DE102010027811B4 - Method for operating a fuel-operated heater, preferably a vehicle heater, in a warm start phase - Google Patents

Abstract

Method for operating a fuel-operated heating device (10), preferably a vehicle heating device, preferably with an evaporator burner, in a warm start phase, in which method a temperature variable (T _G ; T _M ) associated with the temperature of a medium (M) heated by the heating device (10) is compared with a first temperature threshold (T _S ; T _S' ) assigned thereto and then, when the temperature variable (T _G ; T _M ) falls below the first temperature threshold (T _S ; T _S' ), a combustion air conveying arrangement (16) is put into operation and is operated during a cold blowing phase (I _KB ') to convey air (L), wherein during the cold blowing phase (I _KB ') a fuel conveying arrangement (18) is not operated to convey fuel (B), wherein the temperature variable (T _G ) and/or the first temperature threshold (Ts') are dependent on the rate of change of the temperature (T _M ) of the medium (M) is determined.

Description

The present invention relates to a method with which a fuel-operated heater, in particular a vehicle heater, preferably with an evaporator burner, can be operated or put into operation again in a warm start phase.

Fuel-operated vehicle heaters can be operated at different heating output levels to adapt the amount of heat energy provided during combustion to the actual heat energy requirement. Depending on the required heat requirement, an adapted amount of combustion air and a correspondingly adapted amount of fuel are fed into a combustion chamber of the vehicle heater. If the vehicle heater is designed with an evaporator burner, a porous evaporator medium is generally provided into which the liquid fuel supplied via a supply line is introduced, distributed therein by capillary action and possibly also by gravity, and then evaporated in the direction of the combustion chamber. The heat provided during combustion is transferred in a heat exchanger arrangement to a medium to be heated. If the vehicle heater is integrated into a coolant circuit of an internal combustion engine, this medium to be heated is a liquid, generally water. If the vehicle heater is designed as an air heater, the medium to be heated is the air to be introduced into a vehicle interior, for example.

If the medium heated by the vehicle heater, for example the liquid circulating in a coolant circuit or the air in a vehicle interior, has a comparatively high temperature which does not require additional heating in the heat exchanger arrangement, then if the vehicle heater has already been operated at the lowest heating output level, for example, the vehicle heater can be brought into a control state, i.e. combustion operation can be stopped at least temporarily by stopping the fuel supply and the combustion air supply. Since in this control state essentially no heat is transferred in the area of the heat exchanger arrangement, the temperature of the heated medium, for example the temperature of the coolant or the temperature of the air already introduced into a vehicle interior, will gradually decrease. By monitoring this temperature, it can be determined when it is again necessary to provide heat in the area of the vehicle heater in order to avoid the temperature dropping too much. In this case, a criterion for resuming combustion operation in the vehicle heater is generally specified by comparing this temperature with an assigned temperature threshold.

When combustion operation is resumed, the various system areas of the vehicle heater that were previously heated by the ongoing combustion are generally still relatively warm. This particularly applies to the area into which the liquid fuel is fed, for example the porous evaporator medium or the end area of a fuel supply line. However, a temperature that is too high in this area can result in relatively spontaneous fuel evaporation, which in turn leads to the pulse-like release of fuel vapor towards the combustion chamber and thus endangers the start of combustion or can lead to unfavorable combustion conditions. To avoid this, when the temperature of the heated medium falls below the assigned threshold, air is initially fed into the vehicle heater in a cold blowing phase through a combustion air feed arrangement. This air cools the system areas around which it flows and thus helps to ensure that favorable conditions can be created more quickly for restarting combustion.

From the DE 35 17 953 A1 A method for controlling the heating output of a fuel-operated vehicle heater is known in which a control pause occurs when the temperature of water as the medium to be heated reaches a predetermined temperature threshold. During the control pause, only a water pump and a vehicle fan continue to operate. If the temperature of the water falls below another predetermined threshold, heating operation is resumed at full load.

The DE 196 05 324 C1 discloses a method for operating a fuel-operated vehicle heater, in which after a flameout or after an unsuccessful start attempt, the fuel supply is initially interrupted while the combustion air blower continues to operate. After a predetermined period of time, which can be determined depending on the temperature of a heat exchanger of the vehicle heater, the fuel supply is resumed.

