Heat of Combustion

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Measurement Seminar Presentation PPRE
2005-2007 G. Pechlivanoglou
Heat of Combustion Heat of Evaporation
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Combustion is an exothermic reaction between a
fuel and an oxidizer (usually O2), which releases
heat. Fuel Oxygen ? Heat Water Carbon
Dioxide General Chemical Reaction of Organic
substance Combustion CxHy (x y/4)O2 ? xCO2
(y/2)H2O
Example Propane Combustion C3H8 5O2 ? 3CO2
4H2O
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Combustion phases Preheating phase unburned fuel
is heated up to its flash point. Gaseous phase
the mix of evolved flammable gases with oxygen is
ignited. Solid phase the charred fuel does not
burn rapidly anymore but just glows and later
only smoulders.
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Heat of combustion is the energy released as heat
when a compound undergoes complete combustion
with oxygen
HHV (Higher Heating value) Maximum Potential
Energy of Combustion exhaust H2O is in liquid
form LHV (Lower Heating Value) Total Energy of
combustion (-) Energy for produced water
vaporization exhaust H2O is in vapour form GHV
(Gross Heating Value) Energy of combustion (-)
Total Energy of ALL water vaporization (includes
also liquid water in the fuel prior to combustion)
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Evaporation The process whereby atoms or
molecules in a liquid state gain sufficient
energy to enter the gaseous state. Heat of
Vaporization (Latent Heat) The heat demand for
complete evaporation of a unit mass of a
substance at its boiling point.
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Both Combustion Evaporation take place the same
time in all the Thermal Engines and Combustors

• Theoretically, water evaporation reduces the
  engines output.
• With some types of fuel the evaporation is even
  higher due to water existence in the fuel prior
  to combustion (e.g. wood, coal)
• Water Evaporation has also positive effects (in
  ICE) as is lowers the comb. chamber temperature
  preventing pre-ignition
• Studies show that water evaporation has also some
  even more advantageous, but poorly studied
  characteristics (e.g. high ratio water injection)

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• Water Injection (5-25 of fuel mass) reduces the
  combustion chamber temperature due to evaporation
  and prevents pre-ignition
• (compression ratio efficiency power will be
  increased)
• Water Injection (40 of 106octane fuel mass)
  increases the power output by 52!
• Water Injection (150 of 80octane fuel mass)
  increases the power output by 251!!!

Tremendous increase in Fuel Efficiency and Power
Output
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Homogeneous Charge Compression Ignition (HCCI)
combustion
Homogeneous Charge Compression Ignition (HCCI)
combustion

• Efficiency as high as for Diesel.
• Very low emissions of soot and NOx.
• Fuel consumption benefit of 20-25 compared to
  SI.
• Auto-ignition by compression.
• Ignition and combustion controlled by chemistry.
• Mixture not perfectly homogeneous!

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• Advantages
• HCCI combustion is extremely fast so it is closer
  to the ideal Otto cycle than SI combustion.
• Lean operation leads to higher efficiency than in
  spark ignited gasoline engines
• Lower emissions due to the fact that peak
  temperatures are significantly lower than in SI
  engines, NOx levels are almost negligible.
• Since HCCI runs throttleless, it eliminates
  throttling losses

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SI HCCI Comparison Animation
Note the high HCCI combustion speed and the
almost spontaneous combustion of the whole
chamber volume
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• Disadvantages
• High peak pressures
• High heat release rates
• Difficulty of control
• Difficulty of increasing power

Honda HCCI test engine
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Combustion Characteristics Behavior of Flames
In Zero Gravity Environment
Combustion Characteristics Behavior of Flames
In Zero Gravity Environment

• The lack of gravitational forces prevents the
  buoyancy effects.
• The combustion is more efficient and symmetric.
• There are also differences in the heat transfer
  mechanisms from the flame to the environment

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Flame Patterns in Zero Gravity
Flame Patterns in Zero Gravity
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Diesel auto-ignition in normal gravity
Diesel auto-ignition in zero gravity
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• References
• NACA Resource Center
• Wikipedia (www.wikipedia.org)
• Univ. Berkeley HCCI (http//www.me.berkeley.edu/
  cal/HCCI/)
• Chalmers Univ. of Technology HCCI
  (http//www.tfd.chalmers.se/ogink/)
• Stanford Univ. HCCI (http//www-cdr.stanford.edu/d
  ynamic/hcci_control/)
• NASA Glenn Research Center (http//microgravity.gr
  c.nasa.gov/)