[Syty] Passed emissions test, but why?

Wed Jan 18 12:21:37 CST 2006

Sorry about the delay,

I've just been hit by a dozen alligators all at once.  Hot-rodding has gone to the back-back burner.

I'm less familiar with the SyTy than the Grand National/TTA ECM, but every cold engine needs to go through some sort of "choke cycle" before it gets sufficiently warmed-up to go into lean-burn, CLOSED-LOOP operation.  So if you were to be parked overnight at your local tailpipe-sniffer's... in hopes of being first in line the next morning... and then started up and pulled right into the inspection station, YOUR CAR WOULD FLUNK at least unburned-HC in a big, big way!  And the colder the outside air-temp, the worse your "F" would be and the longer the interval to get into closed loop.

The TTA's algorithm for this "choke sequence" is such that running a 160° t-stat in winter conditions (IAT reading correlated with CTS) could delay closed loop indefinitely.  Chip-reprogramming or soldering 300Ω switched-resistors into the harness was the only way around this as I recall, but unfortunately all my priceless TTA notes have been either mislaid or stolen in my mega-move from Maryland, and my own senility has warped the rest of my memory of midnight surgeries performed over a decade ago....

I haven't read about this extended-choke problem with 160° t-stats in our particular vehicles so I can only assume the fuel-enrichment algorithm is different.  JIM SLOAN should be able to tell us about this from a position of superior experience, I told.  I've never even gotten around to swapping my 180° thermostat in the Syclone (which doesn't "ping" in any case and won't "choke" properly at all these cooler days... as you've likely been reading lately) and I haven't driven the Radar-Blue 'Phoon enough to observe rich-running problems or flunking emissions.  We don't have such tyranny in Texas! ...at least not in San Antonio!

So as usual, the answer to your question remains:  "It depends...."

Mr. Wizard
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----- Original Message ----- 
From: Peter 
To: DOlivares ; syty 
Sent: Saturday, January 14, 2006 7:14 PM
Subject: Re: [Syty] Passed emissions test, but why?

Dennis,

Thanks for the insight.  I think few readers will disagree with anything you've said.

Since I don't have a hood scoop, I do like the 160F thermostat for keeping things just a bit cooler under the hood for the general benefit of the components as well as performance.  If I do ever get a vented hood, I would look for one with the opening at the front instead of at the windshield, since the windshield is a high pressure area and actually flows in the reverse direction when at speed.

One person responded with the possibility that at 160F, the engine might think it is not fully warmed up and will deliberately idle rich.  He mentioned that normally we assume a rich mxiture would make the converter run hotter.  I'm not sure that applies at idle.  I have heard of engine controls programmed to intentionally run extra fuel to cool the converter during extended idle longer than 45 seconds.  Where I read this, they mentioned the emissions test will pass the idle portion if either the idle at the beginning or at the end of the test pass.  Sometimes the idle at the beginning will be screwed up by the extended idling in line.

Unless you know enough to idle at 1500 to 2000 rpm while waiting for your turn on the treadmill.  Then hope they don't take too long testing the gas cap.

Anyway, do you think the rich idle theory could be a plausible component of the 160/195F emissions delta?

Peter

DOlivares <seravilo at netzero.net> wrote:
  It's "basic" thermodynamics!  The HOTTER the cylinder-inlet temperature of the (hopefully) oxygen-rich air and gasoline mist, the higher will be the peak cylinder pressure/temperature after combustion.  Hotter inlet-fuel/air temperature, i.e., "initial conditions" from your ol' physics days, is the near-exclusive function of cylinder-wall/head-temps and thus the hotter thermostat you run the better your emissions (and gas mileage) up to the point where the safety/reliability/drivability algorithms in your ECM will start backing-out ignition-advance, enrich the mixture (injectors' pulse-width) or whatever to prevent piston-meltdown, radical preignition or some other illusory fatality that almost never happens if you've kept your motor in good shape and done your build-up homework correctly.

  The trade-off for "running hot" goes beyond simply trying to outfox your onboard computer (which is why I got into chip-reprogramming years ago...) and includes dealing with harsh realities.  For example, running high underhood temps will degrade your plastics and rubbers (!) at an accelerated rate, especially after hard-run summer heat-soaks without a hood-scoop for venting.  (On the other hand, you can roast weenies-- even your own if you're not careful-- with that car-heater set on High!)  Elevated intake-manifold/filter/pipe temperatures also tend to offset all that nice, cold, oxygen-dense air you went through all that trouble to ram-in from outside that hostile underhood environment-- not optimal for peak-power production and racing.  High-heat/lean-burn conditions also drives up NOx emissions, which used to be a don't-care (and which is why I got away with so many emisso-tricks in Virginia years ago!) but may now place a hard-limit on how low you want to drive your CO and unburned-HC readings in "sensitive" jurisdictions such as yours appears to be....

  This topic is quite a bit more complex than I'm letting on, but you got your basic answer, I hope.  Even I could find fault with nearly everything I said above, depending on context and circumstances!  But then, I like arguing with myself best of all... helps keep me young at heart.

  And now that I've pulled the pin, let's see what colorful responses (and the usual personal attacks... from the usual Peanut Gallery) this Grenade provokes!  I can almost predict what I'll see....

  Your Faithful Servant 
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  ----- Original Message ----- 
  From: Peter 
  To: syty 
  Sent: Saturday, January 14, 2006 9:34 AM
  Subject: [Syty] Passed emissions test, but why?

  Why does thermostat temperature have so large an influence on passing emissions?

  Twice now my Ty has failed emissions testing here in St. Louis, just barely failing HC, NOx, and CO.  The third attempt followed swapping the 160F thermostat for a 195F thermostat.  It "quick passed" with emissions at about 10% of the allowable limit.

  Exact same story as the last test two years ago.

  I presume the catalytic converter was not firing up with the 160F and came alive with the 195.  So why does the thermostat influence exhaust gas temperature so much?

  Or is there another emissions control mechanism at work?

  Best regards,

  Peter

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