It is the object of the present invention to provide a vehicle for operating a fuel-operated vehicle heater in a warm start phase, with which, when a resumption of combustion operation is required, It is important to reduce delays in the start of combustion.

According to the invention, this object is achieved by a method for operating a fuel-operated vehicle heater, preferably with an evaporator burner, in a warm start phase, in which method a temperature variable associated with the temperature of a medium heated by the heater is compared with a first temperature threshold assigned to it and then, when the temperature variable falls below the first temperature threshold, a combustion air conveying arrangement is put into operation and is operated to convey air during a cold blowing phase, wherein a fuel conveying arrangement is not operated to convey fuel during the cold blowing phase, wherein the temperature variable and/or the first temperature threshold are determined as a function of the rate of change of the temperature of the medium.

In the procedure according to the invention, not only is the temperature of the heated medium, for example the coolant temperature or the temperature in a vehicle interior, compared with an assigned threshold and then, when the temperature falls below the threshold, the cold blowing phase is started. Rather, by taking into account the rate of change of the temperature of this medium, a temperature development is anticipated. This makes it possible to start the cold blowing phase earlier, since, for example, a comparatively strong change in temperature, i.e. a strong drop in temperature, will quickly lead to thermal conditions that make it necessary to resume combustion operation and therefore also allow the cold blowing phase to begin at an earlier point in time.

For example, it can be provided that the temperature variable is determined on the basis of the temperature of the medium and a correction term representing the rate of change of the temperature of the medium.

wobei gilt:

• T_G = Temperaturgröße,
• T_M(x) = Temperatur des Mediums bei einem letzten Ermittlungszeitpunkt x,
• T_M(x-1) = Temperatur des Mediums bei einem vorletzten Ermittlungszeitpunkt x-1,
• B = Temperaturänderungsbewertungsfaktor.

The temperature value can be determined, for example, as $${\text{T}}_{\text{G}}={\text{T}}_{\text{M}}\left(\text{x}\right)+\left[{\text{T}}_{\text{M}}\left(\text{x}\right)-{\text{T}}_{\text{M}}\left(\text{x}-1\right)\right]\times \text{B}$$

where:

• T _G = temperature quantity,
• T _M (x) = temperature of the medium at a last determination time x,
• T _M (x-1) = temperature of the medium at a penultimate determination time x-1,
• B = temperature change rating factor.

Since, in the sense of the present invention, the rate of change of the temperature of the medium is to be taken into account in such a way that the cold blowing phase can be started earlier if the temperature trend is expected to be normal, i.e. if the temperature is falling, the correction term generally has a negative sign, which is already evident from the fact that the temperature measured at an earlier point in time was greater than the temperature measured at a later point in time and thus the difference calculated from these values will have a negative sign. In this case, the temperature change evaluation factor therefore has a positive sign. Of course, a temperature change evaluation factor with a negative sign could also be used if this temperature difference is set up differently.

In order to ensure that conditions favourable for restarting combustion can be achieved by supplying air in the area of a combustion chamber, it is proposed that the cold blowing phase lasts at least until the temperature of the medium falls below the first temperature threshold.

If, for example due to the fact that the air conveyed in the cold blowing phase has a comparatively high temperature, a situation arises in which, when the temperature of the medium falls below the first temperature threshold, the temperature of the air leaving the heater is above a second temperature threshold associated with this, the cold blowing phase can be continued until the temperature of the air leaving the heater falls below the associated second temperature threshold. In this way, it can be ensured that the cold blowing phase is continued for a sufficiently long time to ensure conditions favorable for the resistance to combustion.

It is further proposed that if the temperature of the medium does not fall below the first temperature threshold within a predetermined period of time after the cold blowing phase has begun, the cold blowing phase is terminated. If the temperature of the medium does not fall sufficiently quickly, this is an indication that there is comparatively little heat energy requirement. In this case, the combustion can be delayed before it is restarted and the cold blowing phase can also be terminated, which helps to save electrical energy. For example, the cold blowing phase can be restarted if the temperature of the medium falls below the first temperature threshold.

