Measuring transformer for detecting hydrocarbons in gases

Details Measuring transformer for detecting hydrocarbons in gases Measuring transformer for detecting hydrocarbons in gases

2001-07-20

2003-03-04

Warden, Jill (Department: 1743)

Chemical apparatus and process disinfecting, deodorizing, preser

Analyzer, structured indicator, or manipulative laboratory...

Means for analyzing gas sample

C422S083000, C422S088000, C422S098000, C422S050000

Reexamination Certificate

active

06528019

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a measure transformer for detecting hydrocarbons in gases.
DISCUSSION OF THE BACKGROUND
Increasingly strict environmental legislation is forcing automobile manufacturers to develop and use exhaust emission control systems, usually catalytic converters, with ever better conversion rates in order to maintain the government-specified maximum values of emitted emissions such as nitrogen oxides, carbon monoxide, or unburned hydrocarbons. At the same time, it is required that the function of the exhaust emission control systems be monitored continuously during operation and that defective function be indicated, ranging from exceeding the limits as a result of aging phenomena of the catalyst to a total failure of the &lgr;-probe that controls the combustion stoichiometry. For this so-called on-board diagnosis (OBD), an exhaust sensor located downstream of the exhaust emission control system is required that monitors the function of the exhaust emission control system during operation and whose sensor signal serves as a basis for determining the state of the exhaust emission control system.
With a four-stroke engine operated at &lgr;=1 (1 represents the fuel-air mixture), the emissions are drastically reduced by a three-way catalyst. While it is not difficult to meet the nitrogen oxide and carbon monoxide limits, theoretically unburned hydrocarbons (HC) pose the greatest problems. Malfunction of the exhaust emission control system is indicated only when the HC concentration in the exhaust increases.
There are many ways to diagnose an exhaust emission control system. Several patents such as DE 34 13 760, U.S. Pat. No. 5,740,676, U.S. Pat. No. 5,467,594, and DE 42 09 136 for example as well as the literature references [1] and [2] cited as examples propose providing &lgr;-probes upstream and downstream of the catalytic converter. The oxygen storage capacity and hence indirectly the function of the catalytic converter can be determined from a comparison of the amplitude fluctuations in the probe output signals upstream and downstream of the catalytic converter. Such methods are already used in mass production. Another frequently discussed method is diagnosis of the exhaust emission control system by means of one or more temperature sensors. In this case, it is the reaction heat resulting from the conversion of the hydrocarbons in the untreated exhaust that is detected. Examples will be found in [3]-[7] or in DE 42 01 136. Direct determination of the hydrocarbon concentration in the purified exhaust by means of an HC sensor would be much simpler and more precise than determination of values that depend only indirectly on emissions.
Such direct HC sensors can incorporate for example HC measurement by means of a surface ionization detector [8], but this method depends to a significant degree on the gas throughput, the type of hydrocarbons, and the oxygen content of the exhaust.
Another type of HC sensor is the familiar catalytic sensor (also known as the pellistor) described here using the example in EP 0 608 122. For such sensors, oxygen is always necessary to burn the hydrocarbons so that the output signal depends largely upon the oxygen content of the exhaust. In addition, very exact temperature control and measurement are required since the electrical resistance of a temperature-dependent part is measured. Therefore, such a sensor principle is unsuited for use in the exhaust line.
A sensor design that consists of an oxygen generator, oxygen diffusion zone, HC sensor zone, and at least two temperature control zones and is therefore very complex is described in U.S. Pat. No. 5,689,059. This sensor is suitable for exhaust but requires electrical terminals in considerable numbers. In addition, this sensor, which in reality a sensor system, requires very complex and costly control and regulating electronics so that it cannot be used for the broad mass market.
Hydrocarbon sensors using planar technology are less expensive to manufacture.
DE 0 046 989 proposes an HC sensor based on tungsten oxide made by the planar technique which can be used only at room temperature.
Pt-MOSiC sensors based on silicon carbide are proposed in [9] for use in motor vehicles. However, the operating mechanism is not easy to understand and the signals are dependent not only on the hydrocarbon but also on oxygen and temperature. Manufacture of planar structures on SiC is also costly and therefore cannot be used for the motor vehicle mass market.
Widely used, inexpensive sensors are made on a ceramic substrate from SnO
2
. Examples include EP 0 444 753 or EP 0 603 945. In this sensor principle, the electrical sensor resistance changes with the HC concentration in the gas. Sensors of this type are used in large numbers as sensitive elements in gas warning systems and their functional mechanism is widely known. An attempt to use such sensors in an automobile is described in [10] and [11]. Unfortunately, these sensors lose their gas-sensitive properties at temperatures above several hundred degrees Celsius and change their resistance only with the oxygen partial pressure of the gas. The long-term stability of these sensors is not guaranteed either.
Resistive sensors based on metal oxides which have been proposed more frequently as an oxygen-detecting element but not as an HC sensor are likewise manufactured using planar technology and are suitable for use in exhaust. In the resistive principle, the electrical resistance of the sensitive material is used as a measured value. For example, DE 37 23 051 proposes doped titanates, zirconates, or stanates as resistive oxygen-sensitive materials which are applied according to DE 37 23 052 using thick film technology to a ceramic substrate. DE 42 02 146, DE 42 44 723, and DE 43 25 183 propose compositiones based on cuprates manufactured using thick film technology as oxygen-sensitive materials. DE 44 18 054 mentions lanthanum ferrites doped with alkaline earths for the same purpose. Such multiple metal oxides which are usually present in the perovskite structure have the advantage of increased chemical stability and greater long-term stability over the sensors made of simple metal oxides, TiO
2
[10] for example, that have been in use for a long time. All of these oxygen sensors however have a typical temperature curve of the electrical resistance according to an exponential function typical for semiconducting metal oxides, in other words the sensor output signal depends not only on the oxygen partial pressure of the exhaust but on the sensor temperature as well. Hence, for such oxygen exhaust sensors, an exact temperature measurement is linked to a costly electronic regulation or a reference that is not exposed to the exhaust and is kept at a constant temperature must be integrated in the substrate which is also expensive and leads to problems with long-term stability.
The series connection of two resistive oxygen sensors which in addition to their oxygen dependence have a temperature dependence with different temperature coefficients of the specific electrical resistance, is proposed in DE 38 33 295. In addition, a compensating resistance must also be included. However, in this method, despite the high costs, a temperature independence of the sensor resistance can be achieved only in a very narrowly delimited oxygen partial pressure range.
A resistive oxygen sensor that is temperature-independent only at a certain oxygen partial pressure is described in a portion of EP 0 062 994. In DE 19 744 316, an oxygen sensor is proposed made of a material in which the oxygen partial pressure range of temperature independence can be varied by deliberate variation (doping) of the layer material.
Typical HC sensors manufactured using planar technology are characterized by the following typical arrangement. On the underside of an electrically insulating substrate a heater and/or a temperature measuring device in the form

Affiliated with

Also associated with

Rating

Measuring transformer for detecting hydrocarbons in gases does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Measuring transformer for detecting hydrocarbons in gases, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Measuring transformer for detecting hydrocarbons in gases will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3039272