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LSPI – lubricant formulation effect on fuel
pre-ignition in engines
Presented by: Dr Cecile Pera, Senior Engineer
Performance you can rely on.
UNITI Mineral Oil Technology Congress
Introduction
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• LSPI has been studied by researchers for more than a decade
• It happens at low-speed and high-load
• Very sporadic and ‘random’ occurrences
IntroductionWhat do we know about LSPI?
High speed pre-ignition
Turbocharged engineLow speed pre-ignition
Knocking
Natural aspirated engine
Auto ignition
Engine speed (rpm)
To
rqu
e (
Nm
)
October 2014 edition of SRI news
2012-01-1276
IntroductionWhat do we know about LSPI?
Normal
combustion
Knock
Super-knock
Pre-ignition
Spark-timing
Pcy
l
Crank
• Problem exacerbated with high power-density engines (downsizing)
• Contrary to ‘classical knock’, it cannot be eliminated by spark-timing delay
• Contrary to ‘classical knock’, it can instantaneously damage the engine
• Lubricant and its additive package are involved
• Fuel blending and its properties also play a crucial role
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Normal
combustion
Knock
Super-knock
Pre-ignition
Spark-timing
Pcy
l
Crank
• LSPI synopsis:• Pre-ignition stage (prior to spark timing)• Slow heat release stage due to flame propagation which is similar to
normal combustion• Super-knock: fast heat release due to large amount of unburnt
mixture auto-igniting in one go
IntroductionDifference between LSPI, pre-ignition and super-knock?
Although sometimes interchangeable,
‘pre-ignition’ and ‘super-knock’ represent
2 different phenomena
– Pre-ignition: combustion of fuel/air mixture
triggered by ‘hot-spot’ prior to the spark
timing
– Super-knock: severe engine knock (fuel
auto-ignition) resulting of in-cylinder
conditions (pressure, temperature)
Contents
• Introduction
• Influencing parameters
• LSPI mechanism – focus on combustion
• LSPI – combustion process
• Ignition Quality Tester (IQT)
• Conclusion
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Influencing parameters
Influencing parameters on LSPIOverview
• Many factors can affect LSPI
• Engine hardware design (injection system, piston rings, etc.)
• Engine operating conditions (temperature, pressure, mixture composition,
etc.)
• Gasoline fuel properties
• Deposits
• Lubricant
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Influencing parameters on LSPIAdditives
0
2
4
6
8
10
12
14
0
20
40
60
80
100
120
LSP
I e
ven
ts
% Calcium
SwRI HSHL Average GM Engine
ILSAC GF-5
products
Influencing parameters on LSPIAdditives
0
2
4
6
8
10
12
14
0
20
40
60
80
100
120
LSP
I e
ven
ts
% Magnesium
GM engine
SwRI engine
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Influencing parameters on LSPIAdditives
0
20
40
60
80
100
120
140
160
180
200
LSP
I E
ven
ts
% ZDDP
SwRI engine
Influencing parameters on LSPIBase stocks
Gp I Gp II Gp III PAO PAO(High Visc)
LSP
I fr
eq
ue
ncy
Data from [SAE 2012-01-1615]
Gp I Gp II Gp III PAO PAO(High Visc)
Ign
itio
n f
req
ue
ncy
Data from [SAE 2014-01-1213]
LSP
I e
ven
t
GM engine Ford engine
Gp II Gp III Gp III+ Gp IV Gp V
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LSPI mechanism – focus on combustion
Oil film
Injector
Fuel + Oil
Spray
Step 1: pre-ignition
Hot-spot ignition triggered by oil /
additive combustion properties
LSPI mechanism: Proposed synopsis
Fuel liner wetting
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Step 2: Following combustion
process
(flame propagation)
Oil film
Injector
Fuel + Oil
Spray
Step 1: pre-ignition
Hot-spot ignition triggered by oil /
additive combustion properties
Fuel liner wetting
LSPI mechanism: Proposed synopsis
Step 2: Following combustion
process
(flame propagation)
Oil film
Injector
Fuel + Oil
Spray
Step 1: pre-ignition
Hot-spot ignition triggered by oil /
additive combustion properties
Step 3:
Super-knock
(fuel auto-ignition)
•Hardware (small vs large
bore, etc.)
•Operating conditions (spark
timings, etc.)
•Fuel/oil mixture and mixing
– Lube formulation
– Fuel properties
Fuel Liner wetting
LSPI mechanism: Proposed synopsis
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LSPI – combustion process
Combustion processWhy is pre-ignition not always followed by super-knock?
– Flame propagation development
Auto-ignition
Unburnt
Gas
Flame
propagation
• LSPI synopsis:
– Early auto-ignition
– 2nd auto-ignition(s) or spark event is (are) followed by:
a) Combustion extinction (no flame development)
Normal
combustion
(envelope)
Pre-ignition + flame
propagationPre-ignition
Spark timing
Pcyl
°V
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Combustion processWhy is pre-ignition not always followed by super-knock?
– Flame propagation development
Unburnt
Gas
Flame
propagation
• LSPI synopsis:
– Early auto-ignition
– 2nd auto-ignition(s) or spark event is (are) followed by:
a) Combustion extinction (no flame development)
b) Flame propagation
Normal
combustion
(envelope)
Pre-ignition + flame
propagationPre-ignition
Spark timing
Pcyl
°V
Flame
fronts
Combustion processWhy is pre-ignition not always followed by super-knock?