In a procedure according to the invention, which also allows the temperature development of the heated medium to be taken into account, it is proposed that the first temperature threshold is determined on the basis of a base temperature threshold and a correction term representing the rate of change of the temperature of the medium.

wobei gilt:

• Ts' = erste Temperaturschwelle,
• T_GS = Grund-Temperaturschwelle,
• T_M(x) = Temperatur des Mediums bei einem letzten Ermittlungszeitpunkt x,
• T_M(x-1) = Temperatur des Mediums bei einem vorletzten Ermittlungszeitpunkt x-1,
• B = Temperaturänderungsbewertungsfaktor.

For example, the first temperature threshold can be determined as: $${\text{T}}_{{\text{S}}^{\prime }}={\text{T}}_{\underset{\_}{\text{G}}\text{S}}-\left[{\text{T}}_{\text{M}}\left(\text{x}\right)-{\text{T}}_{\text{M}}\left(\text{x}-1\right)\right]\times \text{B}$$

where:

• Ts' = first temperature threshold,
• T _GS = basic temperature threshold,
• T _M (x) = temperature of the medium at a last determination time x,
• T _M (x-1) = temperature of the medium at a penultimate determination time x-1,
• B = temperature change rating factor.

• 1 In prinzipieller Darstellung ein Fahrzeugheizgerät, bei welchem die erfindungsgemäße Vorgehensweise implementiert sein kann;
• 2 ein Zeit-Temperatur-Diagramm zum Veranschaulichen der erfindungsgemäßen Vorgehensweise.

The present invention is described in detail below with reference to the accompanying figures.

• 1 A schematic representation of a vehicle heater in which the procedure according to the invention can be implemented;
• 2 a time-temperature diagram to illustrate the procedure according to the invention.

In 1 a vehicle heater is generally designated with 10. The vehicle heater 10 comprises a burner area 12 designed as an evaporator burner with a combustion chamber 14. Combustion air L is conveyed into the combustion chamber 14 by means of a combustion air conveying arrangement 16, for example a side channel blower. Liquid fuel B is first introduced into a porous evaporator medium 20 by means of a fuel conveying arrangement 18, for example a metering pump. The fuel B is distributed in the inner volume area of the porous evaporator medium by means of a capillary conveying effect and is then evaporated in the direction of the combustion chamber 14. The exhaust gas flow S resulting from the combustion leaves the vehicle heater 10 via a corresponding exhaust channel area.

In a heat exchanger arrangement 22, heat generated during combustion is transferred to a medium M to be heated. This medium can be, for example, the coolant circulating in a coolant circuit of an internal combustion engine, i.e. generally water. If the vehicle heater 10 is designed as an air heater, the medium can also be the air to be introduced into a vehicle interior.

In order to start the combustion operation, an ignition element 24, for example a glow plug, is provided. By energizing the same, temperatures are generated locally that are so high that the mixture of combustion air and fuel is ignited and the combustion spreads from the combustion chamber 14.

The combustion air conveying arrangement 16, the fuel conveying arrangement 18 and, for example, also the ignition element 24 are controlled by a control arrangement 26. This controls or regulates the operation of the various system areas on the basis of various specifications. Temperature information can also be used for this purpose, for example. A temperature sensor 28 can, for example, detect the temperature of the medium M in the heat exchanger arrangement 22, wherein positioning at the inlet area and/or the outlet area of the heat exchanger arrangement 22 is possible here. A temperature sensor 30 can, for example, measure the temperature of the stream S leaving the heater 10. The temperature detected by the temperature sensor 28 is generally used as a control variable for the amount of thermal energy to be provided by the heater 10. If the temperature of the medium is comparatively low, the heater 10 is operated at a higher heating power level; if the temperature of the medium M is comparatively high, the heater 10 is operated at a lower heating power level. The temperature detected by the temperature sensor 30 can be used as an indicator of whether combustion is taking place or has started in the combustion chamber 14. The temperature sensor 30 can therefore be used as a flame sensor, since when combustion starts or is taking place, the stream S leaving the heater 10, in this state the exhaust gas stream, will have a comparatively high temperature.

