The use of a lambda probe as measuring element in the exhaust of a combustion engine with fuel injection is known. The control signal coming from the lambda probe is used by a central electronic control unit to always provide the ignition and injection systems of the engine with an optimal amount of fuel in relation to the con¬ sumption and composition of the exhaust gases. A very considerable elimination of harmful components in the exhaust gases, i.e. hydrocarbons, carbon monoxide and nitrogen oxides, generally designated HC, CO and NO _χ respectively, is achieved with a three-way catalytic converter. This selective catalytic converter has the property of being able to break down HC, CO and NO _χ in comparatively very large quantities, up to more than 90%, on condition that the engine functions in a range with very slight variations (less than about 1%) relative to stoichiometric air-fuel ratio lambda = 1.

Unfortunately it has not been possible up until the present to apply a lambda control with comparable effect for engines not based on the use of fuel injection but on the use of a carburettor. Since large scale use is still made of such engines, for example for motorcycles, opeds, cars and tools with combustion engines, it is desirable to be able to provide such engines with a control such that the exhaust gases can comply with the emission norms already imposed or to be imposed in the future.

In respect of the above the invention provides an auxiliary device intended for addition to a combustion engine with at least one carburettor, for example a motorcycle engine, which engine comprises:

an engine block with at least one cylinder, into which a mixture of fuel and air is supplied via one or the carburettor; and a discharge pipe added to the or each cylinder for discharging combustion gases, to which discharge pipe or converging discharge pipes connects an outlet which comprises in the flow direction: an inlet pipe, a cata¬ lytic converter, a silencer and a discharge tail pipe; which auxiliary device comprises: a lambda probe for placing in the inlet; an air feed conduit for connecting to the or each discharge pipe; an air supply device with an air inlet, an air discharge connected to the or each air feed conduit and an air flow rate adjustment unit which is connected between the air inlet and the air discharge and is adjustable by means of an adjusting signal supplied via an input connection; and an electronic control unit, of which an input is connected to the lambda probe and the output is connected to the input connection of the air supply device; which control unit is adapted to control the air supply device such that the lambda value is maintained substantially at the value "l". The control takes place such that, if the air- fuel mixture is too rich, the air supply device will supply air to the discharge pipe or pipes, which form the intake zone of the exhaust. Due to this supply of extra air the value of lambda will be adjusted back to the value "1". In general the air supply device will admit more or less air into the exhaust system, depending on whether the mixture is rich or less rich. The lambda probe, which is placed in the transition zone between the discharge pipe and the catalytic converter, generates a signal corresponding therewith. The electronic control unit is the active component in the hereby created control circuit.

A comparison between the known injection engine with lambda control and an engine with an auxiliary device according to the invention shows that the known system optimizes the amount of fuel in advance, while the auxiliary device according to the invention is adapted to adjust afterwards the amount of extra air supplied to the exhaust system.

The air supply system can be based on allowing through more or less of an air flow created by suction. In such an embodiment the auxiliary device according to the invention can display the feature that the air flow rate adjustment comprises a valve with adjustable passage.

The air flow rate adjustment unit is preferably embodied such that the air flow is not turbulent or only slightly so. For this purpose the auxiliary device can have the characteristic that the valve is a butterfly valve controlled by an actuator, for example a stepping motor. In another embodiment use is made of actively generating an air flow. Such an embodiment can have the feature that a fan is arranged between the air inlet and the or each air discharge. It will be apparent that such an embodiment can be combined with a valve with adjustable passage.

In an embodiment which has the characteristic that the air flow rate adjustment unit comprises an adjustable fan, a valve with adjustable passage can be omitted. Air is effectively prevented from flowing back with an embodiment which has the feature that a non¬ return valve is arranged between the air inlet and the or each air discharge.

As is known, explosions may occur in the exhaust system when the fuel supply is deactivated. To prevent such explosions, the auxiliary device according to the invention can have the feature that the electronic con¬ trol unit comprises a second input to receive a vacuum signal coming from the engine such that the flow of the

air generated by the air supply device is set at zero when the fuel supply is deactivated.

A preferred embodiment has the feature that the electronic control unit is programmable. During the experimental phase use was made in the first instance of a conventional technique, a PID con¬ trol. However, the problem occurred here that it was very difficult, in practice impossible, to find a good balance between the response time and the stability. An overshoot occurred regularly. These problems were partly caused by diverse variable parameters relating to the extra air flow. These were found to depend on the nature and con¬ struction of the system, the nature of the engine used and the rotation speed thereof. In practice a balance was only found to occur when the engine was adjusted to a constant rotation speed for a relatively long time, a minimum of 30 seconds.

In respect of these observed problems, an embodiment is recommended in which the electronic control unit is an intelligent type.

