Air-fuel ratio calculation method for hydrogen engine

Abstract

To provide an air-fuel ratio calculation method for a hydrogen engine for calculating an air-fuel ratio that reflects an effect of other gas components excluding oxygen on an O2 concentration sensor.SOLUTION: An air-fuel ratio calculation method for a hydrogen engine includes steps of: measuring each of an oxygen concentration and a hydrogen concentration; correcting the oxygen concentration by using an oxygen concentration correction approximate expression for calculating an oxygen concentration correction ratio corresponding to an unburned hydrogen concentration in exhaust gas; and calculating an air-fuel ratio by using a conversion formula determined on the basis of relation between a change amount of a hydrogen reaction amount along with a change of the air-fuel ratio and a change amount of the oxygen concentration corresponding to the change amount of the hydrogen reaction amount. These steps enable calculation of the air-fuel ratio suitable to the case where fuel is hydrogen.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for calculating the air-fuel ratio of a hydrogen engine, and more particularly to a method for calculating the air-fuel ratio suitable for hydrogen fuel. [Background technology]

[0002] Conventionally, in gasoline-fueled engines (hereinafter simply referred to as gasoline engines), the air-fuel ratio (hereinafter sometimes referred to as λ) used to control the amount of fuel injected from a fuel injection valve is generally calculated using the so-called oxygen balance method based on the detected oxygen concentration detected in the engine's exhaust gas by an O2 concentration sensor.

[0003] Recently, development has been progressing on engines that use hydrogen as fuel (hereinafter simply referred to as hydrogen engines), such as that disclosed in Patent Document 1. Patent Document 1 discloses feedback control of the amount of hydrogen fuel supplied to the engine, taking into consideration changes in the responsiveness of the air-fuel ratio sensor in response to changes in the target air-fuel ratio, with the aim of quickly converging the air-fuel ratio to the target air-fuel ratio, so that the actual air-fuel ratio detected by an air-fuel ratio sensor provided in the exhaust passage of the engine becomes the target air-fuel ratio. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-332782 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the system disclosed in Patent Document 1 applies the λ calculation method used in gasoline engines to hydrogen engines, and therefore does not take into consideration the effect of hydrogen on the O2 concentration sensor. In other words, it is not possible to calculate an air-fuel ratio suitable for hydrogen engines. This is because, until now, the effect of gas components other than oxygen on the O2 concentration sensor in hydrogen engines has not been clearly defined, and it has not been possible to calculate an air-fuel ratio that reflects this effect.

[0006] The present invention has been made in consideration of the above points, and its purpose is to provide a method for calculating the air-fuel ratio of a hydrogen engine that can calculate the air-fuel ratio while reflecting the effects of gas components other than oxygen on the O2 concentration sensor. [Means for solving the problem]

[0007] The solution of the present invention for achieving the above object is a method for calculating the air-fuel ratio of a hydrogen engine using measured values ​​of the oxygen concentration and hydrogen concentration in the exhaust gas of the hydrogen engine, which method comprises the steps of measuring the oxygen concentration and the hydrogen concentration, correcting the oxygen concentration using an oxygen concentration correction approximation equation for calculating an oxygen concentration correction factor corresponding to the concentration of unburned hydrogen in the exhaust gas, and calculating the air-fuel ratio using a conversion equation determined based on the relationship between the amount of change in hydrogen reaction quantity accompanying a change in the air-fuel ratio and the amount of change in oxygen concentration corresponding to the amount of change in the hydrogen reaction quantity.

[0008] This specification allows the calculation of the air-fuel ratio to reflect the influence of gas components other than oxygen on the O2 concentration sensor, making it possible to calculate the air-fuel ratio appropriate for hydrogen fuel. [Effects of the Invention]

[0009] In the present invention, the air-fuel ratio is calculated using an oxygen concentration correction approximation formula for calculating an oxygen concentration correction factor corresponding to the concentration of unburned hydrogen in the exhaust gas, and a conversion formula determined based on the relationship between the amount of change in the hydrogen reaction amount accompanying a change in the air-fuel ratio and the amount of change in the oxygen concentration corresponding to the amount of change in the hydrogen reaction amount, making it possible to calculate an air-fuel ratio that is suitable for hydrogen as fuel. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram showing a schematic configuration of an air-fuel ratio calculation system for realizing an air-fuel ratio calculation method for a hydrogen engine according to an embodiment; [Figure 2] FIG. 2 is a flowchart showing the steps of a method for calculating an air-fuel ratio of a hydrogen engine according to an embodiment. [Figure 3] Figure 3(a) shows an example of the relationship between the hydrogen concentration in the exhaust gas and the oxygen concentration, which is the sensor output; Figure 3(b) shows an example of the relationship between the unburned hydrogen concentration at each λ and the oxygen concentration, which is the sensor output; Figure 3(c) shows an example of the relationship between the unburned hydrogen concentration and the oxygen concentration correction factor; and Figure 3(d) shows an example of the relationship between the oxygen concentration in the exhaust gas and λ. DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment of the present invention will now be described with reference to the drawings. Fig. 1 is a diagram showing the schematic configuration of an air-fuel ratio calculation system 1 for realizing the air-fuel ratio calculation method for a hydrogen engine according to this embodiment. Fig. 2 is a flowchart showing the steps of the air-fuel ratio calculation method for a hydrogen engine according to this embodiment.