As explained above, the heating output of the vehicle heater 10 is set so that it is adapted to the actual heat energy requirement. If the temperature of the medium M is comparatively high, the vehicle heater 10 can be brought into a control state, i.e. into a state in which neither fuel B nor combustion air L are supplied and thus no further heat energy is provided by stopping the combustion. The vehicle The vehicle heater 10 can nevertheless remain in a standby state in order to prevent an excessive drop in temperature or to enable renewed heating by resuming combustion operation when the temperature of the medium M, for example the temperature of the coolant circulating in the coolant circuit or the temperature in a vehicle interior, drops too sharply.

When combustion is resumed, various system areas of the vehicle heater will generally still have a relatively high temperature. For example, the porous evaporator medium 20 or a housing enclosing the combustion chamber 14 will be at a relatively high temperature level, as will the area of the fuel supply line which is connected to this housing or runs through it. Especially when using low-boiling fuels, there is a risk that spontaneous, pulse-like fuel evaporation will occur when the supply of fuel B is resumed, which can lead to delays in ignition or to unfavorable combustion conditions. In order to avoid pulse-like fuel evaporation when the vehicle heater 10 is to be put into operation again after a control break, without excessive time delays occurring, a method can be used in a manner described below with reference to the 2 The procedure described above must first be used to cool down the vehicle heater 10.

The 2 shows, plotted against time t, the development of the temperature T or of various temperature variables. It is initially assumed that at a point in time t a previously operated vehicle heater was brought into the control state, i.e. combustion was stopped by ending the fuel supply and the combustion air supply. The curve T _M illustrates the temporal development of the temperature of the medium M heated by the vehicle heater 10. Because no more heat is transferred to this medium M in the area of the heat exchanger arrangement 22, its temperature T _M gradually decreases. In the case of an air heater, the air temperature recorded in a vehicle interior, for example, decreases due to the fact that no more heated air is fed in.

Ts denotes a temperature threshold, falling below which indicates a comparatively strong cooling of the medium M and therefore requires the resumption of combustion and the provision of combustion heat. It is assumed here that at a time ts the temperature T _M of the medium falls below the temperature threshold Ts. In order to avoid the preceding effect of too spontaneous fuel evaporation, the combustion air conveying arrangement 16 can be operated in a cold blowing phase I _KB following the time ts in order to feed air L into the vehicle heater 10. In this cold blowing phase I _KB the fuel conveying arrangement 18 is not operated, so that only the air flowing through the vehicle heater 10 absorbs heat and carries it away from the area of the vehicle heater 10. The duration of the cold blowing phase I _KB can be fixed, but can also be set, for example, depending on the temperature of the air flow S detected by the temperature sensor 30.

The temperature of the air flow S in the cold blowing phase I _KB depends to a large extent on the temperature of the various system areas of the vehicle heater 10 around which this air flows. If their temperature has dropped sufficiently, this is reflected in a corresponding drop in the temperature of the flow S at the output of the vehicle heater 10, so that when the temperature detected by the temperature sensor 30 falls below an assigned temperature threshold, the cold blowing phase I _KB is ended. In a subsequent preheating phase I _VG, it can then be ensured by energizing the ignition element 24 that sufficiently high temperatures are generated in its surroundings to start combustion. After this preheating phase has ended, the fuel feed arrangement 18 can then be put into operation at a time t _DP in order to actually start combustion again.

The procedure described above can ensure that the fuel feed only starts again when the occurrence of spontaneous fuel evaporation is no longer to be expected. However, there is a comparatively long delay until after the combustion operation has ended at time to, i.e. the transition to the control state, the cold blowing phase is started at time ts and finally at time t _DP the fuel feed is started again and thus the combustion is started.

wobei gilt:

• T_G = Temperaturgröße,
• T_M(x) = Temperatur des Mediums bei einem letzten Ermittlungszeitpunkt x,
• T_M(x-1) = Temperatur des Mediums bei einem vorletzten Ermittlungszeitpunkt x-1,
• B = Temperaturänderungsbewertungsfaktor.

