Rapid prediction method for heat-work conversion process in diesel engine cylinder under variable working conditions

Abstract

The invention discloses a rapid prediction method for detailed heat-work conversion process and performance in a diesel engine cylinder under variable working conditions. Based on the rotating speed, the air inlet pressure, the circulating oil injection quantity, the injection pressure and other parameters in the rack scanning point test, the quantitative relation between the injection duration and the injection characteristic parameters and the quantitative relation between the combustion duration and the injection duration are established; in combination with the cylinder pressure, the cylinder temperature and the excess air coefficient when the air inlet valve is closed, the full-cycle cylinder pressure and cylinder temperature change is solved by applying energy conservation, mass conservation and a gas state equation; the compression and expansion processes are solved in one step by adopting a variable process control equation, and the combustion process is solved by combining the equation; and post-processing cylinder pressure data to obtain performance parameters such as circulating work, average indicated pressure and heat efficiency and evaluation parameters such as the highest explosion pressure and the maximum pressure rise rate, and establishing a zero-dimensional simplified calculation model, so that a rapid and accurate method which can be realized on the existing ECU is provided for on-line simulation optimization of the in-cylinder heat-work conversion process and performance under the vehicle-mounted dynamic working condition.

Description

A method for rapid prediction of in-cylinder heat-work conversion process in diesel engines under varying operating conditions Technical Field

[0001] This invention relates to the field of diesel engine technology, and in particular to a method for rapid prediction of the in-cylinder heat-work conversion process of a diesel engine under varying operating conditions. Background Technology

[0002] The calibration and parameter optimization of diesel engine combustion and heat conversion systems, cooling systems, and lubrication systems are often based on data analysis from steady-state bench tests. However, in real-world diesel engine applications, they operate under variable conditions, where control and operating parameters are constantly changing. For example, parameters affecting the working process and performance, such as intake pressure, cyclic fuel injection quantity, excess air coefficient, injection pressure, and injection advance angle, may all fluctuate. When deviations occur between the actual control and operating parameters and the steady-state bench calibration results under variable operating conditions, the diesel engine's fuel economy typically deteriorates. Therefore, real-time adjustment of key operating and control parameters during actual vehicle operation is crucial. When these key operating and control parameters change, not only do the engine's power and fuel economy change, but parameters affecting NVH, emissions, and thermal load, such as maximum combustion pressure, pressure rise rate, maximum temperature, and average temperature, also change accordingly. Therefore, a comprehensive evaluation of these parameters is necessary.

[0003] Current methods for rapid prediction of in-cylinder processes and performance of diesel engines under varying operating conditions and their online application in vehicles refer to model building methods that target different cyclic operating conditions and application scenarios (environmental conditions, diesel engine loading / unloading conditions, and hot / cold engine states), using bench test steady-state calibration results as the tracking target. However, due to the numerous and interdependent control and operating parameters of diesel engines, multi-parameter optimization based on detailed diesel engine mechanism models requires significant computational power and time, and can only be performed offline on workstations, making it difficult to apply to vehicle ECUs. This fails to meet the control requirements of transient operating conditions in automotive diesel engines, making current rapid prediction models for in-cylinder processes and performance insufficient for real-time optimization of operating and control parameters under transient conditions. Currently, no method has been developed that can establish a rapid prediction model for in-cylinder processes and performance of diesel engines under varying operating conditions based on easily measurable parameters, and whose computational power is sufficient for vehicle ECUs. Therefore, it is necessary to develop a computational model capable of rapidly predicting in-cylinder processes and performance of diesel engines and suitable for application on vehicle ECUs. Summary of the Invention

[0004] This invention aims to develop a rapid numerical model and calculation method that uses the quantitative relationship between key operating, process, and performance parameters obtained under steady-state operating conditions of a diesel engine and characteristic parameters such as engine speed, intake pressure, cyclic fuel injection quantity, injection pressure, and main injection advance angle known or easily obtainable by the ECU as the basis for analysis. This model is capable of predicting the in-cylinder process and performance of a diesel engine in real time under varying operating conditions, and requires memory and computing speed suitable for online use by vehicle ECUs.

[0005] This invention discloses a method for rapidly predicting the in-cylinder heat-work conversion process of a diesel engine under varying operating conditions. The method includes the following steps:

[0006] Step 1: First, obtain the current main control and operating parameters of the diesel engine from the ECU, including speed, intake pressure, fuel injection quantity, injection pressure, and main injection advance angle;

[0007] Step 2: Based on the quantitative relationship between the measured circulating intake volume, speed, and intake pressure under steady-state test conditions, predict the current circulating intake volume in the cylinder; then, combine this with the known circulating fuel injection volume to calculate the current excess air coefficient.

[0008] Step 3: Based on the intake air temperature and the excess air coefficient in the cylinder, predict the polytropic index of the compression process;

[0009] Step 4: Based on the cylinder volume, cylinder pressure, and temperature when the intake valve is closed, use the multivariate exponential equation to calculate the cylinder pressure, cylinder temperature, and work done during the compression phase from intake valve closure to the start of combustion.

[0010] Step 5: Based on the quantitative relationship between combustion efficiency, engine speed, and excess air coefficient obtained from the analysis under steady-state conditions on the test bench, the combustion efficiency is calculated based on the excess air coefficient obtained in step 2.

[0011] Step 6: Predict the injection duration based on the known injection pressure and cyclic injection quantity, and then update the combustion duration;

[0012] Step 7: Update the phase of the combustion heat release rate based on the combustion duration and the main injection advance angle;

[0013] Step 8: Based on the in-cylinder conservation equations, with 1 o CA is used to calculate cylinder pressure, cylinder temperature, and work output point by point during the combustion process at time steps.

[0014] Step 9: Predict the polytropic index of the expansion process based on the temperature at the end of the combustion process and the excess air coefficient;

[0015] Step 10: Based on the volume, pressure, and temperature at the end of the combustion process, calculate the pressure, temperature, and work done during the expansion process according to the polytropic process.