– Flame propagation development
Unburnt
Gas
Flame
propagation
• LSPI synopsis:
– Early auto-ignition
– 2nd auto-ignition(s) or spark event is (are) followed by:
a) Combustion extinction (no flame development)
b) Flame propagation
Normal
combustion
(envelope)
Pre-ignition + flame
propagationPre-ignition
Spark timing
Pcyl
°V
Flame
fronts
-auto-ignition heat release is too weak
-initiated initial flame kernel is too small
-fuel properties }
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Combustion processWhy is pre-ignition not always followed by super-knock?
– Flame propagation development
• LSPI synopsis:
– Early auto-ignition
– 2nd auto-ignition(s) or spark event is (are) followed by:
a) Combustion extinction (no flame development)
b) Flame propagation
c) Detonation (succession of auto-ignitions)
Normal
combustion
(envelope)
Pre-ignition + flame
propagationPre-ignition
Spark timing
Pcyl
°V
Super-knockFlame
fronts
Succession
of auto-ignition
Combustion processWhy is pre-ignition not always followed by super-knock?
– Flame propagation development
• LSPI synopsis:
– Early auto-ignition
– 2nd auto-ignition(s) or spark event is (are) followed by:
a) Combustion extinction (no flame development)
b) Flame propagation
c) Detonation (succession of auto-ignitions)
Normal
combustion
(envelope)
Pre-ignition + flame
propagationPre-ignition
Spark timing
Pcyl
°V
Super-knockFlame
fronts
Succession
of auto-ignition
- acoustic/combustion coupling
- hardware design
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Combustion processCombustion theory: deflagration vs detonation
Sound speed
‘reaction’ front
velocity
�
�ξ =
• Two conditions are needed to initiate a coupling between pressure waves
and the 2nd AI front
– An auto-ignition propagation velocity close to the one of pressure waves (called here ξ)
Combustion processCombustion theory: deflagration vs detonation
Subsonic deflagration = classical flame
propagationSound speed
‘reaction’ front
velocity
Time for maximal
heat releaseTime for pressure wave to go
out from the end gas region
�
�ξ =
��
��ε =
Supersonic reaction wave propagation
Developing and developed detonation
• Two conditions are needed to initiate a coupling between pressure waves
and the 2nd AI front
– An auto-ignition propagation velocity close to the one of pressure waves (called here ξ)
– A smooth gradient in the fresh gases surrounding the initial auto-ignition spot (called
here ε)
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IQT – Ignition Quality Tester
Ignition Quality TesterObjectives
• In auto-ignition mechanism, there are two driving mechanisms:
– Creation of radicals through branching reactions which is an important
mechanism for low temperature chemistry
– Run-away due to large heat release and
temperature increase which is more important
for high temperature kinetics
– IQT has the potential to focus on both mechanisms
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Ignition Quality TesterPrinciple of measurements
• Either pure oil or mixture of oil and fuel were tested
Injection nozzle
body
ττττig
Injector
displacement
Bomb pressure
EN228 Surrogate
RON 95.0 95.0
MON 85.0 85.0
H/C ratio 1.801 1.801
O/C ratio 0.011 0.011
Density (g/mol) 94.3 94.3
Surrogate
Ignition Quality TesterGasoline surrogate design
Real EN228 Gasoline
Linear
paraffins
Branched
paraffins
Cyclic
paraffins
Aromatics
OlefinsOxgenated
n-Heptane
Isooctane
Cyclohexane
Toluene
CyclohexeneEthanol
Gasoline surrogate has been designed to target auto-ignition properties following methodology defined by Pera and Knop[1,2,3]
[1] Pera and Knop, Fuel 2012 – [2] Knop et al, Combust Flame 2013 – [3] Knop et al Fuel 2014
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Ignition Quality TesterResults
50
60
70
80
90
100
10
20
30
40
RO
ND
CN
Mass Oil A in Sur95f
DCN RON (Estimated)
IQT
DCN ↔ RON Correlation
Pure Fuel Fuel + oil
• Various Ca and Mg concentrations showed no statistically measurable
difference in the IQT
• Addition of peroxide increased oil/fuel reactivity but showed again no
sensitivity to the IQT to Ca / Mg content
– This result seems to indicate that Ca does not play a role either in
the initiation or the branching reactions of the oil auto-ignition
in the gaseous phase
DC
N
Mass oil in fuel
Gasoline
Various Ca and /or
Mg concentration
Ignition Quality TesterResults
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• Largest effect on IQT ignition appears to be the general chemical
identity of the base oil
– Increased hydrocracking as group number increases through Group I
→ Group IV
10
15
20
25
30
35
40
45
Group I Group II Group III Group IV Group V
DC
N
25% mass Oil in
Sur95f
Sur95f
Average
+
Ignition Quality TesterResults
Conclusion
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• Base oil and additives play a role on LSPI mechanisms subject to test
conditions (hardware, operating conditions, others)
• LSPI mechanism involves intricate sub-mechanisms
– Combustion is one of them
Conclusion
• No discernible impact observed for Ca and Mg additives in the IQT
– No catalytic effect of Ca on auto-ignition in the homogeneous gaseous phase
• Large impact by base oil chemistry (Groups I-V) on auto-ignition in the IQT
– Reactivity ranking is: Gp I < Gp II < Gp III < Gp IV (Gp IV is the one that auto-ignite the more easily)
– Some Gp V base oils are very resistance to auto-ignition
Conclusion
Step 2:
Flame propagation
Oil film
Injector
Fuel + Oil
Spray
Step 1:
pre-ignition
Step 3:
Super-knock
Fuel Liner
wetting
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Talk to us at exhibition stand 10
Dr. Cecile Pera, Senior [email protected]
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