ENERGY CONTENT OF GASOLINE

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 Net 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:

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)

The Antiknock Index is (RON+MON)/2, ("Pump Octane"), or the Research Octane
Number + Motor Octane Number) divided by two.  Fuels with an Antiknock index
of 87, 89, 91 (Unleaded), and 88 (Leaded) are listed as typical for the US
at sea level, however higher altitudes will specify lower octane numbers.

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 (indicates aromatic content).

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

The mass or volume of air required to provide sufficient oxygen (O2) to
achieve this complete combustion is the "stoichiometric" amount 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.

Typical stoichiometric air-fuel ratios for various fuel components are:

	 6.4  methanol
	 9.0  ethanol
	11.7  MTBE
	12.1  ETBE, TAME
	14.6  gasoline without oxygenates

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
            Net 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 energy values for commercial fuels in Megajoules/Kilogram are:

                            Gross       Net
Hydrogen                    141.9      120.0
Natural Gas                  53.1       48.0
Liquefied petroleum gas      49.8       46.1
Aviation gasoline            46.0       44.0
Automotive gasoline          45.8       43.8
Kerosene                     46.3       43.3
Diesel                       45.3       42.5

Obviously, for automobiles, the net calorific value is appropriate, as the
water is emitted as vapor. The engine cannot 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 (Spark Ignition) 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 (Compression Ignition) engines can achieve higher thermal efficiencies,
but even though combustion efficiencies are high, 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).


DRIVABILITY ISSUES

Most suppliers of quality gasolines will formulate similar additives into
their products, and cheaper product lines are less likely to have such
additives added. As different brands of gasoline use different additives
and oxygenates, it is probable that important fuel parameters, such as
octane distribution, are slightly different, even though the pump octane
ratings are the same.

So, if you know your car is well-tuned, and in good condition, but the
drivability is pathetic on the correct octane, try another brand.
Remember that the composition will change with the season, so if you
lose drivability, try yet another brand. As various Clean Air Act
changes are introduced over the years, gasoline will continue to change.


THE EFFECT OF COMPRESSION RATIO ON ENGINE EFFICIENCY

Most people know that an increase in Compression Ratio will require
an increase in fuel octane for the same engine design. Increasing the
compression ratio increases the theoretical thermodynamic efficiency
of an engine according to the standard equation:

	Efficiency = 1 - (1/compression ratio)^gamma-1

where gamma = ratio of specific heats at constant pressure and constant
volume of the working fluid (for most purposes air is the working fluid,
and is treated as an ideal gas). There are indications that thermal
efficiency reaches a maximum at a compression ratio of about 17:1 for
gasoline fuels in an SI engine.

The efficiency gains are best when the engine is at incipient knock, that's
why knock sensors are used. Low compression ratio engines are less efficient
because they can not deliver as much of the ideal combustion power to the
flywheel. For a typical carbureted engine, without engine management:


   Compression       Octane Number    Brake Thermal Efficiency
     Ratio            Requirement          (Full Throttle)
      7:1                 81                     28 %
      8:1                 87                     30 %
      9:1                 92                     32 %
     10:1                 96                     33 %
     11:1                100                     34 %
     12:1                104                     35 %


TYPICAL GASOLINE CONTENTS:

* Oil-soluble Dye, initially added to leaded gasoline at about 10 ppm
  to prevent its misuse as an industrial solvent, and now also used
  to identify grades of product.

* Antioxidants, typically phenylene diamines or hindered phenols, are
  added to prevent oxidation of unsaturated hydrocarbons.

* Metal Deactivators, typically about 10ppm of chelating agent such as
  N,N'-disalicylidene-1,2-propanediamine is added to inhibit copper,
  which can rapidly catalyze oxidation of unsaturated hydrocarbons.

* Corrosion Inhibitors, about 5ppm of oil-soluble surfactants are added
  to prevent corrosion caused either by water condensing from cooling,
  water-saturated gasoline, or from condensation from air onto the
  walls of almost-empty gasoline tanks that drop below the dew point.

