METHOD OF CONTROLLING AN AIR COMPRESSOR OF A VEHICLE

Abstract

A method of controlling an air compressor of a vehicle. The absolute humidity is determined for atmospheric air entering the compressor as well as for the compressed air exiting the compressor. A liquid water mass formed or evaporated inside the compressor during a defined period of time is calculated. The above steps are repeated in order to calculate a cumulated liquid water mass inside the compressor. The compressor is stopped when the calculated cumulated liquid water mass has returned to zero and the control unit no longer receives a compressed air request.

Claims

1. A method of controlling an air compressor of a vehicle, comprising: starting the compressor, at the time of starting the compressor: determining, for the atmospheric air entering the compressor, a value H.sub.abs,atm of the absolute humidity, and determining, for the compressed air exiting the compressor, a value H.sub.abs,comp of the absolute humidity, based on the determined values H.sub.abs,atm and H.sub.abs,comp, calculating by means of a control unit a liquid water mass formed or evaporated inside the compressor during a defined period of time, repeatedly performing the determining steps and the calculating step, for each repetition, calculating by means of the control unit a cumulated liquid water mass inside the compressor, and stopping the compressor when the calculated cumulated liquid water mass has returned to zero and the control unit no longer receives a compressed air request.

2. The method of claim 1, wherein the formation or evaporation of liquid water mass is calculated for a series of consecutive periods of time, wherein the start of a next period of time in the series coincides with the end of the previous period of time in the series, wherein the cumulated liquid water mass inside the compressor is calculated by summarizing the calculation for the series of consecutive periods of time.

3. The method of claim 1, comprising storing the most recently calculated value of the cumulated liquid water mass in an electronic memory.

4. The method of claim 3, further comprising, when the vehicle is turned on after having been turned off: restarting the compressor, repeatedly performing the determining steps and the calculating steps, for each repetition, calculating the cumulated liquid water mass inside the compressor and updating the stored value in the electronic memory with a new calculated value of the cumulated liquid water mass, and stopping the compressor when the new calculated value of the cumulated liquid water mass is zero.

5. The method of claim 1, wherein the step of calculating a liquid water mass formed or evaporated inside the compressor for a defined period of time comprises determining the flow rate through the compressor for the defined period of time, wherein the formed or evaporated liquid water mass is calculated based on the determined flow rate during the defined period of time.

6. The method of claim 5, wherein the liquid water mass formed or evaporated in the compressor for the defined period of time is calculated using the formula: $M_{t_n}=\left(\frac{H_{\mathrm{abs},{\mathrm{atm}}_{t_{n-1}}}+H_{\mathrm{abs},{\mathrm{atm}}_{t_n}}}{2}-\frac{H_{\mathrm{abs},{\mathrm{comp}}_{t_{n-1}}}+H_{\mathrm{abs},{\mathrm{comp}}_{t_n}}}{2}\right)\times \frac{Q_{t_{n-1}}+Q_{t_n}}{2}\times p$ where M.sub.t.sub.n is the liquid water mass formed or evaporated from time t.sub.n−1 to time t.sub.n, where a positive number represents formation and a negative number represents evaporation, n is a natural number, t.sub.n=t.sub.n−1+p, p is a time step, natural number, $H_{\mathrm{abs},{\mathrm{atm}}_{t_{n-1}}}$ is the value of the absolute humidity for atmospheric air determined at time t.sub.n−1, $H_{\mathrm{abs},{\mathrm{atm}}_{t_n}}$ is the value of the absolute humidity for atmospheric air determined at time t.sub.n, $H_{\mathrm{abs},{\mathrm{comp}}_{t_{n-1}}}$ is the value of the absolute humidity for compressed air determined at time t.sub.n−1, considering 100% relative humidity for compressed air, $H_{\mathrm{abs},{\mathrm{comp}}_{t_n}}$ is the value of the absolute humidity for compressed air determined at time t.sub.n, considering 100% relative humidity for compressed air, Q.sub.t.sub.n−1 is the flow rate through the compressor determined at time t.sub.n−1, and Q.sub.t.sub.n is the flow rate through the compressor determined at time t.sub.n.

