Good Idea, or not? - Page 4 - PeachParts Mercedes-Benz Forum

#46 11-14-2005, 10:13 AM

Quote:

Originally Posted by P.E.Haiges

Taking the example to the extreme, if you had no oxygen, the mixture would be extremly rich but the EGT would be very low because the fuel would not burn.

So where is the "knee" in the curve?

P E H

I think PEH asked the right question. BHD's rule of thumb is based on the results of adding more fuel the "normal" air resulting in increased EGT. I think everyone agrees that adding more fuel will increase EGT. The question is whether or not you will get the same effect by keeping the amount of fuel the same and reducing O2, while increasing CO2. It is not obvious that this will have the same result, and our experience is not helpful because, AFAIK, this has not been tested.

I understand the argument that having inadequate O2 will lead to slow/incomplete combustion in the cylinder, resulting in some damaging combustion in the exhaust. That may cause an increase of EGT over some range of depleted O2. However, at some point the reduction of O2 has to cause an overall decrease in combustion, reducing the EGT. I don't know where this "knee" is, but if BHD is correct it has to exist someplace. In addition, the CO2 would tend to reduce EGT by absorbing some of the heat of combustion within the cylinder, similar to an EGR valve. This is an interesting question, but I don't believe we have enough info to get the full answer.

boneheaddoctor · #47 11-14-2005, 10:26 AM

Quote:

Originally Posted by Craig

I think PEH asked the right question. BHD's rule of thumb is based on the results of adding more fuel the "normal" air resulting in increased EGT. I think everyone agrees that adding more fuel will increase EGT. The question is whether or not you will get the same effect by keeping the amount of fuel the same and reducing O2, while increasing CO2. It is not obvious that this will have the same result, and our experience is not helpful because, AFAIK, this has not been tested.

I understand the argument that having inadequate O2 will lead to slow/incomplete combustion in the cylinder, resulting in some damaging combustion in the exhaust. That may cause an increase of EGT over some range of depleted O2. However, at some point the reduction of O2 has to cause an overall decrease in combustion, reducing the EGT. I don't know where this "knee" is, but if BHD is correct it has to exist someplace. In addition, the CO2 would tend to reduce EGT by absorbing some of the heat of combustion within the cylinder, similar to an EGR valve. This is an interesting question, but I don't believe we have enough info to get the full answer.

Adding more fuel will not increase EGT as long as enough O2 is present in the intake charge to support a full burn....

You can have a Turbo OM617 tweeked and pushing 200HP (stock is 120hp) and still have lower EGT than a stock OM617 non turbo so the total BTU thing is a non-issue.

THe knee would be the point when enough O2 required for a complete burn is not there....and that point will vary depending on the condition of your IP...the closer to original spec it is running the less leeway you have...and this is assuming your Boost is at original spec too...

Like I have repeatedly said...its all about the ratio of O2 for a given quantity of fuel...

Anything you do to offset the percentage of O2 in the air will effect that ratio....as you see O2 is not the majority of the air we breath..nitrogen is.which is why I always reference O2..

Air Composition

The sea-level composition of air (in percent by volume at the temperature of 15°C and the pressure of 101325 Pa) is given below.

Name Symbol Percent by Volume
Nitrogen N2 78.084 %
Oxygen O2 20.9476 %
Argon Ar 0.934 %
Carbon Dioxide CO2 0.0314 %
Neon Ne 0.001818 %
Methane CH4 0.0002 %
Helium He 0.000524 %
Krypton Kr 0.000114 %
Hydrogen H2 0.00005 %
Xenon Xe 0.0000087 %

#48 11-14-2005, 12:41 PM

Quote:

Originally Posted by boneheaddoctor

Like I have repeatedly said...its all about the ratio of O2 for a given quantity of fuel...

Ok, I think we may have gotten to the bottom of this. What you are talking about is the fuel/air ratio, not the fuel/O2 ratio. These are not the same thing. The high EGT associated with high fuel/air ratios has to do with the ratio of BTUs (amount of fuel) to the amount of air available to "absorb" the heat of combustion. In other words, each pound of fuel contains a specific amount of potential energy, and it takes a specific amount of energy to heat a pound of air by a fixed amount. Therefore, EGT is directly related to the ratio of fuel to air. If I double both the amount of fuel and the amount of air, EGT stays the same. If I increase the amount of fuel by 20% and keep the amount of air the same, I will heat the EGT by an additional 20%. The fuel/O2 ratio has nothing to do with EGT in this case. The ratio of fuel to O2 only becomes important if there is not sufficient O2 to support efficient combustion.

