DESCRIPTION ENGINE MISFIRE DETECTION AND ENGINE EXHAUST SYSTEMS The present invention relates to a method of detecting misfire in a multicylinder internal combustion engine and to an apparatus for protecting an exhaust system of a multicylinder internal combustion engine operable with fuel injection and spark ignition.
It is known that exhaust gas catalysers and even the exhaust manifolds of internal combustion engines can be damaged or destroyed due to overheating. The danger of overheating arises in the event of a misfire, that is to say, the fuel/air mixture in one or more of the combustion chambers fails to ignite through faulty ignition. The uncombusted mixture when it reaches the cataiyser whose working temperature is around 550°C is immediately ignited, rapidly heating the cataiyser to a temperature at which the expensive catalyst is destroyed. There is even the danger of the motor vehicle in which the engine is installed being set alight.
It is an object of the invention to provide a method and apparatus whereby faulty combustion or misfire can be immediately identified so that steps can be taken to prevent consequential overheating of the exhaust system, particularly of the cataiyser, and also to remove the risk of the vehicle itself catching fire. Advantages of the Invention
These disadvantages are avoided by the method claimed in claim 1 and the apparatus claimed in claim 7. Not only is a misfire promptly detected through the irregularity or rough running of the engine, but also the faultily operating cylinder is immediately identified. In the case of petrol injection individual to the cylinder or to groups of cylinders, the feeding of further fuel to the misfiring combustion chamber can be prevented. The response is much more rapid than can be achieved by monitoring the temperature in the cataiyser and monitoring the cataiyser temperature cannot identify which cylinder is misfiring, unless several catalysers were to be provided, individual to the cylinders or groups of cylinders.
Further advantageous features are identified in the dependant claims. Drawings The invention is further described, by way of example, with reference to the accompanying drawings, in which:-
Fig.l is a block schematic diagram of an electronic fuel injection and ignition system incorporating apparatus according to the invention for protecting the engine exhaust system from overheating in the event of misfire;
Fig.2 is a graph showing angular acceleration of the crankshaft both in correct operation of the engine and when misfire occurs in one of the cylinders;
Fig.3 is a diagrammatic sectional view of part of a cylinder block and head fitted with a strain gauge stri ; and
Fig.4 is a diagrammatic plan view of the cylinder block with the head removed.
Description of the Preferred Embodiment
A four-cylinder petrol engine has electronically controlled fuel injection and ignition systems. The four injection valves 10,12,14,16 (shown diagra matically) are opened and closed in timed relation to the crankshaft rotation under the control of the computer 18 and via end stage amplifiers 20. The injection valves 10-16 are individual to the four cylinders of the engine in that they inject the fuel into the engine inlet manifold branches adjacent the respective engine inlet valves. The computer 18 also controls the four spark plugs 22,24,26,28 (shown diagrammatically) via an ignition coil and distributor 30. The computer 18 controls the injection valves 10-16 and the ignition trigger voltages supplied to the ignition coil 30 in accordance with operating parameters to provide the optimum injected fuel quantities, injection timing and ignition timing in conventional manner. The operating parameters include a reference pulβe BM, inlet pressure (vacuum) p, load L (pedal position), engine speed n and engine cooling water temperature T. The reference pulses BM are obtained in timed relation to rotation of the crankshaft and are for synchronization purposes. If so-called lambda control is incorporated, the air number λ , as measured by a lambda sensor in the engine exhaust system, is also fed to the computer 18. Lambda control is control in accordance with the composition of the exhaust gases, particularly the oxygen content, whereby the fuel/air ratio is that which provides for optimum combustion, i.e., close to stoichiometric.
The engine exhaust system (not shown) includes a cataiyser for eliminating harmful components (pollutants) from the exhaust gases. The cataiyser runs at an optimum temperature, e.g., about 550"C, for catalysing the reduction of nitrogen oxides and the simultaneous oxidation of carbon monoxide and hydrocarbons, using any residual oxygen in the exhaust gases.
