CN112304623B - Effective thermal efficiency prediction method of marine diesel engine based on fuel components - Google Patents

Effective thermal efficiency prediction method of marine diesel engine based on fuel components Download PDF

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CN112304623B
CN112304623B CN202011170193.7A CN202011170193A CN112304623B CN 112304623 B CN112304623 B CN 112304623B CN 202011170193 A CN202011170193 A CN 202011170193A CN 112304623 B CN112304623 B CN 112304623B
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刘龙
唐元亨
梅齐昊
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Harbin Engineering University
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Abstract

The invention aims to provide a method for predicting the effective thermal efficiency of a marine diesel engine based on fuel components, which comprises the following steps: determining boundary conditions and initial conditions, selecting the type of fuel to be combusted, dividing the working process in a cylinder into three stages of compression, combustion and expansion, calculating the compression process, the combustion process and the expansion process, performing integral calculation on the obtained pressure, calculating the effective work in the working process, and calculating the effective thermal efficiency according to the ratio of the effective work to the total input heat. The method can realize the prediction of the effective thermal efficiency of any constant volume combustion ratio, so that the application range of the model is not limited to a specific model diesel engine. The method can realize the prediction of the effective thermal efficiency when the diesel engine burns different types of fuel, adapts to the development trend of replacing fuel by the current marine diesel engine, and provides a numerical prediction means for the optimal design of the thermal efficiency of the diesel engine in a wider range.

