GB2277594A

GB2277594A - Estimating the temperature of a catalytic converter

Info

Publication number: GB2277594A
Application number: GB9309098A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1993-05-01
Filing date: 1993-05-01
Publication date: 1994-11-02
Anticipated expiration: 2013-05-01
Also published as: GB9309098D0; GB2277594B

Abstract

An internal combustion engine 12 having an exhaust system fitted with a catalytic converter 10, 11 and a heated exhaust gas oxygen (HEGO) sensor 40 downstream of the catalytic converter, an indication of the temperature of the catalytic converter 11 during and shortly after engine start-up is provided by monitoring the resistance of the heating element of the HEGO sensor 40. A further HEGO sensor, located upstream of the catalytic converter 10, 11 may be used in mixture control and the HEGO sensor 40 may also be used to detect any error in operation of the other HEGO sensor. <IMAGE>

Description

ESTIMATING THE TEMPERATURE OF A CATALYTIC CONVERTER Field of the invention The invention is concerned with estimating the temperature of a catalytic converter in the exhaust system of an internal combustion engine.

Background of the invention The efficiency of a catalytic converter drops dramatically when it is operating below its so-called light off temperature. For this reason, various systems have been devised for heating the catalyst of a catalytic converter during cold starts but as it is also important to avoid overheating a reliable method is required to assess the operating temperature of the catalyst.

One can use elements like thermistors and thermocouples to measure catalyst temperature directly, but the reliability of such devices cannot be guaranteed over 100,000 miles of vehicle operation, as currently proposed legislation would require.

One has therefore to resort to indirect but more durable methods of estimating the temperature of the catalytic converter and this invention aims to do so without adding significantly to the complexity and cost of the engine.

Summarv of the invention According to the present invention, there is provided an internal combustion engine, having an exhaust system fitted with a catalytic converter and a heated exhaust gas oxygen (HEGO) sensor downstream of the catalytic converter, wherein means are provided for monitoring the resistance of the heating element of the HEGO sensor during and shortly after engine start-up to provide an indication of the temperature of the catalytic converter.

It is common in order to control the air/fuel ratio, to provide a HEGO sensor in an internal combustion engine exhaust system. This sensor is normally upstream of the catalytic converter but control systems have also been proposed with HEGO sensors arranged both upstream and downstream of the catalytic converter, the latter serving to detect long term drift in the first HEGO sensor. Such a second sensor is not vital to the fuelling system and one can dispense with its output signal indicating exhaust oxygen for the first few minutes of engine operation.

The preferred embodiment of the present invention provides this second sensor with the dual role of acting as a catalyst temperature sensor during startup. However, more importantly, the invention allows an element of known durability (a HEGO sensor) to act as a temperature sensor.

The invention not only therefore solves the problem of estimating catalyst temperature but does so at no extra cost by using a reliable element that may already present be in the exhaust system.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which the single figure shows diagrammatically an engine having an exhaust gas ignition system for heating its catalytic converter and a temperature sensor for estimating the catalyst temperature.

Description of the preferred embodiment An engine 12 having an exhaust gas ignition (EGI) system comprises an intake system having an air mass meter 22, a butterfly throttle 24 and fuel injectors 20. The exhaust system includes a down pipe 14 leading to a catalytic converter that comprises two matrices 10, 11 disposed within a common housing and separated from one another by an afterburner chamber 16 containing an igniter 18. An air pump 30 serves to introduce additional air into the exhaust system through a flow control valve 32. The engine also includes two heated exhaust oxygen (HEGO) sensors 38, 40 the former being positioned near the exhaust ports and the latter downstream of the catalytic converter.

A HEGO sensor, as is already known, may be formed of a zirconia tube that has a heating element on it inner surface and platinum electrodes on its external surface. The sensor generates a signal dependent upon the oxygen concentration in the exhaust gases and the heater is provided because it only operates efficiently above a certain temperature.

Under normal operating conditions, in other words when the engine is warm, the pump 30 does not introduce additional air into the exhaust system and the first HEGO sensor 38 is used to set the air to fuel ratio supplied to the engine at stoichiometry. The catalytic converter, which acts as a three-way catalyst, operates most efficiently under these conditions to reduce hydrocarbons, carbon monoxide and nitrogen oxides in the exhaust gases.

