1 Calculation model In practical calculations, the use of appropriate assumptions can greatly simplify the thermal process. The calculation of the combustion heat release rate described in this paper is based on the following assumptions: 1 The pressure, temperature, and concentration differences at various points in the cylinder are not considered. That is, at each moment, the working fluid at each point in the cylinder is the same in terms of temperature, pressure, and composition; 2 the combustion exothermic effect is the result of producing the final product without considering a series of intermediate reactions; The ideal gas to deal with, the specific heat and heat energy only with the gas temperature and the composition of the gas components, with the ideal gas state equation pV = MRT to express its relationship between the state parameters. The calculation model is based on these assumptions, based on the change in the measured cylinder pressure, iterative method to calculate the combustion heat release rate and the percentage of heat release.
1.1 Basic equations The heat released from fuel combustion can be divided into three main parts: one part is used for external work, one part is used to increase the internal energy of the working medium, and another part is lost to the cylinder wall.
Q = ΔU + W + Qw where ΔU is the internal energy increment of the working medium, ΔU = U-U0dQd According to the ideal gas state equation pV = MRT available internal energy U = pV 8.314 cV work W = pV is done by two neighbors The work dWi = d; R is the molar gas constant, R = 8.314J/(kg).
1.2 Calculation of heat transfer quantity The amount of heat dissipation to the surrounding parts of the working medium is less than the other items in the energy equation. The combustion heat release rate mainly depends on the internal energy of the working medium and the work done, according to the author’s Calculations using different heat transfer coefficient formulas have little effect on the heat release rate results. This article selects Woschni's heat transfer coefficient formula.
Heat transfer coefficient Hg = 6.777 × D-0.214 × p0.786 × C0.786m where D is the cylinder diameter (m); Cm is the piston running speed (m/s).
Using as and ae as the crank angle for calculating the start and end points, the heat transfer amount is Qw=∫aeas∑3k=1d 1.3 The calculation of the composition and the specific heat of the mixture 1.3.1 The composition of the gas in the cylinder The main components of the combustion chamber Including air, CH4, and gas. Since it is a homogeneously mixed gas, the gas in the combustion chamber can be regarded as being composed of a homogeneous mixture of air and CH4 and gas.
In the homogeneous mixture of air and CH4, the proportion of CH4 at the beginning is rCH4=(1+γ)×mfc16M1M1=π4D2×S×μps8.314Ts×(1+γ) where M1 is in the cylinder The total number of moles of initial gas; mfc is the cyclical oil supply (kg/cycle); S is the stroke (m); Ts is the initial temperature of the gas in the cylinder (K); ps is the initial gas pressure in the cylinder (kPa); Residual exhaust gas coefficient; μ is the coefficient of inflation.
The proportion of fresh air in the cylinder is rair=1-rCH4. The composition of the gas is mainly composed of carbon dioxide CO2, water vapor H2O, excess oxygen O2, and nitrogen N2.
When the fuel is completely combusted, the content of the above-mentioned components in the combustion product of 1 kg of liquid fuel is MCO2=gC12 (kmol) MH2O=gH2 (kmol) MO2=0.21 (α-1)L0 (kmol) MN2=0.79αL0 (kmol) In the formula, gC is the content of C in 1 kg of fuel, gH is the content of H in 1 kg of fuel, α is the excess air ratio, and L0 is the stoichiometric air-fuel ratio, that is, the kmol of the air quantity theoretically required for burning 1 kg of fuel.
After the complete combustion of 1 kg of fuel, the total number of moles of gas generated is M2 = MCO2 + MH2O + MO2 + MN2 The instantaneous total number of moles of gas in the cylinder is calculated based on the change in the number of moles after combustion of the working medium.
The instantaneous total number of moles of gas in the cylinder M=M1+QLHu×(116+αL0)×(β-1)β=1+(gH4+gO32-116)(L0×α+116) where QL is the start of fuel The cumulative heat released after combustion (kJ); Hu is the low heating value of the fuel (kJ/kg); β is the molecular change coefficient when the working fluid is completely burned; gO is the content of O in 1 kg of fuel.
The proportion of gas in each instantaneous cylinder is γt=QLHu×(116+αL0)×βM1.3.2 The specific heat of different gases in the cylinder is calculated The average specific heat of different gases is based on the discrete data given in the literature. The least squares method was used to fit, so that the quadratic trinomial equation of the average constant specific heat of different gases with temperature was obtained.
