Patent Application
20130337388
The present invention relates to a method of controlling a mobile heating device and to a mobile heating device.
Heating devices for mobile use or mobile applications (in the following: mobile heating devices) are particularly used in vehicles as parking heaters or additional heaters.
In such heating devices, typically fuel and combustion air is combusted in order to generate heat. To achieve this, mobile heating devices typically comprise a combustion chamber in which combustion takes place. With the requirement to develop vehicles showing reduced emissions, a demand for developing mobile heating devices releasing fewer emissions comes along. In general, the amount of emissions correlates with the ratio of combustion air to fuel. The air-fuel-ratio AFR is the ratio between the mass of air and the mass of fuel in the mixture which is combusted. Commonly a combustion process is categorized in relation to a stoichiometric combustion in which exactly the mass of air is present which is need for entire combustion of the fuel. To this end, the ratio λ (called λ value in the following) of the actual air-fuel-ratio to the stoichiometric air-fuel-ratio (λ=AFRactual/AFRstoichiometric) is used to characterize the actual combustion taking place. Thus, λ>1 defines a combustion with excess of combustion air, λ<1 defines a combustion with deficient combustion air, and λ=1 defines stoichiometric combustion. In particular, a combustion process with λ<1 comes along with increased emissions, since the fuel cannot be completely combusted. Further, the temperature which can be attained in the combustions chamber has a maximum at λ=1 and becomes lower to higher or lower values of λ. Thus, it is an aim to operate mobile heating devices in a range of λ>1 in order to achieve an efficient combustion with low emissions. Typically mobile heating devices are designed to operate in the range 1.2≦λ≦1.8. However, in applications of mobile heating devices, problems in the supply of combustion air can occur which result in reduced mass flow of combustion air and thus undesired air-fuel-ratios. Mobile heating devices typically do not comprise a lambda sensor (air-fuel ratio gauge) and typically the mass flow of fuel and the mass flow of combustion air are not directly measured either. As a consequence, undesired air-fuel ratios cannot easily be detected.
It is an object of the present invention to provide an improved method of controlling a mobile heating device and an improved mobile heating device.
The object is attained by a method of controlling a mobile heating device which is adapted for generating heat by combustion of fuel with combustion air, the method comprising the steps: operating the mobile heating device with a predetermined fuel supply rate and a predetermined combustion air supply rate; monitoring the signal of at least one temperature sensor over time; temporarily changing at least one of the fuel supply rate and the combustion air supply rate; analyzing the response in the signal of the at least one temperature sensor; and based on the response, operating the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate.
By temporarily changing at least one of the fuel supply rate and the combustion air supply rate and analyzing the response in the signal of the temperature sensor, information about the actual air-fuel ratio can easily be deduced without requiring a costly lambda sensor and without measuring the mass flow of fuel and the mass flow of combustion air. Thus, if it is found based on the analyzed response that the mobile heating device is operated with an undesired λ value, the mobile heating device can be operated with at least one of an adapted fuel supply rate and an adapted combustion air supply rate such that combustion is shifted to the desired air-fuel ratio. If analysis of the response results in that the mobile heating device is already operated with the desired air-fuel ratio, of course neither the fuel supply rate nor the combustion air supply rate is changed. As a consequence, the method of controlling a mobile heating device enables operating a mobile heating device more reliable and with reduced emissions. Further, plugging of a combustion air supply line can quickly and easily be detected in which case the mobile heating device can be shut down, for example. Preferably, temporarily changing at least one of the fuel supply rate and the combustion air supply rate comprises temporarily changing the air-fuel ratio. Preferably, the fuel supply rate is temporarily changed while the combustion air supply rate is maintained unchanged. In particular, the method can be realized such that, based on the response, the mobile heating device is operated with at least one of an adapted fuel supply rate and an adapted combustion air supply rate only if analyzing the response results in that an undesired air-fuel ratio is present.
According to a further development, operating the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate comprises operating the mobile heating device with an adapted air-fuel ratio. In this case, the mobile heating device can conveniently be operated with a desired X, value.
According to a further development, analyzing the response comprises attributing the response to one of an initial λ value which is equal to or higher than 1 and an initial λ value lower than 1. In this case, based on the result it can easily be decided in which way a ratio of the combustion air supply rate to the fuel supply rate should be changed in order to operate the mobile heating device with low emissions.