Description

Effective thermal efficiency prediction method of marine diesel engine based on fuel components
Technical Field
The invention relates to a thermal efficiency prediction method, in particular to a thermal efficiency prediction method for a marine diesel engine.
Background
With the increasing demand for energy conservation and emission reduction in society, various emission regulations of the shipping industry are continuously proposed, and requirements are provided for the control of various emissions of marine diesel engines. Among the emissions, carbon dioxide, which is one of the greenhouse gases, has been more severely limited in recent years.
The effective thermal efficiency of the diesel engine directly influences the emission level of carbon dioxide, and the diesel engine with high thermal efficiency can not only meet the regulation and restriction requirements of carbon dioxide emission, but also realize good power performance and improve the economy. The experimental research aiming at the optimization of the effective thermal efficiency usually needs high-precision measuring equipment and a complex combustion simulation device, and has high cost and long period, so that the method has important significance for predicting the effective thermal efficiency of the diesel engine by adopting calculation software.
At present, the simulation prediction of the effective thermal efficiency is mainly divided into two categories of finite time thermodynamic derivation and zero-dimensional model simulation. The finite time thermodynamic derivation calculation method is complex, the result is too strong in theory, and the actual reference is weak. The existing zero-dimensional model mainly uses a weber formula or a double weber formula and other semi-empirical formulas to simulate the combustion process, and the model is weak in combustion adjustability and difficult to predict the universal effective thermal efficiency for different diesel engine models. In addition, most of the existing zero-dimensional models only consider the condition that diesel oil is used as fuel for combustion, and are difficult to adapt to the development trend of replacing fuel by the existing diesel engine.
Disclosure of Invention
The invention aims to provide a method for predicting the effective thermal efficiency of a marine diesel engine based on fuel components, which uses a constant-volume combustion and constant-pressure combustion ratio value to replace a heat release rate formula and calculates the specific heat capacity and specific heat capacity ratio of a working medium through fuel components.
The purpose of the invention is realized as follows:
the invention discloses a method for predicting effective thermal efficiency of a marine diesel engine based on fuel components, which is characterized by comprising the following steps of:
(1) determining boundary conditions and initial conditions according to geometric parameters and initial parameters of the diesel engine, and setting a constant volume combustion ratio X in the combustion process, wherein the constant volume combustion ratio is the input heat Q of constant volume combustion cv With total heat input Q of combustion process total The ratio of:
Figure BDA0002747064810000011
wherein the constant volume combustion ratio is experimentally measured or artificially set according to predicted requirements;
(2) selecting the type of fuel for combustion, and determining the components of the working medium after combustion according to the selected fuel components and the following chemical equation:
Figure BDA0002747064810000021
according to the excess air coefficient a, the number of carbon molecules n, the number of hydrogen molecules m and the number of oxygen molecules l of the fuel, obtaining CO in the combustion product 2 Number of molecules n 1 、H 2 Number of O molecules n 2 、N 2 Number of molecules n 3 、O 2 Number of molecules n 4 Determining the specific heat capacity of the working medium in the working process by combining the Dalton partial pressure theorem and the formula;
(3) dividing the working process in the cylinder into three stages of compression, combustion and expansion, wherein each stage is divided into step lengths according to a 0.01-degree crankshaft rotation angle, and each step length is subjected to iterative solution by adopting a first thermodynamic law;
(4) calculating the compression process according to the heat transfer amount in one step of the compression process
Figure BDA0002747064810000022
Volume work
Figure BDA0002747064810000023
Mass m of working medium and specific heat capacity c of working medium v1 The temperature change in one step is obtained by the first thermodynamic law
Figure BDA0002747064810000024
Figure BDA0002747064810000025
Temperature t according to initial conditions 1 Solving the temperature t of the first step end 2 Will t 2 Performing iterative calculation on initial temperature serving as the next step to obtain the temperature of each step in the compression process, knowing the temperature of each step, and obtaining the pressure of each corresponding step according to an ideal gas state equation pV (RT);
(5) calculating the combustion process based on the heat transfer in one step of the combustion process
Figure BDA0002747064810000026
Volume work
Figure BDA0002747064810000027
Mass m of working medium and specific heat capacity c of working medium v2 Input heat of combustion g of fuel f (Hu-u) determining the temperature change within one step by the first thermodynamic law
Figure BDA0002747064810000028
Figure BDA0002747064810000029
Repeating the temperature iteration step in the step (4), and solving to obtain the temperature and the pressure of each step;
(6) calculating the expansion process, repeating the calculation process of the step (4), and substituting the calculation process into the working medium specific heat capacity c after the combustion process v3 Solving to obtain the temperature and pressure of each step length in the expansion process;
(7) performing integral calculation on the pressure obtained in the steps (4), (5) and (6) to obtain effective work W in the working process, and then calculating the total input heat Q according to the effective work W total The effective thermal efficiency is found as the ratio of:
Figure BDA00027470648100000210
the invention has the advantages that: the method is based on the first law of thermodynamics, takes the input constant volume combustion ratio as a combustion rule, iteratively solves the temperature and the pressure of the initial state according to the working sequence of compression, combustion and expansion according to the step length, can realize the prediction of the effective thermal efficiency of any constant volume combustion ratio, and ensures that the application range of the model is not limited to the diesel engine with a specific model. Secondly, according to the preset fuel component parameters, the specific heat capacity and specific heat capacity ratio parameters of the working medium before and after combustion in the cylinder are calculated through the Dalton partial pressure theorem of different gas components, the influence of different fuels on the working process of the diesel engine is reflected, the effective thermal efficiency prediction of the diesel engine when different types of fuels are combusted can be realized, the development trend of replacing fuels by the existing marine diesel engine is adapted, and a numerical prediction means is provided for the optimal design of the thermal efficiency of the diesel engine in a wider range.
Drawings
FIG. 1 is a flow chart of the present invention;
FIG. 2 is a comparison of cylinder pressure calculations and experimental results for a model of diesel engine;
FIG. 3 is a parameter for a model diesel engine;
FIG. 4 is a result of prediction of effective thermal efficiencies for different fuels;
fig. 5 shows the physicochemical parameters of the respective fuels.
Detailed Description
The invention will now be described in more detail by way of example with reference to the accompanying drawings in which:
with reference to fig. 1-5, the invention provides an effective thermal efficiency prediction method based on any fuel component, aiming at the problems that the existing effective thermal efficiency prediction method of the diesel engine is too complex, is not suitable for combustion of different types of diesel engines, cannot be popularized to combustion of various fuels and the like.
The object of the invention is achieved by the following steps:
step 1, determining boundary conditions and initial conditions according to geometric parameters and initial parameters of a diesel engine, and setting a constant volume combustion ratio X in a combustion process. The constant volume combustion proportion is the constant volume combustion input heat Q cv With total heat input Q of combustion process total The ratio of:
Figure BDA0002747064810000031
the constant volume combustion ratio can be measured experimentally or set manually according to predicted requirements.
And 2, selecting the type of fuel to be combusted, and determining the components of the working medium after combustion according to the selected fuel components and the following chemical equation.
Figure BDA0002747064810000032
Based on the excess air ratio a, the number of carbon molecules n, the number of hydrogen molecules m, and the number of oxygen molecules lCO in the combustion products 2 Number of molecules n 1 、H 2 Number of O molecules n 2 、N 2 Number of molecules n 3 、O 2 Number of molecules n 4 . The specific heat capacity of the working medium in the working process can be determined by the Dalton partial pressure theorem in combination with the formula.
And 3, dividing the working process in the cylinder into three stages of compression, combustion and expansion, dividing each stage into smaller steps according to a 0.01-degree crankshaft angle, and performing iterative solution on each step by adopting a first law of thermodynamics.
Step 4, calculating the compression process according to the heat transfer quantity in one step length of the compression process
Figure BDA0002747064810000041
Volume work
Figure BDA0002747064810000042
Mass m of working medium and specific heat capacity c of working medium v1 The temperature change in one step is obtained by the first thermodynamic law
Figure BDA0002747064810000043
Figure BDA0002747064810000044
Temperature t according to initial conditions 1 Solving the temperature t of the first step end 2 Will t 2 And performing iterative calculation on the initial temperature serving as the next step to obtain the temperature of each step in the compression process. Knowing the temperature of each step, the pressure of each step can be obtained from the ideal gas state equation pV ═ RT.
Step 5, calculating the combustion process according to the heat transfer quantity in one step length of the combustion process
Figure BDA0002747064810000045
Volume work
Figure BDA0002747064810000046
Mass m of working medium and specific heat capacity c of working medium v2 Input heat of combustion g of fuel f (Hu-u) determining the temperature change within one step by the first thermodynamic law
Figure BDA0002747064810000047
Figure BDA0002747064810000048
And repeating the temperature iteration step of the step four, and solving to obtain the temperature and the pressure of each step.
Step 6, calculating the expansion process, repeating the calculation process of step 4, and substituting the calculated working medium specific heat capacity c after the combustion process v3 And solving to obtain the temperature and pressure of each step in the expansion process.
Step 7, performing integral calculation on the pressure obtained in the steps 4, 5 and 6 to obtain effective work W in the working process, and then calculating the total input heat Q according to the effective work W total The effective thermal efficiency is found from the ratio of (A) to (B).
Figure BDA0002747064810000049
The method is based on the first law of thermodynamics, takes the input constant volume combustion ratio as a combustion rule, iteratively solves the temperature and the pressure of the initial state according to the working sequence of compression, combustion and expansion according to the step length, can realize the prediction of the effective thermal efficiency of any constant volume combustion ratio, and ensures that the application range of the model is not limited to the diesel engine with a specific model. Secondly, according to the preset fuel component parameters, the specific heat capacity and specific heat capacity ratio parameters of the working medium before and after combustion in the cylinder are calculated through the Dalton partial pressure theorem of different gas components, the influence of different fuels on the working process of the diesel engine is reflected, the effective thermal efficiency prediction of the diesel engine when different types of fuels are combusted can be realized, the development trend of replacing fuels by the existing marine diesel engine is adapted, and a numerical prediction means is provided for the optimal design of the thermal efficiency of the diesel engine in a wider range.