[0016] Step 11: Accumulate the work done in the three processes obtained in steps 4, 8, and 10, and calculate the high-pressure cycle indicated work, indicated thermal efficiency, and average indicated pressure;

[0017] Step 12: Compare the cylinder pressure, cylinder temperature, and cylinder pressure change rate obtained in steps 4, 8, and 11 to obtain the maximum combustion pressure, maximum pressure rise rate, and maximum combustion temperature of the cycle;

[0018] Step 13: Update PMEP and FMEP based on engine speed and cyclic injection quantity.

[0019] Step 14: Calculate BMEP, effective thermal efficiency, torque, and effective fuel consumption rate.

[0020] Furthermore, in step 2, the current circulating intake air volume in the cylinder is predicted:

[0021] The circulating intake air volume of a diesel engine mainly depends on the intake air pressure, and the relationship can be expressed by the following equation. The influence of engine speed is mainly reflected in the values ​​of A, B, and C:

[0022] ;

[0023] In the formula: : Recirculating air intake volume; : Intake pressure; A, B, C: Constants that vary with engine speed;

[0024] Based on the obtained intake air volume, combined with the fuel injection volume provided by the ECU, the current excess air coefficient can be calculated. The equation for the excess air coefficient is:

[0025] ;

[0026] In the formula: Excess air coefficient; : Recirculating air intake volume; : Circulating fuel injection quantity; AFR0: Equivalent air-fuel ratio.

[0027] Furthermore, in steps 3 and 9, the process of determining the polytropic index during compression and expansion includes:

[0028] During compression, the gas inside the cylinder only experiences changes in pressure and temperature, while its composition remains unchanged. The polytropic index during compression is... It should be close to the specific heat ratio g of air;

[0029] The equation for solving the polytropic index of the combustion process is:

[0030] ;

[0031] The equation for solving the polytropic index after the combustion process is completed is:

[0032] ;

[0033] In the formula, The ratio of the air-fuel mixture to complete combustion, ranging from 0 to 1; and : These are the specific heat ratios of combustion products and fresh air, respectively, both of which are functions of the mixture temperature.

[0034] Furthermore, in step 4, the cylinder gas pressure and temperature when the intake valve is closed...

[0035] The in-cylinder gas pressure and temperature at the moment of intake valve closing, i.e., the start of compression, are closely related to the intake manifold pressure and engine speed, and are determined based on bench steady-state test results:

[0036] ;

[0037] ;

[0038] In the formula: Cylinder pressure when intake valve is closed; :constant; The gas temperature when the intake valve is closed; :constant; Engine speed; Intake air temperature, This refers to the intake pressure.

[0039] Furthermore, in step 5, a dataset is established based on the bench steady-state measured data, and the combustion efficiency is obtained by rapid interpolation based on the excess air coefficient. The functional relationship is summarized as follows:

[0040]

[0041] For combustion efficiency, Engine speed, This is the excess air coefficient.

[0042] Furthermore, in step 6, when the injection pressure of the diesel engine changes, the injection duration will change. The relationship between the injection duration and the cyclic injection quantity and injection pressure is as follows:

[0043] ;

[0044] In the formula, This is the current duration of the jetting process; The measured spray duration under steady-state test bench conditions; This is the current cycle fuel injection quantity; This refers to the cyclic fuel injection quantity under steady-state test bench conditions. The injection pressure under steady-state test bench conditions; The current injection pressure;

[0045] When the cyclic injection quantity and injection pressure change, under the same engine speed and similar cyclic injection quantity conditions, the combustion heat release rate has the following relationship with the cyclic injection quantity and injection pressure:

[0046] ;

[0047] In the formula, This represents the current combustion heat release rate; The measured combustion heat release rate under steady-state bench conditions;

[0048] The change in the combustion initiation point is as follows:

[0049] ;

[0050] In the formula, This is the current point of combustion initiation; The measured combustion initiation point under steady-state test bench conditions; This is the current main jet advance angle; This refers to the advance angle of the main spray under steady-state test bench conditions.

[0051] Furthermore, in step 8, based on the conservation of energy and mass within the closed container and the ideal gas law, the rate of increase in pressure of the gas inside the cylinder can be expressed as:

[0052] ;

[0053] The cylinder pressure at any given time can be calculated using the following formula:

[0054] ;

[0055] After obtaining the cylinder pressure, the gas temperature can be solved based on the ideal gas law:

[0056] ;

[0057] In the formula, : The rate of increase in cylinder gas pressure when the crankshaft rotates; : Gas pressure at crankshaft rotation angle; : Cylinder volume at crankshaft rotation angle; : Gas temperature at crankshaft rotation angle; : Circulating fuel injection quantity; Total gas volume in the cylinder; Fuel has a low calorific value; Combustion efficiency; : In-cylinder combustion heat release rate; Instantaneous heat transfer rate within the cylinder; : The specific heat ratio at constant pressure and constant volume of a gas;

[0058] The Woschini equation can be used to calculate the heat transfer rate between the gas in the cylinder and the combustion chamber wall.

[0059]

[0060] In the formula: The heat transfer coefficient between the gas inside the cylinder and the wall of the combustion chamber; Engine speed; The heat transfer area of ​​the engine combustion chamber wall; Instantaneous temperature of the gas; The average temperature of the walls inside the combustion chamber;

[0061] Solving for the heat transfer coefficient:

[0062]

[0063] In the formula: D: piston diameter; : Gas pressure at crankshaft rotation angle; : Gas temperature at crankshaft rotation angle; The average speed of the piston;

[0064] Cylinder pressure control equation for pure compression process:

[0065] ;

[0066] ;

[0067] ;

[0068] Cylinder pressure control equation for pure expansion process:

[0069] ;

[0070] ;

[0071] In the formula: : Gas pressure when the intake valve is closed; .: Cylinder volume when the intake valve is closed; The variability index of the compression process; : Gas pressure at the end of combustion; Cylinder volume at the end of combustion; The polytropic index of the expansion process.

