Download Combustion Fundamentals

Combustion Fundamentals
Presentation Outline
 Average Combustion Process
 Cylinder Pressure vs. Crank Angle
 Rate of Pressure Change vs. Crank Angle
 Cylinder Pressure vs. Cylinder Volume
 Mass Fraction Burned vs. Crank Angle
 Cycle-to-Cycle Variability
 Combustion Limits
 Final Thoughts
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Average Combustion Process
Cylinder Pressure Based Performance
 Cylinder pressure based parameters commonly used to characterize
the average combustion performance:
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Cylinder pressure vs. crank angle
Peak cylinder pressure and its location
Maximum rate of pressure rise and its location
Cylinder pressure vs. cylinder volume
Log cylinder pressure vs. log cylinder volume
Indicated mean effective pressure (IMEP)
Pumping mean effective pressure (PMEP)
Flame formation period and bulk burn duration
Location of 50 % burned
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Cylinder Pressure vs. Crank Angle
General Information
 The fundamental measurement of cylinder pressure based
combustion diagnostics
 Also known as the P-T diagram (archaic)
 Required to determine the peak cylinder pressure and its location
with respect to TDC
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Cylinder Pressure vs. Crank Angle
Correlation To Engine Events
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Cylinder Pressure vs. Crank Angle
Fundamental Parameters
 Peak Cylinder Pressure
 The maximum pressure due to combustion. Occurs when the
pressure rise due to combustion equals the pressure drop due to
volume change. Can be reduced by reducing the load, diluting the
charge, or retarding the spark timing. Values can range from 600
kPa to 10,000 kPa for naturally aspirated spark ignition engines
 Location of Peak Cylinder Pressure (LPP)
 The location, with respect to TDC, of the occurrence of peak cylinder
pressure. A measure of spark timing relative to MBT. Advancing the
spark timing will move the LPP closer to TDC. At MBT spark timing,
the LPP is generally between 15 deg ATDC and 20 deg ATDC
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Cylinder Pressure vs. Crank Angle
Peak Cylinder Pressure and Location
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Cylinder Pressure vs. Crank Angle
Firing vs. Motoring
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Cylinder Pressure vs. Crank Angle
Influence of Load
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Cylinder Pressure vs. Crank Angle
Influence of Spark Timing
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Cylinder Pressure vs. Crank Angle
Spark Knock
Spark knock begins with the autoignition of a portion of the charge
ahead of the advancing flame. The rapid release of energy creates a
sonic pressure wave which propagates back and forth within the
combustion chamber, creating vibrations within the engine. The noise
which is transmitted through the engine structure as a result of these
vibrations is known as KNOCK
Because the amplitude and occurrence of spark knock is seemingly
random, a fundamental measure of its intensity is difficult. However,
of all the methods of quantifying knock, the most precise measure is
the maximum amplitude of the pressure oscillation results from the
sonic wave. These oscillations are sinusoidal and, therefore, they
can be analyzed by band-pass filtering the pressure vs. crankangle
waveform. The frequency of knock is proportional to the bore size
and the speed of sound in the combustion gases.
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Cylinder Pressure vs. Crank Angle
Spark Knock
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Rate of Pressure Change vs. Crank Angle
General Information
 The first derivative of the pressure-crank angle waveform
 Also known as dP/dq
 Required to determine the maximum rate of pressure rise
 A very useful tool for the diagnosis of electrical and mechanical noise
in the pressure signal
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Rate of Pressure Change vs. Crank Angle
Correlation To Engine Events
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Rate of Pressure Change vs. Crank Angle
Fundamental Parameters
 Max. Rate of Pressure Rise
 The maximum increase of pressure within one degree of crankshaft
rotation. Its levels are increased and decreased by the same
parameters influencing peak cylinder pressure. Generally used as a
relative measure of the impact loading due to combustion. Typical
values range from 20 kPa/ deg CA at very light loads to over 600
kPa/ deg CA at wide open throttle
 Location of Max. Rate of Pressure Rise
 The location, with respect to TDC firing, of the occurrence of the
maximum rate of pressure rise. As with LPP, advancing the spark
timing will advance the location of the maximum rate of pressure rise
relative to TDC. Not a widely utilized performance metric.