In order to avoid this comparatively long delay before the cold blowing phase begins, a temperature value represented by the curve T _G is determined according to the invention. When determining this temperature value, a trend value of the actual temperature of the medium T _M is taken into account by taking into account not only the actual recorded temperature T _M but also its rate of change. In particular, the temperature value T _G can be determined from the following relationship: $${\text{T}}_{\text{G}}={\text{T}}_{\text{M}}\left(\text{x}\right)+\left[{\text{T}}_{\text{M}}\left(\text{x}\right)-{\text{T}}_{\text{M}}\left(\text{x}-1\right)\right]\times \text{B}$$

where:

• T _G = temperature quantity,
• T _M (x) = temperature of the medium at a last determination time x,
• T _M (x-1) = temperature of the medium at a penultimate determination time x-1,
• B = temperature change rating factor.

Thus, for each calculation or determination cycle, the temperature T _M (x) present or recorded at this cycle or point in time as well as a change compared to the temperature T _M (x-1) present during the last or a previous recording or calculation cycle are taken into account, whereby the temperature change evaluation factor ultimately defines the extent to which this temperature change is actually taken into account.

If the temperature of the medium changes relatively slowly, the curve T _G will be very close to the actual temperature profile T _M. However, if the temperature changes more quickly, which indicates a relatively strong dissipation of heat energy from the medium, the curve T _G will fall much more steeply and thus fall below the temperature threshold Ts earlier, namely at time t _S' . This in turn means that the cold blowing phase I _KB ' can start earlier and therefore also end earlier. After the subsequent preheating phase I _VG ' , the dosing pump can then be put into operation at time t _DP ' and combustion can be started.

It should be noted here that in the preheating phase I _VG ' the combustion air feed arrangement 16 can of course be throttled in its feed rate or deactivated, since on the one hand sufficient cooling has already been achieved and on the other hand the reduced or stopped air flow prevents excessive heat loss from the ignition element 24, which then has to be brought to operating temperature. When the fuel feed begins, a quantity of combustion air adapted to the quantity of fuel fed is then of course introduced into the combustion chamber 14.

The cold blowing phase I _KB ' can extend over a basically fixed time interval. In order to ensure sufficient cooling, it should be continued at least until not only the temperature T _G but also the actual temperature T _M of the medium falls below the temperature threshold Ts. If at this point in time, i.e. in the representation of the 2 If it is determined at time ts that the temperature of the stream S detected by the second temperature sensor 30 is still above an associated temperature threshold, which is an indication that the air conveyed in the cold blowing phase still absorbs a comparatively large amount of heat, the cold blowing phase can be continued until the temperature of the stream S falls below the associated temperature threshold.

If it is further determined that after the start of the cold blowing phase at the time T _M the temperature of the medium has not fallen below the temperature threshold Ts within a maximum period of time specified for this purpose, the cold blowing phase can be terminated or interrupted and then continued or restarted when the temperature T _M has actually fallen below the threshold Ts.

wobei gilt:

• Ts' = Temperaturschwelle,
• T_GS = Grund-Temperaturschwelle,
• T_M(x) = Temperatur des Mediums bei einem letzten Ermittlungszeitpunkt x,
• T_M(x-1) = Temperatur des Mediums bei einem vorletzten Ermittlungszeitpunkt x-1,
• B = Temperaturänderungsbewertungsfaktor.

In an alternative approach, with which the temperature development can be taken into account when determining the start of the cold blowing phase, the temperature threshold below which the cold blowing phase begins is determined depending on the temporal development of the temperature T _M. For example, the following relationship can be chosen: $${{\text{T}}^{\prime }}_{\text{S}}={\text{T}}_{\text{GS}}-\left[{\text{T}}_{\text{M}}\left(\text{x}\right)-{\text{T}}_{\text{M}}\left(\text{x}-1\right)\right]\times \text{B}$$

where:

• Ts' = temperature threshold,
• T _GS = basic temperature threshold,
• T _M (x) = temperature of the medium at a last determination time x,
• T _M (x-1) = temperature of the medium at a penultimate determination time x-1,
• B = temperature change rating factor.

By taking into account the correction term for determining the temperature threshold Ts', this can be shifted further upwards in the event of a comparatively rapid drop, so that in fact the same state can be achieved as previously explained, namely the temperature variable to be taken into account, in this case for example the temperature T _M of the medium, falls below the then assigned threshold Ts' at an earlier point in time, in the example shown at time ts', and thus the start of the cold blowing phase is triggered.