[0072] Furthermore, in step 11, the high-pressure indicated cycle thermal efficiency is calculated:

[0073] The heat released by fuel is equal to the amount of fuel injected in each cycle multiplied by the lower heating value of the fuel.

[0074] ;

[0075] In the formula, Indicates thermal efficiency for high-pressure cycling;

[0076] To simplify the calculation process, this patent employs a piecewise solution:

[0077] ;

[0078] The amount of work done in each stage:

[0079] From bottom dead center to intake valve closed, -180 o CA to IVC:

[0080] ;

[0081] Power output when the intake valve is closed to the start of combustion, IVC-BOC:

[0082] ;

[0083] From the start of combustion to the end of combustion, BOC-EOC:

[0084] ;

[0085] From the end of combustion to the beginning of exhaust valve opening, EOC-EVO:

[0086] ;

[0087] From the start point of exhaust valve opening to the bottom dead center, EVO-180 o CA:

[0088] ;

[0089] Prediction of mean pressure in high-pressure cycle indicators:

[0090] The average indicated average pressure of the high-pressure cycle can be based on the above-mentioned segmented work results. Solve based on the following formula:

[0091]

[0092] This indicates the average indicated average pressure of the high-pressure cycle. This represents the average cylinder volume at each stage.

[0093] Furthermore, in step 14, the prediction of effective performance indicators: Based on the engine speed and cyclic injection quantity, the average pumping loss pressure PMEP and average friction loss pressure FMEP of the engine can be found in the pre-stored bench steady-state test data table. Based on this, the effective average pressure, effective torque, and effective fuel consumption rate can be calculated.

[0094] ;

[0095] ;

[0096] ;

[0097] ;)

[0098] In the formula, The average effective pressure; For torque; PMEP is the average effective pressure due to pumping losses; FMEP is the average effective pressure due to friction losses; For effective thermal efficiency; Effective fuel consumption rate; It is a fuel with a low calorific value;

[0099] By performing secondary processing on the changes in cylinder pressure and temperature along the crankshaft angle within a known range, the maximum burst pressure, maximum temperature, and maximum pressure rise rate required for evaluating engine NVH performance are determined; as well as the average pressure and temperature of the in-cylinder gas during combustion and expansion required for evaluating cooling system performance.

[0100] ;

[0101] ;

[0102] ;

[0103] ;

[0104] ;

[0105] Indicates the highest burst pressure. express Pressure during crankshaft rotation Indicates the highest combustion temperature. express Temperature at crankshaft rotation angle This indicates the average pressure of the gas inside the cylinder during combustion and expansion. This indicates the crankshaft angle at which the exhaust valve begins to open. Indicates the crankshaft angle at the point of combustion initiation. This indicates the average temperature of the gas inside the cylinder during combustion and expansion.

[0106] The beneficial effects achieved by this invention are:

[0107] Using the method of this invention, cylinder pressure, cylinder temperature, and indicated performance of a diesel engine can be predicted quickly and accurately. This method has the potential for direct application to engine ECUs. Attached Figure Description

[0108] Figure 1 shows a typical relationship between the circulating intake volume and the intake pressure.

[0109] Figure 2 shows the change of combustion efficiency with excess air coefficient at various speeds;

[0110] Figure 3 shows the relationship between the specific heat ratio and polytropic index of the gas during the compression and expansion process of the gas in the cylinder of the diesel engine, as well as the temperature of the gas in the cylinder.

[0111] Figure 4 shows a comparison between measured and simplified numerical simulation results: cylinder pressure and pressure rise rate;

[0112] Figure 5 shows a comparison between the measured and simplified numerical simulation results: gas temperature and temperature rise rate;

[0113] Figure 6 shows a comparison between the measured and simplified numerical simulation results: in-cylinder expansion efficiency. Detailed Implementation

[0114] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as a result. However, these embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0115] This invention provides a method for rapidly predicting the in-cylinder heat-work conversion process of a diesel engine under varying operating conditions, comprising the following steps:

[0116] Step 1: First, obtain the current main control and operating parameters of the diesel engine from the ECU, including speed, intake pressure, fuel injection quantity, injection pressure, and main injection advance angle;

[0117] Step 2: Based on the quantitative relationship between the measured circulating intake volume, speed, and intake pressure under steady-state test conditions, predict the current circulating intake volume in the cylinder; then, combine this with the known circulating fuel injection volume to calculate the current excess air coefficient.

[0118] Step 3: Based on the intake air temperature and the excess air coefficient in the cylinder, predict the polytropic index of the compression process;

[0119] Step 4: Based on the cylinder volume, cylinder pressure, and temperature when the intake valve is closed, use the multivariate exponential equation to calculate the cylinder pressure, cylinder temperature, and work done during the compression phase from intake valve closure to the start of combustion.

[0120] Step 5: Based on the quantitative relationship between combustion efficiency, engine speed, and excess air coefficient obtained from the analysis under steady-state conditions on the test bench, the combustion efficiency is calculated based on the excess air coefficient obtained in step 2.

[0121] Step 6: Predict the injection duration based on the known injection pressure and cyclic injection quantity, and then update the combustion duration;

[0122] Step 7: Update the phase of the combustion heat release rate based on the combustion duration and the main injection advance angle;

[0123] Step 8: Based on the in-cylinder conservation equations, with 1 o CA is used to calculate cylinder pressure, cylinder temperature, and work output point by point during the combustion process at time steps.

[0124] Step 9: Predict the polytropic index of the expansion process based on the temperature at the end of the combustion process and the excess air coefficient;

[0125] Step 10: Based on the volume, pressure, and temperature at the end of the combustion process, calculate the pressure, temperature, and work done during the expansion process according to the polytropic process.