If, in time, the calibration of the HEGO sensor 38 should drift, the catalytic converter will no longer function with maximum efficiency. The second HEGO sensor 40 will detect a permanent absence or presence of oxygen and this signal can be used to trim the calibration of the first HEGO sensor 38 to allow for long term drift.

The above description is given only as an example of a system that uses two HEGO sensors in the exhaust system for the purpose of engine control. The method of control of the engine is not of fundamental importance to the present invention.

During cold starts, the catalytic converter does not operate until it reaches its light-off temperature and the EGI system is intended to minimise the light-off time of the catalytic converter. During cold start, the mixture supplied to the engine is enriched to the point where a significant concentration of hydrogen is present in the exhaust gases. Air is added into the exhaust system by means of the pump 30 and the control valve 32 to restore the stoichiometry of the gases reaching the catalytic converter.

Within the afterburner chamber 16, this mixture is burnt using the igniter 18 and the heat generated rapidly heats the front face of the second matrix 11 to its light off temperature. The EGI system is then switched off to extinguish the afterburner flame.

At this point, there is a risk that the matrix could be cooled down again by the cold exhaust gases and this is prevented by switching to a catalyst temperature management (CTM) regime. In this regime, a mixture is supplied to the engine that it not rich enough to allow ignition of the cold exhaust gases but nevertheless contains carbon monoxide and hydrocarbons that can react exothermically with additional air supplied by the pump 30 in the presence of the partly operational catalytic converter to continue heating the remainder of the catalyst.

During the time that the engine intake charge is controlled to allow exhaust gas ignition or catalyst temperature management, it is advantageous to be able to determine the temperature of the catalyst so that control can be carried out by closed loop rather than open loop. Temperature sensors based on thermistors have been proposed for this purpose but they have not been used commercially because their long term reliability has not been proven.

Such closed loop control requires an accurate and fast response sensor, which it is not believed that the present invention can provide. However, a less accurate form of feedback can still provide useful data on whether the EGI system has been successful in lighting-off the catalytic converter and whether the converter is sufficiently warm during start-up not to require the EGI system to be brought into operation. Such limited data can be achieved by the temperature sensor of the present invention.

The preferred embodiment of the present invention uses the heating element of the second HEGO sensor as a temperature sensor. As the HEGO sensor body is heated by the gases flowing over it, its temperature will rise to that of the exhaust gases and the resistance its heating element will vary as a function of temperature. Because of the large surface area over which the exhaust gases contact the catalyst matrix, the temperature of the gases will also be close to the temperature of the catalyst. If therefore the resistance of the heating element is monitored without passing a current through the heating element that will affect its temperature, the measurement will provide a signal sufficient to indicate if the EGI and CTM systems are operating effectively. Furthermore the rate at which the HEGO sensor cools down when the engine is turned off will be related to the rate at which the catalytic converter cools down and its signal can therefore be used as a basis for deciding whether or not to activate the EGI and CTM systems.

The resistance of the heating element can be monitored in a variety of ways which are well known per se and need not be described in detail in the context of the present application. For example, one may use a derivative of Wheat stone bridge circuit or one may measure the voltage developed across the heating element when known quiescent current flows through it, the latter method be represented diagrammatically in the drawing by the positive and negative leads connected to the heating element of the HEGO sensor 40.

In a system having a second HEGO sensor in the position illustrated, it is not recommended to heat it when the engine is cold because water condensing on the zirconia tube from one side while it is being heated from the other can cause the tube to crack. The use of the heating element of the HEGO sensor as a temperature probe need not therefore interfere in any way with the intended use of the sensor 40.

Claims

1. An internal combustion engine, having an exhaust system fitted with a catalytic converter and a heated exhaust gas oxygen (HEGO) sensor downstream of the catalytic converter, wherein means are provided for monitoring the resistance of the heating element of the HEGO sensor during and shortly after engine start-up to provide an indication of the temperature of the catalytic converter.

2. An internal combustion engine as claimed in claim 1, wherein two HEGO sensors are provided in the exhaust system position upstream and downstream of the catalytic converter, respectively, and wherein the upstream sensor is used by a control system of the engine to set the mixture strength of the charge supplied to the engine combustion chambers and the HEGO sensor downstream of the catalytic converter serves not only to provide an indication of the temperature of the catalytic converter during engine start-up but also to detect any error in the operation of the upstream HEGO sensor.

3. An internal combustion engine constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.