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Old 02-09-2015, 03:57 PM   #21 (permalink)
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Originally Posted by stovie View Post
... if the fuel requires 14.7-1 a/f ratio at atmospheric pressure then it souls only need about 147-1 at a 10-1 compression ratio do to the reduction in the fuel molecules distance apart....
The physical spacing of the molecules has nothing to do with it. What matters is the number of molecules of air, and the number of molecules of fuel. That's why the 14.7:1 ratio is in weights, because weight is a very good proxy for the number of molecules.

Chemical reactions use a certain amount of reactants, and create a certain number of products. For instance, when you burn hydrogen in an oxygen atmosphere, you get 2 * (H2) + 1 * (O2) => 2* (H2O). To make up the two H and one O from water, you need twice as much O2 as H2. Similarly, when you burn gasoline in air, you need very close to 14.7 times as much air as gasoline in order for the reaction to be complete. (That's the "stoichiometric" ratio.) More fuel means you have un-reacted fuel left, more air means you have un-reacted air.

When you have excess air, the combustion reaction changes. It can produce more heat, then as you go even leaner it produces less heat. It also produces more oxides of nitrogen, and can cause mis-fires where the mixture doesn't burn at all. It can even cause incomplete burning if the flame front gets too weak.

Many of those problems can be solved in different ways, but the NOx one is rather more difficult. Since NOx produces stuff like acid rain, it's generally a good thing to try to limit its production. Keeping the mixture to right around 14.7:1 makes the current catalytic converters work well, which reduces all of the emissions from the tailpipe.

I believe that there are catalysts that can work at leaner ratios, but they are more expensive. And the higher MPG is now being produced less expensively (or at least more profitably) with hybrid cars rather than lean-burn cars.

-soD

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Old 02-12-2015, 01:51 PM   #22 (permalink)
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I understand that you need the 14.7-1 a/f ratio (or the stoichiometric ratio) to burn the fuel at atmospheric pressure to achieve a "2 C8H18 + 25 O2 → 16 CO2 + 18 H2O". However, once you get further away from that ratio the issue becomes weither or not the "reaction" will continue after exceeding that ratio is the distance of the fuel molecules. If there to far a part then some will "react", but others will not braking the reaction and stopping the burning of the fuel.

Now in a internal combustion engine where it compresses the a/f mixture it's a bit less of an issue. However the issue after that is the increased heat produced as you lean it out, because if the engine is running pretty rich then your already running at the other end of the stoichiometric ratio. Basically what i'm trying to say is that the only real way to get the a/f ratio to the lowest you can, you have to start with no fuel and slowly add it tell the engine starts to run properly, and with my experience with doing that so far it is far less then the 14.7-1 a/f ratio that the car manufacturers say they are using.

A car can handle the "instant explosion" caused by going so low because it's a very small explosion when you get it to the right amount. It's like lighting a small balloon of gas in a closed room, to big and it will blow the windows out. If it's just the right size though it will explode but just enough to raise the pressure in the room without blowing out the windows. In an engine if the "instant Explosion" is too high it just increases the RPM's.
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Old 02-12-2015, 02:17 PM   #23 (permalink)
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Actually, what happens inside the cylinder is not really an explosion. It is a burn, though a rapid one. Explosions are what you hear as detonation or pinging, and those are uncontrolled and can damage the engine. They're not more powerful than the normal operation, they are not controlled burns.

The proximity of the molecules to each other has some effect on the burn, but it does not have any effect on what ratio of reactants produces a complete reaction. That has purely to do with the number of molecules. You also seem to have been assuming a linear relationship between compression ratio and burnable mixture, and that is certainly not the case.

If the mixture is not close to the stoichiometric one, undesirable things happen. If it's too rich, you wind up with unburned fuel going out the exhaust, resulting in high HC emissions. If it's too lean, you wind up with elevated temperatures in the chamber and high NOx emissions. Those are both undesirable, though some of each can be tolerated as a necessary evil. Too far in either direction will also result in a loss of power, which is also generally undesirable.

The really big thing that happens when you stray from the stoich mixture is that standard (relatively inexpensive) catalytic exhaust converters stop working as effectively. So all levels of pollutants will potentially increase, or at the very least the total amounts will increase. Again, this is undesirable.

There are expensive catalysts and more sophisticated engine management strategies that can work better with leaner-running engines, but they cost more and are only moderately effective. They could likely be improved to where they do meet the current emissions regulations, but there isn't really any incentive for the manufacturers to do so. The cost will be significant, and basically the same benefits can be had for less money by using a smaller engine running more or less conventionally paired up with an electric motor.

