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Phenomenological numerical simulation of diesel engine working process

abstract an improved Phenomenological Combustion model was used to simulate the working process of a small bore high-speed direct injection diesel engine and compared with the measured results. The results show that the model is in good agreement with the measured results, so it has good practicability and can be used to guide the research and design of the working process of similar diesel engines

Description: simulation and calculation of combustion heat release rate of diesel engine NOx emission

0 preface

as the combustion heat release law determines the economy and power of diesel engine, and has an important impact on combustion noise and emission, the research on combustion heat release law and emission generation process is a research content most valued by researchers. Researchers have been trying to find a combustion model to express the combustion phenomenon - to express the influence of some main factors on the combustion rate and the formation process of some harmful components. Using combustion model to calculate and predict combustion heat release law is not only an important supplement to experimental research, but also a means to deepen the understanding of combustion mechanism

so far, combustion models of internal combustion engines can be divided into three types: zero dimensional model, quasi dimensional model and multi-dimensional model. Both zero dimensional model and quasi dimensional model analyze the combustion process based on thermodynamic principle, and their governing equations are ordinary differential equations with time as the only independent variable. The zero dimensional model regards the whole cylinder as a uniform field, and does not consider the change of parameters with spatial position (single zone). The quasi dimensional model deals with the space in different regions, and the parameters are different among the regions. However, the assumption of zero dimensional model is still used in each district. Due to the zoning treatment, emissions can be predicted to a certain extent. The multidimensional combustion model is a set of conservation equations describing the mass, momentum, energy and chemical components of the combustion process. For such an interdisciplinary subject that covers a wide range of areas and has basically the same functions of these products, it is still quite difficult to move towards practical application under the current test, testing and technical conditions

a four sample pinch zone phenomenological model for combustion rate calculation is proposed, which divides the gas in the cylinder into air zone (g zone), spray zone (J zone), combustion zone (B zone) and combustion product zone (p zone). The model makes a good compromise between zero dimension and multi dimension. Because it considers many factors related to combustion process, it is not only suitable for simulation calculation, but also can be used to guide experimental research. However, because the model is based on the static state, when it is applied to the small cylinder diameter high-speed direct injection diesel engine with strong intake swirl, it will produce large errors. The mass transfer coefficient will also change with the change of inlet swirl. This paper develops this model based on the above two points

1 simulation calculation method

1.1 simulation calculation of combustion rate

the calculation is simulated according to two combustion division methods, and the empirical formulas of instantaneous combustion rate are formula (1) and (2) respectively

the first combustion segment division method shown by the dotted line in Figure 1, that is, the diffusion combustion segment lags behind the premixed combustion segment, and the expression of combustion rate is

the combustion segment division method in Figure 1 and the second combustion segment division method shown by the dotted line in Figure 1, that is, the diffusion combustion and premixed combustion start at the same time, The expression of combustion rate is

where - instantaneous mass combustion rate

xp - proportion of premixed combustion

φ P -- crankshaft angle during premixed combustion duration (℃ a)

mp -- shape coefficient of premixed combustion

xd -- proportion of diffusion combustion, xd=1-xp

φτ—— Crankshaft angle (℃ a) of diffusion combustion lag period

φ D -- crank angle during diffusion combustion duration (℃ a)

md -- diffusion combustion shape exerts market clearing effect coefficient

subscript: P -- premixed D D -- diffusion τ—— Time

1.2 NOx simulation calculation

no formation mechanism has been studied and discussed by many researchers, the most important is Zeldovich mechanism, whose reaction formula is shown as follows

(3)

it is assumed that other components except no and N appear in local equilibrium concentration. Based on this assumption and equations (3) and (4), the equation of no generation rate expressed by crankshaft angle can be derived, that is, the mass concentration of nitric oxide in zone P (combustion product zone) ρ No is

, where RF1, RF2 -- unidirectional reaction rate

rf1=k1 ρ e,NO ρ e,N Rf2=k2 ρ e,N ρ e. O2

k1, K2 -- positive reaction rate constant

k1=3.10 × 1010exp(168/Tp)

k2=6.43 × 106exp (-3145/tp)

tp - temperature in zone P (k) ρ—— Concentration

n -- rotating speed VP -- volume of p zone

ρ E -- equilibrium concentration

a -- ratio of no mass concentration to its equilibrium concentration

2 simulation calculation results

2.1 simulation calculation and correction of combustion heat release rate

the combustion rate calculated by the above two formulas is in good agreement with the measured combustion rate (see Figure 1). The results derived from the phenomenological model have higher early heat release rate and lower medium heat release rate. This is because the model assumes that the combustion is carried out locally, and its rapid early combustion and medium-term anoxia are more in line with the actual situation of diesel engine combustion. Therefore, it can be considered that the combustion rate obtained from it is closer to the actual situation. In terms of the two combustion division methods, the error of the second one is smaller. When considering the mass transfer between zones, it is necessary to introduce the added air volume M1 due to eddy current. M1 is determined by the following formula

where IC - swirl ratio at the end of compression

n - number of spray holes

v - cylinder volume at each crankshaft angle

mt - total mass of working medium in the cylinder at the beginning of fuel injection

