Crude oil, as removed from the earth, is a
mixture of hydrocarbon compounds ranging from gases to heavy tars and waxes. The
crude oil can be refined into products, such as lubricating oils, greases,
asphalts, kerosene, diesel fuel, gasoline, and natural gas. Before its
widespread use in the internal combustion engine of automobiles, gasoline was an
unwanted by product of refining for oils and kerosene.
Two important factors affect the power and
efficiency of a gasoline engine - compression ratio and detonation (abnormal
combustion). The higher the compression ratio, the greater the engine's power
output and efficiency. The better the efficiency, the less fuel consumed to
produce a given power output. To have a high compression ratio requires an
engine of greater structural integrity. Due to the use of low-octane unleaded
gasoline in post-1975 models, compression ratios now generally range from 8:1 to
10:1. High performance cars may have higher compression ratios.
Normal combustion occurs gradually in each
cylinder. The flame front (the edge of the burning area) advances smoothly
across the combustion chamber until all the air/fuel mixture has been burned.
Detonation occurs when the flame front fails to reach a pocket of mixture before
the temperature in that area reaches the point of self-ignition. Normal burning
at the start of the combustion cycle raises the temperature and pressure of
everything in the cylinder. The last part of the air/fuel mixture is both heated
and pressurized, and the combustion of those two factors can raise it to the
self-ignition point. At that moment, the remaining mixture burns almost
instantaneously. The two flame fronts create a pressure wave between them that
can destroy cylinder head gaskets, break piston rings, and burn pistons and
exhaust valves. When detonation occurs, a hammering, pinging, or knocking sound
is heard. But, when the engine is operating at high speed, these sounds cannot
be heard because of motor and road noise.
Many of the performance characteristic of
gasoline can be controlled in refining and blending to provide proper engine
function and vehicle driveability. The major factors affecting fuel performance
are antiknock quality, volatility, sulfur content, and deposit control.
An octane number or rating was developed by the
petroleum industry so the antiknock quality of a gasoline could be rated. The
octane number is a measure of the fuel's tendency not to produce knock in an
engine. The higher the octane number, the less tendency to knock. By itself, the
antiknock rating has nothing to do with fuel economy or engine efficiency.
Two commonly used methods for determining the
octane number of motor gasoline are the motor octane number (MON) method and the
research octane number (RON) method. Both use a laboratory single-cylinder
engine equipped with a variable head and knock meter to indicate knock
intensity. The test sample is used as fuel, and the engine compression ratio and
air/fuel mixture are adjusted to develop a specified knock intensity. There are
two primary standard reference fuels, isooctane and heptane. Isooctane does not
knock in an engine but is not used in gasoline because of its expense. Heptane
knocks severely in an engine. Isooctane has an octane number of 100. Heptane has
an octane number of zero.
A fuel of unknown octane value is run in the
special test engine, and the severity of knock is measured. Various proportions
of heptane and isooctane are run in the test engine to duplicate the severity of
the knock of the fuel being tested. When the knock caused by the heptane/isooctane
mixture is identical to the test fuel, the octane number is established by the
percentage of isooctane in the mixture. For, if 85 percent isooctane and 15
percent heptane produce the same severity of knock as the fuel in question, the
fuel is assigned an octane number of 85. Factors that affect knock follow.
LEAN FUEL MIXTURE A lean mixture burns
slower than a rich mixture. The heat of combustion is higher, which promotes the
tendency for unburned fuel in front of the spark-ignition flame to detonate.
IGNITION TIMING OVERADVANCED Advancing
the ignition timing induces knock. Slowing ignition timing suppresses knock.
COMPRESSION RATIO Compression ratio
affects knock because cylinder pressures are increased with the increase in
VALVE TIMING Valve timing that fills the
cylinder with more air/fuel mixture promotes higher cylinder pressures,
increasing the chances for detonation.
TURBOCHARGING Turbocharging or
supercharging forces additional fuel and air into the cylinder. This induces
higher cylinder pressures and promotes knock.
COOLANT TEMPERATURE Hotspots in the
cylinder or combustion chamber due to inefficient cooling or a damaged cooling
system raise combustion chamber temperatures and promote knock.
CYLINDER-TO-CYLINDER DISTRIBUTION If an
engine has poor distribution of the air/fuel mixture from cylinder to cylinder,
the leaner cylinders could promote knock.
EXCESSIVE CARBON DEPOSITS The
accumulation of carbon deposits on the piston, valves, and combustion chamber
causes poor heat transfer from the combustion chamber. Carbon accumulation also
artificially increases the compression ratio. Both conditions cause knock.