7. The method of claim 6, wherein the cumulated liquid water mass at time t.sub.n is calculated using the formula:
M.sub.c,t.sub.n=M.sub.c,t.sub.n−1+M.sub.t.sub.n where M.sub.c,t.sub.n is the cumulated liquid water mass at time t.sub.n, and M.sub.c,t.sub.n−1 is the cumulated liquid water mass at time t.sub.n−1.

8. The method of claim 5, wherein the step of determining the flow rate comprises measuring the compressor speed and determining the flow rate based on the measured compressor speed.

9. The method of claim 1, wherein the step of determining the value H.sub.abs,atm of the absolute humidity for the atmospheric air entering the compressor comprises: measuring the relative humidity of the atmospheric air with a humidity sensor, measuring the temperature of the atmospheric air with a first temperature sensor, and determining the value H.sub.abs,atm based on the measured relative humidity and measured temperature of the atmospheric air.

10. The method of claim 1, wherein the step of determining the value H.sub.abs,comp of the absolute humidity for the compressed air exiting the compressor comprises: measuring the temperature of the atmospheric air with a first temperature sensor, determining the atmospheric pressure, such as by measuring the pressure of the atmospheric air with a first pressure sensor or estimating the pressure from the vehicle altitude or estimating the pressure to be 1.013 bar, determining the pressure of the compressed air, such as by measuring the pressure of the compressed air with a second pressure sensor or fixed parameter depending on system pressure, measuring the relative humidity of the atmospheric air with a humidity sensor, measuring the temperature of the compressed air with a second temperature sensor, and determining the value H.sub.abs,comp based on the measured temperature of the atmospheric air, the determined atmospheric pressure, the determined pressure of the compressed air, the measured relative humidity and measured temperature of the compressed air.

11. The method of claim 10, wherein the second temperature sensor is placed in the coldest area of the compressor.

12. The method of claim 1, wherein the steps of determining the value H.sub.abs,atm further comprises: determining the vehicle's current altitude over sea level by means of a navigation system, determining the atmospheric air pressure based on the determined altitude, and determining the value Habs,comp based on the determined atmospheric air pressure.

13. The method of claim 1, further comprising: using a compressor map to select, by means of the control unit, a compressor speed for which the temperature increase gradient relative to electric power consumption is optimized, and operating the compressor at the selected compressor speed.

14. A computer program comprising program code means for performing the steps of the method of claim 1 when the program is run on a computer.

15. A computer readable medium carrying a computer program comprising program code means for performing the steps of the method of claim 1 when the program product is run on a computer.

16. A control unit for controlling an air compressor of a vehicle, the control unit being configured to perform the steps of the method of claim 1.

17. A vehicle comprising the control unit of claim 16.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0096] With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

[0097] In the drawings:

[0098] FIG. 1 illustrates a vehicle according to at least one exemplary embodiment, for which the method of the present disclosure may be implemented.

[0099] FIG. 2 illustrates schematically an example of components that may be used for carrying out the method of the present disclosure.

[0100] FIG. 3 illustrates schematically a method in accordance with at least one exemplary embodiment of the present disclosure.

[0101] FIG. 4 illustrates schematically a graphical representation of the implementation of at least one exemplary embodiment of the method of the present disclosure.

[0102] FIG. 5 illustrates schematically another graphical representation.

[0103] FIG. 6 illustrates schematically yet another graphical representation.

[0104] FIG. 7 illustrates schematically a control unit according to at least one exemplary embodiment of the present disclosure.

[0105] FIG. 8 illustrates schematically a computer program product according to at least one exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

[0106] The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain aspects of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments and aspects set forth herein; rather, the embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Accordingly, it is to be understood that the present invention is not limited to the embodiments described herein and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims. Like reference numerals refer to like elements throughout the description.

