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  #16  
Old 07-09-2004, 01:00 PM
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Peter, I know just enough about cars to be dangerous. I will go and buy a can of ether and after I put the stuff together I will start it up and spray ether at the air possible leaking places and hope for it to speed up. I remember using that tecnique in the past.

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  #17  
Old 07-15-2004, 09:02 PM
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One more question regarding the MR2.
The spark plugs were white instead of tan color, if the car is using more oil than it should 1qt per 1k miles, shouldn't the color of the plugs be black from burnt oil instead of white?

Hope the oil usage is other than through the rings or valve guides.
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  #18  
Old 07-16-2004, 02:41 PM
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High NOx often indicates lean mixture, as you are suspecting. The mixture can be adjusted on many FI systems. May not get at the root cause, but may get you past the test.
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  #19  
Old 07-17-2004, 06:19 PM
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today it was tested again, NOx 822 max 825.
changed plugs, and put a tank of 91 octane gas.
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  #20  
Old 07-17-2004, 11:51 PM
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Congrats, Juan,
I'm sure the high octane slower burning fuel helped reduce NOX because of the lower combustion temps. Hey, I'm not making this up. It is well documented.

Peter
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  #21  
Old 07-18-2004, 10:01 AM
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Thanks Peter, the car runs better with the better gas, may need to keep fueling with 89 octane from now on.
Now I can concentrate on the 300E. Finally.
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  #22  
Old 07-18-2004, 12:34 PM
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Quote:
Originally posted by autozen
Congrats, Juan,
I'm sure the high octane slower burning fuel helped reduce NOX because of the lower combustion temps. Hey, I'm not making this up. It is well documented.

Peter
I'd like to see some of the "documentation" and sources for this claim.

Duke
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  #23  
Old 07-18-2004, 05:11 PM
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Duke,

If you happen to have a friend in the smog business, ask if you can curl up with some of his books that come with his machine or any of the course books he may have that helped him get his license. These books are loaded with theory, explanations and graphs showing relationships beween the 5 gas readings and temperature. Used to be CO and HC were enough. Now you have to look at CO, HC, O2, CO2, and NOX to diagnose cars.

Peter
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  #24  
Old 07-18-2004, 06:16 PM
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No, but I have a MS from the U. of Wisconsin Engine Research Center where I did emssions related research, and I refute an claims that commercially available gasolines have a significantly different flame propagation rate.

It's a myth!

Duke
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  #25  
Old 07-19-2004, 01:27 AM
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Then why are there different grades of gasoline? why not just one? Why crack gas at different grades?

Peter
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  #26  
Old 07-19-2004, 01:47 PM
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The basic difference between commercial gasoline grades is octane number, which is a measure of the fuel's resistance to detonation. Higher compression ratios need higher octane, but deliver higher power across the rev range and greater fuel efficiency.

During "normal combustion" flame propagation rates for fuel-air mixtures of a given ratio and density are about the same. Detonation is "abnormal combustion", and since the unburned portion of the F-A mix ignites spontaneously and completely rather than being consumed by normal flame propogation this may be the source of the myth than "premium gas burns slower".

Duke
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  #27  
Old 07-19-2004, 02:20 PM
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Engine operating temperature is often overlooked as a contributing factor to high NOX output. A ten degree increase in engine temperature will affect NOX output by a certain percentage. Duke2.6 may be able to shed more light on this particular area of combustion science. Suppose your radiator has low flow, on your temp gauge you see no real visible signs until you decide to pull a steep grade. When you begin your ascent you begin to notice the temperature increase and in most cases you may dismiss it because you are climbing a hill. The radiator may be flowing 70% to 80% of the volume it was designed to flow. Because it is so gradual over time as a driver you begin to unknowingly adapt to this gradual increase in operating temperature. In some cases a new t-stat, a new temp switch, or a new radiator (maybe a combo of any or all) will have more impact at reducing NOX than what is often considered.

