OW! MY BRAIN!
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From: Thornhill, Ontario, Canada
OW! MY BRAIN! I've been mulling over this little problem for some time now and I'm still not getting it figured out; my brain is on the verge of leaping out of my skull and splattering all over the wall like some horrible impressionist painting....
Ok, the optimum stoichiometic air/fuel ratio for the most complete combustion is 14.7:1- yes? So does it not stand to reason that this "most complete" burn wouldn't also yeild maximum power? So, why is it that optimum power is acheived with 13.3:1? Why does left-over fuel produce more power???
Please- save my brain.
-Mike
Ok, the optimum stoichiometic air/fuel ratio for the most complete combustion is 14.7:1- yes? So does it not stand to reason that this "most complete" burn wouldn't also yeild maximum power? So, why is it that optimum power is acheived with 13.3:1? Why does left-over fuel produce more power???
Please- save my brain.
-Mike
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From: Thornhill, Ontario, Canada
Re: OW! MY BRAIN! Yes, it does but that doesn't mean higher output: Cool is good for the air comming into the cylinder bcause it means higher density, but once it's IN the cylinder you want it as hot as possible (just before detonation). Cooling the intake charge once it's already in the chamber is counter productive.
Good point though...
-Mike
Good point though...
-Mike
Re: OW! MY BRAIN! I don't have exact terms here, but here goes:
That 14.7 is really the intersection point of two lines on a graph. Adding more fuel increases power but more fuel is wasted in that process. The opposite is true as well, leaning it out increases the MPG at the cost of power. So the 14.7:1 ratio is the optimum mix for power and efficency. Adding more fuel to get that 13.3:1 mix gives you max power at the cost of poor mileage.
Hope that made sense.
Richard
That 14.7 is really the intersection point of two lines on a graph. Adding more fuel increases power but more fuel is wasted in that process. The opposite is true as well, leaning it out increases the MPG at the cost of power. So the 14.7:1 ratio is the optimum mix for power and efficency. Adding more fuel to get that 13.3:1 mix gives you max power at the cost of poor mileage.
Hope that made sense.
Richard
On The Tree
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From: Atlanta
Re: OW! MY BRAIN! Also not ever molecule of fuel will find a molecule of air, by increasing the amount of fuel you are increasing the chance that all the air will be used up in the combustion chamber. While leaning it out is increasing the chance that all the fuel will be used up at the expense of additional air left over. In a perfect world with perfect combustion 14.7 might make the best power, maybe?
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From: Thornhill, Ontario, Canada
Re: OW! MY BRAIN! Chris,
So, are you suggesting that in the air/fuel combustion reaction Air is the limiting factor? That is to say that the ammount of combustion is *more* limited by the ammount of air available than my the ammount of fuel available? That doesn't sit right with me. Perhaps I'm mis-interpreting you?
-Mike
So, are you suggesting that in the air/fuel combustion reaction Air is the limiting factor? That is to say that the ammount of combustion is *more* limited by the ammount of air available than my the ammount of fuel available? That doesn't sit right with me. Perhaps I'm mis-interpreting you?
-Mike
Re: OW! MY BRAIN! </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by MikeTheNewbie:
<strong>Chris,
So, are you suggesting that in the air/fuel combustion reaction Air is the limiting factor? That is to say that the ammount of combustion is *more* limited by the ammount of air available than my the ammount of fuel available? That doesn't sit right with me. Perhaps I'm mis-interpreting you?
-Mike</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">That's right. That's exactally what N2O does. Introduces more "air" (oxygen) so you can burn more fuel.
As far as the original question, I'm going to go with Chris on this one. Stoichiometry simply means that there is the perfect amount of air to burn the included fuel -- no more, no less. At first it would make sense that max power would be at the stoich A/F ratio but that's been proven to not be the case. Here's my opinion (and it goes along with Chris'): and I/C (internal combustion) engine is typically about 35% efficient. That means that about 35% of the energy produced is converted to mechanical energy but the other 65% is lost as heat energy. Perhaps the cooling allows more of the energy produced to be used as mechanical energy. That would also explain why a forced induction motor typically likes an even richer mixture. They produce even more heat. I'm not sure if it's right, but it sounds good to me.
<small>[ March 21, 2002, 09:56 AM: Message edited by: sross ]</small>
<strong>Chris,
So, are you suggesting that in the air/fuel combustion reaction Air is the limiting factor? That is to say that the ammount of combustion is *more* limited by the ammount of air available than my the ammount of fuel available? That doesn't sit right with me. Perhaps I'm mis-interpreting you?
-Mike</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">That's right. That's exactally what N2O does. Introduces more "air" (oxygen) so you can burn more fuel.
