Stoichiometric air/fuel ratio of gasoline vapor

[Mech_Engineer]

The Stoichiometric ratio comes from the amount of oxygen needed to combine with all the carbon (to make CO2) and hydrogen (to make H2O) in the fuel on a theoretical basis, then since air is 22% oxygen, 78% nitrogen, the amount of air needs to be adjusted accordingly.

It's my understanding that the stoich. ratio for gasoline is actually experimentally derived due to the complex combination of hydrocarbons in gasoline.

Regarding different mixtures vs. power vs. efficiency, I found this chart interesting:

https://en.wikipedia.org/wiki/Air–fuel_ratio

I'm not sure of the original data source, it's not specified in Wikipedia.

 

[Ali Durrani]

Correct me if i am wrong but stoichiometric AFR only quantifies the amount of fuel and air needed for complete combustion, i don't see any relation between AFR and the inlet state.

 

[Ketch22]

Correct me if i am wrong but stoichiometric AFR only quantifies the amount of fuel and air needed for complete combustion, i don't see any relation between AFR and the inlet state.

This is a very good point. The AFR is actually a perfect ratio to achieve perfect combustion. There is also as Mech Engineer posted a variance where a little richer produces more power (at the expense of efficiency) and leaner produces the best efficiency (at the expense of power).
A modern gasoline engine using an Engine Control Module uses either a Mass Air Flow sensor to determine intake air charge or a Manifold Air Pressure sensor either of these when combined with a throttle position sensor allows the ECU to calculate the intake air volume and inject the proper amount of fuel. Also the rpm the engine is rotating at compared to the throttle position allows the ECU to calculate the power/economy balance the driver is requesting.
The last stage is a pre catalytic converter O²sensor, by measuring the exhaust gas leaving the engine the ECU can identify the actual AFR achieved. This is then used as a correction feedback to the ECU to adjust injection. Also for variations in fuel such as "oxygenated" fuel which is 10% alcohol or E85 which is 15% the addition of alcohol and the reduction in total octane changes the Stoichiometric ratio. The O²sensor can than assist in the ECU adapting to a completely new fuel. High performance vehicles often push the limits of AFR and one of the significant changes that is done is to go to a wide range sensor so that the ECU can continue to read with a significantly changed AFR map.
On a fuel injected engine this is all controlled through the ECU. On a carbureted engine the jets are in the venturi prior to the throttle plates. The pressure drop is proportional to the air flow. Thus wider opening relates to more fuel. The engine is essentially self regulating. There is also a small mechanical pump to inject additional fuel when the accelerator is opened quickly to create the richer condition required. There is no actual feedback mechanisms to best operating condition this is all adjusted by the person tuning the engine. Their arsenal is lever rate changes and metering orifice changes. In more complex setups it also can include multiple carburetors operating in sequence. Mechanical fuel injection is actually more similar to carburetors as the feedback and calculation is very limited. The lack of feedback and significantly reduced adaptation is one of the large causes of the increase in both fuel efficiency and power when you compare a nearly identical engines of modern design and much older ones.

The goal is always to achieve Stoich or best match of performance and economy. The mechanism must sense or calculate the inlet volume to make this happen.

 

[Rx7man]

There's a reason that there has been so much research on camshafts, valvetrains, and combustion chamber designs over the last years..

Better intake systems yield more air into the engine, reducing pumping losses on that side, and increasing volumetric efficiency, thus power.
Better camshafts (VVT, etc) that tailor overlap and duration to the current requirements, as well as late-closing intake valve systems making the engine essentially follow the Atkinson cycle (especially at low loads)
Better combustion chamber designs yield higher detonation resistance and CLEANER BURNS from turbulence and minimized "dead air" volume that is in too close proximity to the cold walls to allow it to burn.

Heating the intake charge will yield minimal improvements in efficiency.. increasing engine coolant temperature can help to an extent, but the design of the internals is going to be where any real gains are made.