[0126] Step 11: Accumulate the work done in the three processes obtained in steps 4, 8, and 10, and calculate the high-pressure cycle indicated work, indicated thermal efficiency, and average indicated pressure;

[0127] Step 12: Compare the cylinder pressure, cylinder temperature, and cylinder pressure change rate obtained in steps 4, 8, and 11 to obtain the maximum combustion pressure, maximum pressure rise rate, and maximum combustion temperature of the cycle;

[0128] Step 13: Update PMEP and FMEP based on engine speed and cyclic injection quantity.

[0129] Step 14: Calculate BMEP, effective thermal efficiency, torque, and effective fuel consumption rate.

[0130] In step 2, the current circulating intake air volume in the cylinder is predicted:

[0131] Modern diesel engines do not use externally cooled EGRs; therefore, the circulating intake air volume of a diesel engine mainly depends on the intake air pressure. The relationship between the two can be expressed by the following equation, and the influence of engine speed is mainly reflected in the values ​​of A, B, and C:

[0132] (1)

[0133] In the formula: : Circulating air volume (mg); : Intake pressure (N / m2); A, B, C: Constants that vary with engine speed.

[0134] Based on the circulating intake volume obtained from equation (1), the current excess air coefficient can be calculated by combining it with the circulating fuel injection volume provided by the ECU.

[0135] The equation for solving the excess air coefficient is:

[0136] (2)

[0137] In the formula: Excess air coefficient (-); : Recirculating air volume (kg); : Circulating fuel injection quantity (kg). AFR0: Equivalent air-fuel ratio (-).

[0138] In step 3, the process of determining the polytropic index during compression and expansion includes:

[0139] During compression, the gas inside the cylinder only experiences changes in pressure and temperature (increase), while its composition remains unchanged. Because the temperature difference between the gas and the combustion chamber walls is small during this stage, the heat exchange between them is negligible. Therefore, the polytropic index during compression is relatively small. It should be close to the specific heat ratio g of air.

[0140] During combustion, not only does the temperature rise sharply, but the composition of the gas also changes from entirely diatomic gases (O2, N2) to triatomic gases (H2O, CO2). Therefore, the specific heat ratio of the gas changes not only with temperature but also with the composition of the gas. As a result, the specific heat ratio of the gas during the expansion process is quite different from that during the compression process.

[0141] Since the isobaric and isovolumetric specific heat ratios of both fresh air and combustion products decrease with increasing gas temperature, but because their gas compositions differ, the specific heat ratio of the in-cylinder gas is related to both the excess air coefficient and the in-cylinder gas temperature, i.e.:

[0142] (3)

[0143] The specific solution process for the combustion process is as follows:

[0144] (4)

[0145] The equation for solving the polytropic index after the combustion process is completed is:

[0146] (5)

[0147] In the formula, The ratio (-) for the complete combustion of the air-fuel mixture, ranging from 0 to 1; and : These are the specific heat ratios of combustion products and fresh air, respectively (-), both of which are functions of the mixture temperature.

[0148] The relationship between the specific heat ratio and polytropic index of the gas during the compression and expansion of the gas in the cylinder of a diesel engine and the changes in λ and the temperature of the gas in the cylinder;

[0149] Figure 3 shows the relationship between the specific heat ratio and polytropic index of the gas during the compression and expansion of the diesel engine cylinder and the changes in the in-cylinder gas temperature. Figure 3A shows the curves of the isobaric and isochoric specific heat ratio g of air and combustion products with l=1.0 as a function of gas temperature. It can be seen from the figure that the specific heat ratio of both fresh air and combustion exhaust gas decreases with increasing temperature, but the specific heat ratio of fresh air is always higher than that of combustion exhaust gas. Figure 3B shows a comparison of the in-cylinder gas temperature (based on the conversion of measured cylinder pressure) under two engine loads (excess air coefficient): It can be seen that the gas temperature is low when the excess air coefficient is large, and conversely, the gas temperature is high when the excess air coefficient is small.

[0150] Figure 3C shows a comparison between the measured in-cylinder polytropic index K and the specific heat ratio obtained by interpolation using equations (3)-(5) based on the relationship model of measured gas temperature (Figure 3B) and excess air coefficient (Figure 3A). The polytropic index K shown in the figure is obtained by inverse calculation based on the measured cylinder pressure and cylinder volume variation curves:

[0151] (6)

[0152] As can be seen in Figure 3:

[0153] 1. During the compression process before combustion begins, the polytropic index K and the specific heat ratio g of the gas basically coincide. That is to say, the polytropic index during compression can be replaced by the specific heat ratio of the gas. The reason why the polytropic index K decreases as the crankshaft angle approaches top dead center during compression is due to the decrease in the specific heat ratio of the gas in the cylinder caused by the increase in temperature due to compression. The slight difference between the K and g values ​​during this process is due to the heat transfer process in the cylinder.

[0154] 2. After combustion, the polytropic index K and specific heat ratio measured in the cylinder show a certain difference, and the K value is always lower than the g value. This difference is due to the heat loss of the in-cylinder gas to the outside. As the crankshaft angle moves backward, the pressure and temperature of the in-cylinder gas decrease, and the heat transfer coefficient and temperature difference between the in-cylinder gas and the combustion chamber wall decrease. Therefore, the heat loss rate in the cylinder decreases, and the difference between the two values ​​also decreases accordingly. Around 90 degrees after top dead center, the two values ​​basically coincide again. It is worth noting that the difference between the polytropic index K values ​​in the later stage of expansion under two different excess air coefficients is the same as the difference between the specific heat ratios of the gas.

[0155] 3. Therefore, we can conclude that the polytropic index of a gas during both compression and expansion is primarily determined by its specific heat ratio. For compression, the specific heat ratio can be used directly; however, for the polytropic index after combustion, a value slightly lower than the specific heat ratio can be used initially, until approximately 90 degrees Celsius after top dead center, at which point the specific heat ratio can be used directly.

[0156] In step 4, the cylinder gas pressure and temperature when the intake valve is closed...