Average constant specific heat of methane: average constant volume ratio of carbon dioxide = 25.810 + 2.9893 x 10 -2 T - 3.5948 x 10 -6 T2 air, average constant heat ratio cVair = 1.29 + 3.4631 x 10-3T -3.9517 x 10 -7 T2 carbon dioxide Heat cVCO2=16.044+1.7459×10-2T-3.4031×10-6T2 The average constant heat capacity of water vapor cVH2O=14.697+6.1965×10-3T-2.7768×10-7T2 Average constant heat capacity of oxygen cVO2=11.274+ The average constant specific heat of nitrogen of 5.0828×10-3T-8.0410×10-7T2 is cVN2=8.2321+3.0325×10-3T-2.8717×10-7T2 The instantaneous average temperature of the gas in the cylinder is obtained from the ideal gas state equation Ti=piViMiR The specific heat of the homogenous gas mixture cVc=rCH4×cVCH4+rair×cVair The specific heat of the gas is calculated as follows: cVg=MCO2×cVCO2+MH2O×cVH2O+MO2×cVO2+MN2×cVN2M2 represents the instantaneous average specific heat of the gas mixture in the cylinder. Calculate and analyze the heat release rate for cV=cVg×γt+cVc×(1-γt)2. The calculation start and end points are selected after the intake Valve is closed and before the exhaust valve is opened. The program has the function to calculate multiple fuels. . Before calculating the heat release rate, the pressure of the working fluid in the combustion chamber is accurately measured and the pressure is smoothed. This article takes an internal combustion engine with natural gas modified by S195 diesel engine as an example. The output torque is 12N.
Based on the dynamometer diagram measured at m, the multiple cycles of the dynamometer diagram are averaged, and smoothing is applied to the pressure using a five-point smoothing method. This is combined with the structural parameters and operating parameters of the internal combustion engine. The calculation procedure gives the heat release rate and heat release percentage curve (Figure 1) and the temperature curve (Figure 2). This program calculates a 1° crank angle in steps. The mixture gas is formed by injecting preheated natural gas into the intake port to form a uniform mixture of air and natural gas in the cylinder. The main parameters of the internal combustion engine: cylinder diameter D = 95mm; stroke S = 115mm; compression ratio ε = 18.5; connecting rod length L = 210mm; speed n = 1160r/min.
Analysis of calculation results: From the above two sets of graphs, it can be seen that at 17° after top dead center, the in-cylinder pressure reaches a maximum value of 4.152 MPa; at a 22° after top dead center, the maximum temperature within the cylinder reaches 1876 K. 22 ° after top dead center, the combustion process ends. Combustion is completed within approximately 20° crankshaft angle after top dead center, and the combustion rate is fast, close to the ideal Alto cycle; while the heat dissipation of ordinary compression ignition combustion is mainly concentrated within 40° after top dead center, so the homogeneous pressure The burning rate of burning combustion is much greater than the burning rate of ordinary compression ignition.
The reason why the HCCI internal combustion engine has a high combustion rate is determined by its combustion characteristics. In this example, natural gas is injected in the intake passage so that the natural gas and the air can be completely and evenly mixed before combustion, and the combustion can be performed simultaneously at multiple points, so the combustion reaction is rapid.
3 Conclusions 1) The calculation of the combustion heat release rate of a homogeneous compression ignition natural gas internal combustion engine is presented in this paper. The specific heat of different substances is accurately fitted. Based on this, the use of Visual Basic language is based on Windows. The operating system's heat release rate calculation program for a homogeneous compression-ignition natural gas internal combustion engine, and this program generates an executable file. This procedure is calculated using the difference formula. The starting point of combustion is determined based on the dynamometer graph, so that a certain combustion exothermic effect can be felt as the starting point for judging significant combustion. The interface of this calculation program is friendly, and the operation is simple and quick. Input known data and click to execute. The result can be obtained and the graphics can be output. It is more intuitive.
2) In this paper, the parameters of the combustion heat release rate and the heat release percentage of a natural gas internal combustion engine modified by S195 diesel engine under partial load conditions are successfully calculated by this program. It provides a valuable reference for improving and improving the performance of natural gas internal combustion engines.
3) The calculation model presented in this paper is applicable to homogeneous compression ignition internal combustion engines for various fuels.
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