I have done extensive work on 2 stroke engines, and as different as they are, they suffer from many of the same problems.
Going back to combustion chamber design, a little bit of machine work on a chainsaw engine can yield DRASTIC improvements.. Moving the crankshaft up, or lowering the head so that "squish" (the distance between the piston and head at TDC) is reduced from a factory "safe" setting of .050" (~2mm) to about .020" and uniform is nothing less than surprising... I can't remember the estimated boundary layer air/fuel needs to be away from metal in order for combustion, It was about .020" I think, so reducing the squish from .050" to .020" pretty much HALVES the unburnt fuel, In a 70cc engine this can be about 2cc, and is certainly noticeable.

Completely evaporating the fuel in the intake is an exercise in futility the way I see it.. The problem isn't that the fuel doesn't have the TIME to evaporate, it's that the compression following it works to re-condense the fuel. Heating the intake charge works toward increased detonation, which requires higher A/F mixtures to combat, once again working against your end goal.

If a diesel engine can burn diesel (with a higher boiling point than gasoline) injected IN LIQUID FORM, that's proof enough for me that the evaporation of gasoline isn't where the losses are.

 

[HowlerMonkey]

Any possible gain of vaporizing is lost by the fact that the "air" has been heated and now has already expanded quite a bit so you must operate at much higher temperatures to gain back expansion.

Look up Smokey Yunicks hot vapor cycle engine.

The problem is materials handling the necessary temperatures.

 

[Rx7man]

Here's something far more promising that a vaporized fuel system... I was contemplating how feasible something like this would be so I googled it.. apparently Mahle has a lot of brain power and beat me to it, though I had envisioned a few other styles of doing it as well
https://crcao.org/workshops/2014AFEE/Final%20Presentations/Day%202%20Session%204%20Alt%20Fuels%20Presentations/4-4%20Blaxill,%20Hugh/4-4%20Blaxill,%20Hugh%20-%20CRC%202014%20Lean%20Combustion%20of%20Liquid%20and%20Gaseous%20Fuels.pdf

 

[RogueOne]

Hello again Mech_Engineer,
Thank you for taking the time to talk to me, appreciated. Just to let you know what I am doing, I am in the process of building a Gasoline Vapor System for an automobile, (1978 Chevy Camaro, 350 cu.in. SBC, automatic), I am on my 9th prototype and I am getting very close to success. The system does work and run.
I am using a stainless steel Shell & Tube Heat Exchanger, (3" X 14"), and I have plumbed exhaust tubes into the shell sides, with the system having it's own independent exhaust system. I am using a motorcycle carburetor to supply air and fuel. The Air/Fuel mixture flows through the 30 straight tubes, and then through a 2.5" 90,(elbow), into the Open Plenum Edelbrock intake manifold. The Stoichiometric ratio remains mostly in the 14:7 range, but goes down to the 12:1 range at lower RPM, (by Air/Fuel Gauge). I have been doing a lot of studying and research, and it seems that their are many of my predecessor's who believe that the 14.7:1 Stoichiometric is not correct for Vaporized Gasoline, and should be much leaner. What are your thoughts? Thanks, Mike

14.7:1 is correct. This is basic chemistry. If you have one molecule of gasoline, you need 14.7 molecules of the chemicals that make atmospheric air in order to burn all of the gasoline. You can work this out through what is called a "balanced equation". This is high-school level chemistry, frankly.

Also, I am not sure why you are going to such great lengths to "vaporize" the fuel before it goes into your engine. The fuel is already atomized as soon as it is drawn into the engine because it is very hot inside the engine's cylinder.

Did you know there are engines that already run on gaseous fuels? Natural GAS, Propane GAS, Hydrogen GAS, etc etc. If you want a gaseous fuel for some reason, I don't know why you are trying to vaporize liquid gasoline.

What do you plan to gain from it?