What we were discussing here involves adding inert gas (CO2) to the process. This will not affect the fuel/air ratio, but will affect the fuel/O2 ratio. If there is still sufficient O2 available to support complete combustion, this should have no effect on EGT (because the specific heat of CO2 is about the same as O2, or close enough anyway). If we displace enough O2 (with CO2) to affect combustion efficiency, the EGT may be affected. This may tend to slow combustion (potentially allowing more unburned fuel mixture through the exhaust valves and increasing the EGT, as suggested by BHD). At the limit, there would not be sufficient O2 left for complete combustion (in which case EGT would probably decrease). Eventually the engine wouldn't run at all.

boneheaddoctor · #49 11-14-2005, 12:59 PM

Quote:

Originally Posted by Craig

Ok, I think we may have gotten to the bottom of this. What you are talking about is the fuel/air ratio, not the fuel/O2 ratio. These are not the same thing. The high EGT associated with high fuel/air ratios has to do with the ratio of BTUs (amount of fuel) to the amount of air available to "absorb" the heat of combustion. In other words, each pound of fuel contains a specific amount of potential energy, and it takes a specific amount of energy to heat a pound of air by a fixed amount. Therefore, EGT is directly related to the ratio of fuel to air. If I double both the amount of fuel and the amount of air, EGT stays the same. If I increase the amount of fuel by 20% and keep the amount of air the same, I will heat the EGT by an additional 20%. The fuel/O2 ratio has nothing to do with EGT in this case. The ratio of fuel to O2 only becomes important if there is not sufficient O2 to support efficient combustion.

What we were discussing here involves adding inert gas (CO2) to the process. This will not affect the fuel/air ratio, but will affect the fuel/O2 ratio. If there is still sufficient O2 available to support complete combustion, this should have no effect on EGT (because the specific heat of CO2 is about the same as O2, or close enough anyway). If we displace enough O2 (with CO2) to affect combustion efficiency, the EGT may be affected. This may tend to slow combustion (potentially allowing more unburned fuel mixture through the exhaust valves and increasing the EGT, as suggested by BHD). At the limit, there would not be sufficient O2 left for complete combustion (in which case EGT would probably decrease). Eventually the engine wouldn't run at all.

It has nothing to do with air absorbing anything...there is not enough of it to do so...If you have insufficient O2 to complete the burn you will have high EGT...the only way you are going to drop EGT is to effectively lean the mixture out, IE More O2 or less fuel....there is a small buffer in the O2 in excess of whats needed being present for full combustion.

You guys are trying to make a mountain out of a molehill....

#50 11-14-2005, 01:41 PM

Quote:

Originally Posted by boneheaddoctor

It has nothing to do with air absorbing anything...there is not enough of it to do so...If you have insufficient O2 to complete the burn you will have high EGT...the only way you are going to drop EGT is to effectively lean the mixture out, IE More O2 or less fuel....there is a small buffer in the O2 in excess of whats needed being present for full combustion.

Maybe I didn't explain this very well. Let me try again, a diesel engine operates on a gas cycle. Cold air is heated by combustion, and the heated air does the work. The energy is provided by the fuel, which contains a specific amount of potential energy (BTU/pound). That energy is used to heat the air. Therefore, the EGT is normally determined by the ratio of fuel to air. As long as there is sufficient O2 for complete combustion, the fuel to O2 ratio has nothing to do with EGT. Diesels are designed to operate with significant excess air, so insufficient O2 is not a problem under normal operating conditions. Everyone's day-to-day experience with "more fuel = higher EGT" is a function of fuel/air ratio, not fuel/O2 ratio. The only time fuel/O2 may be an issue is at full throttle.

Based on the above, adding additional inert gas to the combustion process should have no effect on EGT unless the combustion process has insufficient O2. This should only happen with full throttle, or nearly full throttle conditions, where O2 is limiting. Remember that air is only about 20% O2, the rest of the air is already inert. It's primary function is to operate as the "working fluid" of the engine, absorbing energy from the combustion process.

