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Old 01-15-2013, 09:53 AM
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Example of adiabatic compression

Let's now look at a common example of adiabatic compression- the compression stroke in a gasoline engine. We will make a few simplifying assumptions: that the uncompressed volume of the cylinder is 1000cc's (one liter), that the gas within is nearly pure nitrogen (thus a diatomic gas with five degrees of freedom and so = 7/5), and that the compression ratio of the engine is 10:1 (that is, the 1000 cc volume of uncompressed gas will compress down to 100 cc when the piston goes from bottom to top). The uncompressed gas is at approximately room temperature and pressure (a warm room temperature of ~27 degC or 300 K, and a pressure of 1 bar ~ 100,000 Pa, or about 14.7 PSI, or typical sea-level atmospheric pressure).

so our adiabatic constant for this experiment is about 1.58 billion.
The gas is now compressed to a 100cc volume (we will assume this happens quickly enough that no heat can enter or leave the gas). The new volume is 100 ccs, but the constant for this experiment is still 1.58 billion:

so solving for P:

or about 362 PSI or 24.5 atm. Note that this pressure increase is more than a simple 10:1 compression ratio would indicate; this is because the gas is not only compressed, but the work done to compress the gas has also heated the gas and the hotter gas will have a greater pressure even if the volume had not changed.
We can solve for the temperature of the compressed gas in the engine cylinder as well, using the ideal gas law. Our initial conditions are 100,000 pa of pressure, 1000 cc volume, and 300 K of temperature, so our experimental constant is:

We know the compressed gas has V = 100 cc and P = 2.50E6 pascals, so we can solve for temperature by simple algebra:

That's a final temperature of 751 K, or 477 C, or 892 F, well above the ignition point of many fuels. This is why a high compression engine requires fuels specially formulated to not self-ignite (which would cause engine knocking when operated under these conditions of temperature and pressure), or that a supercharger and intercooler to provide a lower temperature at the same pressure would be advantageous. A diesel engine operates under even more extreme conditions, with compression ratios of 20:1 or more being typical, in order to provide a very high gas temperature which ensures immediate ignition of injected fuel.


Here ya go guy's...the science behind it. Theory works to get you something useful in practical applications but stray too far from theory and the practical won't last too long!
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Old 01-15-2013, 10:00 AM
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Look up Adiabatic process. The wikipedia one is ok, it is accurate enough for this conversation. If that is not enough for you look up the thermodynamic laws and the heat engine. This is a great start to understanding the how it is and why it is then take it from there.
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Old 01-15-2013, 10:03 AM
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Quote:
Originally Posted by steeny View Post
Example of adiabatic compression

Let's now look at a common example of adiabatic compression- the compression stroke in a gasoline engine. We will make a few simplifying assumptions: that the uncompressed volume of the cylinder is 1000cc's (one liter), that the gas within is nearly pure nitrogen (thus a diatomic gas with five degrees of freedom and so = 7/5), and that the compression ratio of the engine is 10:1 (that is, the 1000 cc volume of uncompressed gas will compress down to 100 cc when the piston goes from bottom to top). The uncompressed gas is at approximately room temperature and pressure (a warm room temperature of ~27 degC or 300 K, and a pressure of 1 bar ~ 100,000 Pa, or about 14.7 PSI, or typical sea-level atmospheric pressure).

so our adiabatic constant for this experiment is about 1.58 billion.
The gas is now compressed to a 100cc volume (we will assume this happens quickly enough that no heat can enter or leave the gas). The new volume is 100 ccs, but the constant for this experiment is still 1.58 billion:

so solving for P:

or about 362 PSI or 24.5 atm. Note that this pressure increase is more than a simple 10:1 compression ratio would indicate; this is because the gas is not only compressed, but the work done to compress the gas has also heated the gas and the hotter gas will have a greater pressure even if the volume had not changed.
We can solve for the temperature of the compressed gas in the engine cylinder as well, using the ideal gas law. Our initial conditions are 100,000 pa of pressure, 1000 cc volume, and 300 K of temperature, so our experimental constant is:

We know the compressed gas has V = 100 cc and P = 2.50E6 pascals, so we can solve for temperature by simple algebra:

That's a final temperature of 751 K, or 477 C, or 892 F, well above the ignition point of many fuels. This is why a high compression engine requires fuels specially formulated to not self-ignite (which would cause engine knocking when operated under these conditions of temperature and pressure), or that a supercharger and intercooler to provide a lower temperature at the same pressure would be advantageous. A diesel engine operates under even more extreme conditions, with compression ratios of 20:1 or more being typical, in order to provide a very high gas temperature which ensures immediate ignition of injected fuel.


Here ya go guy's...the science behind it. Theory works to get you something useful in practical applications but stray too far from theory and the practical won't last too long!
Please repost.This one didn't go though and I can't read the formulas or the pix's did show up.

Thanks.
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Old 01-15-2013, 10:06 AM
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type in adiabatic process and read it from the wikipedia. The part I copy and pasted from starts as "Example of adiabatic compression".
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Old 01-15-2013, 10:10 AM
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1gary, did you find it? I tried resizing it and repost but keep getting a file error.
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Old 01-15-2013, 10:34 AM
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Ignoring carbs for the momment
we can see how tbi will not tolerate as high of compression as direct port injection.

and there are many many reasons for fuel to not ignite just because it reaches an ignition temperature that is just so far beyond any even caring,when some of us cannot even figure out the displacement of our engines when we bore and stroke them.
Im very happy I have Tech here to do the simple DCR math for me.