[0157] The in-cylinder gas pressure and temperature at the moment the intake valve closes, i.e., the start of compression, are closely related to the intake manifold pressure and engine speed, and can be determined based on bench steady-state test results.

[0158] (7)

[0159] (8)

[0160] In the formula: Cylinder pressure (bar) when the intake valve is closed; : constant (bar / rpm); Gas temperature when intake valve is closed ( o K); :constant( o K / rpm); Engine speed (rpm); Intake air temperature ( o K), This refers to the intake pressure (bar).

[0161] In step 5, the combustion efficiency is calculated.

[0162] Combustion efficiency should ideally be calculated from measured emission data. However, installing an emission analyzer in a vehicle is impractical, and the current calculation method is cumbersome and difficult to quickly apply to predictive model calculations. Research shows that combustion efficiency is almost entirely related to engine speed and excess air coefficient. Therefore, a dataset can be established based on bench steady-state measured data, and combustion efficiency can be quickly obtained by interpolation using the excess air coefficient. The functional relationship is summarized as follows:

[0163] (9)

[0164] Figure 2 shows the relationship between the measured combustion efficiency and the excess air coefficient.

[0165] In step 6, the prediction of the injection duration.

[0166] When the injection pressure of a diesel engine changes, the injection duration will also change. Analysis of the measured results shows that the relationship between the injection duration and the cyclic injection quantity and injection pressure is as follows:

[0167] (10)

[0168] In the formula, The current injection duration (°CA); The measured spray duration (°CA) under steady-state bench conditions. The current cyclic injection quantity (mg); The cyclic injection quantity (mg) is under steady-state test bench conditions. The injection pressure (bar) under steady-state test bench conditions. This represents the current injection pressure (bar).

[0169] In step 6, the combustion heat release rate is updated.

[0170] When the cyclic injection quantity and injection pressure change, the injection duration also changes accordingly, and consequently, the combustion heat release rate also changes. Our research team's results show that, under the same engine speed and similar cyclic injection quantities, the combustion heat release rate is related to the cyclic injection quantity and injection pressure as follows:

[0171] (11)

[0172] In the formula, The current combustion heat release rate (1 / °CA); The measured combustion heat release rate (1 / °CA) under steady-state test conditions.

[0173] The change in the combustion initiation point is as follows:

[0174] (12)

[0175] In the formula, The current combustion initiation point (1 / °CA ATDC); The measured combustion initiation point (1 / °CA ATDC) under steady-state test bench conditions. The current main injection advance angle (°CA BTDC); The main injection advance angle (°CA BTDC) is the angle under steady-state test bench conditions.

[0176] In step 8, the instantaneous heat transfer rate between the in-cylinder gas and the combustion chamber wall is solved.

[0177] The heat release from combustion of the air-fuel mixture and the heat exchange between the gas and the combustion chamber walls during combustion mean that equation (10) can no longer be described by a polyvariable process, but can only be solved one by one along the crankshaft rotation based on equations (10) and (11). However, to solve equation (10), in addition to substituting the updated combustion heat release rate, it is also necessary to update the instantaneous heat transfer rate between the in-cylinder gas and the combustion chamber walls. For piston internal combustion engines, the Woschini formula is usually used to solve for the heat transfer rate between the in-cylinder gas and the combustion chamber walls:

[0178] (13)

[0179] In the formula:

[0180] Heat transfer coefficient between the gas inside the cylinder and the combustion chamber wall (W / m²) 2o K); Engine speed (r / min); Heat transfer area of ​​engine combustion chamber wall (m²) 2 ); Instantaneous temperature of the gas ( o K); The average temperature of the combustion chamber walls ( o K).

[0181] The difficulty in equation (13) lies in solving for the heat transfer coefficient. A formula proposed by a German scientist, Woschini, is now widely used:

[0182] (14)

[0183] In the formula:

[0184] D: Piston diameter (m); : Gas pressure (kPa) at crankshaft rotation angle; : Gas temperature at crankshaft rotation angle ( o K); : Average piston speed (m / s);

[0185] Solving for cylinder pressure and temperature during combustion:

[0186] The in-cylinder combustion heat release rate is updated in real time based on the cyclic injection quantity, injection pressure, and main injection advance angle. Equation (11); Based on the cylinder pressure and cylinder temperature calculated in the previous time step, the instantaneous heat transfer rate between the gas and the combustion chamber wall is updated. After equation (13), the pressure and temperature of the in-cylinder gas at each crankshaft angle during the combustion process can be solved one by one based on equations (15) and (16). Combining the calculation results of the compression section according to the polytropic process (equations (17) and (18)) and the calculation results of the expansion section according to the polytropic process (equations (21) and (22)), the calculation of the in-cylinder pressure and temperature of the entire high-pressure cycle is completed. It is worth mentioning that the calculation of the compression section and the expansion section is completed in one step, and only the calculation of the combustion section needs to be done according to equation (13). 0 The time step of CA is calculated; therefore, this method will use -180 from the traditional method. o CA to +180 o CA's 360 o Detailed calculations of the in-cylinder processes using CA (360 time steps) have been shortened to only the combustion process, which is approximately 60 seconds. o The detailed calculation of the in-cylinder process in CA, combined with the simplified calculation of the two-stage (two-step) variable process, greatly accelerates the calculation speed and reduces the computing power requirements.

[0187] Control equations for in-cylinder gas pressure and temperature

[0188] Based on the conservation of energy and mass within a closed container, as well as the ideal gas law, the rate of pressure increase of the gas inside the cylinder can be expressed as:

[0189] (15)

[0190] The cylinder pressure at any given time can be calculated using the following formula:

[0191] (16)

[0192] After obtaining the cylinder pressure, the gas temperature can be solved based on the ideal gas law:

[0193] (17)

[0194] In the formula, : Crankshaft rotation angle: Cylinder gas pressure rise rate (Pa / ) o CA); : Gas pressure (Pa) at crankshaft rotation angle; : Cylinder volume (m³) at crankshaft rotation angle 3 ); : Gas temperature at crankshaft rotation angle ( o K); : Circulating fuel injection quantity (kg); Total gas volume in cylinder (kg); Low calorific value of fuel oil (J / kg); Combustion efficiency (-); In-cylinder combustion heat release rate (1 / o CA); Instantaneous heat transfer rate in the cylinder (J / o CA); : The specific heat ratio of a gas at constant pressure and constant volume (-).

