WO1994011622A1

WO1994011622A1 - Catalytic converters

Info

Publication number: WO1994011622A1
Application number: PCT/GB1993/002368
Authority: WO
Inventors: Thomas Tsoi Hei Ma; David Nicholas Ball
Original assignee: Ford Motor Company Limited; Ford Werke A.G.; Ford France S.A.; Ford Motor Company; Kidde Technologies Inc.
Priority date: 1992-11-18
Filing date: 1993-11-17
Publication date: 1994-05-26
Also published as: GB9224145D0

Abstract

A catalytic converter is provided incorporating an elongate temperature sensor (8) in contact with the catalyst block (4) at several points along its length. Such a sensor (8) has the ability to detect either uniform temperatures of the catalyst block (4) or hot spots which may occur within the catalyst block (4).

Description

CATALYTIC CONVERTERS

Field of the invention

This invention relates to a catalytic converter and to a method of monitoring a temperature of a catalytic converter.

Background of the invention

Catalytic converters are being increasingly used to reduce carbon-monoxide and other noxious gas emissions produced by engines for motor vehicles. Before such converters become fully effective, the catalyst contained within them has to reach a minimum operating temperature, termed the light-off temperature. Given that a large number of journeys undertaken by motor vehicles are of a short duration, even vehicles equipped with such converters can produce significant quantities of the unwanted emissions.

It has been proposed to alleviate the problem by rapidly heating the catalyst to its light-off temperature in a way that does not rely upon heat transfer from the engine exhaust. This approach in itself, or other conditions such as an engine misfire, may also cause problems in that they may result in the catalyst attaining a temperature above its safe maximum operating temperature resulting in damage to or destruction of the catalyst material.

It has already been proposed in US 4,102,127 to sense the temperature of the catalyst material directly by means of detector, such as a thermistor, and to discontinue operation of a glow plug, which acts as an auxiliary heat source for the catalytic converter, when the sensed temperature of the catalyst reaches a predetermined level . In the prior art proposal, however, temperature sensing is localised within the catalyst material and it is assumed that the temperature within the catalyst material is uniform. This assumption is not however justified under all operating conditions. For example, when an afterburner is used to heat the catalytic converter during warm-up, the front face of a catalyst block exposed to the flame of the afterburner will be heated more rapidly than the interior of the block. Consequently, a detector located in the interior of the block would not prevent the front face of the block from being damaged by overheating.

Object of the invention

The invention seeks to mitigate the foregoing disadvantage by enabling the temperature at several points in a catalyst block of a catalytic converter to be monitored.

Summary of the invention

According to a first aspect of the invention there is provided a catalytic converter having a catalyst block and a sensor located within the block for producing an output signal indicative of the temperature of the block, characterised in that the temperature sensor is elongate and is mounted in the block in such a manner as to sense to the temperature at several points along the length of the block.

In the invention, the output signal of the sensor will vary as a function of the temperature at different points along the length of the block, the length being the dimension of the block parallel to the direction of flow of exhaust gases.

Conveniently, the output signal may vary as the integral of the weighted temperature over the length of the block. The weighting serves to accentuate high temperatures so that local a hot spot at a high temperature may cause a significant change in the output signal of the sensor even when it does cover an extended region of the catalyst block. Preferably, the sensor is a thermistor and its temperature responsive characteristic is the integral of conductance over the length of the sensor.

According to a second aspect of the invention, a method of monitoring temperature in a catalytic converter comprises providing a sensor within the block for producing an output signal indicative of the temperature of the block and monitoring the output signal of the sensor, the method being characterised by the steps of forming the sensor as an elongate sensor and mounting the sensor in the block in such a manner as to sense to the temperature at several points along the length of the block.

In an exhaust system fitted with auxiliary heating means, such as an afterburner, for rapidly raising the temperature of the catalytic converter during warm-up to achieve early light-off, the output signal of the sensor in the present invention may suitably be used to control the heating of the catalytic converter as it will not only reflect uniform heating of the catalytic converter but also local high temperatures resulting from the localisation of the auxiliary heating means.

Brief description of the drawings

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which : -

Figure 1 shows an engine exhaust system incorporating a catalytic converter according to the invention, and

Figure 2 shows graphically the characteristics of a sensor used in the system of Figure 1. Detailed description of the preferred embodiment

Referring to Figure 1, an engine exhaust system 1 incorporates a catalytic converter 2 comprising a generally cylindrical casing 3 containing two substantially identical blocks 4 of catalytic material, sometimes also termed bricks. These are typically matrices of ceramic material having a honeycomb-like structure having a coating that contains particles of a catalyst, such as platinum.