Claims (1)

1. A method for predicting effective thermal efficiency of a marine diesel engine based on fuel components is characterized by comprising the following steps:
(1) determining boundary conditions and initial conditions according to geometric parameters and initial parameters of the diesel engine, and setting a constant volume combustion ratio X in the combustion process, wherein the constant volume combustion ratio is the input heat Q of constant volume combustion cv With total heat input Q of combustion process total The ratio of (A) to (B):
Figure FDA0002747064800000011
wherein the constant volume combustion ratio is experimentally measured or artificially set according to predicted requirements;
(2) selecting the type of fuel for combustion, and determining the components of the working medium after combustion according to the selected fuel components and the following chemical equation:
Figure FDA0002747064800000012
according to the excess air coefficient a, the number n of carbon molecules, the number m of hydrogen molecules and the number l of oxygen molecules of the fuel, obtaining CO in the combustion product 2 Number of molecules n 1 、H 2 Number of O molecules n 2 、N 2 Number of molecules n 3 、O 2 Number of molecules n 4 Determining the specific heat capacity of the working medium in the working process by combining the Dalton partial pressure theorem and the formula;
(3) dividing the working process in the cylinder into three stages of compression, combustion and expansion, wherein each stage is divided into step lengths according to a 0.01-degree crankshaft rotation angle, and each step length is subjected to iterative solution by adopting a first thermodynamic law;
(4) calculating the compression process according to the heat transfer amount in one step of the compression process
Figure FDA0002747064800000013
Volume work
Figure FDA0002747064800000014
Mass m of working medium and specific heat capacity c of working medium v1 The temperature change in one step is obtained by the first thermodynamic law
Figure FDA0002747064800000015
Figure FDA0002747064800000016
Temperature t according to initial conditions 1 Solving the temperature t of the first step end 2 Will t 2 Performing iterative calculation on initial temperature serving as the next step length to obtain the temperature of each step length in the compression process, knowing the temperature of each step length, and obtaining the pressure of each corresponding step length by using an ideal gas state equation pV (RT);
(5) calculating the combustion process based on the heat transfer in one step of the combustion process
Figure FDA0002747064800000017
Volume work
Figure FDA0002747064800000018
Mass m of working medium and specific heat capacity c of working medium v2 Input heat of combustion g of fuel f (Hu-u) determining the temperature change within one step by the first thermodynamic law
Figure FDA0002747064800000019
Figure FDA00027470648000000110
Repeating the temperature iteration step in the step (4), and solving to obtain the temperature and the pressure of each step;
(6) calculating the expansion process, repeating the calculation process of the step (4), and substituting the calculation process into the working medium specific heat capacity c after the combustion process v3 Solving to obtain the temperature and pressure of each step length in the expansion process；
(7) Performing integral calculation on the pressure obtained in the steps (4), (5) and (6) to obtain effective work W in the working process, and then calculating the total input heat Q according to the effective work W total The effective thermal efficiency is found as the ratio of:
Figure FDA0002747064800000021
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CN113466691B (en) * 2021-06-18 2022-02-22 哈尔滨工程大学 Prediction method for power generation efficiency of two-stage compression expansion generator
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