I contacted 76 with a couple ,what I thought was simple questions about fuel/octane etc,and they offered to fax a simple answer to me. I got around 70 sheets of formulas from them. I understood less than 1/2.
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Old 01-15-2013, 10:52 AM
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Then slap components together to see if it lives and for how long. Make up theories and excuses after the fact to support your failure or triumphs. The people that could have told you that your thinking is either good to go or hmmmm, got a problem are really just trying to help. Whether you believe them or not is your choice.
I am done with this subject....knock yourselves out!
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Old 01-15-2013, 01:43 PM
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steeny,welcome to the forum.I see you just joined,hope you enjoy the venue
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Old 01-15-2013, 01:46 PM
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Thanks, I will be around. By the way I have very thick skin.
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Old 01-15-2013, 06:10 PM
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X2

Last edited by Croz; 01-16-2013 at 07:56 AM. Reason: Removing quoted post that was deleted.
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Old 01-16-2013, 08:23 AM
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Understand you theory completely and appreciate it.Thing is when you start calling about "real world situations" we both know full well the mechanics in the upper rpm regions in the real world are far,far,from perfect.Valves that are supposed to be completely closed aren't etc.

Now you can experiment on your wallet all you want with pinging engines.But please for the sake of the rep of the forum,the newbies that come on it for advise,re-frame from your risky behavior advise.

I can without doubt tell you that Tech has a good history with hundreds,no thousands of readers.And I have trailing not far behind him involvement in hot rods agree with what he has said.
I am not really sure this makes any sense to me. It sounds like you are saying you essentially agree with landshark about how it works, but that its alleviated by the valves not being closed all the way when they are supposed to be. Which, sounds to me like you are relying on the engine not working the way it should to compensate for the high compression ratio. Well, engines don't normally operate in a friction-less vacuum as the physicists say they like to operate in, so that is probably true. Where I have a serious problem though is when you insinuate that running a lower compression ratio is the risky behavior that is being recommended to a newbie. The way this thread reads to me, is one person asking if a base compression ration of 13:1 is ok on pump gas if he puts enough cam in. Some people say its fine, others say it isn't and that you really shouldn't run more than 10-10.5:1 on pump gas. I fail to see how the 13:1 crowd is the conservative, non-risk taking side of the group. I think if we have newer, less experienced people asking questions, it shouldn't come off that a higher compression ratio is the safer, less risky option...

On an unrelated note... My debate experiences in college force me to point out, that if one side of an argument is demanding source citations from the other side, they better have some of their own...
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Old 01-16-2013, 02:25 PM
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I know I said I would not say anymore on this subject but I can't contain myself.

To the guy's that say it is ok to run "pump" gas ( of any grade) on an engine with 13-1 and compensate with a large duration, high overlap cam is good to go then why would one use a better composition of gasoline to run this same engine at the track. If there is no need then why do it.....cause it smells good?

To the guy's that were like me back in the early 70's building hot rods and high compression engines (even over what you could get already from the factory) to cruise the blvd and race out on the lonely road we definitely understood (even before we were off to college for higher learning) the need. Sunoco 260 was our friend!!

To the stubborn crew that does not want to listen to experience?......go ahead and try your combo on the street with the quality of gas these days. Let us all know of your real world experience learned. Bet I know the answer if your feedback is honest!!

To the guy's that do it successfully.....kudo's for the knowledge and understanding that there is more to it than just huge domes, small chambers and wild camshafts!!!!
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Old 01-16-2013, 02:40 PM
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Quote:
Originally Posted by steeny View Post
I know I said I would not say anymore on this subject but I can't contain myself.

To the guy's that say it is ok to run "pump" gas ( of any grade) on an engine with 13-1 and compensate with a large duration, high overlap cam is good to go then why would one use a better composition of gasoline to run this same engine at the track. If there is no need then why do it.....cause it smells good?

To the guy's that were like me back in the early 70's building hot rods and high compression engines (even over what you could get already from the factory) to cruise the blvd and race out on the lonely road we definitely understood (even before we were off to college for higher learning) the need. Sunoco 260 was our friend!!

To the stubborn crew that does not want to listen to experience?......go ahead and try your combo on the street with the quality of gas these days. Let us all know of your real world experience learned. Bet I know the answer if your feedback is honest!!

To the guy's that do it successfully.....kudo's for the knowledge and understanding that there is more to it than just huge domes, small chambers and wild camshafts!!!!
I don't think any of us are willing to risk good money on a "what-if" of this kind. This was originally a question posed by Vinnie and I chimed in to put my 2 cents worth in. I've never tried the combination and never will. Advancements in the sport are not made by those who sit back and are comfortable with the status quo, they are made by those who buck the system and try different things. Again, like I said though, I'm not foolish enough to try this, it was just an exercise in fantasy land.

You seem to be a level-headed fellow and I appreciate all your input. Thanks.
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Old 01-16-2013, 03:24 PM
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Techinspector1,

thanks.
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Old 01-16-2013, 04:56 PM
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Understanding compression on a pump gas street motor - Engine Masters

Read this by Vizard. He touches this subject. It will point you in the correct direction in understanding basic's.
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