[0012] -Outline of the air-fuel ratio calculation system- As shown in Fig. 1, the air-fuel ratio calculation system 1 includes an O2 concentration meter 2, an H2 concentration meter 3, a calculation unit 4, and an output unit 5. As shown in Fig. 2, the air-fuel ratio calculation system 1 measures the oxygen concentration (O2 concentration) using the O2 concentration meter 2 (step ST1), measures the hydrogen concentration (H2 concentration) using the H2 concentration meter 3 (step ST2), corrects the oxygen concentration using the calculation unit 4 (step ST3) and converts the oxygen concentration to λ (λ value) (step ST4), and outputs λ using the output unit 5 (step ST5).

[0013] The configuration of the air-fuel ratio calculation system 1 and the air-fuel ratio calculation method performed by the air-fuel ratio calculation system 1 will be specifically described below.

[0014] The O2 concentration meter 2 includes an O2 concentration sensor 21 and a sensor signal output unit 22. The O2 concentration sensor 21 detects the oxygen concentration in the exhaust gas of a hydrogen engine. This O2 concentration sensor 21 is configured, for example, as a concentration cell type (electromotive force type). While the O2 concentration sensor 21 is not limited to this, it is preferable that it be highly water-resistant (for example, equipped with a heater) in consideration of the large amount of water vapor in the exhaust gas of a hydrogen engine. Furthermore, it is preferable that the O2 concentration sensor 21 have a wide combustion gas temperature measurement range so that it can be used in the λ value range of "λ = 1 to 3." Furthermore, hydrogen engines are often controlled with lean burn compared to gasoline engines, so combustion temperatures tend to be lower than those of gasoline engines. For this reason, it is preferable that the O2 concentration sensor 21 be capable of detecting oxygen concentration even in a relatively low temperature range.

[0015] The sensor signal output unit 22 transmits the output from the O2 concentration sensor 21 to the calculation unit 4 as oxygen concentration information.

[0016] The H2 concentration meter 3 detects the hydrogen concentration in the exhaust gas of the hydrogen engine. The H2 concentration meter 3 transmits this hydrogen concentration information to the calculation unit 4.

[0017] The calculation unit 4 includes a processor such as a CPU (Central Processing Unit), a ROM (Read-Only Memory) for storing control programs, a RAM (Random-Access Memory) for temporarily storing data, and input / output ports. The calculation unit 4 has functions of correcting the oxygen concentration received from the O2 concentration meter 2 (the function of performing the process of step ST3 in FIG. 2) and calculating λ (air-fuel ratio) according to the corrected oxygen concentration (the function of performing the process of step ST4 in FIG. 2). These functions will be explained in detail below.

[0018] -Oxygen concentration correction processing- The O2 concentration sensor 21 has a characteristic that the higher the concentration of unburned hydrogen in the exhaust gas, the lower the oxygen concentration output value. Figure 3(a) shows an example of the relationship between the hydrogen concentration in the exhaust gas and the oxygen concentration output from the sensor at a certain λ. The open circles in Figure 3(a) represent the oxygen concentration output when an exhaust gas analyzer is used (the oxygen concentration obtained by analyzing the exhaust gas), while the black circles represent the oxygen concentration output when the O2 concentration sensor 21 is used (the oxygen concentration output value). Thus, the higher the concentration of unburned hydrogen in the exhaust gas, the lower the oxygen concentration output value output from the O2 concentration sensor 21. Furthermore, the higher the concentration of unburned hydrogen, the greater the deviation of the actual oxygen concentration output value from the oxygen concentration output value that should be output (the oxygen concentration obtained using the exhaust gas analyzer). Therefore, to obtain an appropriate oxygen concentration output value, the oxygen concentration output value must be corrected.

[0019] 3(b) shows an example of the relationship between the unburned hydrogen concentration and the oxygen concentration, which is the sensor output, at each λ. This figure shows the relationship between the unburned hydrogen concentration and the oxygen concentration, which is the sensor output, at λ=1.5, λ=2, λ=2.5, and λ=3. Thus, for the O2 concentration sensor 21, even if the value of λ changes, the rate at which the oxygen concentration output value decreases with an increase in unburned hydrogen concentration remains approximately constant.

[0020] Taking this into consideration, the inventors of the present invention have developed the following formula (1) as an approximate formula for oxygen concentration correction for calculating an oxygen concentration correction factor y corresponding to the unburned hydrogen concentration x in exhaust gas.

