The present invention relates to a device for limiting

A the exhaust temperature in an internal combustion engine, comprising means for supplying a cooling medium to the combustion chamber of the engine. 5

When designing modern internal combustion engines, especially supercharged engines, the exhaust tempera¬ ture is an important parameter to be taken into con¬ sideration, due to firstly the relatively large gas

10 flow in supercharged engines relative to the cylinder volume, secondly the higher exhaust counter-pressure when using catalytic exhaust converters leading to elevated exhaust temperatures, and thirdly the tendency to knock resulting from the desire to keep a relatively

15 high compression ratio for preserving good engine suction characteristics.

A known method of limiting the maximum level of the exhaust gas temperature is to inject a cooling medium

20 into the combustion chamber of the engine at those engine load conditions where there is risk that the temperature may exceed the maximum permissible level. Water can be used as a cooling medium injected into the combustion chamber through special injectors. The

25 most common method is however to use extra fuel as a cooling medium and to quite simply use the ordinary engine injection system for making the engine fuel-air mixture richer.

iλ 30 Known systems for limiting exhaust gas temperature by injecting a cooling agent lack feedback circuits, and this means that one must assume a worst case situation when calculating the amount to be injected. The system

must be designed with the assumption that the engine is operated with low octane fuel at high ambient tempera¬ ture with maximum load and that these conditions shall prevail for a long period of time. This results in poor fuel economy since when driving normally even with a high load there is seldom any need to add extra fuel and temporary increases in load, such as when passing, do not bring the exhaust temperature to a critical value. Another disadvantage is that the exhaust gas temperature will be unnecessarily low when driving on high octane fuel at normal ambient temperature, and this results in a non-optimal fuel-air mixture ratio.

The purpose of the present invention is to provide a device of the type described by way of introduction which makes it possible to optimize the supply of cooling medium in such a manner that the medium is only supplied when there is actual need for cooling.

This is achieved according to the invention by means of a temperature-sensing means protruding into an exhaust-conducting conduit and which is coupled to a control unit for sending an exhaust gas temperature- dependent signal to the control unit, which in turn is coupled to said cooling medium-conducting means for controlling the supply of cooling medium as a function of the exhaust gas temperature.

The invention provides feedback or reaction control which makes it possible to optimize the engine for high octane fuel and normal driving conditions. When using supercharged engines, the gain will be lower fuel consumption under precisely those operating conditions where supercharged engines normally have very high fuel consumption.

The device according to the invention can be used to control injection of cooling medium in the form of fuel or water through a separate valve in the engine induc¬ tion pipe, e.g. the start valve in an engine with fuel injection, but in a preferred embodiment for engines with fuel injection, the control unit is coupled to the fuel injection system to direct it to inject an excess of fuel through the ordinary injectors at exhaust gas temperatures above a certain level.

The invention is described below with reference to the accompanying drawings, where Figure 1 shows a block diagram of a preferred embodiment, Figure 2 shows a diagram illustrating the duty cycle of the control unit as a function of exhaust gas temperature, Figure 3 shows a diagram illustrating the shape of the pulse at several selected exhaust gas temperatures, and Figure -. shows a block diagram of a second embodiment.

In Fig 1, 1 designates the exhaust manifold of an internal combustion engine. Into the manifold and preferably as close to the exhaust valve as possible, there protrudes one end of a thermoelement 2, which can be of the encapsulated type with a diameter of 3 mm and a length of about 200 mm. The thermoelement 2 is coupled to a control unit 3, in the form of an electronic unit built up by so-called hybride techno¬ logy, i.e. the components are applied to a ceramic substrate to make the control unit able to withstand high temperatures. The thermoelement 2 is suitably embedded in the electronic unit 3 and this unit should be mounted as close to the manifold as temperature considerations permit and on a component which vibra'tes in the same manner as the manifold at the point of measurement.

The control unit 3 electronics contain circuits for converting the weak electrical signal from the thermo¬ element to a pulse width-modulated output signal which in the embodiment shown in Fig 1 is fed to the control electronics in an electronic fuel-injection system 4 which is known per se with an injector for each cylin¬ der. The output signal has a duty cycle in which the "on-time" increases with increasing exhaust gas tempe¬ ratures above a certain level. The injection system 4 measures the duty cycle of the signal from the control unit 3 and uses it as an extra parameter when calcula¬ ting the open time for the fuel injection valves.

Fig 2 illustrates the duty cycle as a non-linear func- tion of the exhaust gas temperature between 940°C and 980°C. A minimum duty cycle of 2 is generated by a special circuit in the control unit 3. This pulse is a so-called "diagnostic pulse", which is superimposed on the temperature-dependent pulse and enables faults to be detected in the system, e.g. when the device according to the invention is used in a turbocharged engine together wit a turbocontrol system 5 (Fig 1) of the type described in PCT/SE 88/00283. The diagnostic - pulse indicates only that the unit 2,3 is functioning correctly at exhaust gas temperatures below 940°C. It is too short to affect the fuel injection.

A duty cycle of for example 100 % or 0 % can be selected as indication that a fault has arisen in the system. In the diagram in Fig 3 the signal frequency is 10 Hz and the 0 % criteria can be that the measured pulse length is equal to or less than 1 ms. The criteria of 100 % DC can be that the measured pulse length is greater than 99 ms. In both cases this can suitably result in a warning light being lit on the instrument panel.

Instead of as in the example described above, allowing the unit 3 to affect the supply of fuel through the ordinary injection valves of an injection system via the control electronics of the injection system, it is possible within the scope of the invention to connect the unit 3 directly to a single injection valve 6 (se Fig 4) disposed in the engine induction pipe. This valve can be specially designed for injection of cooling medium. Alternatively, the start valve in a fuel injection system can be used for this purpose. The valve 6 can be controlled to inject fuel intermit¬ tently in time with the control unit 3 pulses and is completely open at 100 % DC. In Fig 4, 7 designates extra equipment for diagnosis of faults in the signal from the unit 3.