zilspeed
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| posted on 12/9/07 at 05:26 PM |
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Stoichiometric - what's all that about ?
Oo - err, that's a big word
Now that my daily car is fitted with LPG, I continue to hone the fuel mapping. Ideally, the Lambda sensor should be switching from lean to rich
continually and under all operating conditions from idle to WOT within the working range. (I won't mention red lines or any such nonsnse,
because the engine never sees it).
Anyway, my latest tweak involved reducing the base gas pressure and increasing injection times which have made a happier engine. I have added fuel and
the engine is much happier.
Now, my question.
I am advised that a lean mixture (on petrol ) will lead to poor running and probably overheating. I understand that much. It is the accompanying
increased fuel consumption which I am confused about. Is it to do with the lean mixture leading to poor running which in turn leads to a need to give
it more gas for the same net result.
LPG - to get back to the point, sort of, is 110 octane, so I 'think' that is what made the engine so tolerant of a lean mixture.
Lastly, if there is one correct air / fuel ratio, why do we speak about mapping for power or mapping for economy ?
[Edited on 12/9/07 by zilspeed]
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NeilP
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| posted on 12/9/07 at 05:56 PM |
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As I simply understand it because running rich generates more power if more airflow is available.
If you are out of gear running smoothly at 3000rpm with an EFI setup then you will be at stoichiometric ratio. If you are under load but at the same
engine speed then you are running slightly richer to cater for the additional power requirements (for the same flow of air).
The Lambdas are only really used at lower revs / smooth running - Deck the pedal and you go way rich to get the engine revved up as fast as possible
(hence more injectors on serious race engines).
If you pay peanuts...
Mentale, yar? Yar, mentale!
Drive it like you stole it!
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snapper
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| posted on 12/9/07 at 06:15 PM |
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Stoichiometric or Theoretical Combustion is the ideal combustion process during which a fuel is burned completely. A complete combustion is a process
which burns all the carbon (C) to (CO2), all hydrogen (H) to (H2O) and all sulfur (S) to (SO2). If there are unburned components in the exhaust gas
such as C, H2, CO the combustion process is uncompleted. Related topics:
Excess Air is defined as the amount of air in excess of the stoichiometric amount. It is common to use more air than the stoichiometric amount in the
combustion chamber. It increases the chances of complete combustion. Excess air can also be used to control the temperature of combustion chamber.
When we open and close the throttle butterfly's we create an air fuel ratio that is not the ideal stoichiometric and the engine tries to reach
that ideal ratio.
Looking further Stoichiometric as a word means the preservation of matter after a reaction... you take fuel and air burn it and whats left CO2 water
and sulfer dioxide must equal what there was before less heat, i think
[Edited on 12/9/07 by snapper]
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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t.j.
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| posted on 12/9/07 at 06:51 PM |
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As far as I know ideal petrol is 14.7:1 litres (don't know in pints  )
For LPG is this above 15
that's why your consumption is a little bit higher.
the octane isn't so high as you think is around 101-104 RON.
(this also means you could change the compression ratio)
Your Lambda -signal should go normal lean-rich-lean-rich-lean-rich-lean-rich-lean-rich-lean-rich-lean-rich-
grtz
[Edited on 12/9/07 by t.j.]
Please feel free to correct my bad English, i'm still learning. Your Dutch is awfull! :-)
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tks
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| posted on 12/9/07 at 07:01 PM |
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quote:
Originally posted by t.j.
As far as I know ideal petrol is 14.7:1 litres (don't know in pints )
For LPG is this above 15
that's why your consumption is a little bit higher.
the octane isn't so high as you think is around 101-104 RON.
(this also means you could change the compression ratio)
Your Lambda -signal should go normal lean-rich-lean-rich-lean-rich-lean-rich-lean-rich-lean-rich-lean-rich-
grtz
[Edited on 12/9/07 by t.j.]
there will always be a small margin...