AIR INLET TEMPERATURE The higher the air
temperature when it enters the cylinder, the greater the tendency to knock.
COMBUSTION CHAMBER SHAPE The optimum
combustion chamber shape for reduced knocking is hemispherical with a spark plug
located in the center. The hemi-head allows for faster combustion, allowing less
time for detonation to occur ahead of the flame front.
OCTANE NUMBER Only when an engine is
designed and adjusted to take advantage of the higher octane gasoline can the
value of the fuel be obtained. Most modern engines are designed to operate
efficiently with regular grade gasoline and do not require a high-octane premium
As stated earlier, gasoline is very volatile.
It readily evaporates so its vapor adequately mixes with air for combustion.
Only vaporized fuel supports combustion. To ensure complete combustion, complete
vaporization must occur.
The volatility of gasoline affects the
following performance characteristics or driving conditions.
COLD STARING AND WARMUP A fuel can cause
hard starting, hesitation, and stumbling during warmup if it does not vaporize
readily. A fuel that warmup if it does not vaporize readily. A fuel that
vaporizes too easily in hot weather can form vapor bubbles in the fuel line and
fuel pump, resulting in vapor lock or loss of performance.
TEMPERATURE Because a highly volatile
fuel vaporizes at a lower temperature than a less volatile fuel, winter-grade
gasoline is more volatile than summergrade gasoline.
ALTITUDE Gasoline vaporizes more easily
at high altitudes, so volatility is controlled in blending according to the
elevation of the place where fuel is sold.
CARBURETOR ICING PROTECTION Carburetor
icing is not as common in modern engines as in older engines. It can occur when
ambient temperatures reach between 28˚ and 55˚F and the relative humidity rises
above 65 percent. The humid air enters the carburetor and mixes with drops of
fuel. When the fuel evaporates, it removes heat from the air and surrounding
metal parts. When this occurs, the throttle temperature is rapidly lowered to
below 32˚F (if the ambient temperature is within the range indicated), and
condensing water vapor forms ice. The ice causes the engine to stall if it is
idling during this phase.
CRANKCASE OIL DILUTION
A fuel must vaporize well to prevent diluting
the crankcase oil with liquid fuel. If parts of the gasoline do not vaporize,
droplets of liquid break down the oil film on the cylinder wall, causing
scuffing or scoring. The liquid eventually enters the crankcase oil and results
in the formation of sludge, gum, and varnish accumulation as well as decreasing
the lubrication properties of the soil.
Poor vaporization can also affect the
distribution of fuel from cylinder to cylinder since vaporized fuel travels
farther and faster in the manifold.
Gasoline can contain some of the sulfur present
in the crude oil. Sulfur content is reduced at the refinery to limit the amount
of corrosion it can cause in the engine and exhaust system.
When the hydrogen in the hydrocarbon of the
fuel is burned with air, one of the products of combustion is water. Water
leaves the combustion chamber as steam but can condense back to water when
passing through a cool exhaust system. When the engine is shut off and cools,
steam condenses back to a liquid and forms water droplets. Steam present in
crankcase blowby also condenses to water.
When the sulfur in the fuel is burned, it
combines with oxygen to form sulfur dioxide. This sulfur dioxide can then
combine with water to form highly corrosive sulfuric acid. This type of
corrosion is the leading cause of exhaust valve pitting and exhaust system
deterioration. With catalysts, the sulfur dioxide can cause the obnoxious odor
of rotten eggs during vehicle warmup. To reduce corrosion caused by sulfuric
acid, the sulfur content in gasoline is limited to less than 0.01 percent.
Several additives are put in gasoline to
control harmful deposits, including gum or oxidation inhibitors, detergents,
metal deactivators, and rust inhibitors.
For many years, lead compound such as
tetraethyl lead (TEL) and tetramethyl lead (TML) were added to gasoline to
improve its octane ratings. However, since the mid-1970s, vehicles have been
designed to run on unleaded gasoline only. Leaded fuels are no longer available
as automotive fuels. The main reason for the change to unleaded gasoline was to
provide a fuel for cars with special antipollution devices - catalytic
converters. These systems must have unleaded fuel to work properly.
Because of the deactivating or poisoning effect
lead has on the catalyst, gasolines are limited to a lead content of 0.06 gram
per gallon. Since TEL or TML is not added to unleaded gasolines, the required
octane number is obtained by blending compounds of the required octane quality.
Methylcylopentadienyl manganese tricarbonyl (MMT) is a catalyst-compatible
octane improver. Vehicles with catalytic converters are labeled at both the fuel
gauge and fuel filler - unleaded fuel only.