[0107] FIG. 1 illustrates a vehicle 1 according to at least one exemplary embodiment, for which the method of the present disclosure may be implemented. In this example, the vehicle 1 is a heavy-duty vehicle in the form of a tractor unit. However, the teachings of the present disclosure may also be implemented in other types of vehicles which use an air compressor for providing compressed air to various other parts of the vehicle.

[0108] FIG. 2 illustrates schematically an example of components that may be used for carrying out the method of the present disclosure. An air compressor 2 is provided for sucking ambient air 4 and to increase the pressure of the air. Compressed air 6 leaves the compressor 2 and may be provided to other parts of the vehicle, such as to a service brake, a parking brake, air suspensions, a connected trailer, auxiliaries, etc. The compressor 2 may be operated in response to control signals 8 from a control unit 10. Thus, the control unit 10 controls the operation of the compressor 2. The control unit 10 can turn the compressor 2 on and off. Furthermore, the control unit 10 can control the rotational speed of the compressor 2. The control unit 10 may be used for implementing the method of this disclosure. As such, the method of the present disclosure may be a computer-implemented method performed by the control unit 10. The control unit 10 may receive various sensor signals 12 from different sensors 14 and may process the sensor signals 12 to determine appropriate controlling of the air compressor 2. Furthermore, the control unit 10 may receive request signals 16 from various other parts or subsystems of the vehicle that demand compressed air to be provided from the compressor 2. In the present illustration a pressurized air tank 18 is illustrated as sending a request signal 16 to the control unit 10. For instance, the tank 18 may act as a storage for pressurized air to be distributed to other components, and when the pressure is low, a request signal 16 may be sent to the control unit 10. However, it is conceivable to allow the control unit 10 to receive request signals 16 from other parts of the vehicle as well. Furthermore, although three sensors 14 are illustrated, it should be understood that this is just made for explanatory purposes, and it should be understood that the specific number of sensors 14 may be varied according to the desired implementation of the method disclosed in here and its various exemplary embodiments. Examples of sensors 14 include temperature sensors, humidity sensors, pressure sensors, flow sensors, etc. Further details of the control unit 10 will be briefly discussed later in connection with FIG. 7.

[0109] FIG. 3 illustrates schematically a method 100 in accordance with at least one exemplary embodiment of the present disclosure. More specifically, FIG. 3 illustrates a method 100 of controlling an air compressor of a vehicle. The method 100 comprises: [0110] in a step S1, starting the compressor, [0111] at the time of starting the compressor: [0112] in a step S2, determining, for the atmospheric air entering the compressor, a value H.sub.abs,atm of the absolute humidity, [0113] in a step S3, determining, for the compressed air exiting the compressor, a value H.sub.abs,comp of the absolute humidity, [0114] in a step S4, based on the determined values H.sub.abs,atm and H.sub.abs,comp, calculating by means of a control unit a liquid water mass formed or evaporated inside the compressor during a defined period of time, [0115] in a step S5, repeatedly performing the determining steps and the calculating step, [0116] for each repetition, in a step S6, calculating by means of the control unit a cumulated liquid water mass inside the compressor, and [0117] in a step S7, stopping the compressor when the calculated cumulated liquid water mass has returned to zero and the control unit no longer receives a compressed air request.

[0118] Step 1 may, for instance, be initiated by a compressed air request from another part of the vehicle. For example, as illustrated in FIG. 2, the compressed air request may come as a request signal 16 from a tank 18. However, Step 1 may also be initiated if the control unit knows that there is condensed liquid in the compressor, i.e., without a compressed air request having been received. For instance, the control unit may access an electronic memory which stored the latest calculated cumulated liquid water mass before the compressor was shut off. The fact that the compressor was shut off before all liquid had evaporated may be because the vehicle was turned off, or it may be because there was no compressed air request and that the conditions were not adequate for enabling the compressed air temperature to exceed the pressure dew point (Tdpres).

