Tonys96

It is important to note that the theoretical energy content of gasoline when burned in air is only related to the hydrogen and carbon contents. The energy is released when the hydrogen and carbon are oxidised (burnt), to form water and carbon dioxide. Octane rating is not fundamentally related to the energy content, and the actual hydrocarbon and oxygenate components used in the gasoline will determine both the energy release and the antiknock rating.

Two important reactions are:
C + O2 = CO2
H + O2 = H2O

The mass or volume of air required to provide sufficient oxygen to achieve this complete combustion is the "stoichiometric" mass or volume of air. Insufficient air = "rich", and excess air = "lean", and the stoichiometric mass of air is related to the carbon:hydrogen ratio of the fuel. The procedures for calculation of stoichiometric air-fuel ratios are fully documented in an SAE standard.

Atomic masses used are:- Hydrogen = 1.00794, Carbon = 12.011, Oxygen = 15.994, Nitrogen = 14.0067, and Sulfur = 32.066.

The composition of sea level air ( 1976 data, hence low CO2 value ) is
Gas Fractional Molecular Weight Relative
Species Volume kg/mole Mass
N2 0.78084 28.0134 21.873983
O2 0.209476 31.9988 6.702981
Ar 0.00934 39.948 0.373114
CO2 0.000314 44.0098 0.013919
Ne 0.00001818 20.179 0.000365
He 0.00000524 4.002602 0.000021
Kr 0.00000114 83.80 0.000092
Xe 0.000000087 131.29 0.000011
CH4 0.000002 16.04276 0.000032
H2 0.0000005 2.01588 0.000001
---------
Air 28.964419

For normal heptane C7H16 with a molecular weight = 100.204
C7H16 + 11O2 = 7CO2 + 8H2O
thus 1.000 kg of C7H16 requires 3.513 kg of O2 = 15.179 kg of air.

The chemical stoichiometric combustion of hydrocarbons with oxygen can be written as:
CxHy + (x + (y/4))O2 -> xCO2 + (y/2)H2O

Often, for simplicity, the remainder of air is assumed to be nitrogen, which can be added to the equation when exhaust compositions are required. As a general rule, maximum power is achieved at slightly rich, whereas maximum fuel economy is achieved at slightly lean.
The energy content of the gasoline is measured by burning all the fuel inside a bomb calorimeter and measuring the temperature increase. The energy available depends on what happens to the water produced from the combustion of the hydrogen. If the water remains as a gas, then it cannot release the heat of vaporization, thus producing the Nett Calorific Value. If the water were condensed back to the original fuel temperature, then Gross Calorific Value of the fuel, which will be larger, is obtained.
The calorific values are fairly constant for families of HCs, which is not surprising, given their fairly consistent carbon:hydrogen ratios. For liquid ( l ) or gaseous ( g ) fuel converted to gaseous products - except for the 2-methylbutene-2, where only gaseous is reported. * = Blending Octane Number as reported by API Project 45 using 60 octane base fuel, and the numbers in brackets are Blending Octane Numbers currently used for modern fuels. Typical Heats of Combustion are [36]:

Fuel State Heat of Combustion Research Motor
MJ/kg Octane Octane
n-heptane l 44.592 0 0
g 44.955
i-octane l 44.374 100 100
g 44.682
toluene l 40.554 124* (111) 112* (94)
g 40.967
2-methylbutene-2 44.720 176* (113) 141* (81)

Because all the data is available, the calorific value of fuels can be estimated quite accurately from hydrocarbon fuel properties such as the density, sulfur content, and aniline point (which indicates the aromatics content).
It should be noted that because oxygenates contain oxygen that can not provide energy, they will have significantly lower energy contents. They are added to provide octane, not energy. For an engine that can be optimized for oxygenates, more fuel is required to obtain the same power, but they can burn slightly more efficiently, thus the power ratio is not identical to the energy content ratio. They also require more energy to vaporize.

Energy Content Heat of Vaporization Oxygen Content
Nett MJ/kg MJ/kg wt%
Methanol 19.95 1.154 49.9
Ethanol 26.68 0.913 34.7
MTBE 35.18 0.322 18.2
ETBE 36.29 0.310 15.7
TAME 36.28 0.323 15.7
Gasoline 42 - 44 0.297 0.0<>
Typical values for commercial fuels in megajoules/kilogram are [37]:

Gross Nett
Hydrogen 141.9 120.0
Carbon to Carbon monoxide 10.2 -
Carbon to Carbon dioxide 32.8 -
Sulfur to sulfur dioxide 9.16 -
Natural Gas 53.1 48.0
Liquified petroleum gas 49.8 46.1
Aviation gasoline 46.0 44.0
Automotive gasoline 45.8 43.8
Kerosine 46.3 43.3
Diesel 45.3 42.5

Obviously, for automobiles, the nett calorific value is appropriate, as the water is emitted as vapour. The engine can not utilize the additional energy available when the steam is condensed back to water. The calorific value is the maximum energy that can be obtained from the fuel by combustion, but the reality of modern SI engines is that thermal efficiencies of only 20-40% may be obtained, this limit being due to engineering and material constraints that prevent optimum thermal conditions being used. CI engines can achieve higher thermal efficiencies, usually over a wider operating range as well. Note that combustion efficiencies are high, it is the thermal efficiency of the engine is low due to losses. For a water-cooled SI engine with 25% useful work at the crankshaft, the losses may consist of 35% (coolant), 33% (exhaust), and 12% (surroundings).