 

[RogueOne]

Hello Again Mech_Engineer,
Thanks for your response, though I do not agree with you. As pointed out previously in this topic, Liquid Gasoline doesn't burn, only Gasoline Vapor Burns in a Gasoline powered internal combustion engine. The inefficiency lies in the gasoline that does not become vaporized, and therefore is not combusted. Their is simply not enough time in the milliseconds of the power stroke, to completely burn a atomized gasoline / air mixture that must be converted into vapor from the heat that exists in the combustion chamber in order to combust. It is a fact that gasoline engines are very inefficient, and waste fuel through various different ways. This wasted Gasoline not only represents inefficiently, and less power, it is a major cause of air pollution. It is possible to create a safe mechanical system to pre-vaporize Gasoline in the area of the Intake Manifold, and through engine vacuum, duct the Gasoline Vapor into the combustion chamber of the engine. It has been done successfully many times before. I would also respectfully disagree with you regarding the transport of liquid gasoline through the intake tract, while simultaneously keeping the air / liquid in suspension. This as opposed to moving a gas,(vapor) through the intake tract. In my opinion, moving a gas,(vapor) would be no different than simply moving only the air. May I also point out the existence of successful vehicles that currently run, on a daily basis, on Compressed Natural Gas and on Propane.
Regards, Mike

You will be disappointed to learn that the Brake Specific Fuel Consumption of LPG engines is no more efficient than gasoline powered engines. Also, the "wasted" gasoline that you speak of does not happen. There is more than enough time to burn all of the gasoline, especially with the 14.7:1 stoich ratio. Look at CO and HC emissions from modern engines. They are very low, which indicates that the fuel is being burned completely. I've explained this to many people like you. They do not believe me, but they can't prove me wrong. I know this topic intimately from when I worked on a combustion engineering team for gasoline engines.

 

[Work Hard Play Hard]

Also, the "wasted" gasoline that you speak of does not happen. There is more than enough time to burn all of the gasoline, especially with the 14.7:1 stoich ratio. Look at CO and HC emissions from modern engines. They are very low, which indicates that the fuel is being burned completely.

Gutsy but good for you.

I can't agree with the "completely" but it's darn minimal or someone is driving around with the MIL--check engine soon--light on. There is also a lot more to evaporative emission systems than has been discussed. For example, the secondary air injection pump system controlled by the secondary air injection system monitor. The pump injects air into the exhaust to promote oxidation of VOC's such as unburned fuel. Except for cold starts if the pump is working there is a fault code to go along with it. There is a long list of similar monitors and systems.


These are the gases that the 4 or 5-gas Gas analyser sees in a petrol engine:

• HC = Hydrocarbons, concentration of the exhaust in parts per million (ppm). = Unburned Petrol, represents the amount of unburned fuel due to incomplete combustion exiting through the exhaust. This is a necessary evil. We don't want it so try to keep it as low as possible. An approximate relationship between the percentage of wasted fuel through incomplete combustion and the ppm of HC is about 1/200 ( 1.0% partially burned fuel produces 200 ppm HC, 10%=2000 ppm HC, 0.1%=20 ppm HC )
• CO = Carbon Oxide, concentration of the exhaust in percent of the total sample. = Partially Burned Petrol, This is the petrol that has combusted, but not completely. This gas is formed in the cylinders when there is incomplete combustion and an excess of fuel. Therefore excessive CO contents are always a sign of an overly rich mixture preparation. ( The CO should have become CO2 but did not have the time or enough O2 to became real CO2 so it is exhausted as CO instead.) CO is HIGHLY POISONOUS ODORLESS GAS! Always work in well ventilated areas!
• CO2 = Carbon Dioxide, concentration of the exhaust in percent of the total sample. = Completely Burned Petrol, represents how well the air/fuel mixture is burned in the engine ( efficiency ). This gas gives a direct indication of combustion efficiency. It is generally 1-2% higher at 2500 RPM than at idle. This is due to improved gas flow resulting in better combustion efficiency. Maximum is around 16%. At night the trees convert CO2 into Oxygen. Preserve them!
• O2 = Oxygen, concentration of the exhaust in percent of the total sample. Free O2 occurs in the exhaust when there is an excess of air in the mixture. The O2 content increases sharply as soon as Lambda rises above 1. Taken with the CO2 maximum, the oxygen content is a clear indicator of the transition from rich to lean mixture range, or leaks in the manifold or exhaust systems or combustion failures. With rich mixture most of the oxygen is burned during combustion. Whit very lean mixture more O2 escapes "un-combusted" so the level rises.
• NOx = Oxides of Nitrogen (This is only seen by a 5-gas analyser) Only seen with dynamometer or engine under load. NOx emissions rise and fall in a reverse pattern to HC emissions. As the mixture becomes leaner more of the HC's are burnt, but at high temperatures and pressures (under load) in the combustion chamber there will be excess O2 molecules which combine with the nitrogen to create NOx. NOx increases in proportion to the ignition timing advance, irrespective of variations in A/F ratio. This gas is related to the exhaust gas detoxification systems ( in conjunction with Co and HC) , exhaust gas recirculation systems. Those systems bring some of the inert (processed) exhaust gas back into the engine to be burned again. This time around this gas has no O2 extra molecules and prevents high combustion temperatures and further increase in NOx formation. NOx is Very Dangerous Lethal Gas and air pollutant!
• A/F ratio or Lambda = Calculated Air/Fuel Ratio or Lambda value based on the HC, CO, CO2 and O2 concentrations. Remember the ideal (Stoichiometric) A/F is 14.7 liters air to 1 liter fuel or 14.7/1. The ideal Lambda value is 1(one) below that the A/F mixture is rich and above - lean. For example, lambda=0.8 corresponds to an air/fuel ratio of (0.8x14.7):1=11.76:1 ( e.g. lambda 0.8 = A/F ratio of 11.76/1 or very rich air fuel mixture )