If there is a misfire in one of the engine cylinders, the unburnt fuel/air mixture would reach the cataiyser which would immediately catalyse combustion and lead to overheating and rapid destruction of the catalyst. This is avoided, according to the invention, by detecting the irregular or rough running of the engine which occurs upon misfire. The engine mass, together with its resilient mounting on the body or chassis of the vehicle in which the engine is installed, forms an oscillatory system. When all cylinders are firing properly, this oscillatory system oscillates in a regular or uniform manner. If one cylinder misfires, the oscillatory system oscillates in a manner which is irregular or non-uniform within a working cycle of the engine. The oscillation of the engine can be monitored by means ofa suitably disposed acceleration sensor. The optimum mass of the acceleration sensor and its optimum position on the engine block and direction of sensitivity (orientation) can be found by trial and error. Fig.2 shows how the acceleration of the engine mass varies with respect to crankshaft rotation, the ignition trigger pulses also being shown so that the timing can be appreciated. The curve A represents the instantaneous acceleration of the engine block with respect to time with the engine running at a nominal constant speed of 1000 rpm. The curve B represents the ignition trigger pulses of which there are four for every two revolutions of the engine crankshaft, in the case of a four stroke engine. All cylinders are firing normally up to the point 32. It can be seen that the oscillations of the engine block are uniform. At the point 32, one cylinder starts to misfire which results in an immediate perceptible irregularity in the oscillations. By comparing the position of an exaggerated acceleration peak or trough with the individual ignition trigger pulses, the cylinder that is misfiring can be identified. Thus, referring again to Fig.l, the signal a from an acceleration sensor is fed to a misfire detecting circuit 34 within the computer 18. The detecting circuit 34 notes the presence of an exaggerated acceleration peak or trough in the sequence of normally four peaks or troughs within a complete working cycle, i.e., over two crankshaft revolutions, and compares the instants of such exaggerated peaks or troughs with the instants of ignition as set by the trigger pulses. Each acceleration peak or trough lies approximately 90° after the corresponding ignition trigger pulse. Thus, the detecting circuit 34 is able to identify the cylinder which is mis-firing and delivers an inhibit signal to the injection system to prevent the corresponding one of the fuel injection valves 10-16 from opening, whereby no more fuel is delivered to the mis-firing cylinder. This prevents any significant quantity of unburnt fuel from reaching the exhaust system and in particular the cataiyser, so that the risk of the cataiyser being destroyed is much reduced.
Instead of using an acceleration sensor, it is possible to use one or more modified "knock" sensors. Such a sensor comprises a quartz crystal which is capable of vibrating or oscillating under mechanical stimulation and converting the mechanical oscillation into corresponding electrical signals. A vibrating mass is attached to the engine at a suitable plane via the quartz crystal.
Another possibility is to use one or more strain gauges which are disposed on parts of the engine which are subjected to stress due to the pressure of combustion. A misfire in one cylinder will result in a peak strain at the corresponding strain gauge not being reached.
Figs. 3 and 4 show a suitable disposition for two strain gauges on a four-cylinder engine. Sleeves 40 are disposed on two of the cylinder head screws 34 by -6- which the cylinder head 36 is attached to the cylinder head 38 with the interposition of the head gasket 42. Strain gauge strips 44 are disposed on the sleeves 40 so as to detect changes in the compressive stresses in these sleeves due to the tension in the corresponding cylinder head bolts 34. It can be seen from Fig. 4 that each of the two sleeves 40 having a strain gauge strip 44 thereon is disposed between two adjacent cylinders whereby each of the sleeves 40 is subjected to additional stresses due to the pressure of combustion in each of the two adjacent cylinders. The signals from the strain gauges are combined to produce an output signal comparable with that shown at A in Fig. 2 which can then be processed as desired with reference to Fig. 2. For a six-cylinder engine, three strain gauge strips would be needed, since one strain gauge strip must be disposed adjacent each cylinder.
In principal, with the use of quartz crystal sensors as described above, there must be one such sensor adjacent each cylinder so that sensors could be placed in position close to those shown in Fig. 4 for the strain gauge strips. The signals from the sensors would have to be processed by filtering and sampling with respect to time (time window) and then combined to form a composite output signal comparable with that at A in Fig. 2.
The method and apparatus of the invention enable a misfire at one or more cylinders to be detected immediately and the misfiring cylinder or cylinders toQ be identified so that the fuel supply to the faulty cylinder or cylinders can be stopped without delay. This is achieved by monitoring the combustion by means of suitable detectors which detect movement or combustion individual to the cylinders.