Thermodynamic Properties

GASOLINE GRADES AND OCTANE NUMBER

Gasoline is a term used to describe a complex
mixture of various hydrocarbons refined from
crude petroleum oil.
Gasoline and air burns in the cylinder of the
engine and produces heat and pressure, which is
converted to rotary.
When combustion occurs, carbon dioxide and
water are produced If the process is perfect and all
of the air and all of the fuel are consumed in the
process.
 STOICHIOMETRIC AIR-FUEL RATIO
The ideal mixture or ratio at which all of the fuel combines
with all of the oxygen in the air and burns completely is
called the stoichiometric ratio, a chemically perfect
combination. In theory, this ratio for gasoline is an air-
fuel mixture of 14.7 to 1.
The octane rating of gasoline is the measure
of its antiknock properties. Engine knock (also
called detonation, spark knock, or ping ) is a
metallic noise an engine makes, usually during
acceleration, resulting from abnormal or
uncontrolled combustion inside the cylinder.

Gasoline with a low-octane rating burns fast and

may explode rather than burn when introduced to a
high compression ratio, which can cause pre-
ignition. The higher a gasoline’s octane rating, the
less likely it is to explode.
Detonation is usually caused by excessively
advanced ignition timing, excessively lean
mixtures, or the use of gasoline with too low of an
octane rating.
Another condition that also causes pinging or
spark knocking is called pre-ignition, which occurs
➢hot spot within the combustion
➢Pre-ignition are in-candescent carbon deposits in
combustion chamber
➢ faulty cooling system
➢ too hot a spark plug
➢ poor engine lubrication
 OCTANE RATING
• Antiknock standard or basis of comparison is the
knock resistant hydrocarbon isooctane,
chemically called trimethylpentane (C 8 H 18 ),
also known as 2-2-4 trimethylpentane.
• Exactly same antiknock characteristics as
isooctane, it is rated as 100-octane gasoline.
• If the gasoline tested had only 85% of the
antiknock properties of isooctane, it was rated as
85 octane.
Remember, octane rating is only a
comparison test!
Two basic methods used to rate gasoline for
antiknock properties (octane rating) are:
1. The research method
2. The motor method

Each uses a model of the special Cooperative Fuel

Research (CFR) single cylinder engine

Variation as to temperature of air, spark advance,

and other parameters.
Research method typically results in readings that are
6 to 10 points higher than those of the motor
method.

For example, a fuel with a Research Octane Number

(RON) of 93 might have a Motor Octane Number
(MON) of 85.

Octane rating: (RON + MON)/2 = (93 + 85)/2 = 89

The pump octane is called the Anti-Knock Index
(AKI)!
OCTANE IMPROVER ADDITIVES
• 1. Aromatic hydrocarbons (hydrocarbons containing the
benzene ring) such as xylene and toluene
• 2. Alcohols such as ethanol (ethyl alcohol), methanol
(methyl alcohol), and tertiary butyl alcohol (TBA)
• 3. Metallic compounds such as methylcyclopentadienyl
manganese tricarbonyl (MMT)
• Oxygenated fuels contain oxygen in the
molecule of the fuel itself.
• Examples: methanol, ethanol, methyl tertiary
butyl ether (MTBE),tertiary-amyl methyl ether
(TAME), and ethyl tertiary butyl ether (ETBE).
 Cetane Rating
• Diesel fuel contains 12% more heat energy
than the same amount of gasoline.
• The fuel in a diesel engine is ignited by the
heat generated by high compression.
• The pressure of compression (2,800 to 4,800
kPa) generates temperatures of 700°C to
900°C, which speeds the pre-flame reaction to
start the ignition of fuel injected into the
cylinder.
• Cetane number is measured by comparing the
“ignition delay time” of the sample fuel with a
mixture of cetane (C16H34) and alphamethyl
naptane (C10H7CH3). The Cetane percentage
in the mixture gives the CN of the sample fuel.
• CN of the reference fuel cetane is arbitraryly
set at 100, and of alphamethyl naptane at 0.
• The cetane number for diesel fuel is the
opposite of the octane number for gasoline. The
cetane number is a measure of the ease with
which the fuel can be ignited. The cetane rating
of the fuel determines, to a great extent, its
ability to start the engine at low temperatures
and to provide smooth warm-up and even
combustion.
• The cetane rating of diesel fuel should be
between 45 and 50. The higher the cetane
rating, the more easily the fuel is ignited.
Two grades of diesel fuels:
Grade1 and Grade2.
Grade2 diesel fuel is the most popular and widely
distributed.
Grade1 diesel fuel is less dense than Grade2, with
lower heat content.
Grade1 diesel fuel is blended with Grade 2 to
improve starting in cold weather.
In the winter, diesel fuel is likely to be a mixture of
Grade1 and 2 fuels.
In moderately cold climates, the blend may be 90%
Grade2 to 10% Grade1.
The drawbacks with normal diesel are:
• Diesel fuel is a high-sulfur content fuel.
• Diesel engines emit particulates, commonly
called soot.
• Diesels have high combustion temperatures,
which result in excessive nitrogen oxide emissions.
• Diesel fuel is also prone to “waxing” in cold
weather, a term for the solidification of diesel oil
into a crystalline state.
 Advantages of biodiesel fuel
• It can help reduce the nation’s dependency on
imported oil.
• Because it is carbon neutral, it can reduce CO2
emissions by 20% to 60%.
• HC emissions can be reduced by 50%.
• Sulfur emissions can essentially be eliminated.
• CO emissions can be reduced by an average of 48%.
• PM emissions can be reduced by 47%.
• It enables diesel engines to run smoother and
quieter and have a longer life.