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

Average Combustion Process Cylinder Pressure Based Performance Cylinder pressure based parameters commonly used to characterize the average combustion performance: 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

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

Cylinder Pressure vs. Crank Angle Correlation To Engine Events

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

Cylinder Pressure vs. Crank Angle Peak Cylinder Pressure and Location

Cylinder Pressure vs. Crank Angle Firing vs. Motoring

Cylinder Pressure vs. Crank Angle Influence of Load

Cylinder Pressure vs. Crank Angle Influence of Spark Timing

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.

Cylinder Pressure vs. Crank Angle Spark Knock

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

Rate of Pressure Change vs. Crank Angle Correlation To Engine Events

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.

Rate of Pressure Change vs Rate of Pressure Change vs. Crank Angle Maximum Rate of Pressure Rise and Location

Rate of Pressure Change vs. Crank Angle Firing vs. Motoring

Rate of Pressure Change vs. Crank Angle Influence of Load

Rate of Pressure Change vs. Crank Angle Influence of Spark Timing

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

Cylinder Pressure vs. Cylinder Volume Correlation To Engine Events - Linear

Cylinder Pressure vs. Cylinder Volume Correlation To Engine Events - Logarithmic

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

Cylinder Pressure vs. Cylinder Volume Polytropic Coefficients

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.

Cylinder Pressure vs. Cylinder Volume Mean Effective Pressures

Cylinder Pressure vs. Cylinder Volume Firing vs. Motoring

Cylinder Pressure vs. Cylinder Volume Influence of Load

Cylinder Pressure vs. Cylinder Volume Influence of Spark Timing

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)

Mass Fraction Burned vs. Crank Angle Combustion Events

Mass Fraction Burned vs. Crank Angle Influence of Spark Timing a. k. a Mass Fraction Burned vs. Crank Angle Influence of Spark Timing a.k.a. Combustion Phasing

Mass Fraction Burned vs. Volume Fraction Burned General Information Mass Volume Fraction 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 50% Mass Burned 75% Volume Burned 90% Mass Burned 96% Volume Burned r = 0.5 x R r = 0.87 x R r = 0.98 x R

Cycle-to-Cycle Variability Cylinder Pressure Based Performance Cylinder pressure based parameters commonly used to characterize the cyclic variability of the combustion process: 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

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

Cycle-to-Cycle Variability Cylinder Pressure vs. Crank Angle

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.

Cycle-to-cycle Variability IMEP vs. Cycle

Cycle-to-cycle Variability IMEP vs. Cycle - Partial Burning

Cycle-to-cycle Variability IMEP vs. Cycle - Misfire

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

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

Combustion Limits Combustion Limits Diagram - 3% COV of IMEP Unstable combustion is defined as having a COV of IMEP > 3%

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.