According to a further development, operating the mobile heating device with an adapted air-fuel rate comprises: if the initial λ value is found to be lower than 1, operating the mobile heating device with an increased air-fuel ratio. In this case, the mobile heating device can conveniently be operated with reduced emissions. As an alternative, if the initial A, value is found to be lower than 1 it is also possible to shut down the mobile heating device in order to prevent operation with increased emissions.
According to a further development, temporarily changing the at least one of the fuel supply rate and the combustion air supply rate comprises abruptly changing the at least one of the fuel supply rate and the combustion air supply rate for a predetermined time. In this context, temporarily changing means that the respective supply rate is changed only for a predetermined period and thereafter is returned to the previous value. Abruptly changing the fuel supply rate or the combustion air supply rate results in a clear response in the signal of the temperature sensor which can easily be detected and analyzed.
According to a further development, temporarily changing the at least one of the fuel supply rate and the combustion air supply rate comprises changing one of the fuel supply rate and the combustion air supply rate in the manner of a square wave signal. In this case, a particularly clear response is attained. A particularly clear response can be attained if the fuel supply rate is changed in this manner.
According to a further development, temporarily changing the at least one of the fuel supply rate and the combustion air supply rate comprises abruptly changing the air-fuel ratio. An abruptly changed air-fuel ratio results in a particularly easily detectable response.
According to a further development, the at least one temperature sensor comprises an exhaust gas temperature sensor for measuring the temperature of exhaust gas discharged from a combustion chamber of the mobile heating device. Since the signal of an exhaust gas sensor correlates with and follows the temperature in the combustion chamber, analyzing the response in the signal of an exhaust gas sensor allows particularly reliable analysis. For example, the exhaust gas sensor may be arranged directly downstream of the combustion chamber, e.g. at an entry of a heat exchanger. However, it is also possible that the exhaust gas sensor is arranged further downstream, e.g. even downstream of the heat exchanger, for example in an exhaust gas outlet of the mobile heating device.
According to a further development, temporarily changing at least one of the fuel supply rate and the combustion air supply rate comprises temporarily reducing one of the fuel supply rate and the combustion air supply rate by a factor of 0.67 or lower. Reducing by a factor of 0.67 means that a supply rate is applied which corresponds to the initial supply rate multiplied by a factor of at most 0.67. In this case, a pronounced response which can clearly be attributed to an initial λ value above or below 1 is achieved.
According to a further development, temporarily changing at least one of the fuel supply rate and the combustion air supply rate comprises temporarily enhancing one of the fuel supply rate and the combustion air supply rate by a factor of 1.3 or higher. Again, preferably the fuel supply rate is temporarily changed in order to achieve a particularly pronounced response. Temporarily enhancing the respective supply rate by a factor of 1.3 or higher means that a supply rate is applied which corresponds to the initial supply rate multiplied by a factor of at least 1.3.
The object is also attained by a mobile heating device comprising: a combustion chamber for combusting fuel with combustion air; a fuel supply for supplying fuel to the combustion chamber with a fuel supply rate; a combustion air supply for supplying combustion air to the combustion chamber with a combustion air supply rate; at least one temperature sensor for detecting a temperature of the mobile heating device; and a control unit for controlling operation of the mobile heating device, wherein the control unit is adapted such that it: operates the mobile heating device with a predetermined fuel supply rate and a predetermined combustion air supply rate; monitors the signal of the at least one temperature sensor over time; temporarily changes at least one of the fuel supply rate and the combustion air supply rate; analyzes the response in the signal of the at least one temperature sensor; and based on the response, operates the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate. The thus-adapted mobile heating device achieves the advantages which have been described above with regard to the method of controlling a mobile heating device. In particular, by temporarily changing at least one of the fuel supply rate and the combustion air supply rate and analyzing the response in the signal of the temperature sensor, information about the actual air-fuel ratio can easily be deduced without requiring a costly lambda sensor and without measuring the mass flow of fuel and the mass flow of combustion air. Thus, based on the analyzed response, the mobile heating device can be operated with at least one of an adapted fuel supply rate and an adapted combustion air supply rate such that combustion is shifted to the desired air-fuel ratio. As a consequence, the mobile heating device can be operated more reliable and with reduced emissions. Further, plugging of a combustion air supply line can quickly and easily be detected in which case the mobile heating device can be shut down. Preferably, temporarily changing at least one of the fuel supply rate and the combustion air supply rate comprises temporarily changing the air-fuel ratio. Preferably, the fuel supply rate is temporarily changed while the combustion air supply rate is maintained unchanged.