General Rules of Emission Analysis

• If CO goes up, O2 goes down, and conversely if O2 goes up, CO goes down. Remember, CO readings are an indicator of a rich running engine and O2 readings are an indicator of a lean running engine.
• If HC increases as a result of a lean misfire, O2 will also increase
• CO2 will decrease in any of the above cases because of an air/fuel imbalance or misfire
• An increase in CO does not necessarily mean there will be an increase in HC. Additional HC will only be created at the point where rich misfire begins (3% to 4% CO)
• High HC, low CO, and high O2 at same time indicates a misfire due to lean or EGR diluted mixture
• High HC, high CO, and high O2 at same time indicates a misfire due to excessively rich mixture.
• High HC, Normal to marginally low CO, high O2, indicates a misfire due to a mechanical engine problem or ignition misfire
• Normal to marginally high HC, Normal to marginally low CO, and high O2 indicates a misfire due to false air or marginally lean mixture

Evaporative Emissions
Up to now, we've only discussed the creation and causes of tailpipe or exhaust emission output. However, it should be noted that hydrocarbon (HC) emissions come from the tailpipe, as well as other evaporative sources, like the crankcase, fuel tank and evaporative emissions recovery system. In fact, studies indicate that as much as 20% of all HC emissions from automobiles comes from the fuel tank and carburetor (on carbureted vehicle, of course). Because hydrocarbon emissions are Volatile Organic Compounds (VOCs) which contribute to smog production, it is just as important that evaporative emission controls are in as good a working order as combustion emission controls. Fuel injected vehicles use an evaporative emissions system to store fuel vapors from the fuel tank and burn them in the engine when it is running. When this system is in good operating order, fuel vapor cannot escape from the vehicle unless the fuel cap is removed.
And finally remember: In nature nothing is lost or gained, only converted! Same rule applies to emission analysis.

Michael,

Your question, "What is the optimum AFR for gasoline as a vapor?", is like asking what's the optimum way to strike out, lose your car keys or leave your wallet at home while on a hot date. There is more to gasoline then the 500+ hydrocarbons. As a liquid, atomized or aerosol the additives are suspended or in solution. Is that true with gasoline as a vapor? Is this an "All things being equal," question you've asked? Just a thought but what would those participating at Chemical Engineering have to say? I would also suggest you go to the EPA website and look at the CFR links found there. You might even find regulations identifying the amount of allowable vapor. If you become involved with engine design you will get intimate with the CFR.

 

[DrClaude]

Closed pending moderation.

 

[fresh_42]

It appears that everything reasonable and unreasonable has been said or corrected.
Since it's been quite a while the OP has been participated for the last time, the thread remains closed.