This assumes that you have eliminated primary contributors and or emission components/devices that are responsible for either increasing or decreasing the overall NOX output. (Engine timing, EGR Valve & control, Catalyst, etc warrant that a technician may need to verify their correct operation before reaching a conclusion)
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  #28  
Old 07-19-2004, 05:19 PM
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Coolant temperature has little to do with NOx. NOx is produced in the 4500 degree flame front, and there is essentially no difference in flame front temperature (for a given set of operating conditions) within the normal range of coolant temperatures. The higher the flame front temperature, the greater the NOx produced, and there are strategies to decrease engine-out NOx to a level that will be acceptable at the tailpipe after further reduction reaction in the catalyst. (I use the word "reduction" in the chemistry context. It is the opposite of oxidation.)

Higher coolant temperature mean hotter combustion chamber boundaries, which reduces flame quenching at the boundaries, so engine out HC is usually lower with high coolant temperatures.

Spark timing has a big impact on both engine out NOx and tailpipe HC emissions. For every operating condition (speed and load), there is a timing value that both maximizes torque and minimizes fuel consumption. This "ideal" timing will also produce the greatest flame front temperature and NOx, especially under "lean" i.e. stoiciometric condtions, so timing under many operating conditions is decreased below the ideal to decrease engine out NOx. Retarded timing also increases EGT, which helps keep the converter hot enough to generate good oxidation and reduction efficiency to decrease engine-out emissions to acceptable tailpipe values.

The "ideal" timing is that which produces the greatest thermal efficiency, and greater themal efficiency requires adding heat at higher temperature, but if NOx is too high, timing must be retarded. This essentially reduces the fuel's contribution to useful energy at the crankshaft and throws more energy out the exhaust, which, of course, increases fuel consumption.

The other strategy to control engine-out NOx is EGR. By diluting the fresh charge with an inert gas, peak flame front temperature is reduced.

A simple stategy that car owners can use to reduce both NOx and HC on a car that may have marginal emission test performance is to retard initial timing. Retarding the timing reduces peak flame front temperature, which reduces engine out NOx. Also, the higher EGT from retarded timing will keep the converter hotter, which will increase its oxidation and reduction efficiency.

Unfortunately this is not an option on many Mercedes models because the intial timing is not adjustable - like my M103 engine.

Duke
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  #29  
Old 07-19-2004, 05:40 PM
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Let me share a real world experience with you and you may have a different perspective.

1979 450SL-Failed NoX, you name it was done. Any and all items that you could possibly install to correct the matter simply did little to bring the NoX down. After my friend spent many hours on the vehicle I suggested he take for a nice long drive to see how it would clean up. As luck would have it we have a good selection of decent grades here in the bay area, so off to Santa Cruz. Half way up the hill the engine temp was hovering at 100 C, way too hot for a 65 degree day.

Long story short- the radiator was flowing less than 60%. Installed a new raditator and the NoX fell to the basement.

2100ppm before the rad, <400 after.

Engine temp is something I now consider based on this experience.
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  #30  
Old 07-19-2004, 11:49 PM
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Interesting anecdote, but there had to be some other factor at work. For example, above a certain high coolant temperature, some emission control configuratations will restore vacuum advance at idle and low speed/load and/or suspend EGR, both of which will increase NOx. That's just an example. Since I'm not intimately familiar with the emission control equipment or configuration of that engine, I could only hazard a guess, but engines of that era had all kinds of klugy emission control hardware that can be a nightmare to sort out.

As a general rule for emission testing you want the engine as hot as possible. A change in operating temperature within the normal range - say 80 to 110 C will not cause that dramatic a change in NOx unless the high temp disabled the NOx control features; 2100 ppm would be representative of a non-emission controlled engine.

The high temperatures were effecting the NOx part of the emission control system in some way, but without a detailed understanding off all the emission control hardware and control systems it would be tough to sort out. Catalysts of that era were oxidizing only. NOx was controlled either with EGR, excess designed in valve overlap, or the spark timing map or a combination up to all three on on big, heavy cars. Excess valve overlap creates a full time EGR system, and GM used this strategy in the seventies, but it really killed low end torque and in town fuel economy.

Duke


Last edited by Duke2.6; 07-20-2004 at 12:07 AM.
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