Hey, it's what I do :lol: :lol: :lol:

Marlin in Minn

Okey dokey, I read all of these posts ( ***fingers crossed behind back*** ) and just have one question.

Which gas station or convenience store sells the best gas? :)

Bob86ZZ4

Hay Tony, that's exactly what I think about going through turn 2 at Brainerd, how did you know?

Stingray1967

I was talking to Bruce Brown (THE C5 repair guy in town) over at Village Chev about this when my fuel sending unit failed last year. He says that Mobil is one of the consistently better brands, AND to stay away from SA. Apparently they put an additive in their gas that isn't as nice to the spark plugs.

Tonys96

All the gas stations in town buy some gas from us, they all have their own additive package. SA has their own refinery and buys only a small amount of gasoline from us, I can smell the difference.

That being said, the federal government regulates the heck out of gasoline, they are all pretty much the same.

VetNutJim

I ALWAYS consult the Periodic Tables before and while I'm driving.
It allows me to properly apply Kirchoff's Law and Thevinems Theorem to obtain the maximum BTU efficiency.
Judiciously applied, you may expect an increase of several Microts Per Parsec.
Of course you must take E=MC`` and the effect of gravity waves in maximizing your Microts.

Any specific MPP (Microts Per Parsec) must be substantiated by sound Thessalonian Thesis.
By taking into account the third dimensional order of magnitude of the developing boundary layer one can surmise there will be a considerable rise in the DCP (dynamic cylinder pressure) at the time of maximum ER. (energy release)
Once achieved, max ER may be calculated to be the result of E02 * Xy/15.032 to the tenth power of Magnamous.
As a result of this formula the dynamic pulse value in KV (kilovolts) can be computed by the following formula: P=I*E and the energy released can be assumed to be P * Zn(47/Pi)/237.00231 & %%7@^(73).

The likelyhood of spontaneous combustion increases in proportion to the value of Z in the above formula.
Therefore it is good sound practice to dynamically modify the value of Zn by applying Hungarian Notation to the exponent of Pi which yields 4.883%(Zn#) .
This always provides some degree of protection from the Alpha and Gamma ray components of the pulsar beam.
The galvanic strength of the sidelobes may be computed thusly:

Gs=9*(square root*Pi)/117.87321

Once the value of Gs is calculated the maximum Gs likely to be encountered can be computed as:

GsMax=W*M/pF or Weight * Mass divided by the Pulsar Force spread spectrum value.

CorvetteZ51Racer

Two quick questions/comments:

True, but have you guys looked at all at lean burn engines? We have done research in lean burn engines as well as alternative fuels engines, as well as the neural network programming that goes into running an engine that will run at lean burn during low-load conditions and slightly rich during high load conditions to provide a more flexible system.

My understanding on oxygenates is that their whole purpose is to introduce oxygen into the combustion chamber FOR combustion purposes...the more oxygen you can burn the more power you'll make. This is why Top Fuel cars burn Nitromethane and everybody and their dog uses nitrous (2 oxygen atoms per molecule in nitro...enough for it to burn in an inert gas environment). The added fuel, as my understanding goes, is to bring the AFR back in line, since the fuel metering system cannot detect the oxygen present in the fuel itself. I do know that some race fuels will use oxygenates as an alternative to lead for increasing the octane rating of the fuel, but at least here in Florida and back in Texas, where I'm originally from, those fuels are few and far between.

Tonys96

Do you work in a refinery ?

:lol: :lol: :lol:

Those are good ones.

Tonys96

Not up here in the corn belt.

Remember:

For an engine that can be optimized for oxygenates, more fuel is required to obtain the same power, but they can burn slightly more efficiently, thus the power ratio is not identical to the energy content ratio. They also require more energy to vaporize


[Modified by Tonys96, 9:07 PM 2/21/2003]

Flame Red

I was told this about various gasoline brands as far as gas goes here in central Florida (perhaps other areas are different).

There is one pipeline that runs from Tampa Bay to Southern Orlando (Taft). The ships come from the refineries to Tampa Bay and offload the gasoline into the pipeline. Truck tankers come to Taft and deliver the fuels to the local stations.

I was told that except for various octane, they just mix it up in the pipeline. All the same, it just varies from ship to ship and what contaminates it happens to pick up in the bottom of the ship/tank/etc.

I donno if this is just an urban legend, but that is what one of the guys that operate the pipeline told me.

Let the flames begin!

tkrussell

Tony,
Great exposition of the thermodynamics.
You singlehandedly raised the IQ of this Forum to Mensa levels.
Will this be on the test?
Thanks
Thomas

grapeknutz

:leaving:

Tonys96

Pipelines are magic stuff, usually tankers unload into huge storage tanks, grades are kept separate. When it goes into a pipeline there usually is an interface between products, this has to be downgraded to be sold.

As far as contaminants,(water) they usually settle out in the tanks and are removed.


[Modified by Tonys96, 11:16 AM 2/22/2003]

Tonys96

Thanks Thomas. no test, was just a local discussion on tuning that got carried away, look at the rest of the [Octane] posts.