14.7 is the ratio sow as long as its in pints or in gallons or in litres it doesn't matter....
lets call 14.5 lean and 15 rich..do you get the point???
on overrun you could go leaner if the engine can cope with the heat, thats for running an engine rich (on the rich side) cools down the piston and
valves etc..
a labda should read always a good (stable) figure, there is no reason wy it should alternate that much.
The above comments are always meant to be from the above persons perspective.
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adithorp
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| posted on 12/9/07 at 07:23 PM |
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..."there will always be a small margin...
14.7 is the ratio sow as long as its in pints or in gallons or in litres it doesn't matter.... "
Well if I remember correctly its 14.7/1 BY MASS so that would be Kgs or Lbs. By volume would use one hell of a lot of fuel!
adrian
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Bob C
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| posted on 12/9/07 at 09:43 PM |
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14.5:1 is a ratio it doesn't have units. I always thought lower number meant rich. - i.e.it's 14.7 wights of air to 1 weight of
petrol.
Bob
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piddy
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| posted on 12/9/07 at 10:09 PM |
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Bob Is correct.
1.0:1 Very rich
14.7:1 stoichiometric for a petrol engine
17.0:1 very lean
Lower means richer higher means leaner.
[Edited on 12/9/07 by piddy]
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mark chandler
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| posted on 12/9/07 at 10:54 PM |
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If I monitor my car through the OBDII port it bounces rwrwrwrwrwrwr every second or so, its when it takes a few seconds to do this that the Lambda
sensor has beome lazy an needs replacing.
I run LPG but its the same when on gas.
The OBDII tool is laptop based so I get nice graphs, you may not see this if using a lesser tool that just gave figures as it could avaerge.
Regards Mark
[Edited on 12/9/07 by mark chandler]
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piddy
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| posted on 12/9/07 at 10:58 PM |
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Here's a write up I found on the subject.
A stoichiometric or ideal air/fuel ratio is 14.7:1 which means that there are 14.7 parts of air and one part fuel. The reaction of this mixture would
produce emissions that contained only carbon dioxide and water, no fuel or oxygen would be left. At an air/fuel ratio of 14.7:1 the oxygen sensor
voltage signal would read 1.0 volt on a narrow band sensor and 2.5 volts on a wide band sensor. If the air/fuel ratio mixture is rich (more fuel than
air) than the sensor would read fewer volts or conversely if the mixture is lean (more air than fuel) the O2 sensor would read a larger voltage. If
the air/fuel ratio is richer than 11.7:1 (0.8 volts) or leaner than 18:1 (1.22 volts) the engine won't run.
Sometimes it is desirable to run either leaner or richer than 14.7:1. If your goal is to produce maximal power output of the engine, as with forced
induction, you'll want to run a richer air/fuel mixture around 12.5:1. The additional fuel in the cylinder will not burn. For example if we
added 2 parts fuel to 14.7 parts air only one part of fuel will burn and do work. This is because there is not enough air to burn the second part of
the fuel, remember that one part fuel combines with 14.7 parts air in a stoichiometric reaction. This unburnt fuel takes up valuable space in the
cylinder but in forced induction applications cylinder temperatures can skyrocket and cause detonation. This extra fuel provides an anti-knock or
cooling effect allowing for more power to be produced.
If you ran at an air/fuel ratio of 12.5:1 all the time you would really waste a lot of fuel but fortunately you don't need to. Normally it is
only necessary to enrich the fuel mixture under load i.e. when boosting or engaging nitrous at all other times it is better to run a little leaner
than 14:1. Running a lean air/fuel ratio will improve your gas mileage but be careful if you run too lean you will cause your combustion chambers to
overheat. Running lean under power destroys engines because the combustion chambers get so hot that the pistons fall apart under the stress which is
mostly due to pre-ignition. This will defiantly make for a bad day very fast.
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tks
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| posted on 13/9/07 at 06:35 AM |
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yeah it true....
its the ratio of air particles to fuel particles...
The above comments are always meant to be from the above persons perspective.
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