Edit: Just a clarification for others reading this post - When I said, " That energy is used to heat the air." I was referring to the process of heating the exhaust gases after combustion begins. Prior to combustion, the fresh air is heated by compression only. I hope that's not too confusing.

boneheaddoctor · #51 11-14-2005, 02:00 PM

Quote:

Originally Posted by Craig

Maybe I didn't explain this very well. Let me try again, a diesel engine operates on a gas cycle. Cold air is heated by combustion, and the heated air does the work. The energy is provided by the fuel, which contains a specific amount of potential energy (BTU/pound). That energy is used to heat the air. Therefore, the EGT is normally determined by the ratio of fuel to air. As long as there is sufficient O2 for complete combustion, the fuel to O2 ratio has nothing to do with EGT. Diesels are designed to operate with significant excess air, so insufficient O2 is not a problem under normal operating conditions. Everyone's day-to-day experience with "more fuel = higher EGT" is a function of fuel/air ratio, not fuel/O2 ratio. The only time fuel/O2 may be an issue is at full throttle.

Based on the above, adding additional inert gas to the combustion process should have no effect on EGT unless the combustion process has insufficient O2. This should only happen with full throttle, or nearly full throttle conditions, where O2 is limiting. Remember that air is only about 20% O2, the rest of the air is already inert. It's primary function is to operate as the "working fluid" of the engine, absorbing energy from the combustion process.

When one talk air/fuel ratio you are reallg talking O2 fuel ratio in a sense because air is assumed to be 20% O2 which is what does the job being the oxidizer...

Adding additional inert gas will effect the EGT because you are diluting the O2 compoment of the air and making it a smaller percentage. Now how much that effect is depends on the state of tune of the car...

however what one does not want is to reduce the O2 content of the charge.....LOW egt = low power...high egt is bad only due to the physical limitations of the engine...the materials used in contruction of the engine are what determine that point. The weakest link being the aluminum alloy of the pistons.

An engine makes it power based on thermal expansion of gases...less heat = less expansion = less power and vice versa up to the physical limits of the engine and materials used in its construction.

#52 11-14-2005, 02:14 PM

BHD,

I agree with what you've just posted. There is no good reason to add inert gas to the process. Adding enough inert gas to result in insufficient O2 for complete combustion would result in a loss of power and could be detrimental to the engine. Fuel/air ratio is proportional to fuel/O2 ratio as long as we are talking about "normal" air, without additional gasses added.

Craig

leathermang · #53 11-14-2005, 02:55 PM

BHD, I agree with your last statement also....

t walgamuth · #54 11-16-2005, 07:52 AM

i think the original poster should at this time be required to try it out on his engine to see if the egt really will rise.

... and how long to blow it sky high.

tom w

boneheaddoctor · #55 11-16-2005, 07:59 AM

Quote:

Originally Posted by t walgamuth

i think the original poster should at this time be required to try it out on his engine to see if the egt really will rise.

... and how long to blow it sky high.

tom w

not likely to blow......anything except a hole in at least one piston....adn thats more of a MELT a hole than blow a hole.

t walgamuth · #56 11-16-2005, 08:03 AM

definately call a holed piston a blown engine.

tom w

boneheaddoctor · #57 11-16-2005, 08:16 AM

Quote:

Originally Posted by t walgamuth

definately call a holed piston a blown engine.

tom w

true............

whunter · #58 11-16-2005, 08:17 PM

I suspect the turbo would be first to suffer a catastrophic failure from the raised EGT.
In durability testing I have seen a turbo glow white hot in daylight, something you never want to see on a vehicle you own.

boneheaddoctor · #59 11-16-2005, 08:35 PM

Quote:

Originally Posted by whunter

I suspect the turbo would be first to suffer a catastrophic failure from the raised EGT.
In durability testing I have seen a turbo glow white hot in daylight, something you never want to see on a vehicle you own.

Well cumbustian temps elevated to put piston temps to the plasticity state of aluminum...add boost either the pistons, vlaves or even the urbo will fail...if you don't induce heat stress to the head and housings causing them to crack etc...long list of possible failure modes and it only takes one.

fireman1073 · #60 11-29-2005, 06:03 PM

If you inject C02 into your motor it will die because it does not sustain combustion.
It will put fire out.
No horsepower will be gained and That is the reason for the experiment is it not?
c02 does not make horsepower when injected into the motor, it will stall it.
Runaway diesels are often shut down by dumping a c02 extinguisher into the intake.

Try it!

Steve