Limits in Combustion Systems

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• Limits in Combustion Systems
• Knock-imposed limits to spark ignition and HCCI
  engine operation
• C G W Sheppard et al.
• Combustion Institute Meeting, Oxford, 11th April
  2006

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• Mostly based on,
• Leeds SAE Paper Numbers
• 2004-01-2998
• 2002-01-2831
• 982616
• 942060
• 902135
• 902136

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KNOCK WHAT IS IT ? (a) in-cylinder
pressure oscillation (b) noise
Tsurushima et al - SAE 2002-01-0108
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KNOCK - WHY IS IT A PROBLEM ? (a) noise (b)
damage


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KNOCK -WHAT LIMITATIONS DOES IT IMPOSE?

• Spark Ignition Engine
• limits compression ratio, hence expansion
  ratio and thermal efficiency.
• HCCI/CAI Engine
• limits turn down ratio, power modulation
• by dilution.

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CAI Combustion Map
Ricardo E6 at 1500rpm, Tin 320 oC, CR11.51,
WOT, unleaded gasoline

• Knock limit
• violent combustion
• Partial burn limit
• low combustion temperature
• Misfire limit
• retarded timing and duration due to EGR

Courtesy of Brunel University (Zhao and
Ladommatos)
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KNOCK WHY DOES IT OCCUR ?

• Local rate of energy (heat) release fast
  relative to pressure equalisation
• SI Engine
• 1) overly fast normal flame propagation
• 2) autoignition (normally of end-gas)
• HCCI/CAI Engine
• (inhomogeneous) autoignition

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AUTOIGNITION

• Spontaneous chemical reaction accelerating to
  ignition
• High heat release
• High temperature (gt1000K)
• Strong light emission.

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AUTOIGNITION IN RCM (vs fuel type)
measured in Leeds RCM at gas density 130 mol m-3
(pc 0.7- 1.0) at F 1 (courtesy Prof J F
Griffiths)
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Negative Temperature region
From Nishiwaki et al. SAE 2000-01-1897
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CAI Combustion (41 IEGR, 6-17o ATDC)
1
2
3
5
4
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Direct Images of CAI Combustion
Courtesy of Brunel University (Zhao and
Ladommatos)
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AUTOIGNITION IN SI ENGINES

• Without knock
• Followed by knock
• (of increasing severity)

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Weak Knock Cycle
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Severe Knock Cycle
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Knock severity associated withmodes of
autoignition? 1. Deflagration 2. Developing
Detonation 3. Thermal Explosion In turn
associated with temperature gradient away from
autoignition centre.

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Hot Spot
Unburned mixture
Hot Spot/Autoignition Centre
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Mode Boundaries
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Reference condition test fuels
Previous studies of in-cylinder flow, turbulence,
breathing flame propagation Bulk of this study
taken at/close to ref. condition
PRFs and commercial gasolines Fuels selected had
similar RON
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General behaviour- Cyclic variation

• 30 cycles

• Cyclic variation

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General behaviour- Combustion rate
The test fuels had different burning velocities
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General behaviour- Autoignition/knock onset
Gasolines are more resistant to knock than the
PRF of same RON
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Autoignition/knock onset models
Chemical schemes selected as representative of
model genre
Model Type
Source
Reactions
Species
Application
Douaud and Eyzat (1978)
Single step
SI engine
1
-
Shell Halstead, et al. (1977)
Representative
RCM
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5
Reduced
SI engine
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Cowart, et al. (1990)
More detailed
Zheng, et al. (2002)
HCCI engine
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ON term correlated using 80 100PRFs
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Comparison of knock model performance

• Any differences
• thermodynamic code
• residual

DE expression optimised at reference condition,
B3378
Predicted Knock Onset Time (C.A.)
Similar agreement over range of speed, ? and
intake/head T
Experimental Knock Onset Time (C.A.)
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Results- Residual free operation

• PRFs

Modified DE expression
ON term as RON of fuel
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Results- Residual free operation
Gasolines
Modified DE expression
ON term as RON of fuel
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Effect of NO concentration

• With 5 residual and variable overall in-cylinder
  NO concentrations
• Change in knock onset time by 4 CA equivalent to
  8 ONs

• 95PRF- influence less significant

• Reverse effect for gasolines same trend
  observed with 98ULG and 76TRF

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Effect of Residual- Iso-octane
Influence of dilution
Influence of NO
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Effect of Residual- 95ULG

• From 0 to 5 residual, dilution influences
  autoignition reaction

• Addition of NO, reverse effect to iso-octane
• Future autoignition models must consider PRFs
  and gasoline fuels differently

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Discussion
Single stage region
RON Test 95ULG, PRF95
MON Test 95ULG, PRF86
Kalghatgi OI OI RON- K (RON-MON)
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Kalghatgi K modified Octane Index
Single stage region
RON Test 95ULG, PRF95
MON Test 95ULG, PRF86
Kalghatgi OI OI RON- K (RON-MON)
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Some observations (1)

• Knock limits SI engine efficiency and HCCI/CAI
  operation range via unacceptable noise and (in
  extremis) engine damage.
• Knock is generally associated with inhomogeneous
  autoignition, however it is possible to have
  knock without autoignition and autoignition
  without knock
• Autoignition may occur some time ahead of knock
  onset, particularly for weak knock.

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Some observations (2)

• The most damaging form of knock is likely to be
  associated with a developing detonation mode of
  autoignition development this may occur at lower
  knock severity indices lower than for less
  damaging thermal explosion-like events.
• Developing detonation is likely to occur in a
  wider range of temperature gradients, and induce
  greater peak pressure and temperature , with
  charge dilution reduction.

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Some observations (3)

• Autoignition delay is a function of the
  pressure-temperature-time history as well as the
  fuel type.
• For identical engine operating conditions, burn
  rate (and associated end gas pressure-temperature-
  time history for autoignition events) varies with
  fuel type (including, and not necessarily related
  to, octane rating).
• Even for identical RON value knock onset time
  varies with fuel specification, residual gas
  concentration and concentration of NO is that
  residual the effects of
• NO in various fuels may be contrary.

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Some observations (4)

• The influence of negative temperature
  coefficient behaviour may be more significant
  for some modern engine autoignition
  pressure-temperature-time histories than others
  the Kalghatic K octane index parameter may be
  useful in characterising engine specific effects
  on autoignition/knock behaviour.

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Some observations (5)

• With uncertainties in modelling normal flame
  propagation and cyclic variation (fast cycles
  are more likely to knock than mean cycles),
  uncertainties in real fuel composition (and
  associated chemical kinetic mechanisms and
  rates), unknown residual gas concentrations and
  their effects on autoignition for particular
  fuels, then simple 1-D autoignition/knock models
  may prove no less satisfactory than more complex
  chemical kinetic formulations

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Acknowledgements

• The work reported here was mostly funded by a
  series of EPSRC and Commission of the European
  Community projects, most latterly Gasoline
  Engine Turbocharging Advanced Gasoline
  Powertrain for reduced Fuel Consumption and CO2
  Emissions (GET-CO2). Thanks are due to the
  Commission for its support, and to project leader
  Dr Afif Ahmed and other colleagues at Renault,
  VW, Garrett and Ricardo for assistance and
  discussions.
• Discussions with Prof G. Kalghatgi (Shell) and
  Prof John Griffiths (Leeds University) are
  acknowledged, as is the vital input and
  contribution of colleagues Prof Derek Bradley,
  Drs Lawes, Burluka and Woolley, as well as many
  current and past Leeds research students.

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