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汽车工程本科课程作业:关于奥托循环和实际循环--英国论文代写范文精选

2016-01-17 | 来源:51Due教员组 | 类别:更多范文

51due英国论文代写网精选essay代写范文:关于奥托循环和实际循环”。这篇论文的目的是对理想的周期(奥托循环)和内燃发动机的实际的四冲程循环的比较分析。首先根据相关数据和式内燃机计算奥托循环。

1. Abstract
The purpose of this paper is a comparative analysis on the ideal cycle (Otto cycle) and the actual four-stroke cycle of an internal combustion engine. It first calculates Otto cycle of the internal combustion engine in accordance with the relevant data and formula. Calculate the volume, temperature and pressure on the four pistol strokes of compression, ignition, power and exhaust on an Otto cycle PV diagram respectively. Hence the paper concludes that its engine efficiency is 60.9%. The actual cycle of PV and P-Ə chart is made using excel spreadsheet. Comparing the two cycles, it can be concluded that actual efficiency of the engine cycle is much smaller than Otto cycle.
2.Introduction
Otto cycle, also known as four-stroke cycle is a thermodynamic cycle of internal combustion engine. It is the ideal thermodynamic cycle for constant volume heating. The theoretical analysis and calculation are based on the assumption that the cycle is consisted of adiabatic, isochoric and isobaric processes, and the composition, the nature and quality of the system are maintained. In fact the combustion and explosion will inevitably result in the change of system composition and , so the actual efficiency of the gasoline engine is much lower than Otto cycle, only about a half or even less (25%). The main content of this paper is divided into two parts: first is the calculation on Otto cycle and actual cycle; followed by a comparative analysis on the results of the two cycles. This paper has made many assumptions on the calculation of the actual cycle times which will be mentioned later.
3.Methodology
3.1. Overview of Calculation for the Otto Cycle
Figure 1. P-V digram of Otto Cycle
From the known data, it can be derived the temperature in the cylinder after the intake completion was 300K, pressure of 90KPa, with the compression ratio of 10.5.
Through the above equation:
The following formula is used in 1 → 2 in the compression process:
Where k is a constant and k = 1.4 in this paper, the following result can be obtained:
The following formula is used in 2 → 3 ignition processes:
Wherein, LCV is the Lower calorific value of 43.4 MJ / kg, 30% is the fuel usage rate, is a constant of value 0.718KJ/kg K. The above equation can be rewritten as:
Among them, is the Air-fuel ratio (AFR) of known value 14.7, can be obtained.
Re-use of the following equation:
is obtained.
The following formula is used in in 3 → 4 power stroke process:
the following result is obtained:
For this engine, the efficiency calculated using Otto cycle is as follows:
3.2. Overview of Calculation for the Real Cycle
The interval between crank angle positions is assumed as 5° in this paper。
For the selected engine, the ignition process, wiebe equation is used to express a percentage of quantity of burning gasoline over total gasoline:
Given data shows has gas was ignited when crank turned to -30 °, the combustion process lasted at 70 °, and end at 40 °. and from the above formula, can be Substituted into wiebe formula and calculate value from -30 ° to 40 °.
The volume change of the actual cycle is calculated by the following formula:
Among them, refers to the length of connecting rod, a is the crank radius, refers to the distance from the piston to the center (i.e., a crank fixed point) of crank rotation. Hence calculates the cylinder volume of every angle from -360 ° to 360 °.
The known data shows that the intake air temperature is 300K, while it is 313K or is 40 ° at the beginning of the compression. The following equation is used at the beginning of the compression to ignition process:
A temperature between -175 ° to -30 °is obtained as each corresponding value to is determined from previous work.
The following formula is used in the ignition process of -30 ° to 40 °:
Wherein the air density is 1.283g / L, air mass and gasoline quality can be derived. And can be obtained using . A temperature between -30 ° to 40 ° can be obtained using the above equation.
Power stroke process from 40 ° to 180 ° may use the following formula:
After the 180 °is exhaust process. The temperature shall be constant by theory, hence it has a constant value afterwards.
The pressure is 90KPa from the intake process, becomes 100KPa at the moment of compression. Hence it is 90KPa from -360 ° to -175 °, and 100KPa at -180 °. -180 ° to -30 ° is the compression process; the pressure of each angle can be calculated by the following formula:
-30 ° to 40 °is the ignition and combustion process, the pressure is calculated for each angle as follows:
Formula:PV=nRT, can be rewrite as:。
hence:
40 ° to 180 ° is the power stoke process, the pressure at each angle is obtained by the following equation;
In the exhaust process from 180 ° to 360 °, the pressure of cylinder after gas exhaust is -110KPa, so the pressure between 180 ° to 360 ° on each angle is 110KPa.
3.3. Assumption
In the Otto cycle, the assumptions include cylinder insulation, same volume and pressure, composition of the system remains the same, and the nature and quality are maintained. In the actual cycle, although various values can truly reflecting the changes in temperature and pressure of the engine, but there are also a variety of assumptions. For instance, we assume no energy losses during pumping work, and the energy loss in the combustion process is 25%. Note that even in the actual cycle, there are assumptions that the intake and exhaust valves are opened and closed at the desired time. However, in practice the intake valve will open at 5 ° before the piston reaches top dead centre, and the exhaust valve reaches the bottom dead center in front of the piston at 52 ° open, and will close after reaching top dead center at 1 °. The temperature and pressure changes in these processes are without correlation calculation.
4.Results and Discussions
4.1. Otto Cycle
When Otto cycle is not at 1 °, the temperature is 300K and pressure is 90KPa; at 2°, the temperature is 768K, pressure is 2420KPa; at 3°, the temperature is 1234K, the pressure is 6310KPa; at 4°, the temperature is 782K, pressure is 234KPa.
4.2. Real Cycle
In the actual cycle, the temperature at the start of compression is 313K, pressure is 100KPa. When the compression piston moves from top dead center to 30 °, ignition is started with temperature 631K and pressure 1161KPa; ignition combustion process lasted till 70 °. At the end of the combustion, cylinder temperature is 1423K, pressure is 1972KPa. In the beginning of power stroke, the temperature is 1084K, and the pressure is 3638KPa. In the exhaust stroke process the temperature becomes 791K, pressure is 252KPa. Below is the actual cycle PV diagram.
Figure 2. The P-V digram of the Real Cycle
4.3. Comparison
Otto cycle assumes complete combustion of gasoline from ignition to complete combustion process which happens in a flash, so there is no energy loss, and the pressure reached 6310KPa in the beginning of power stroke. In actual cycle, when the piston reaches 30 ° and ignition started. At this point power stroke has not started; the energy produced by combustion will lose certain part. Also, because after the piston reaches top dead point, and turn the crank of about 40 ° then will combustion end, hence there is no complete combustion of gasoline in the beginning of the power stroke, some energy is not used. Therefore, in the actual cycle, the pressure at the beginning of power stroke is only 3637KPa, which is nearly half of Otto cycle.
Conclusion: The engine efficiency of Otto cycle is far below the actual cycle. The most important reason is the great energy loss in the combustion process.

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