does this mean,for diesel and petrol,temperature changes during phase change(since they are mixtures)?
But flash point is well below the boiling point for diesel and petrol.So how does vapourisation occurs below the boiling point?
 
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  • #8
dRic2
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Does this mean,for diesel and petrol,temperature changes during phase change(since they are mixtures)?
Yeah, in mixtures Temperature changes during a phase change
 
  • #9
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Yeah, in mixtures Temperature changes during a phase change
But flash point is well below the boiling point for diesel and petrol.So how does vapourisation occurs below the boiling point?
 
  • #10
dRic2
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So how does vapourisation occurs below the boiling point?
Water boiling point at atmospheric pressure is 100 *C, but if you leave an open glass of water on a table for a sufficient amount of time you won't see the water anymore inside the glass... Where did it go? Ever thought about that?
 
  • #11
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So how does vapourisation occurs below the boiling point?
Google "vapor pressure, partial pressure(s)."
 
  • #12
Merlin3189
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Vapourisation occurs at temperatures below the BP. It's just that at the BP temperature, the vapour pressure equals atmospheric pressure. Below BP temp, the liquids still evaporate, but have a lower saturated vapour pressure.
 
  • #13
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When I googled it, I understood that vapourisation occurs even below the boiling point.It was explained that the molecules near the surface of the liquid(on the air surface interface),if they gain enough energy they break the liquid attraction force and escape into the surrounding.But this rate of vapourisation is very low(compared with after boiling point).As temperature increases, vapour pressure also increases.So a higher pressure is needed to keep the substance in liquid state.But since the external presaure remains constant,the evapouration rate increases.(these with higher vapour pressure at room temperature are called volatile)

Is this the right interpretation or is there any thing more to it?

Thank you guys.
 
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  • #14
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On the internet, I read that the self ignition temperatures of petrol and diesel are arround 260 and 210 degree celsius respectively.The difference is just about 70degree celsius.The temperature at the end of compression in a CI engine is about(500 to600 degre celsius).

Then why cant diesel engine self ignite petrol.?

In a lot of websites,they claim that since petrol doesnot self ignite easily it cannot be used as a fuel for diesel engine.
Otto cycle was invented first and then diesel cycle was developed.So gasoline engines existed befor the diesel engines.
But there should be some other reason they switched the fuel oil for.Is self ignition the only reason or is there any other reason?
 
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  • #15
Gasoline self ignites just fine in a high compression diesel engine, way too quickly in fact, violently. "does not self ignite easily" makes no sense, maybe it means it is uncontrollable?

There are new gasoline engines working on using controlled autoignition google this "Controlled Auto-Ignition/HCCI Combustion."
 
  • #16
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Gasoline self ignites just fine in a high compression diesel engine, way too quickly in fact, violently. "does not self ignite easily" makes no sense, maybe it means it is uncontrollable?

There are new gasoline engines working on using controlled autoignition google this "Controlled Auto-Ignition/HCCI Combustion."
Thank you for answering.
Can this violent ignition of petrol be a reason for changing the fuel oil when diesel engine was invented?
Because the pressure at the end of compression will be significantly higher in a CI engine.Rapid combustion of petrol may add heat at constant volume(approximately) which will resul in ver high pressures.So diesel(which burns slowly) may be used(so that heat is added approximately at constaant pressure)
Can this be a reason?
 
  • #17
There are probably lots of reasons why. Off the top of my head, higher efficiency due to higher compression, lower pumping losses due to no throttle in intake, diesel is safer and easier to store. I am no engine expert, just an ME who likes engines.
 

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