If, as is desirable, the catalytic converter 2 is required to operate rapidly from cold, heat must be applied externally to bring the blocks 4 to the required operating temperature, typically within a few seconds after engine start-up. A preferred way in which this can be accomplished is by supplying an excess of fuel to the engine cylinders, adding air to the exhaust gases and burning the resultant ignitable mixture upstream of the catalytic converter 2. To that end, the exhaust conduit 5 of the exhaust system 1 is fitted with an air-blowing fan 6 and a suitable ignition device 7, such as a sparking plug. Alternatively, an independently fuelled afterburner or an electrically heated element may be used to bring the block 4 to the required operating temperature.

In order to exercise control over the amount of heat being supplied to the catalytic converter 2 during the start-up period, it is desirable to monitor the temperature of the catalytic material . Each block 4 is therefore fitted with an elongate temperature sensor 8, the operation of which will be described hereafter. Each sensor 8 extends diagonally across its respective block 4, so as to sense the temperature along the entire length of the block. Because it is arranged diagonally rather than parallel to the length of the block leakage of exhaust gases is avoided because gases cannot flow along the hole made for the sensor without passing through the passages in the block. An engine management unit (EMU) 9 controls the heating source to the catalyst material 4 in response to the outputs from the elongate temperature sensors 8.

The elongate temperature sensors 8 are used to monitor a variety of different operating conditions. They are used to sense the presence of the external heat source, and they are also used to sense when the catalyst light-off temperature has been attained, thereby providing an indication that the supply of external heat can be discontinued. In addition, the temperature sensors 8 are used to detect potentially damaging overheating of the catalyst material.

One such damaging condition is engine misfire. The fuel and oxygen that are not burnt in the combustion chamber will on reaching the catalytic converter react exothermically with one another to heat the catalyst block 4. This results in a much greater amount of heat than normal being released progressively as the unburnt fuel passes through the catalyst block 4. An excessively high temperature may therefore be reached in a region inside the catalyst block 4 at some distance from its front face, with the result that damage may occur internally and not be evident from subsequent external examination.

Because of distributed temperature detection over the length of the catalyst block, the invention permits such a hot spot within the block 4 to be detected and as soon as a danger condition is alerted it is possible for the engine management unit 9 to take remedial action by stopping the injection of fuel into the misfiring cylinder.

The temperature sensors should thus be capable of detecting hot spots, where any small region of the block 4 may reach a very high temperature (e.g. 900°C) and of detecting when the entire block or most of it reaches a somewhat lower temperature (e.g. 700°C) . The elongate temperature sensors 8 have a temperature responsive characteristic (resistance or capacitance) which depends on the temperature along the length of the sensor 8.

In effect, each sensor 8 functions as a large number of unit thermistors, exhibiting capacitive characteristics, connected in parallel. The overall resistance (or capacitance) of the sensor is a function of temperature along the length of the sensor 8, but weighted towards lower resistances (or higher capacitances) , corresponding to higher temperatures. An example of a suitable elongate temperature sensor is the continuous fire detector thermistor marketed by Walter Kidde. This particular sensor comprises two elongate electrical conductors embedded in ceramic material, the electrical resistance between the conductors being dependent on temperature. This sensor is also advantageous in being suitably rugged to withstand the environment of the catalytic converter in a motor vehicle and repeated heating and cooling. It is also capable of providing an electrical output compatible with vehicle sensing devices.

The temperature sensors should also be capable of detecting when a small section, say 2 cms or so of the block has attained the required ligh -off temperature (typically 500°C) indicating that the supply of external heat can be discontinued, and that subsequently the entire block is being maintained above a minimum operating temperature (typically 400°C) by the heat of the exhaust gases.

As will become clear, an elongate temperature sensor of this kind is ideally suited to sense the temperature of a catalyst block also to detect overheating of the block.

Curve A in Figure 2 shows how the resistance R in ohms of a temperature sensor 20 cms long varies as a function of temperature T (in °C) when the entire length of the sensor is uniformly heated. Curve B in Figure 2 shows how the resistance R varies as a function of temperature T when only any one short section of the sensor 8 (in this case a section 2 cm long) is heated, the remainder of the sensor 8 being unheated.

The resistance-temperature characteristic of curve A is applicable to the case of a catalyst block which is at a uniform temperature (400°C in this example) over its entire length. In this instance, the elongate temperature sensor will also be heated over its entire length by the material of the block. The resistance-temperature characteristic of curve B, on the other hand, is applicable to the case of a catalytic block which is heated to a higher temperature (500°C) only at the first 2 cms or so of the block. In this instance, only a short section of the temperature sensor will be subjected to heating by the block.