In the present context, the term mobile heating device is used to characterize a heating device which is adapted such that it is suited for mobile applications. This means that the heating device is transportable (e.g. incorporated into a vehicle or placed therein for transport purposes) and is not solely adapted for a permanent stationary use as is the case for a domestic heating device, for example. The mobile heating device can in particular be adapted for heating a vehicle or a temporarily stationary application such as big tents or mobile cabins (welfare cabins). Thus, the term mobile heating device is also used for a heating device which is adapted for fixed installation in a mobile device (such as a vehicle or a mobile cabin). In particular, the mobile heating device can be adapted as a parking heater or additional heater for a motor vehicle.
According to a further development, operating the mobile heating device with at least one of an adapted fuel supply rate and an adapted combustion air supply rate comprises operating the mobile heating device with an adapted air-fuel ratio. In this case, the mobile heating device is adapted to operate with a desired λ value.
According to a further development, analyzing the response comprises attributing the response to one of an initial λ value which is equal to or higher than 1 and an initial λ value lower than 1. In this case, the control unit is adapted to distinguish between operation resulting in undesirably high emissions and the intended low emission operation and can initiate appropriate countermeasures in the case of operation with high emissions.
According to a further development, operating the mobile heating device with an adapted air-fuel ratio comprises: if the initial λ value is found to be lower than 1, operating the mobile heating device with an increased air-fuel ratio. In this case, if combustion with high emissions is found, the operation of the mobile heating device is adjusted to lower emission operation.
Instead of operating the heating device with an increased air-fuel ratio it is also possible to adapt the control unit such that it shuts down the heating device in the case of an initial λ value which is below 1.
According to a further development, the at least one temperature sensor comprises an exhaust gas temperature sensor for measuring the temperature of exhaust gas discharged from the combustion chamber. The exhaust gas temperature sensor allows very fast and efficient analysis since the temperature of the exhaust gas correlates well with the temperature in the combustion chamber.
Further features and advantages will become apparent from the following description of an embodiment which will be described with reference to the enclosed drawings.
An embodiment will now be described with reference to
As schematically shown in
A heat exchanger 6 through which the hot exhaust gases are guided is arranged downstream of the combustion chamber 2. The hot exhaust gases from the combustion chamber 2 are guided in contact with an inner wall of the heat exchanger 6 as schematically depicted by arrows in
The mobile heating device 1 is provided with a control unit 10 which is adapted for controlling operation of the mobile heating device 1. To this end, the control unit 10 is connected to the at least one temperature sensor such that it receives a signal representing a temperature which is characteristic for the combustion process in the combustion chamber 2. The control unit 10 is further connected to the fuel supply 4 such that it can control the fuel supply rate and is connected to the combustion air supply 5 such that it can control the combustion air supply rate.
In the embodiment, in normal operation the control unit 10 operates the mobile heating device 1 at a certain heating power level which can e.g. be defined by a certain predetermined fuel supply rate. For such heating power level, the control unit 10 operates the combustion air supply 5 with a predetermined combustion air supply rate which is set such that—under normal conditions—a λ value in a predetermined range is expected (e.g. 1.2≦λ≦1.8) which corresponds to operation with low emissions.
A method of controlling the mobile heating device 1 according to the embodiment will now be described with reference to
As can be seen in
The signal T of the temperature sensor is monitored by the control unit 10 (step S2). In the present embodiment, the signal T of the temperature sensor is e.g. sampled every 0.1 seconds or every second.
At the time t1, the fuel supply rate sr is abruptly reduced to a lower level srx, with srx≦0.67×sr0, as can be seen in
As can be seen in
In the embodiment shown, in a step S5, based on the response in the signal T of the temperature sensor, the control unit 10 operates the mobile heating device 1 with an adapted fuel supply rate and/or with an adapted combustion air supply rate. For example, if the analysis results in that the initial λ value was below 1, the control unit 10 can preferably operate the mobile heating device 1 with unchanged fuel supply rate but with an adapted combustion air supply rate which is increased as compared to the initial combustion air supply rate in order to shift the λ value to a higher value. Alternatively, if an initial λ value lower than 1 is found, the control unit could also judge that a defect in the combustion air supply is present and shut down operation of the mobile heating device 1. On the other hand, if an initial λ value which is in the desired range is found, the control unit 10 operates the mobile heating device 1 with unchanged combustion air supply rate and unchanged fuel supply rate.