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Rate of Pressure Change vs. Crank Angle
Maximum Rate of Pressure Rise and Location
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Rate of Pressure Change vs. Crank Angle
Firing vs. Motoring
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Rate of Pressure Change vs. Crank Angle
Influence of Load
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Rate of Pressure Change vs. Crank Angle
Influence of Spark Timing
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Cylinder Pressure vs. Cylinder Volume
General Information
 Calculated from the Pressure-crank angle waveform and engine
geometry
 Also known ass the P-V diagram
 Since work is the product of pressure and volume change, analysis
of the P-V diagram will yield displacement specific torque
 Required to determine the polytropic compression and expansion
coefficients
 Required to determine indicated mean effective pressure (IMEP),
pumping mean effective pressure (PMEP), and net mean effective
pressure (NMEP)
 The logarithmic form of the P-V diagram is the most useful tool for
quantifying the quality of the pressure-crank angle measurement
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Cylinder Pressure vs. Cylinder Volume
Correlation To Engine Events - Linear
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Cylinder Pressure vs. Cylinder Volume
Correlation To Engine Events - Logarithmic
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Cylinder Pressure vs. Cylinder Volume
Polytropic Coefficients
 Polytropic Compression Coefficient
 A thermodynamic property of the compression process.
Defined as the slope of the compression process on a log P
vs. log V diagram. Typical values range from 1.26 to 1.36
 Polytropic Expansion Coefficient
 A thermodynamic property of the expansion process. Defined
as the slope of the expansion process on value of the
polytropic expansion coefficient is always greater than the
value of the polytropic compression coefficient
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Cylinder Pressure vs. Cylinder Volume
Polytropic Coefficients
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Cylinder Pressure vs. Cylinder Volume
Mean Effective Pressures
 Indicated Mean Effective Pressure (IMEP)
 IMEP is defined as that theoretical constant pressure which, if expected
during the expansion stroke of the engine to produce work, would produce
the indicated work. IMEP is calculated as the indicated work per engine cycle
divided by the cylinder volume displaced per cycle. Therefore, IMEP is
proportional to indicated work
 Pumping Mean Effective Pressure (PMEP)
 PMEP is defined as that theoretical constant pressure which, if expected
during the expansion stroke of the engine to produce work, would produce
the pumping work. PMEP is calculated as the pumping work per cycle
divided by the cylinder volume displaced per cycle. Therefore, PMEP is
proportional to pumping work.
 Net Mean Effective Pressure (NMEP)
 NMEP is defined as that theoretical constant pressure which, if expected
during the expansion stroke of the engine to produce work, would produce
the net work. NMEP is calculated as the work delivered by the cylinder gases
to the piston (P dV work) during the complete cycle divided by the cylinder
volume displaced per cycle. NMEP id proportional to net work.