[0119] Steps 2 and 3, i.e., determining the absolute humidity of the atmospheric air (H.sub.abs,atm) and the absolute humidity of the compressed air (H.sub.abs,comp) may be accomplished by using the following general method to calculate absolute humidity H.sub.abs [g/m.sup.3]:

[0120] 1/Coefficient

[00006]$\begin{array}{cc}=1-\frac{T}{T_c}& \left(1\right)\end{array}$

[0121] With: [0122] T=air temperature [K] [0123] Tc=critical temperature of water, Tc=647.096 K

[0124] A first temperature sensor may be used to measure the temperature of the atmospheric air, for determining/calculating the absolute humidity of the atmospheric air. A second temperature sensor provided inside the compressor may be used to measure a temperature of the compressed air, the for determining/calculating the absolute humidity of the compressed air.

[0125] 2/ Water Vapour Saturation Pressure Pws [hPa]

[00007]$\begin{array}{cc}{P}_{\mathrm{ws}}=P_c\exp \left(\frac{T_C}{T}\left(C_1+C_2{1.5}^{}+C_3{3}^{}+C_4{3.5}^{}+C_5{4}^{}+C_6{7.5}^{}\right)\right)& \left(2\right)\end{array}$

[0126] With: [0127] Pc=critical pressure of water, Pc=220640 hPa [0128] C1 to C6=constants known from literature

[0129] 3/ Water Vapour Partial Pressure Pw [hPa]

[00008]$\begin{array}{cc}{P}_w=\frac{P_{ws}RH}{100\%}& \left(3\right)\end{array}$

[0130] RH is the relative humidity of the atmospheric air. This may, for instance, be measured by means of a humidity sensor.

[0131] 4/ Absolute Humidity H.sub.abs [g/m3]

[00009]$\begin{array}{cc}{H}_{\mathrm{abs}}=C.Math.\frac{P_w}{T}& \left(4\right)\end{array}$

[0132] With C=2.16679 g.Math.K/J

[0133] Continuing with the method of FIG. 3, the step S4, i.e., calculating by means of a control unit a liquid water mass formed or evaporated inside the compressor during a defined period of time, may be accomplished by using the below formula, which has already been discussed and explained previously in this disclosure.

[00010]$M_{t_n}=\left(\frac{H_{\mathrm{abs},{\mathrm{atm}}_{t_{n-1}}}+H_{\mathrm{abs},{\mathrm{atm}}_{t_n}}}{2}-\frac{H_{\mathrm{abs},{\mathrm{comp}}_{t_{n-1}}}+H_{\mathrm{abs},{\mathrm{comp}}_{t_n}}}{2}\right)\times \frac{Q_{t_{n-1}}+Q_{t_n}}{2}\times p$

[0134] As the compressor is running the above determinations/calculations are repeated (step S5) and the cumulated liquid water mass is calculated in connection with each repetition (step S6). The cumulated liquid water mass may be calculated by using the below formula, which has already been discussed and explained previously in this disclosure.

M.sub.c,t.sub.n=M.sub.c,t.sub.n−1+M.sub.t.sub.n

[0135] When the sum of the above formula results in zero, and the control unit (e.g., the control unit in FIG. 2) does not receive a compressed air request, then in accordance with step S7, the control unit may stop the compressor.

