air - fuel ratio

Virtually all newer vehicles use three-way catalyst exhaust after-treatment systems. For maximum catalyst efficiency the A/F target average is stoichiometric (or the chemically correct mixture ratio). For

100% gasoline this is nominally 14.6:1. Most engine control systems purposefully dither on either side of stoich to maximize catalyst efficiency.

As alcohol(ethanol)is added, the feedback ECU controls will adjust the fuel system to keep the mixture at the mixture's stoichiometric value. Here's a simple table to illustrate how stoich varies with %EtOH

%EtOH Stoich A/F

0 14.6 5 14.16 10 13.74 15 13.35 20 12.98 85 9.55 100 9.00

Excellent info, thanks...

Dyno wrote in news:481dcd15$0$20205 $ snipped-for-privacy@roadrunner.com:

Interesting information. But no road-legal US/Canadian gasoline is 100% gasoline.

How do the above numbers compare to fuels with other oxygenates, such as MTBE? And what if the fuel contains MMT or TEL, neither of which are oxygenates?

Really? The certification fuel used by the automakers does not contain alcohol. And I believe, AMOCO still sells alcohol free fuel.

FWIW, the 0% gasoline shown above IS for pump grade unleaded regular. So, I guess I'm not sure what you are driving at here.

Not sure about MTBE, but MMT and TEL are added in such tiny percentages to do their jobs that they really don't affect the stoichiometric ratio in any measurable way. They're fractions of a percent by volume, not the

10% of ethanol in E10 fuel.

Steve wrote in news:JPidnZ5dYNFk8IPVnZ2dnUVZ snipped-for-privacy@texas.net:

That's the answer I was looking for.

If volume is the determining factor, I'd guess MTBE would have a similar effect on stoichiometric as ethanol, since it's added in concentrations up to 15%.

I dont think so, Tegger... Methyltertiarybutylether has a different ratio of hydrogen to carbon to oxygen.. It should not have the same stoichiometry as ethanol.

Ethanol is almost 35% oxygen by weight. MTBE is only about 18% oxygen.

"HLS" wrote in news:avKTj.1755$ snipped-for-privacy@flpi148.ffdc.sbc.com:

Then MTBE is about twice as efficient as ethanol, so the mixture can be leaner?

Some more fuel blend data.

****** MBTE ****** %MBTE Stoich A/F 0 14.6 5 14.42 10 14.25 15 14.07 20 13.91 100 11.69

****** Methanol ****** %MeOH Stoich A/F

0 14.6 5 13.73 10 12.96 15 12.27 20 11.65 55 8.61 (most energy per unit mass of air) 100 6.45

"Lean", to me, indicates a mixture which does not have an excess of fuel, or may even have an excess of the oxidant.

It requires more oxygen to burn a unit amount of MTBE than it does to burn the same unit amount of ethanol, and the energy released is consequently greater.

Most of the energy is derived from the oxidation of the hydrogen atoms in a hydrocarbon to form water. Less energy is derived from oxidation of the carbon to give CO2.

The oxygen (in oxygenates such as alcohol and MTBE) just occupies space and doesnt contribute to the energy derived from combustion. So, since the ethanol has a higher percentage of oxygen in the molecule, it has a lower amount of energy that it can contribute upon combustion.

The "octane rating" takes into account different properties of the fuel, not directly related to the stoichiometry.

Not according to the EPA. They say it contains less energy than ethanol per gallon. Density might be different but doesn't matter - no one purchases liquid fuel by weight.

But octane is far more important to fuel economy and performance. That is more important to how much energy goes to the wheels.

According to the EPA Ethanol and MTBE are almost the same for oxygen content, octane and energy content. According to the EPA it takes 10 percent ethanol to get the same oxygen as 11 percent MTBE.

-Jim

----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==----

The #1 Newsgroup Service in the World! >100,000Newsgroups

---= - Total Privacy via Encryption =---

the amount of oxygen required to stoiciometrically burn a unit of fuel is not directly related to the energy yielded by that process. those are two different properties of a fuel.

nate

"HLS" wrote in news:q5XTj.14417$ snipped-for-privacy@nlpi070.nbdc.sbc.com:

Then maybe I am using "lean" incorrectly.

My intent in the use of that word was to describe a mixture that contained closer to the traditional emissions-desired stoich of 14.7:1 air/fuel, versus a "rich" mixture that was lower than 14.7:1 (more fuel, less air).

Because MTBE does not contain as much oxygen as ethanol...?

I think you can tell I'm no chemist.

My faulty long-term memory coughs up something here... As I recall, the original purpose of oxygenates was to reduce CO by increasing C02, this to be achieved by making more oxygen available to the engine by packing it directly into the fuel.