* Anti-icing Additives, used mainly with carbureted cars, and usually
  either a surfactant, alcohol or glycol.

* Anti-wear Additives, these are used to control wear in the upper
  cylinder and piston ring area that the gasoline contacts, and are
  usually very light hydrocarbon oils. Phosphorus additives can also
  be used on engines without exhaust catalyst systems.

* Deposit-modifying Additives, usually surfactants.
  1. Carburetor Deposits, additives to prevent these were required when
     crankcase blow-by (PCV) and exhaust gas recirculation (EGR) controls
     were introduced. Some fuel components reacted with these gas streams
     to form deposits on the throat and throttle plate of carburetors.
  2. Fuel Injector tips operate about 100C, and deposits form in the annulus
     during hot soak, mainly from the oxidation and polymerization of the
     larger unsaturated hydrocarbons. The additives that prevent and unclog
     these tips are usually polybutene succinimides or polyether amines.
  3. Intake Valve Deposits caused major problems in the mid-1980s when
     some engines had reduced drivability when fully warmed, even though
     the amount of deposit was below previously acceptable limits. It is
     believed that the new fuels and engine designs were producing a more
     absorbent deposit that grabbed some passing fuel vapor, causing lean
     hesitation. Intake valves operate about 300C, and if the valve is
     kept wet, deposits tend not to form, thus intermittent injectors
     tend to promote deposits.
  4. Combustion Chamber Deposits have been targeted in the 1990s, as
     they are responsible for significant increases in emissions. Recent
     detergent-dispersant additives have the ability to function in both
     liquid and vapor phases to remove deposits that have resulted from
     the use of other additives, and prevent deposit formation.

* Octane Enhancers, these are usually formulated blends of alkyl lead or
  MMT compounds in a solvent such as toluene, and added at the 100-1000 ppm
  levels. They have been replaced by hydrocarbons with higher octanes such
  as aromatics and olefins. These hydrocarbons are now being replaced by a
  mixture of saturated hydrocarbons and and oxygenates.


NITROMETHANE

Nitromethane (CH3NO2) - usually mixed with methanol which helps to reduce
peak flame temperatures - also provides excellent increases in volumetric
efficiency of IC engines - in part because of the lower stoichiometric
air-fuel ratio (1.7:1 for CH3NO2) and relatively high heats of vaporization
(0.56 MJ/kg for CH3NO2) result in dramatic cooling of the incoming charge.

	4CH3NO2 + 3O2 -> 4CO2 + 6H20 + 2N2

The nitromethane Specific Energy at stoichiometric (heat of combustion
divided by air-fuel ratio) of 6.6, compared to 2.9 for iso-octane,
indicates that the fuel energy delivered to the combustion chamber is
2.3 times that of iso-octane for the same mass of air. Coupled with
the higher flame temperature (2400C), and flame speed (0.5 m/s), it has
been shown that a 50% blend in methanol will increase the power output
by 45% over pure methanol, however knock also increased.

NITROUS OXIDE

Nitrous oxide (N2O) contains 33% by volume of oxygen, consequently the
combustion chamber is filled with less useless nitrogen. It is also
metered in as a liquid, which can cool the incoming charge further,
thus effectively increasing the charge density. With all that oxygen,
a lot more fuel can be squashed into the combustion chamber.

The advantage of nitrous oxide is that it has a flame speed, when burned
with hydrocarbon and alcohol fuels, that can be handled by current IC
engines, consequently the power is delivered in an orderly fashion, but
rapidly. Nitrous oxide has also been readily available at a reasonable
price, and is popular as a fast way to increase power in racing engines.

KEY:
CI    Compression Ignition
CO2   Carbon diOxide
H2O   diHydro Oxygenate (water)
HC    HydroCarbons
ETBE  Ethyl Tertiary Butyl Ether
MJ    MegaJoules
MMT   Methylcyclopentadienyl Manganese Tricarbonyl
MTBE  Methyl Tertiary Butyl Ether
PPM   Parts Per Million
SI    Spark Ignition
TAME  Tertiary Amyl Methyl Ether