[0195] During the compression process before combustion begins, and during the expansion process after combustion is complete: the rate of heat release from combustion. If the heat loss between the gas and the wall If it is also 0, equation (15) can be simplified to:

[0196] (15')

[0197] Equation (6') can be solved analytically directly:

[0198] (16')

[0199] (16') characterizes an ideal gas (constant pressure, constant volume, specific heat ratio g = constant) from state 1 adiabatic ( The control equation for cylinder pressure during compression or adiabatic expansion to state point 2. When considering real gas, g is no longer a constant but a variable that varies with temperature and composition; and if the heat transfer term between the gas and the combustion chamber wall is also not negligible, the exponent in equation (16') is no longer the gas specific heat ratio g, but a polytropic exponent K that is slightly different from the value of g, to characterize the effect of heat transfer on cylinder pressure. The process expressed by equation (16') is then called a polytropic process. The reason for using a polytropic process to describe the change in cylinder pressure during pure compression and pure expansion is to simplify the calculation of this process: this process does not need to be calculated point by point according to crankshaft rotation angle as in the combustion process, but can be completed in one step. Therefore, the computational power requirement of the ECU can be greatly reduced.

[0200] Cylinder pressure control equation for pure compression process:

[0201] (18)

[0202] (19)

[0203] (20)

[0204] Cylinder pressure control equation for pure expansion process:

[0205] (twenty one)

[0206] (twenty two)

[0207] In the formula:

[0208] : Gas pressure (Pa) when the intake valve is closed; .: Cylinder volume when intake valve is closed (m³) 3 ); : The polytropic index of the compression process (-); : Gas pressure at the end of combustion (Pa); Cylinder volume at the end of combustion (m³) 3 ); : The polyvariance index of the expansion process (-);

[0209] Because the temperature and composition of the gas differ significantly during compression and expansion, the isobaric and isochoric specific heat ratio and the rate of heat dissipation from the gas to the cylinder wall, which affect the polytropic index, also vary considerably. Therefore, different polytropic indices need to be used. and To describe.

[0210] During the combustion process, the cylinder pressure changes drastically due to the combustion and heat exchange processes, making it difficult to describe as a variable process. Instead, it can only be calculated one by one along the crankshaft rotation position according to equations (15) and (16). To solve equation (15), in addition to the combustion efficiency obtained from equation (9), it is also necessary to update the combustion heat release rate in the cylinder and the heat transfer rate between the gas and the cylinder wall.

[0211] In step 11, the high-pressure indicated cycle thermal efficiency is solved.

[0212] The indicated thermal efficiency of a high-pressure cycle is defined as the indicated work done by the piston in the high-pressure cycle, divided by the heat released by the fuel. The indicated work done by the piston in the high-pressure cycle can be obtained by integrating the product of cylinder pressure and cylinder volume change rate over one cycle; while the heat released by the fuel is equal to the fuel injection quantity in the cycle multiplied by the lower heating value of the fuel.

[0213] (twenty three)

[0214] In the formula, The thermal efficiency of the high-pressure cycle is -.

[0215] To simplify the calculation process, this patent employs a segmented (5-segment) solution, as follows:

[0216] (twenty four)

[0217] The amount of work done in each stage:

[0218] From bottom dead center to intake valve closed (based on isobaric process assumption): -180 o CA to IVC

[0219] (24a)

[0220] Work done from intake valve closed to the start of combustion (based on polytropic process assumptions): IVC-BOC

[0221] (24b)

[0222] From the start of combustion to the end of combustion (based on detailed simulation calculations of the combustion process): BOC-EOC

[0223] (24c)

[0224] From the end of combustion to the start of exhaust valve opening (based on the assumption of a variable process): EOC-EVO

[0225] (24d)

[0226] From the start point of exhaust valve opening to the bottom dead center (based on the assumption of an isobaric process): EVO-180 o CA

[0227] (24d)

[0228] In step 11, the high-pressure cycle indicates the prediction of the mean pressure.

[0229] Based on the definition: the average indicated average pressure of a high-pressure cycle can be based on the above-mentioned segmented work results. Solve based on the following formula:

[0230] (25)

[0231] In step 14, the prediction of effective performance indicators

[0232] Based on the engine speed and cyclic injection quantity, the average pumping loss pressure (PMEP) and average friction loss pressure (FMEP) of the engine can be found in the pre-stored bench steady-state test data table. Based on this, the effective average pressure, effective torque, and effective fuel consumption rate can be calculated.

[0233] (26)

[0234] (27)

[0235] (28)

[0236] (29)

[0237] In the formula, Mean effective pressure (bar); Torque (Nm); PMEP is the average effective pressure of pumping losses (bar); FMEP is the average effective pressure of friction losses (bar). Effective thermal efficiency (-); Effective fuel consumption rate (g / kWh); It has a low calorific value (J / (kg*K)) for fuel oil.

[0238] In step 14, prediction of other performance indicators

[0239] By performing secondary processing on the cylinder pressure and temperature changes along the crankshaft angle obtained by the above method within a known range, the maximum burst pressure, maximum temperature, and maximum pressure rise rate required for evaluating the engine's NVH performance can be determined; as well as the average pressure and temperature of the in-cylinder gas during combustion and expansion required for evaluating the cooling system performance.

[0240] (30)

[0241] (31)

[0242] (32)

[0243] (33)

[0244] (34)

[0245] Since the highest burst pressure usually occurs from the top dead center to 20° after the top dead center. o Between CA; the maximum pressure rise occurs 15 degrees before and after the top dead center. o Between CA; the highest combustion temperature occurs from top dead center to 30°C after top dead center. oTherefore, the search for these parameters only needs to be performed within a narrower range, making the search process faster.