Hmmm. Then it appears to make a difference in combustion whether the oxygen is contained within the intake air or contained within the fuel.

Why?

Jim, Octane rating has very little to do with fuel economy. Performance, yes.

You probably meant the amount of energy "liberated" when burning; it's an exothermic reaction after all.

LHV - Gasoline: 42 MJ/Kg LHV - Ethanol: 26.68 MJ/Kg LHV - Methanol: 19.95 MJ/Kg LHV - MBTE: 35.12 MJ/Kg

For 1 kg of air here are some interesting data. Terms: LHV - lower heating value (liquid fuel into reaction, uncondensed water vapor in exhaust). The blend energy@stoic is the theoretical yield from a reaction using one kg of air and stoichiometric amount of fuel.

Blend Blend Blend EtOH % Stoic LHV energy@stoich A/F MJ/Kg MJ/kg-air

0% 14.60 42.0 2.88 2% 14.42 41.7 2.89 5% 14.16 41.2 2.91 10% 13.74 40.5 2.94 15% 13.35 39.7 2.97 20% 12.98 38.9 3.00 40% 11.69 35.9 3.07 45% 11.41 35.1 3.08 50% 11.14 34.3 3.08 55% 10.88 33.6 3.09 60% 10.63 32.8 3.09 65% 10.40 32.0 3.08 85% 9.55 29.0 3.03 90% 9.36 28.2 3.01 95% 9.18 27.4 2.99 100% 9.00 26.7 2.96 Blend Blend Blend MeOH % Stoic LHV energy@stoich A/F MJ/Kg MJ/kg-air 0% 14.60 42.00 2.88 2% 14.24 41.56 2.92 5% 13.73 40.90 2.98 10% 12.96 39.80 3.07 15% 12.27 38.69 3.15 20% 11.65 37.59 3.23 40% 9.70 33.18 3.42 45% 9.31 32.08 3.45 50% 8.95 30.98 3.46 55% 8.61 29.87 3.47 60% 8.30 28.77 3.46 65% 8.02 27.67 3.45 85% 7.04 23.26 3.30 90% 6.83 22.16 3.24 95% 6.64 21.05 3.17 100% 6.45 19.95 3.09 Blend Blend Blend %MBTE Stoic LHV energy@stoich A/F MJ/Kg MJ/kg-air 0% 14.60 42.00 2.88 2% 14.53 41.86 2.88 5% 14.42 41.66 2.89 10% 14.25 41.31 2.90 15% 14.07 40.97 2.91 20% 13.91 40.62 2.92 40% 13.28 39.25 2.96 45% 13.13 38.91 2.96 50% 12.98 38.56 2.97 55% 12.84 38.22 2.98 60% 12.70 37.87 2.98 65% 12.57 37.53 2.99 85% 12.05 36.15 3.00 90% 11.93 35.81 3.00 95% 11.81 35.47 3.00 100% 11.69 35.12 3.00

While not shown above, in all cases if the amount of air is reduced to maintain constant energy@stoich, more fuel is still needed for all three oxygenates as the percentage increases.

Having AF ratio designed to maximize efficiency in the catalytic converter doesn't have much to do with fuel economy. When fuel economy starts to really matter the catalytic converter is going to be replaced with systems that burn the fuel inside the engine and deliver it to the wheels. And octane will matter.

-jim

----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==----

The #1 Newsgroup Service in the World! >100,000Newsgroups

---= - Total Privacy via Encryption =---

Seeing as how 99%+ of the fuel already IS burned in the engine, I don't agree with your assertion.

Octane rating is related to the susceptibility of the fuel to auto-ignition. This is generally most significant at high-load operating conditions.

When I think of fuel economy, I think of light-load operation; cruising down the highway or driving around town. On a time-weighted basis, under these conditions engine loads (BMEP levels) are relatively low. It is also at these conditions that the octane requirement is also low. Increasing the octane rating of the fuel will not alter the energy inherent in the fuel nor will it boost fuel economy.

Not true. Thermal efficiency aids BOTH performance and fuel economy. A higher CR without detonation will provide more power without any increase in fuel flow rate.

If we had higher octane fuels without serious environmental consequences, we could have better fuel milage. The higher CR that modern fuels allow is one reason fuel milage is better on a ton-mile per gallon basis than it was pre-WW2.

Burning the fuel and using the energy as opposed to shooting it out the back at the guy behind you are 2 completely different things. The average car engine is using less than 30% of the energy in the fuel so talking about the total energy contained in the fuel as if all of it is being used is a complete absurdity. If you want to talk fuel economy in a serious way you need to look at the 70% that is trailing behind every automobile rather than focusing on whether a particular fuel has 1% more or less energy.