[0136] FIG. 4 illustrates schematically a graphical representation of the implementation of at least one exemplary embodiment of the method of the present disclosure. The solid black line shows how the compressor air temperature increases after the compressor has been started (i.e., at time zero). The dotted line shows the pressure dew point temperature (Tdpres), which in this example is approximately 65° C. After compression, if the compressed air temperature is below Tdpres, air becomes saturated (RH=100%). In other words, as long as the compressed air temperature is below Tdpres, the water vapour in the air condenses into liquid water inside the compressor. However, when the compressed air temperature exceeds Tdpres, then the liquid water inside the compressor is vaporized and can be released from the compressor as vapour in the air flow. In the example in FIG. 4, the solid line representing the compressed air temperature crosses the dotted line representing Tdpres at approximately 60 s. When this happens, the dashed line, representing the cumulated liquid water mass, turns downwardly, i.e., the cumulated liquid water mass is steadily decreased as the liquid water evaporates. As indicated by the arrow, at approximately 140 s, the cumulated liquid water mass has returned to zero. Unless the control unit still receives a compressed air request, it can now stop the compressor. Hereby, the liquid water has been successfully evaporated without running the compressor for longer than necessary, thereby saving energy.

[0137] In at least some exemplary embodiments, the control unit may suitably calculate the pressure dew point Tdpres when starting the compressor. This may be based on ambient air temperature, relative humidity, ambient air pressure and compressed air pressure. Furthermore, the control unit may know, or may determine, the maximum temperature, Tmax, that can be reached uniformly and steadily by the air during the compression in the compressor.

[0138] If Tmax>Tdepres, then the control unit may control the compressor according to the above control strategy.

[0139] However, if Tmax≤Tdepres, then there are two different cases, which will here be referred to as Case 1 and Case 2.

[0140] Case 1: Actual relative humidity is known from humidity sensor or other means, (the control strategy of Case 1 can also be used if worst case 100% RH is assumed).

[0141] If Tmax≤Tdepres, this means that no matter for how long time the compressor is running, the compressed air temperature will never exceed Tdpres, so there is no possibility to evaporate liquid water and condensation will occur during all the time that the compressor is running. In this case, the control unit may suitably limit the running time of the compressor to what is needed by the vehicle, and avoid any extra time, as Tdpres cannot be passed and condensation is occurring. Thus, the control unit may suitably stop the compressor when the vehicle no longer needs any more compressed air, i.e., no compress air request received. Accordingly, in this Case 1, the control unit stops the compressor even though there is liquid water, simply because not stopping the compressor would increase the accumulation of liquid water.

[0142] The control unit may store in an electronic memory the cumulated liquid water mass that was created during this running phase when Tmax≤Tdepres. The liquid water mass may then be eliminated the next time conditions allow to have compressed air temperature greater than the pressure dew point temperature, i.e., when conditions allow Tmax>Tdepres and evaporation can occur. The compressor may be restarted to evaporate liquid water either during vehicle needs or whenever the conditions (such as relative humidity, ambient temperature) are such that Tmax>Tdepres.

[0143] Case 1 is illustrated in FIG. 5

[0144] Case 2: Actual relative humidity is not known, and worst-case scenario is assumed, i.e., 100% RH. In this case the control unit may calculates the pressure dew point for worst case 100% RH, Tdpres_100.

[0145] If Tmax<Tdpres_100 this means that no matter for how long time the compressor is running, the air temperature will never reach the dew point Tdpres_100 and therefore there is no possibility to evaporate liquid water inside the compressor and condensation will continue as long as the compressor is running. However, this is the worst-case scenario and since the actual relative humidity (RH) is not known, the actual RH may be lower than 100%.

[0146] In this case the control unit may base the anti-condensation function on the max Tdpres that can be reached by the compressor with Tdpres_max=Tmax−ΔT (where ΔT is strictly positive, e.g., 1° C.).

[0147] From Tdpres_max and the below equations and the above equations [(7).fwdarw.(6).fwdarw.(1).fwdarw.(2).fwdarw.(3)], the control unit can calculate the RH corresponding to Tdpres_max and base the ant-condensation function on this RHmax.

