[Syty] Passed emissions test, but why?

[DOlivares]

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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