In each case, the respective operating temperature i.e. 400°C (Curve A) and 500°C (Curve B) corresponds to a sensor resistance of approximately 10 kΩ.

Accordingly, by setting an upper threshold level corresponding to this resistance value it is possible to detect when the required light-off temperature has been achieved within the entire block 4 or a higher temperature attained at any portion within the block 4.

Similarly, by detecting resistance values less than a lower threshold level, corresponding to a resistance of about 500 Ω in this example, it is possible to detect hot spots which occur when a localised region of the block (2 cms across, say) reaches a temperature of about 900°C or when the entire block or a large part of it starts to exceed the maximum safe operating temperature of about 700°C. This threshold would correspond to the danger alert mentioned previously. If the monitored resistance values of the temperature sensor lie between the upper and lower threshold values, the catalytic converter 2 will be operating correctly.

Curves A and B in Figure 2 represent the extreme operating conditions of the temperature sensor 8, that is to say, the condition when the sensor 8 is heated uniformly along its entire length (Curve A) , and the condition when only a short length of the sensor is heated, the remainder of the sensor being unheated (Curve B) .

Similar considerations apply to operating conditions between these extremes. Curves C, D, E, F and G in Figure 2 show the resistance-temperature characteristics for a sensor in which any one short, 2 cm, section is held at a respective fixed temperature of 400°C, 500°C, 600°C, 700°C and 800°C, and the remainder of the sensor is heated to the temperature shown along the abscissa.

The EMU 9 controls the engine and the exhaust system in the following manner. Starting from cold, the engine 10 produces exhaust gases which pass along the exhaust system 1 to the catalytic converter 2 and through the catalyst blocks 4. The exhaust gases are insufficient in themselves to heat the catalyst rapidly. Thus the temperature experienced by sensors 8 produce outputs higher than 10 kΩ, that is, below the light-off temperature. The EMU 9 increases the fuel/air mixture to the engine 10 thus enriching the exhaust gases with unburnt fuel. The air blower unit 6 is activated to introduce air into the exhaust system and the resulting air and exhaust gas mixture is ignited by activating the sparking plug 7. The resulting combustion rapidly heats the catalyst blocks and the EMU 9 continues to monitor the output from the sensors 8 to ensure that the catalyst blocks 4 remain within their optimum operating temperature range. Should the temperature of the catalyst blocks 4 reach the danger level for any reason, the EMU 9 may undertake remedial action, and may also provide a visual warning to the driver of the motor vehicle within which the system is located.

Claims

1. A catalytic converter having a catalyst block (4) and a sensor (8) located within the block for producing an output signal indicative of the temperature of the block, characterised in that the temperature sensor (8) is elongate and is mounted in the block in such a manner as to sense to the temperature at several points along the length of the block (4) .

2. A catalytic converter as claimed in claim 2, wherein the output signal of the sensor varies as the integral of the weighted temperature over the length of the block, the weighting serving to accentuate high temperatures.

3. A catalytic converter as claimed in claim 2, wherein the sensor (8) is a linear thermistor array, the output signal derived from the sensor varying as the integral of conductance over the length of the sensor.

4. A catalytic converter as claimed in any preceding claim, wherein the sensor is arranged to extend at an angle inclined to the length of the catalyst block.

5. A method of monitoring temperature in a catalytic converter comprising providing a sensor within the block for producing an output signal indicative of the temperature of the block and monitoring the output signal of the sensor, the method being characterised by the steps of forming the sensor (8) as an elongate sensor and mounting the sensor in the block in such a manner as to sense the temperature at several points along the length of the block (4) .

6. A method as claimed in claim 5, including setting upper lower threshold levels and comparing the output signal with the set thresholds .

7. A method as claimed in claim 6 for use with a catalyst having an external heat source, wherein the external heat source is permitted to operate only when the output signal is indicative of a temperature below the temperature of the lower threshold.

8. A method as claimed in claim 6 or 7, wherein a hazard warning is generated in response to an output signal indicative of a temperature greater than the temperature of the upper threshold.

9. An engine control system comprising a catalytic converter as claimed in any of claims 1 to 4 and means for controlling heating of the catalytic converter in response to an output from the elongate temperature sensor of the catalytic converter.

10. An engine control system as claimed in claim 9, wherein the control means is arranged to supply, or inhibit the supply, of heat to the catalytic converter, according to the output signal of the temperature sensor.