Now, the reason for the difference in the response in the signal T of the temperature sensor will be described. First, the response for the case of an initial λ value equal to or higher than 1 will be described with reference to
If the initial λ value is equal to or higher than 1, in step S1 the fuel supplied to the combustion chamber 2 is completely combusted which results in a high temperature in the combustion chamber and thus a high temperature represented by the signal T. At the time t1, the amount of fuel which is supplied to the combustion chamber 2 is abruptly reduced such that less fuel for combustion is made available to the combustion process. As a result, the temperature in the combustion chamber 2 and thus the signal T of the temperature sensor drops in response to the temporary change in the fuel supply rate. After the initial fuel supply rate is restored at time t2, the temperature in the combustion chamber 2 and the signal T of the temperature sensor return to their initial higher values, as can be seen in
If however the initial λ value is lower than 1, as depicted in
Thus, according to the embodiment described above, a method of controlling a mobile heating device 1 and a mobile heating device 1 which is correspondingly adapted are disclosed which enable assessing the air-fuel ratio present in the combustion chamber 2 in an indirect manner without requiring one or more additional sensors. In turn, this enables controlling the mobile heating device 1 such that operation with an increased level of emissions can be reliably prevented.
Although an embodiment has been described in which the fuel supply rate is temporarily reduced by a certain factor while the combustion air supply rate is maintained constant, it is also possible to temporarily increase the fuel supply rate while the combustion air supply rate is maintained constant. This would also result in a response in the signal T of the at least one temperature sensor which can be clearly attributed to either one of an initial λ value of equal to or larger than 1 or an initial λ value smaller than 1. Preferably, in this case the fuel supply rate is abruptly increased by a factor of 1.3 or higher (i.e. by at least thirty percent) in order to achieve a clear response. Again, a temporary change having the shape of a square wave signal is particularly preferred.
Similarly, instead of temporarily changing the fuel supply rate while maintaining the combustion air supply rate unchanged, it is also possible to temporarily change the combustion air supply rate while maintaining the fuel supply rate unchanged. In the case of an increase in the combustion air supply rate, a similar response as in the case of a reduction in the fuel supply rate can be observed. On the other hand, the response to an abrupt decrease in the combustion air supply rate is somehow similar to the response to the abrupt increase in the fuel supply rate. In these cases again, a reduction by a factor of 0.67 or lower or an increase by a factor of 1.3 or higher is preferred.
Further, combined schemes for the temporary change in which both the fuel supply rate and the combustion air supply rate are temporarily changed are also possible.
Although an embodiment has been described in which a rather general distinction is made between, on the one hand, an initial λ value which is equal to or higher than 1 and, on the other hand, an initial λ value which is lower than 1, the response in the signal T of the temperature sensor can be analyzed even more detailed in order to evaluate the value of the air-fuel ratio more in detail. Further, it is also possible to analyze the signal of more than one temperature sensor.
According to a further development, the step of temporarily changing at least one of the fuel supply rate and the combustion air supply rate and analyzing the response can also be repeated several times. In this case the response can be analyzed by statistical methods in order to achieve an even more detailed and/or more reliable result. According to an even further development, the step of temporarily changing at least one of the fuel supply rate and the combustion air supply rate is be repeated several times but with different amounts of change, i.e. with different intensities. By analyzing the responses to such changes, the initial λ value can be assessed with high accuracy more in detail.
The disclosed method can e.g. be conducted each time the mobile heating device 1 is started at a predetermined point in time after start or in regular or irregular intervals during the life-time of the mobile heating device 1.
The described method of controlling a mobile heating device and the correspondingly adapted mobile heating device enable assessing the quality of combustion in the combustion chamber without requiring additional sensors. Defects of the combustion air supply and plugging of the combustion air supply line can reliably be detected. Further, analysis can be integrated easily in the control unit and, if desired, more detailed analysis is also possible.