[0246] This concludes the demonstration of a simplified calculation model for the entire process of heat-work conversion in a high-pressure circulating cylinder. Since all the above equations are calculated explicitly, their computing power and processing speed requirements should be able to meet the online application requirements of the ECU.

[0247] Model Validation

[0248] To verify the simulation accuracy of the proposed numerical model, several typical working conditions were selected for comparison of measured and simulation results. Figure 4 shows the comparison of cylinder pressure and pressure rise rate of diesel engine under the same speed and two different loads; Figure 5 shows the comparison of measured and simulation results of corresponding cylinder temperature and temperature rise rate. Figures 4 and 5 show that the simplified numerical model derived above can well reflect the changes in cylinder pressure and cylinder temperature. The error is small. Figure 6 shows the comparison between the measured results of in-cylinder expansion efficiency corresponding to different main injection advance angles and the simulation results based on (24) prediction. It can be seen that the application of the variable process to replace the calculation of compression and expansion processes has good accuracy for the prediction of in-cylinder indication function. More importantly, this treatment reduces the simulation calculation workload of in-cylinder process to nearly 1 / 6 of the traditional method.

[0249] The above are merely preferred embodiments of the present invention and do not constitute any limitation on the scope of protection of the present invention; all technical solutions formed by equivalent transformations or equivalent substitutions fall within the scope of protection of the present invention; the parts of the present invention not described in detail are well-known technologies to those skilled in the art.

Claims

1. A method for rapid prediction of the in-cylinder heat-work conversion process of a diesel engine under varying operating conditions, characterized in that, The rapid prediction method for the in-cylinder thermo-mechanical conversion process of a diesel engine under varying operating conditions includes the following steps: Step 1: First, obtain the current main control and operating parameters of the diesel engine from the ECU, including speed, intake pressure, cyclic injection quantity, injection pressure, and main injection advance angle; Step 2: Based on the quantitative relationship between the measured cyclic intake quantity, speed, and intake pressure under bench steady-state conditions, predict the current in-cylinder cyclic intake quantity; then, calculate the current excess air coefficient by combining the known cyclic injection quantity; Step 3: Based on the intake temperature and the in-cylinder excess air coefficient, predict the polytropic index of the compression process; Step 4: Based on the intake... When the valves are closed, the cylinder volume, cylinder pressure, and temperature are used to calculate the cylinder pressure, cylinder temperature, and work done during the compression phase from valve closure to the start of combustion using a multivariate exponential equation; Step 5: Based on the quantitative relationship between combustion efficiency, engine speed, and excess air coefficient obtained from the bench steady-state test, the combustion efficiency is calculated based on the excess air coefficient obtained in step 2; Step 6: The injection duration is predicted based on the known injection pressure and cyclic injection quantity, and then the combustion duration is updated; Step 7: The phase of the combustion heat release rate is updated based on the combustion duration and the main injection advance angle; Step 8: Based on the in-cylinder conservation equations, with 1 o CA calculates cylinder pressure, cylinder temperature, and work output point by point during the combustion process for each time step; Step 9: Predict the polytropic index of the expansion process based on the temperature at the end of the combustion process and the excess air coefficient; Step 10: Calculate the pressure, temperature, and work output of the expansion process according to the polytropic process based on the volume, pressure, and temperature at the end of the combustion process; Step 11: Accumulate the work output of the three processes obtained in Steps 4, 8, and 10, and calculate the high-pressure cycle indicated work, indicated thermal efficiency, and average indicated pressure; Step 12: Compare the cylinder pressure, cylinder temperature, and cylinder pressure change rate obtained in Steps 4, 8, and 11 to obtain the cycle's maximum burst pressure, maximum pressure rise rate, and maximum combustion temperature; Step 13: Update PMEP and FMEP based on engine speed and cycle injection quantity; Step 14: Calculate BMEP, effective thermal efficiency, torque, and effective fuel consumption rate.

2. The rapid prediction method for the in-cylinder heat-work conversion process of a diesel engine under varying operating conditions according to claim 1, characterized in that, In step 2, the current in-cylinder circulating intake air volume is predicted: The circulating intake air volume of a diesel engine mainly depends on the intake pressure, and the relationship is expressed by the following equation. The influence of engine speed is mainly reflected in the values ​​of A, B, and C: In the formula: : Recirculating air intake volume; : Intake pressure; A, B, C: constants that vary with engine speed; Based on the obtained cyclic intake air volume, combined with the cyclic fuel injection volume provided by the ECU, the current excess air coefficient can be calculated. The equation for the excess air coefficient is: In the formula: Excess air coefficient; : Recirculating air intake volume; : Circulating fuel injection quantity; AFR0: Equivalent air-fuel ratio.

3. The method for rapid prediction of in-cylinder heat-work conversion process of a diesel engine under varying operating conditions according to claim 1, characterized in that, In steps 3 and 9, the process of determining the polytropic index during compression and expansion includes: during compression, the gas in the cylinder only experiences changes in pressure and temperature while its composition remains unchanged; the polytropic index during compression... It should be close to the specific heat ratio g of air; the equation for solving the polytropic index of the combustion process is: The equation for solving the polytropic exponent after the combustion process is completed is: In the formula, The ratio of the air-fuel mixture to complete combustion, ranging from 0 to 1; and : These are the specific heat ratios of combustion products and fresh air, respectively, both of which are functions of the mixture temperature.

4. The method for rapid prediction of in-cylinder heat-work conversion process of a diesel engine under varying operating conditions according to claim 1, characterized in that, In step 4, the in-cylinder gas pressure and temperature at the moment the intake valve closes (i.e., the start of compression) are closely related to the intake manifold pressure and engine speed, and are determined based on bench steady-state test results. ； In the formula: Cylinder pressure when intake valve is closed; :constant; Gas temperature when the intake valve is closed; :constant; Engine speed; Intake air temperature, This refers to the intake pressure.

