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Limits in Combustion Systems

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Title: Limits in Combustion Systems


1
  • 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

2
  • Mostly based on,
  • Leeds SAE Paper Numbers
  • 2004-01-2998
  • 2002-01-2831
  • 982616
  • 942060
  • 902135
  • 902136

3
KNOCK WHAT IS IT ? (a) in-cylinder
pressure oscillation (b) noise
Tsurushima et al - SAE 2002-01-0108
4
KNOCK - WHY IS IT A PROBLEM ? (a) noise (b)
damage


5
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.

6
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)
7
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

8
AUTOIGNITION
  • Spontaneous chemical reaction accelerating to
    ignition
  • High heat release
  • High temperature (gt1000K)
  • Strong light emission.

9
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)
10
Negative Temperature region
From Nishiwaki et al. SAE 2000-01-1897
11
CAI Combustion (41 IEGR, 6-17o ATDC)
1
2
3
5
4
12
Direct Images of CAI Combustion
Courtesy of Brunel University (Zhao and
Ladommatos)
13
AUTOIGNITION IN SI ENGINES
  • Without knock
  • Followed by knock
  • (of increasing severity)

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18
Weak Knock Cycle
19

20
Severe Knock Cycle
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23
Knock severity associated withmodes of
autoignition? 1. Deflagration 2. Developing
Detonation 3. Thermal Explosion In turn
associated with temperature gradient away from
autoignition centre.

24
Hot Spot
Unburned mixture
Hot Spot/Autoignition Centre
25
Mode Boundaries
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27
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
28
General behaviour- Cyclic variation
  • 30 cycles
  • Cyclic variation

29
General behaviour- Combustion rate
The test fuels had different burning velocities
30
General behaviour- Autoignition/knock onset
Gasolines are more resistant to knock than the
PRF of same RON
31
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
8
5
Reduced
SI engine
19
17
Cowart, et al. (1990)
More detailed
Zheng, et al. (2002)
HCCI engine
29
20
ON term correlated using 80 100PRFs
32
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.)
33
Results- Residual free operation
  • PRFs

Modified DE expression
ON term as RON of fuel
34
Results- Residual free operation
Gasolines
Modified DE expression
ON term as RON of fuel
35
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

36
Effect of Residual- Iso-octane
Influence of dilution
Influence of NO
37
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

38
Discussion
Single stage region
RON Test 95ULG, PRF95
MON Test 95ULG, PRF86
Kalghatgi OI OI RON- K (RON-MON)
39
Kalghatgi K modified Octane Index
Single stage region
RON Test 95ULG, PRF95
MON Test 95ULG, PRF86
Kalghatgi OI OI RON- K (RON-MON)
40
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.

41
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.

42
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.

43
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.

44
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

45
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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