Originally Posted By: itguy08
Originally Posted By: A_Harman
There is a lot wrong in the above paragraph:
1. Poor fuel economy has everything to do with stoichiometry. Lean burn engines are more efficient than stoichiometric engines, which are in turn more efficient than rich burn engines.
According to
http://www.engineeringtoolbox.com/stoichiometric-combustion-d_399.html
Stochiometric combustion is where you use all the fuel you put into the system. Which in a gas engine is 13:1 (14.7:1 because of the cats). At that point, you are at maximum efficiency. You are getting the most power for your gas - you are not wasting it as unburnt or robbing yourself of power because of lean burn.
Lean burn engines are a way to overcome pumping losses in gasoline engines. At part throttle openings and under low load you can feed it less gas to increase the MPGs but you are not going to run lean burn at WOT or anywhere near that as power will be lost.
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2. Not all engine strive for stoichiometric combustion. Ever hear of diesels?
Diesles are a different breed. They have a stochiometric point too. And they also widely vary the A/F ratio. The issue with Diesels is at the stochiometric ratio there is lots of soot and smoke so we dial it back.
Quote:
3. O2 sensors only maintain gasoline engines at stoichiometric A/F up to a set manifold pressure, above which, enrichment is used to make full power and to protect engine parts, turbos, and cats from overheating.
And I believe I read where the EPA is cracking down on the enrichment at WOT and coaxing manufacturers to stay closed loop and stochiometric at all engine speeds. I know you rarely see the puff of black smoke anymore when someone "gets on it".
I also think that Direct Injection helps with a lot of these issues as it does help to reduce detonation (cooling effect), and seems to be more precse as far as getting gas to the cylendars.
After having worked deeply in engine development as a professional engineer since 1985 at GM, John Deere, Cummins, and at a Tier One automotive supplier, I can assure that what I have written is correct. Stoichiometric is just the point where there is exactly enough fuel in the combustion chamber to combine with the oxygen that is available. Stoichiometric air/fuel ratio for gasoline is 14.7:1, not 13:1. Stoichiometric is neither best power, nor best economy in premix charge engines. Best power is achieved in most engines at ~10% richer than stoichiometric. Best economy is achieved at ~10% lean.
But modern engines' emissions systems are designed to operate at stoichiometric for emissions control. Running rich of stoich will increase an engine's CO and HC emissions. Running lean of stoich will increase an engine's NOx emissions. Therefore, running at stoich optimizes and engines' exhaust gas content to allow the 3-way catalyst to operate most efficiently.
"Diesels are a different breed." Good of you to recognize that because many people don't, but I was responding to your original blanket statement that "Engines strive for stoichiometric". Diesel engines operate in the region of 100:1 to 22:1 air/fuel ratio from idle to designed rated power. Yes, stoich for diesel fuel is 14.5, but no engine sold with a warranty gets there in steady-state operation. Once the air/fuel ratio goes lower than ~20, visible smoke sets in, and even though the engine will make more power at the richer mixture, the thermal efficiency drops because there is not enough time for good mixing of air and fuel in the cylinder. If you want to see diesels running at stoich, go to a tractor pull.
I can believe that the EPA would take it upon themselves to crack down on full power enrichment because they currently take a lot of things on themselves that they shouldn't. But auto manufacturers don't put over-rich operation modes at WOT in their ECM's to spite the EPA. They do it to protect their engines. And the EPA is partly to blame for creating this circumstance. They started regulating cold-start emissions in the late '90's, forcing automakers to move catalytic converters closer to the engines, thereby increasing the inlet temperatures and overtemping the cats. The OEM's then had to richen up the mixtures at high load conditions to ~10-11:1 A/F to keep catalyst inlet temperatures down to ~1650F. And by the way, engine power is actually less at these over-rich A/F's; power drops off after 12.5. "Be careful what you ask for, you might just get it."