θ—— Spray beam half angle

vs - the mass transfer between the zones of jet velocity

will dilute and cool the combustible mixture and combustion products, thus affecting the combustion process and emission indicators

another factor affecting the transmission quality between regions is the mixing coefficient β。 According to a large number of measured results, the following empirical formula

is obtained, in which n - speed II - intake swirl ratio

α—— Average excess air coefficient

φ I -- starting point of combustion

Δφ 0.9 - main combustion duration of 90% cycle fuel supply

β Value selection and formula derivation. According to table 1, when the mixing coefficient β When the intake swirl ratio II increases by 20% at a certain time, ρ e. NOx increases by about 10%

Table 1 Effect of intake swirl ratio on NOx

NOx concentration at balance of intake swirl ratio II ρ e,NOx( μ Mol.m-3) NOx concentration in zone p ρ p,NOx/( μ Mol.m-3)

2.2 14131202

2.6 15101276

note: mixing coefficient β= 0.0135

2.2 effect of parameter selection on prediction accuracy

Figure 2 shows the effect of premixed fuel amount xp on each prediction value, which is affected by intake swirl, compression end temperature and mass transfer. The change of XP will affect the maximum combustion pressure and temperature, and then affect the prediction results. The change of XP will also affect the main combustion period φ e. However, the change trend of these two parameters has the opposite effect on the prediction results

Figure 2 Effect of premixed fuel quantity XP

Figure 3 shows the starting point of combustion φ I impact on NOx and performance. There is an optimal ignition time when the operating conditions are constant φ i。 along with φ I maximum forward combustion pressure Pmax and pressure rise rate dp/d φ With the increase of temperature, the flame temperature increases and the high temperature duration lengthens, which makes N2 molecules easier to decompose. Therefore, the emission index of NOx will increase sharply. And with φ I maximum backward combustion pressure Pmax and pressure rise rate dp/d φ As the flame temperature decreases, the flame temperature becomes lower. Although the increase of diffusion combustion will also prolong the high temperature duration, N2 molecules are not easy to decompose due to lack of oxygen. Therefore, the emission index of NOx will be reduced

Figure 3 starting point of combustion φ I effect on performance

Figure 4 shows the mixing coefficient β Influence on mass transfer and NOx generation process. along with β The mass transfer from the air zone to the combustion product zone and from the combustion product zone to the spray zone increases with the increase of the value. In this way, the temperature of the combustion product zone is greatly reduced due to the addition of more cold air and more hot gas. Although this has little impact on combustion simulation, it has a great impact on the production of NOx. The results show that when the temperature of combustion product zone increases from 2000K to 2500K, the reaction rate of NOx formation will increase by 43.5 times. It can be seen from the figure that even if β A change of 0.2% in the NOx value can also cause a change of up to 15% ~ 20%. The current situation shows that only 75% of the working conditions can control the simulation error within ± 1%. The error is mainly caused by temperature error. Therefore, how to accurately determine the mixing coefficient β Further research is needed

Figure 4 β Influence on mass transfer and NOx generation process

gp - from G zone to p zone PJ - from p zone to j zone

2.3 analysis of simulation results of working process

from the above simulation, we can see the influence of four factors on diesel engine performance. Figure 2 and table 2 show the effects of the above four factors on performance (indicated specific fuel consumption Bi, maximum combustion pressure Pmax, maximum pressure rise rate dp/d φ、 Indicates the effect of power PI). It can be seen from Figure 2 that when XP is constant, φ E the lower one has better economy and the power performance of oxyhydrogen compound with calcium phosphate as the composition of hydroxyapatite (HA) is better, but the work is rough, and its performance deteriorates with the increase of XP. This is because the same ignition start point is used in the calculation, and the higher XP must cause greater compression negative work loss before the top dead center. It can be seen from Figure 2 and table 2 that good performance will appear at a specific ignition starting point. Adopting a larger compression ratio and delaying the ignition angle at the same time is an effective measure that can not only ensure good economy, power and emission indicators, but also make its work tend to be gentle. This has been proved in the experiment

Table 2 compression ratio ε Performance impact

compression ratio ε Indicated specific fuel consumption bi/(.H-1) maximum combustion pressure pmax/mpa maximum pressure rise rate/(mpa. ℃ A-1) indicated power pi/kw

16.0 186.9 8.78 0.875 11.41

17.0 186.0 9.34 0.885 11.46

18.5 182.7 10.23 0.940 11.67

20.0 181.4 11.11 1.024 11.75

note: starting point of combustion φ I=350 ℃ a

3 comparison between simulated calculation and measured results

Figure 5 shows different ignition points φ Comparison diagram between simulation calculation results under I and measured fuel consumption rate of sd195a diesel engine. It can be seen from the figure that when selected ε、 After XP, Pmax and other indicators, the calculation error is small. It should be noted that the calculation of the instantaneous friction work of the machine is complex, so the measured fuel consumption rate in the figure has been converted into the indicator index according to 80% of the mechanical efficiency. Such conversion will produce certain errors if the mechanical efficiency of the diesel engine is not accurately measured

Fig. 5 Comparison between simulation calculation and measured results

4 conclusion

(1) there is a small error in calculating the heat release rate of diesel engine by using the second combustion section division method

(2) seek suitable fuel supply law, that is, reduce the amount of premixed fuel and control the main combustion period φ E it is possible to obtain better economic indicators and emission indicators within an appropriate range

(3) properly increasing the compression ratio and delaying the ignition start point can make the diesel engine work softer under better economic, power and emission indicators

(4) the results of the optimization simulation of the working process of the diesel engine using the Phenomenological Combustion model are more accurate, which can provide a good theoretical basis for the model design and improvement research

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