The fact is that the catalytic converter is part of a design that is consuming additional fuel - not because of the small amount that is burned inside it but because of the constraints it places on the entire design that limits efficiency.

Well that is a very naive and simplistic view of a very complex subject. The fact is that almost any gasoline powered car on the road will get better fuel economy if you start fueling with aviation gas. Why is that? The issue of fuel economy and octane can't be reduced to a platitude and one simple number on a pump. Gasoline is not a single chemical with simplistic properties its a complicated mix and that mix is constantly changing.

Motor fuel is not designed for the cars. It's designed for maximizing profits. And cars are designed for maximizing profits. Lead was used in gasoline not because it benefited you car but because it benefited the oil companies bottom line. MTBE was mandated not because it benefited your car or your pocket book or your environment - it was mandated because it benefited the oil industry. The EPA estimates it will cost 3 billion dollars to clean up and remediate the damage done to the environment by MTBE. And obviously when the work is actually done it will cost a lot more. And of course you will pay for it because the oil industry can't be held responsible because they were told by the government to use MTBE instead of lead. And this was all done for octane which you claim has nothing to do with fuel economy.

But with fuel headed to $10 and $20 a gal. The driver might actually start having some say in the process. When the purchasing habits change the motivation is also going to change significantly.

No. Octane and fuel economy are heavily intertwined. The government and the oil companies would prefer you do not look to closely at that fact - because it's money in their pocket and not yours. Octane ratings are based on both high load and light load testing and it is nowhere near an exact science nor are the constituent properties of the fuel constant - they vary a lot with time and place. But more important fuel economy is what you pay per mile and for starters that price on the pump has everything to do with octane.

Did you know that the oil refineries call it "octane give away" if they ship a fuel that is 0.1% higher in octane than it is legally required to be. Giving the public .1% higher than it needs to be octane can cost a large refinery 20 million dollars a year. And you claim it has nothing to do with fuel economy.

The oil companies, car manufactures and government are not interested in fuel economy. No one cares how much your personal transportation costs other than the driver and many of them apparently don't care either. Today the government is asking you to respond to higher gas prices by driving more - they might as well ask you to slit your own throat. Increased fuel consumption will only drive the price of fuel higher. The entire system from top to bottom is designed to use more fuel and for the most part people like you just go with the flow and don't question any of the platitudes handed down from above. It's only when consumers will stop buying it that things will change.

No, incorrect, if you want to maximize fuel economy, the RON requirement is high under light loads. But your fuel supplier isn't telling you what the RON in the fuel you are using is. So you have no way of knowing how the octane in your fuel affects your fuel economy.

That belief is how fools are parted with their money. They would like you to focus on the inherent energy in the fuel. Octane has everything to do with how the oil companies make their money off you. Octane has everything to do with how an engine and its fuel can be designed to maximize fuel economy. The only hope is that as prices skyrocket that fuel economy will become a real issue, instead of a shell game, The realities of octane are going to have to be addressed head on rather than hidden as they are now.

-jim

----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==----

The #1 Newsgroup Service in the World! >100,000Newsgroups

---= - Total Privacy via Encryption =---

You *do* need to account for the energy content of the fuel, because even assuming that your engine is tuned to use the exact A/F ratio for every fuel that goes into it the overall efficiency of the engine itself does not change in any significant manner, so the fuel with the higher energy density will yield higher MPG.

Huh?

False. In fact, the opposite may be true, although to such a small degree that it's hard to say for sure one way or another.

That much is true, but there is absolutely *no* advantage to using a higher octane fuel than the engine is optimized for.

No, TEL was used because it was a cheap and cheerful octane booster, allowing fuel companies to sell higher octane fuel without a significantly higher cost. It also offered other benefits like exhaust valve/seat protection. This allowed mfgrs. to offer more efficient engines with higher compression ratios once TEL-enhanced, higher octane fuels were available.

Now that much is plausible, the MTBE thing does smack of policy set by politics.

Octane *does* have nothing to do with economy, once you are talking about octane ratings higher than that for which your car's engine was designed.

No, they're not.

Of course higher octane fuels are more expensive, but that doesn't affect you if you don't need the higher octane.

That's right. It has nothing to do with fuel economy. Of course the oil co's don't want to make fuel a higher octane than it needs to be, because higher octane fuels are more expensive to produce. That's why they charge more for them.

OK... so what does this have to do with octane?

No, it's fact.

I'd suggest you do a little more research before posting incorrect wild conjecture as fact. Or, if you're going to make claims that contradict what most educated people believe to be the case, at least provide some supporting documentation for your assertions.

nate