[0148] 1/ Compressed Air Absolute Pressure Ppres [hPa]

P.sub.pres=P.sub.amb+P.sub.rel_pres  (5)

[0149] 2/ Water Vapour Partial Pressure of Compressed Air Pwpres [hPa]

[00011]$\begin{array}{cc}{P}_{\mathrm{wpres}}=\frac{P_{\mathrm{pres}}}{P_{amb}}{P}_w& \left(6\right)\end{array}$

[0150] 3/ Pressure Dew Point Tdpres [° C.]

[00012]$\begin{array}{cc}{T}_{\mathrm{dpres}}=\frac{T_n}{\frac{m}{\log \left(\frac{P_{\mathrm{wpres}}}{A}\right)}-1}& \left(7\right)\end{array}$

[0151] where A, m and T.sub.n are constants for calculating the dew point temperature over different temperature ranges and are listed in commercially available lookup tables.

[0152] The control unit stops the compressor when the cumulated liquid water mass (Tdepress_max; RHmax)=0 g. This allows to cover and avoid condensation in cases where the actual RH≤RHmax and actual Tdpres≤Tdpres_max.

[0153] In parallel, the control unit may calculate and keep in memory the cumulated liquid water mass (Tdpres_100; 100% RH) as well as RHmax and the date and time that this occurs. The cumulated liquid water mass (Tdpres_100; 100% RH) is incremented each time Tmax<Tdpres_100, and if it reaches a defined maximum value, the control unit can warn the driver/user with a message. The driver/user can check actual RH and reset the cumulated liquid water mass (Tdpres_100; 100% RH) if actual RH<Hmax, or drain and exchange compressor oil if actual RH>RHmax.

[0154] Case 2 is illustrated in FIG. 6.

[0155] FIG. 7 schematically illustrates a control unit 10 according to at least one exemplary embodiment of the present disclosure. In particular, FIG. 7 illustrates, in terms of a number of functional units, the components of a control unit 10 according to exemplary embodiments of the discussions herein. The control unit 10 may be comprised in any vehicle disclosed herein, such as the one illustrated in FIG. 1, and others discussed above. Processing circuitry 710 may be provided using any combination of one or more of a suitable central processing unit CPU, multiprocessor, microcontroller, digital signal processor DSP, etc., capable of executing software instructions stored in a computer program product, e.g., in the form of a storage medium 730. The processing circuitry 710 may further be provided as at least one application specific integrated circuit ASIC, or field programmable gate array FPGA.

[0156] Particularly, the processing circuitry 710 is configured to cause the control unit 10 to perform a set of operations, or steps, such as the method discussed in connection to FIG. 3, and exemplary embodiments thereof discussed throughout this disclosure. For example, the storage medium 730 may store the set of operations, and the processing circuitry 710 may be configured to retrieve the set of operations from the storage medium 730 to cause the control unit 10 to perform the set of operations. The set of operations may be provided as a set of executable instructions. Thus, the processing circuitry 710 is thereby arranged to execute exemplary methods as herein disclosed.

[0157] The storage medium 730 may also comprise persistent storage, which, for example may be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

[0158] The control unit 10 may further comprise an interface 720 for communications with at least one external device such as the compressor 2, the sensors 14 and the tank 18 discussed herein. As such, the interface 720 may comprise one or more transmitters and receivers, comprising analogue and digital components and a suitable number of ports for wireline or wireless communication.

[0159] The processing circuitry 710 controls the general operation of the control unit 10, e.g., by sending data and control signals to the interface 720 and the storage medium 730, by receiving data and reports from the interface 720, and by retrieving data and instructions form the storage medium 730. Other components, as well as the related functionality, of the control unit 10 are omitted in order not to obscure the concepts presented herein.

[0160] FIG. 8 schematically illustrates a computer program product 800 according to at least one exemplary embodiment of the present disclosure. More specifically, FIG. 8 illustrates a computer readable medium 810 carrying a computer program comprising program code means 820 for performing the methods exemplified in FIG. 3, when said program product is run on a computer. The computer readable medium 810 and the program code means 820 may together form the computer program product 800.