TEMPERATURE MONITORING OF AN EXHAUST SYSTEM HAVING A COOLING LOOP, FOR INTERNAL COMBUSTION ENGINES The invention relates to the temperature monitoring of an exhaust system having a cooling loop in a motor vehicle, particularly one with a front-mounted internal combustion engine wherein the exhaust gases are passed through a catalytic converter and a nitrogen oxide trap to the rear region of the vehicle, the exhaust gas lines of the individual cylinders being combined on one side of the internal combustion engine in a manifold from which two supply lines of different cross-section lead to the catalytic converter and the larger cross-section supply line can be shut off by means of a shut-off valve.

To ensure rapid heating up of the catalytic converter after a start-up of the internal combustion engine while still providing the necessary cooling for the pipe supplying the exhaust gases to the catalytic converter during prolonged operation of the vehicle, the pipes supplying the exhaust gases to the converter are of different lengths. The larger cross-section supply pipe is arranged beneath the internal combustion engine in front of the engine (in the direction of travel of the vehicle) and underneath the engine. It includes a bend, after which the exhaust pipe runs counter to the direction of travel to the catalytic converter mounted underneath the passenger compartment. The smaller cross-section pipe forms a short connection between a region of the manifold near a decoupling element and a region of the exhaust gas line adjacent to the converter.

This arrangement provides a positive solution to the problem of the so-called packaging of the exhaust gas pipes and the catalytic converter within the vehicle: the larger cross-section connecting pipe lies in a region of the vehicle in which optimum cooling is possible. Despite this,

on starting up the vehicle shutting off the larger cross- section pipe enables rapid heating up of the catalytic converter to be effected by the smaller cross-section supply pipe, which in addition can be made shorter.

Both the rapid heating up of the catalytic converter and the cooling of the exhaust gas during the further operation of the vehicle depend on the cooling loop and the shut-off valve functioning properly.

The object of the invention is to provide monitoring of this functioning without the need for additional outlay on sensors.

To this end, the invention provides a method of monitoring the temperature of an exhaust system having a cooling loop, for internal combustion engines, wherein a catalytic converter and a nitrogen oxide trap are fitted in the exhaust system and the cooling area is controllable by a shut-off valve, wherein at specified operating conditions of the internal combustion engine the shut-off valve is adjusted to a predetermined value and at least one temperature in the exhaust gas train is measured and is compared with an intended value As the specified operating conditions of the internal combustion engine an operating condition is defined which, with the shut-off valve in the selected position, would result in a predetermined intended value at the temperature measuring point. The magnitude of this intended value can be determined in advance by tests and stored in the corresponding control unit together with the operating conditions of the internal combustion engine.

In particular, at full load operation of the internal combustion engine and with the shut-off valve completely

open the temperature at the nitrogen oxide trap is determined and compared with an upper limiting value.

According to a preferred embodiment the temperature at the outlet of the catalytic converter is determined as well as the temperature at the nitrogen oxide trap, and both temperatures are compared with intended values.

The double temperature determination increases the reliability of the temperature monitoring. In particular, from the variation of the two temperature signals with time conclusions can be drawn about the cooling function of the cooling loop and about the functioning of the shut-off valve.

According to a further preferred embodiment of the temperature monitoring process a difference value is formed from the temperature at the outlet of the catalytic converter and the temperature at the nitrogen oxide trap and is compared with an initial intended value. This may be done at specific times determined by the operating conditions of the engine, or a difference function may be continuously formed from the said temperatures and compared with an intended value function.

The sensitivity of the temperature monitoring is thereby improved.

It is further preferable for one of the above-mentioned intended values to be fixed adaptively having regard to -the engine speed -the engine load -the vehicle speed -the ambient temperature -the air/fuel ratio or any desired combination thereof.

By having regard to the variables affecting the exhaust gas temperature by means of an adaptation of the intended value the reliability of the temperature monitoring is improved.

The temperature monitoring can not only be used to give a warning to the driver of any malfunction, but is also preferably used as a precaution against overheating of the converter and/or of the nitrogen oxide trap.

A preferred application of the temperature monitoring is therefore to influence the mixture setting so as to enrich the mixture if the intended value is exceeded.

Alternatively or in addition the temperature monitoring can be used to influence the quantity of intake air by influencing the control of the electronic throttle valve if the intended value is exceeded.

Both for the enrichment of the mixture and for the influencing of the throttle valve position the deviation from the intended value can be used as a controlled variable.