As far as the original question, I'm going to go with Chris on this one. Stoichiometry simply means that there is the perfect amount of air to burn the included fuel -- no more, no less. At first it would make sense that max power would be at the stoich A/F ratio but that's been proven to not be the case. Here's my opinion (and it goes along with Chris'): and I/C (internal combustion) engine is typically about 35% efficient. That means that about 35% of the energy produced is converted to mechanical energy but the other 65% is lost as heat energy. Perhaps the cooling allows more of the energy produced to be used as mechanical energy. That would also explain why a forced induction motor typically likes an even richer mixture. They produce even more heat. I'm not sure if it's right, but it sounds good to me.
<small>[ March 21, 2002, 09:56 AM: Message edited by: sross ]</small>
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Re: OW! MY BRAIN! Its pretty simple, actually.
To achieve max power, you want to burn ALL of the oxygen. With a perfect mix (100% homogeneous), the air/fuel ratio is 14.73.....
However, we never achieve a perfect mixture in the cylinder. So, in order to burn all of the oxygen present in the cylinder, we need a little more fuel. That is why we get air/fuel ratios of 13.3.... It helps insure that we burn all of the oxygen.
If the intake/valve/head/piston combo provides better fuel/air mixing, the the air/fuel ratio might be better at 14.0, for example.
To achieve max power, you want to burn ALL of the oxygen. With a perfect mix (100% homogeneous), the air/fuel ratio is 14.73.....
However, we never achieve a perfect mixture in the cylinder. So, in order to burn all of the oxygen present in the cylinder, we need a little more fuel. That is why we get air/fuel ratios of 13.3.... It helps insure that we burn all of the oxygen.
If the intake/valve/head/piston combo provides better fuel/air mixing, the the air/fuel ratio might be better at 14.0, for example.
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Verdad Gallardo Re: OW! MY BRAIN! In addition,
Nitrious oxide adds more oxygen, if you burn more oxygen, you get more power. You need more fuel with nitrious, because you have more oxygen to burn. If you do not add more fuel with nitrious, then you run lean and in time, burn a little hole through the top of your pistons.
Nitrious oxide adds more oxygen, if you burn more oxygen, you get more power. You need more fuel with nitrious, because you have more oxygen to burn. If you do not add more fuel with nitrious, then you run lean and in time, burn a little hole through the top of your pistons.
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From: College Station, Tx
Re: OW! MY BRAIN! </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by MikeTheNewbie:
<strong>So, are you suggesting that in the air/fuel combustion reaction Air is the limiting factor?
-Mike</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Air definitely is the limiting factor in making power. You can simply spray in more fuel, or if you are limited increase your fuel injector size, etc. You can't do the same with air, hence air is the "limiting reagent" in the power equation. Superchargers, nitrous, increased displacement, etc. all serve to get more air in.
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">To achieve max power, you want to burn ALL of the oxygen. With a perfect mix (100% homogeneous), the air/fuel ratio is 14.73.....
However, we never achieve a perfect mixture in the cylinder. </font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">
EXACTLY! The cooling issue that was also mentioned is another factor, but the above is the primary logic behind running richer than stoich ratios'.
<strong>So, are you suggesting that in the air/fuel combustion reaction Air is the limiting factor?
-Mike</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Air definitely is the limiting factor in making power. You can simply spray in more fuel, or if you are limited increase your fuel injector size, etc. You can't do the same with air, hence air is the "limiting reagent" in the power equation. Superchargers, nitrous, increased displacement, etc. all serve to get more air in.
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">To achieve max power, you want to burn ALL of the oxygen. With a perfect mix (100% homogeneous), the air/fuel ratio is 14.73.....
However, we never achieve a perfect mixture in the cylinder. </font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">
EXACTLY! The cooling issue that was also mentioned is another factor, but the above is the primary logic behind running richer than stoich ratios'.
Thread Starter
Teching In
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From: Thornhill, Ontario, Canada
Re: OW! MY BRAIN! Some great thinking guys!
I'm not sure I totally buy the cooling effect being responsible for the greater power output at higher than stoich ratios; that doesn't work: Max heat = Max energy. Once the air charge is in the cylinder it doesn't matter if it gets really hot- afterall that's the point! The cooling effect merely provides a safety margin so your pretty little pistons won't become ugly little splat marks.
Remember: "Lean is mean".
-Mike
I'm not sure I totally buy the cooling effect being responsible for the greater power output at higher than stoich ratios; that doesn't work: Max heat = Max energy. Once the air charge is in the cylinder it doesn't matter if it gets really hot- afterall that's the point! The cooling effect merely provides a safety margin so your pretty little pistons won't become ugly little splat marks.
Remember: "Lean is mean".
-Mike