Automotive Fuels

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Automotive Fuels
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Gasoline (C8H15)

• Gasoline is refined from crude oil
• Performance characteristics of gasoline
• Anti-knock quality
• Volatility
• Sulfur content
• Deposit control

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Anti-knock Quality

• Measured by the octane number
• Research octane number (RON)
• Motor octane number (MON)
• Unknown fuel is compared to a mixture of
  isooctane and heptane
• Isooctane has an octane number of 100
• Heptane has an octane number of 0
• If a mixture of 87 isooctane and 13 heptane is
  burned in a test engine, and create the same
  knock intensity as the unknown fuel, the octane
  number of the unknown fuel is 87

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RON vs. MON

• MON and RON are determined at different preset
  rpm, and air inlet temperature settings - The
  average of these two are what you see on the gas
  pump

MON RON
RPM 900 600
Water Temperature 212 F 212 F
Inlet air Temperature 300 F 130.4 F
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Factors Affecting Octane Requirements

• Compression ratio
• Ignition timing
• Air to fuel ratio
• Combustion temperature
• Inlet air temperature
• EGR rate (exhaust gas recirculation)
• Combustion chamber design
• Barometric pressure
• Ambient temperature
• Humidity
• Combustion chamber deposits

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Cetane Number

• Cetane is the rating system used for diesel
• Similar to the octane ratings of gasoline
• The higher the cetane number, the shorter the lag
  time between fuel injection and ignition
• The higher the cetane number the quicker and more
  easily the diesel fuel burns

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Volatility

• Volatility describes how easily various fuels
  will evaporate
• Measured by Reid vapor pressure
• The higher the RVP the more volatile the fuel,
  which means it will vaporize more easily

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Reid Vapor Pressure
0
0
Fuel A
Fuel B
70 F
70 F
100 F Water
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Reid Vapor Pressure
7
5
Fuel A
Fuel B
100 F
100 F
100 F Water
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Volatility

• During the summer fuel manufacturers blend fuels
  with lower RVP
• This prevents problems such as vapor lock
• During the winter fuel manufacturers blend fuels
  with higher RVP
• This aids in cold startup

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Gasoline Additives

• Tetraethyl lead
• Alcohol
• Methyl tertiary butyl ether (MTBE)
• Ethyl tertiary butyl ether (ETBE)
• Tertiary amyl methyl ether (TAME)
• Deposit control additives

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Tetraethyl Lead

• Used to lubricate valve stems
• And upper cylinder walls
• Restricted in 1973 and completely phased out in
  1996
• Still used in aviation and some racing
  applications
• Used in some foreign countries
• Coats the surface of catalytic converters and
  renders them inoperative

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Alcohol

• Used as an oxygenator
• Used to reduce CO emissions
• Absorbs moisture (hydroscopic)
• Increases the octane rating

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Methyl Tertiary Butyl Ether (MTBE)

• Increases octane by 2.5 to 3.0
• Used as an oxygenator to reduce CO emissions
• Will not affect RVP
• Carcinogen
• Studies have shown groundwater containing
  excessive amounts of MTBE in areas using it as a
  fuel additive

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Ethyl Tertiary Butyl Ether (ETBE)

• Increases octane number up to 3 points
• Lowers RVP

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Tertiary amyl methyl ether (TAME)

• Performance similar to ETBE

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Deposit Control Additives

• Required in gasoline year round as of 1995
• Used to control deposits on or in
• Port fuel injectors
• Intake valves
• Combustion chambers