[0021] y=(3E-21)x 5 -(2E-17)x 4 -(3E-12)x 3 -(5E-8)x 2 +0.0021x-0.5871 …(1) The coefficients of each term in the formula (1) can be calibrated according to the environmental conditions of the O2 concentration sensor 21, making the formula applicable under various conditions.

[0022] The reason for selecting this oxygen concentration correction approximation equation as a quintic equation of the unburned hydrogen concentration x is that it has a smaller error than correction approximation equations of other orders. For example, the error when using a quartic equation of the unburned hydrogen concentration x was approximately 0.42%, the error when using a hexonic equation of the unburned hydrogen concentration x was approximately 0.05%, and the error when using a quintic equation of the unburned hydrogen concentration x was approximately 0.02%. Another reason for selecting the oxygen concentration correction approximation equation as a quintic equation of the unburned hydrogen concentration x is that it has a lower calculation cost than oxygen concentration correction approximation equations of 6th order or higher.

[0023] - λ calculation process - In the combustion mode within the cylinders of a hydrogen engine, as λ changes, the amount of hydrogen reacted (the amount of hydrogen used for combustion) changes accordingly, and the oxygen concentration in the exhaust gas (the oxygen concentration that changes depending on the amount of oxygen used as an oxidizing agent for combustion) also changes accordingly. Specifically, as λ increases, the proportion of hydrogen reacted decreases, and the oxygen concentration in the exhaust gas tends to increase accordingly.

[0024] Therefore, the following equation (2) can be used as a conversion formula for λ corresponding to the oxygen concentration z in the exhaust gas (hereinafter referred to as the λ conversion formula) to calculate λ corresponding to the oxygen concentration z that changes depending on the reaction amount of hydrogen. In other words, this λ conversion formula is determined based on the relationship between the change in the hydrogen reaction amount that accompanies a change in λ and the change in the oxygen concentration that corresponds to the change in the hydrogen reaction amount.

[0025] λ=720495z 6 -238024z 5 +33198z 4 -1942.5z 3 +89.296z 2 +5.1698z+1.0008 …(2) The reason for selecting this λ conversion formula as a sixth-order equation of oxygen concentration z is that it has a smaller error than λ conversion formulas of other orders. For example, the error when using a fourth-order equation of oxygen concentration z was approximately 1.54%, the error when using a fifth-order equation of oxygen concentration z was approximately 0.37%, and the error when using a sixth-order equation of oxygen concentration z was approximately 0.14%.

[0026] The value of λ calculated by the above formula (2) is temporarily stored by the output unit 5 and then output on demand.

[0027] -Effects of the embodiment- As described above, in this embodiment, λ is calculated using the oxygen concentration correction approximation formula (the above formula (1)) for calculating the oxygen concentration correction factor corresponding to the concentration of unburned hydrogen in the exhaust gas, and the conversion formula (the above formula (2)) determined based on the relationship between the change in the hydrogen reaction amount accompanying a change in λ and the change in the oxygen concentration corresponding to the change in the hydrogen reaction amount. Therefore, it is possible to calculate λ appropriate for when the fuel is hydrogen.

[0028] The inventors of the present invention conducted experiments to verify the effects of the present invention. As a result, they confirmed that the error in the output value of λ was less than ±0.6%. They also confirmed that, for all λ values, the decrease in the oxygen concentration output value of the O2 concentration sensor 21 due to the influence of the unburned hydrogen concentration was properly corrected, and λ was calculated with high accuracy.

[0029] -Other embodiments- The present invention is not limited to the above-described embodiments, and all modifications and applications within the scope of the claims and equivalents thereto are possible.

[0030] For example, the value of λ calculated by the air-fuel ratio calculation method according to the present invention is not particularly limited to vehicle engines that can be used, and can be used for various hydrogen engines, such as in-line, V-type, and horizontally opposed types. [Industrial Applicability]

[0031] The present invention is applicable to a method for calculating the air-fuel ratio of a hydrogen engine mounted on an automobile. [Explanation of symbols]

[0032] 1. Air-fuel ratio calculation system 2 O2 concentration meter 3 H2 concentration meter 4 Arithmetic section 5 Output section

Claims

[Claim 1] A method for calculating an air-fuel ratio of a hydrogen engine using measured values ​​of oxygen concentration and hydrogen concentration in exhaust gas of the hydrogen engine, comprising: measuring the oxygen concentration and the hydrogen concentration; correcting the oxygen concentration using an oxygen concentration correction approximation formula for calculating an oxygen concentration correction factor according to the concentration of unburned hydrogen in the exhaust gas; and calculating the air-fuel ratio using a conversion formula determined based on the relationship between the amount of change in hydrogen reaction amount due to a change in air-fuel ratio and the amount of change in oxygen concentration corresponding to the amount of change in hydrogen reaction amount.

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