5. The method for rapid prediction of in-cylinder heat-work conversion process of a diesel engine under varying operating conditions according to claim 1, characterized in that, In step 5, a dataset is established based on the bench steady-state measured data, and the combustion efficiency is obtained by rapid interpolation based on the excess air coefficient. The functional relationship is summarized as follows: ； For combustion efficiency, Engine speed, This is the excess air coefficient.

6. The method for rapid prediction of in-cylinder heat-work conversion process of a diesel engine under varying operating conditions according to claim 1, characterized in that, In step 6, when the injection pressure of the diesel engine changes, the injection duration will change. The relationship between the injection duration and the cyclic injection quantity and injection pressure is as follows: In the formula, This is the current duration of the jetting process; The measured spray duration under steady-state test bench conditions; This is the current cycle fuel injection quantity; This refers to the cyclic fuel injection quantity under steady-state test bench conditions. The injection pressure under steady-state test bench conditions; The current injection pressure is given. When the cyclic injection quantity and injection pressure change, under the same engine speed and similar cyclic injection quantity conditions, the combustion heat release rate has the following relationship with the cyclic injection quantity and injection pressure: In the formula, This represents the current combustion heat release rate; The measured combustion heat release rate under steady-state test bench conditions; the change in combustion initiation point is as follows: In the formula, This is the current point of combustion initiation; The measured combustion initiation point under steady-state test bench conditions; This is the current main jet advance angle; This refers to the advance angle of the main spray under steady-state test bench conditions.

7. The method for rapid prediction of in-cylinder heat-work conversion process of a diesel engine under varying operating conditions according to claim 1, characterized in that, In step 8, based on the conservation of energy and mass within the closed container and the ideal gas law, the rate of increase in pressure of the gas inside the cylinder can be expressed as: The cylinder pressure at any given time can be calculated using the following formula: After obtaining the cylinder pressure, the gas temperature can be solved based on the ideal gas law: In the formula, ： The rate of increase in cylinder gas pressure when the crankshaft rotates; : Gas pressure at crankshaft rotation angle; : Cylinder volume at crankshaft rotation angle; : Gas temperature at crankshaft rotation angle; : Circulating fuel injection quantity; Total gas volume in the cylinder; Fuel has a low calorific value; Combustion efficiency; : In-cylinder combustion heat release rate; Instantaneous heat transfer rate inside the cylinder; The specific heat ratio at constant pressure and volume of the gas; the heat transfer rate between the gas in the cylinder and the combustion chamber wall is calculated using the Woschini formula. In the formula: The heat transfer coefficient between the gas inside the cylinder and the wall of the combustion chamber; Engine speed; The heat transfer area of ​​the engine combustion chamber wall; Instantaneous temperature of the gas; The average temperature of the combustion chamber walls; the solution for the heat transfer coefficient: In the formula: D: piston diameter; : Gas pressure at crankshaft rotation angle; : Gas temperature at crankshaft rotation angle; Average piston velocity; Cylinder pressure control equation for pure compression process: ； ； ； Cylinder pressure control equation for pure expansion process: ； In the formula: : Gas pressure when the intake valve is closed; .: Cylinder volume when the intake valve is closed; : The variability index of the compression process; : Gas pressure at the end of combustion; Cylinder volume at the end of combustion; The polytropic index of the expansion process.

8. The method for rapid prediction of in-cylinder heat-work conversion process of a diesel engine under varying operating conditions according to claim 1, characterized in that, In step 11, the high-pressure indicator cycle thermal efficiency is calculated as follows: the heat released by the fuel is equal to the fuel injection quantity multiplied by the lower heating value of the fuel. In the formula, To indicate the thermal efficiency of the high-pressure cycle, and to simplify the calculation process, this patent employs a segmented solution: ； The work done at each stage: from bottom dead center to intake valve closing, -180 o CA to IVC: ； Power output when the intake valve is closed to the start of combustion, IVC-BOC: From the start of combustion to the end of combustion, BOC-EOC: From the end of combustion to the beginning of exhaust valve opening, EOC-EVO: From the start point of exhaust valve opening to the bottom dead center, EVO-180 o CA: Prediction of the indicated average pressure during high-pressure circulation: The indicated average pressure during high-pressure circulation can be based on the above-mentioned segmented work results. Solve based on the following formula: ； This indicates the average indicated average pressure of the high-pressure cycle. This represents the average cylinder volume at each stage.

9. The method for rapid prediction of in-cylinder heat-work conversion process of a diesel engine under varying operating conditions according to claim 1, characterized in that, In step 14, the prediction of effective performance indicators: Based on the engine speed and cyclic injection quantity, the average pumping loss pressure PMEP and average friction loss pressure FMEP of the engine can be found in the pre-stored bench steady-state test data table. Based on this, the effective average pressure, effective torque, and effective fuel consumption rate can be calculated. ； ； ； In the formula, The average effective pressure; Torque; PMEP is the average effective pressure for pumping losses; FMEP is the average effective pressure for friction losses; For effective thermal efficiency; Effective fuel consumption rate; For fuel with a low calorific value; based on the changes in cylinder pressure and temperature along the crankshaft angle, and within a known range, perform secondary data processing to determine the maximum combustion pressure, maximum temperature, and maximum pressure rise rate required for evaluating engine NVH performance; and the average pressure and temperature of the in-cylinder gas during combustion and expansion required for evaluating cooling system performance: ； ； ； ； ； Indicates the highest burst pressure. express Pressure during crankshaft rotation Indicates the highest combustion temperature. express Temperature at crankshaft rotation angle This indicates the average pressure of the gas inside the cylinder during combustion and expansion. This indicates the crankshaft angle at which the exhaust valve begins to open. Indicates the crankshaft angle at the point of combustion initiation. This indicates the average temperature of the gas inside the cylinder during combustion and expansion.

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