Chemical kinetics

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Chemical kinetics

• Martin Tunér, Fabian Mauss
• Chemical Kinetics group
• Combustion Physics

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Todays topics

• GT-Power/Mini-Ignition
• Modeling of transient effects during load changes
  for an HCCI engine
• 1. Relations between combustion phasing and
  duration
• 2. Transient simulations following experiments

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GT-Power/Ignition

• Mini-Ignition has been integrated with GT-Power
  6.1
• Compared to previous version
• Typically calculation times reduced 80-98
• Memory usage reduced with 90

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Modeling of transient effects during load changes
for an HCCI engine
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Background

• Unaccounted and not clearly understood Transient
  effects in HCCI combustion
• Combustion Chamber Wall Temperature Measurements
  and Modeling During Transient HCCI Operation, SAE
  2005-01-3731, Carl Wilhelmsson, Andreas Vressner,
  Per Tunestål, Bengt Johansson, Gustaf Särner and
  Marcus Aldén
• Natural extension of ongoing Investigation of
  Exothermic Centers
• Formation of Exothermic centers dependent on Wall
  Temperatures

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Transient effects in HCCI combustionPositive
load step (experiments)
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Transient effects in HCCI combustionNegative
load step (experiments)
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Simulation approach

• 1. Relation between combustion phasing, CA50, and
  combustion duration, CA90 CA10
• SRM calculations varying one parameter at the
  time
• Parameters varied Wall temp, Phi, CR, Tau, Ch,
  EGR
• 2. Transient simulations following experiments
• GT-Power / MiniSRM
• Predefined wall temperature from experiments

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Simulation-parameters
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Chemical modelsby Syed Sayeed Ahmed

• ECM PRF
• n-heptane / iso-octane
• 96 species / 647 reactions
• Detailed (NICE)
• iso-octane
• 267 species / 2612 reactions

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Combustion phasing/durationCh variation

• Ch Stochastic heat transfer coefficient
• Increased Ch
• Earliest phasing Ch 15
• Unaffected duration
• Small impact on phasing
• Almost static relation phasing/duration

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Combustion phasing/durationTau variation

• Tau Turbulence mixing time
• Increased Tau
• Slower mixing
• Later phasing
• Longer duration
• Small impact on phasing and duration
• Static relation phasing/duration

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Combustion phasing/durationWall temperature
variation

• Increased wall temperature
• Earlier phasing
• Shorter duration
• Some impact on phasing and duration
• Static relation phasing/duration

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Combustion phasing/durationCompression ratio
variation

• Increased CR
• Earlier phasing
• Shorter duration
• Very strong impact on phasing
• Impact on duration
• Nonlinear relation phasing/duration

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Combustion phasing/durationPhi variation

• Increased Phi
• Earliest phasing Phi 0.35
• Shorter duration
• Strong impact on phasing
• Very strong impact on duration
• Highly nonlinear relation phasing/duration

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Combustion phasing/durationEGR variation

• Increased EGR
• Earlier phasing
• Longer duration
• Strong impact on phasing
• Impact on duration
• Nonlinear relation phasing/duration in opposite
  direction

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Conclusions1. Relation combustion phasing and
duration

• Steady-state simulations were performed to
  investigate the impact on combustion phasing and
  combustion duration as well as on their relation,
  from variation of Tau, Ch, CR, Phi, EGR and wall
  temperature.
• Tau and Ch have small effects on phasing as well
  as on duration
• EGR, compression ratio and phi has strong effect
  on combustion phasing and duration
• Earlier phasing is attributed with a shorter
  duration except for cases where EGR is varied
• Wall temperature has a effect on phasing and
  duration. The effect is not that strong but is
  more pronounced for cases with later phasing.
  These steady-state simulations does not confer
  nor contradict the conclusions of SAE-Paper
  2005-01-3731, Combustion phasing and combustion
  duration depend heavily on combustion chamber
  wall temperature. This might well be so when
  including transient effects.
• Steady-state simulations can not explain why the
  combustion duration change rate is different for
  a positive load change to a negative change rate.

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Transient simulations following experiments

• Coupled simulations GT-Power / MiniSRM
• Reduced Chemical model for iso-octane with NOx
  chemistry under construction (current chemical
  model would yield calculation times of thousends
  of hours)
• Deterministic wall temperature variation, set
  preheated charge temperature

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• Thank You!