The procedure according to the invention makes it possible, in particular by specifying the weighting of the correction term or the temperature change evaluation factor, i.e. the extent to which the change in temperature of the medium being observed over time is to be taken into account, to reduce the delay until the vehicle heater is restarted or to ensure earlier that suitable conditions are present for the restart. This prevents the temperature of the medium from falling too far below a predetermined temperature level, which in the case of a liquid heater can be between 70 and 90°C, for example. At the same time, irregularities in the fuel evaporation at the start of the fuel feed for the restart can be ruled out.

Claims (9)

Method for operating a fuel-operated heating device (10), preferably a vehicle heating device, preferably with an evaporator burner, in a warm start phase, in which method a temperature variable (T _G ; T _M ) associated with the temperature of a medium (M) heated by the heating device (10) is compared with a first temperature threshold (T _S ; T _S' ) assigned thereto and then, when the temperature variable (T _G ; T _M ) falls below the first temperature threshold (T _S ; T _S' ), a combustion air conveying arrangement (16) is put into operation and is operated during a cold blowing phase (I _KB ') to convey air (L), wherein during the cold blowing phase (I _KB ') a fuel conveying arrangement (18) is not operated to convey fuel (B), wherein the temperature variable (T _G ) and/or the first temperature threshold (Ts') are dependent on the rate of change of the temperature (T _M ) of the medium (M) is determined. procedure according to claim 1 , characterized in that the temperature variable (T _G ) is determined on the basis of the temperature (T _M ) of the medium (M) and a correction term representing the rate of change of the temperature (T _M ) of the medium (M). procedure according to claim 1 or 2 , characterized in that the temperature value (T _G ) is determined as: $${\text{T}}_{\text{G}}={\text{T}}_{\text{M}}\left(\text{x}\right)+\left[{\text{T}}_{\text{M}}\left(\text{x}\right)-{\text{T}}_{\text{M}}\left(\text{x}-1\right)\right]\times \text{B}$$

where: T _G = temperature quantity, T _M (x) = temperature of the medium (M) at a last determination time x, T _M (x-1) = temperature of the medium (M) at a penultimate determination time x-1, B = temperature change evaluation factor. procedure according to claim 2 or 3 , characterized in that the cold blowing phase (I _KB ') lasts at least until the temperature (T _M ) of the medium (M) falls below the first temperature threshold (Ts). procedure according to claim 4 , characterized in that when the temperature (T _M ) of the medium (M) falls below the first temperature threshold (Ts) and the temperature of the air leaving the heater (10) is above a second temperature threshold associated therewith, the cold blowing phase (I _KB ') is continued until the temperature of the air leaving the heater (10) falls below the associated second temperature threshold. Method according to one of the Claims 1 until 5 , characterized in that if the temperature (T _M ) of the medium (M) does not fall below the first temperature threshold (Ts) within a predetermined period of time after the start of the cold blowing phase (I _KB '), the cold blowing phase (I _KB ') is terminated. procedure according to claim 6 , characterized in that the cold blowing phase (I _KB ') is started again when the temperature (T _M ) of the medium (M) falls below the first temperature threshold (Ts). Method according to one of the Claims 1 until 7 , characterized in that the first temperature threshold (Ts') is determined on the basis of a base temperature threshold and a correction term representing the rate of change of the temperature (T _M ) of the medium (M). Method according to one of the Claims 1 until 8 , characterized in that the first temperature threshold (Ts') is determined as: $${\text{T}}_{{\text{S}}^{\prime }}={\text{T}}_{\text{GS}}-\left[{\text{T}}_{\text{M}}\left(\text{x}\right)-{\text{T}}_{\text{M}}\left(\text{x}-1\right)\right]\times \text{B}$$

where: Ts' = first temperature threshold, T _GS = basic temperature threshold, T _M (x) = temperature of the medium (M) at a last determination time x, T _M (x-1) = temperature of the medium (M) at a penultimate determination time x-1, B = temperature change evaluation factor.

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