The whole hybrid thing is very well understood, is being widely produced so the economies of scale are working for it, and has effects that are fairly close to those that would be achieved by investing a lot more $$$ into making a lean-burn setup that does everything that the automakers want it to do.

Lean burn may still be the way to go in the future, but at this point it isn't worth the investment for the manufacturers. Maybe the 2025 standards will force them to move that direction, or maybe not...

-soD
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Old 02-12-2015, 03:25 PM   #24 (permalink)
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Not the numbers on these are neccisarily acurate across different engines, but of all the ones i can recall, the hc emmisions low point is richer than the engine is capable of running. I would be surprised if your best economy would be found beyond the point where you start pushing more fuel out the tailpipe.
Only way I could think to find / tune to that concept would be borrowing a shops 5gas analyzer
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Old 02-12-2015, 06:46 PM   #25 (permalink)
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Yes, I understand that it's just that I've heard auto manufacturers say that they can't go too lean because "The engine can't handle a instant detonation of the fuel at thoughs low a levels".

I was just using that as a base figure. I don't honestly believe that the a/f ratio will Chang one to one with the compression ratio, I was just stating that it can change with it. And quit substantially.(though I'm not 100% sure how much!!)

may I ask how inefficient running lean would make the catalytic converter??

I have my Roketa 150cc scooter running using the "vapor" system and I find it weird that to accelerate with it I have to close the intake(tell about half way, then it starts to drop in rpm's) and to idle it I have to open the intake.
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Old 02-12-2015, 07:09 PM   #26 (permalink)
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Quote:
Originally Posted by stovie View Post
may I ask how inefficient running lean would make the catalytic converter??
I think they can't reduce nox if they stay lean very long, I think the 14.7:1 matters more to the cat than the engine

Stolen from Catalytic Converters - Chemwiki
There are two types of "systems" running in a catalytic converter, "lean" and "rich." When the system is running "lean," there is more oxygen than required, and the reactions therefore favor the oxidation of carbon monoxide and hydrocarbons (at the expense of the reduction of nitrogen oxides). On the contrary, when the system is running "rich," there is more fuel than needed, and the reactions favor the reduction of nitrogen oxides into elemental nitrogen and oxygen (at the expense of the two oxidation reactions).
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Old 02-12-2015, 08:51 PM   #27 (permalink)
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They have made 'cats' , like reverse fuel cells .. that use electrical energy to break down NOx instead of the chemical energy from unburned fuel ... that would be a valid option in PHEVs ... get high MPG Lean Burn and low NOx emissions at the same time.

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Old 02-13-2015, 10:58 PM   #28 (permalink)
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Quote:
Originally Posted by stovie View Post
The reason I'm going with this setup is because I've always believed that cars are running way to rich! I've thought for the last 5-10 years that if the fuel requires 14.7-1 a/f ratio at atmospheric pressure then it souls only need about 147-1 at a 10-1 compression ratio do to the reduction in the fuel molecules distance apart. At 10-1 compression ratio and 147-1 a/f ratio the fuel molecules are about as close as they would be at atmospheric pressure. I was able to mess around with my scooter yesterday for about 4-5 hours on 2 ounces of Coleman camping fuel, not sure of its fuel economy yet because I had no way to control it will on it.
Piston engine aircraft are able to adjust the fuel ratio. Best economy is around 16-17:1, if I remember correctly...and at an airspeed not to far above stall. I know a fella on another forum who had a tuner to adjust his Hemi's A/F ratio, and he pushed it to about that point and was getting another 2mpg.

Autospeed article on A/F ratios: AutoSpeed - Tuning Air/Fuel Ratios

Yes, we run 14.7:1 'cuz it's the chemically ideal mixture. True, some sort of by-products go out the pipe at other ratios. From the chart, it looks like running 17:1 (if able) would be about the same amounts as at 14.7:1, as the NOx is on the other side of the peak.

There is a gizmo from Zeitronix that allows mixture adjustment. They recommend you use a wide-band O2 meter with it. (Found it) Zeitronix Zt-3 : Wideband Controller and Datalogging System
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Old 02-14-2015, 01:05 PM   #29 (permalink)
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Well what would be the a/f ratio with a 3/4" intake and idle being at the WOT position??(my scooters idle is at wot position now, and wot is at 2/3 throttle now)
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Old 02-14-2015, 01:46 PM   #30 (permalink)
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Interesting project do you have some pics?

Quote:
Well what would be the a/f ratio with a 3/4" intake and idle being at the WOT position??(my scooters idle is at wot position now, and wot is at 2/3 throttle now)
My rough guess would be around 46 A/F ratio. Based on going from 30kPa to
100kPa

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