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Cylinder Pressure vs. Cylinder Volume
Mean Effective Pressures
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Cylinder Pressure vs. Cylinder Volume
Firing vs. Motoring
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Cylinder Pressure vs. Cylinder Volume
Influence of Load
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Cylinder Pressure vs. Cylinder Volume
Influence of Spark Timing
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Mass Fraction Burned vs. Crank Angle
General Information
 A measure of the rate of fuel energy released. Also known as heat
release analysis
 Determined from one of the following models:
 Approximate (empirical)
Standard Rassweiler & Withrow Technique
Modified Rassweiler & Withrow Technique
 Single zone (thermodynamic)
 Two zone (thermodynamic)
 Allows quantification of the combustion process; including the flame
formation period (0% to 10% mass burned), the bulk burn duration
(10% to 50% mass burned), and the location of the combustion
process with respect to TDC a.k.a. combustion phasing (crank angle
of 50% mass burned)
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Mass Fraction Burned vs. Crank Angle
Combustion Events
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Mass Fraction Burned vs. Crank Angle
Influence of Spark Timing a.k.a. Combustion Phasing
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Mass Fraction Burned vs. Volume Fraction Burned
General Information
Mass
Fraction
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Volume
Fraction
2.5%
5 %
10 %
25 %
25 %
50 %
50 %
75 %
75 %
90 %
90 %
96 %
Mass Fraction Burned vs. Volume Fraction Burned
General Information
Assuming a 2-D disk-shaped combustion chamber
10% Mass Burned
25% Volume Burned
r = 0.5 x R
50% Mass Burned
75% Volume Burned
90% Mass Burned
96% Volume Burned
r = 0.87 x R
r = 0.98 x R
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Cycle-to-Cycle Variability
Cylinder Pressure Based Performance
 Cylinder pressure based parameters commonly used to characterize
the cyclic variability of the combustion process:
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Variation of peak cylinder pressure
Variation of the location of peak cylinder pressure
Variation of maximum rate of pressure rise
Variation of the location of maximum rate of pressure rise
Variation of cylinder pressure at a fixed crank angle
Variation of IMEP
Variation of the flame formation period
Variation of the bulk burn duration
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Cycle-to-Cycle Variability
Performance Statistics
 COV (Coefficient of Variation)
 The Standard deviation of a sample set divided by the mean
of the sample set and multiplied by 100. Therefore, COV is
the standard deviation as a percent of the mean.
 LNV (Lowest Normalized Value)
 The lowest value of a data set divided by the mean of the
data set
 HNV (Highest Normalized Value)
 The highest value of a data set divided by the mean of the
data set
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Cycle-to-Cycle Variability
Cylinder Pressure vs. Crank Angle
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Cycle-to-Cycle Variability
Variation of IMEP
 The cycle-to-cycle variability of IMEP is generally expressed in terms
of the COV of IMEP. No single level of COV is universally accepted
as the onset of poor driveability but 5% COV of IMEP is the most
widely used. The emissions degradation limit is generally reached by
3% COV of IMEP.
 Analysis of the cycle-to-cycle variation of IMEP can indicate partially
burned or misfired cycles. A partially burned cycle yields an IMEP
considerably below the average level of IMEP but greater than zero.
A misfire yields a negative IMEP.
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Cycle-to-cycle Variability
IMEP vs. Cycle
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Cycle-to-cycle Variability
IMEP vs. Cycle - Partial Burning
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Cycle-to-cycle Variability
IMEP vs. Cycle - Misfire
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Combustion Limits
Misfire Limit
 The misfire limit is characterized by a failure of the combustion
process BEFORE 10% of combustible charge mass has burned.
 Conditions that can result in a misfire limit:
 low ignition energy
 small flame kernel area
 advanced combustion phasing
 excessive dilution at the spark plug
 poorly directed flow at the spark plug
 excessive flow velocity at the spark plug
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Combustion Limits
Partial Burn Limit
 The partial burn limit is characterized by a failure of the combustion
process BEFORE 90% of the combustible charge mass has burned.
 Conditions that can result in a partial burn limit:
 slow burn due to excessive dilution (EGR, lean air/fuel ratio)
 slow burn due to the lack of in-cylinder charge motion (swirl
and/or tumble)
 retarded combustion phasing
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Combustion Limits
Combustion Limits Diagram - 3% COV of IMEP
Unstable combustion is defined as having a COV of IMEP > 3%
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Cylinder Pressure Based Engine Diagnostics
Final Thoughts
 Poor cycle-to-cycle and cylinder-to-cylinder combustion performance
is a significant problem with many engines
 Cycle-to-cycle and cylinder-to-cylinder combustion performance is
best studied with cylinder pressure based engine diagnostics.
 Until recently, Cycle-to-cycle combustion variation has been the
primary focus of combustion studies. Cylinder-to-cylinder combustion
variation, while observed, was not widely studied.
 The use of cylinder pressure based engine diagnostics within GM is
currently at an all time high.
 The GM combustion community is aggressively pursuing cycle-to-
cycle and cylinder-to-cylinder combustion improvements to our
current and future powertrains.
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