The embodiment can particularly be such that the electronic control unit is programmable on the basis of "fuzzy logic". Such an algorithm gives a considerably improved flexibility. The algorithm defines a number of operating situations, wherein the manner in which the control must respond in these operating situations is also determined. Thus the rule can be incorporated for instance that when the average lambda value indicates that the mixture is too rich, while the current lambda value gives the same indication, the flow rate of the extra air flow must increase rapidly, for example by opening the valve to the maximum or causing an optional fan to rotate at full power in order to supplement the deficit of air. However, when the current lambda value indicates that the mixture is too poor, the amount of air is already slightly reduced, for instance by turning the butterfly valve through a pre-selected angle, so that for

example 7/8 of the total passage remains available, hereby anticipating an overshoot.

A number of rules are thus formulated, wherein the microprocessor forming part of the electronic control unit determines in each case which of the rules is appli¬ cable. This rule then determines which adjusting signal is sent to the air supply device.

The result of this adjustment is that both a dynamic and a static balance are created particularly rapidly, without instability phenomena and substantially irrespective of the selected type of engine, the rotation speed and the construction.

The auxiliary device according to the invention is preferably characterized by a housing in which the air supply device and the electronic control unit are ar¬ ranged. This embodiment has a number of advantages. There is a reduced danger of electronic malfunction because the electronic unit is arranged in an environment which is not susceptible to malfunction. There are fewer external electrical connections. The system according to the invention comprises fewer individual components, i.e. the integrated auxiliary device with electronics, the lambda sensor and a vacuum switch. Finally, it may be stated that the integrated unit has a more attractive appear- ance.

The invention will now be elucidated with reference to the annexed drawings, in which:

Figure 1 shows a graphic view of the activity of a commercially available lambda probe; Figure 2 shows a schematic view of an engine with an auxiliary device according to the invention;

Figure 3 is a partly broken away perspective view of the air flow rate adjustment unit according to figure 2; and Figure 4 is a graph showing the effect of the invention.

Figure 1 shows the conversion of NO, HC and CO dependent on lambda. The hatched area shows the effective adjustment range.

Figure 2 shows schematically the combination of a combustion engine 1 with an auxiliary device according to the invention. The engine 1 is of the type with four cylinders. These receive combustible mixture from a carburettor 3 via an inlet manifold 2. An air feed is designated schematically with the reference numeral 4, while the fuel supply is designated with the reference numeral 5. An arrow 6 indicates schematically that the user of the engine 1 can adjust the activity of the engine by means of a throttle or accelerator pedal.

A discharge pipe 7 for the combustion gases is added to each cylinder. These discharge pipes come together in the intake zone of an exhaust. This comprises an inlet pipe 8 which transports the combustion gases further to a three-way catalytic converter 9, from where the combustion gases are further transported through a silencer 10 to a discharge tail pipe 11 to be discharged. The inlet pipe 8 carries a lambda probe 12 the active part 13 of which protrudes into the interior of the inlet pipe 8 so that it can measure the oxygen ions in the combustion gases flowing past. The lambda probe generates a lambda signal via a line 14 to an electronic control unit 15. Via a line 16 this electronic control unit 15 also receives a vacuum signal which comes from a vacuum switch (not drawn) present in engine l. The unit 15 controls an electrically adjustable air supply device 17 via a line 18. The device 17 receives ambient air 19 which is filtered through an air filter 20 and, in a manner to be described below and under the control of the control unit 15, is passed to a greater or lesser degree to four air feed conduits 21 which receive air at a determined flow rate from the device 17 via air discharges 22. The pipes 21 debouch into the respective discharge pipes 7. The air flowing through pipes 21 is

added to the combustion gases which flow through these discharge pipes 7.

As figure 2 shows, the auxiliary device 13, 12, 14, 15, 18, 17, 22, 21 forms with the exhaust 7, 8, 10, 11 connected to the available engine 1 a closed control circuit. The electronic control unit is programmed such that it continuously controls the air supply device such that the lambda value measured by the lambda probe 12 is continuously maintained at the nominal value "1". In this way the three-way catalytic converter 9 can function optimally in the manner shown in figure 1 in order to break down the share of NO, HC and CO in the gases sup¬ plied to the catalytic converter 9.

The vacuum signal supplied to the control unit 15 via the line 16 is generated in the case the fuel control 6 is released by a user. This effectively prevents explo¬ sions in the exhaust system.

The electronic control unit 15 comprises a microprocessor which is programmed on the basis of fuzzy logic.

Figure 3 shows the structure of the air supply device 17. This comprises a housing 23, the wall of which drawn on the underside comprises the air filter 20. The air allowed therethrough is admitted via pipes 24 into respective non-return valves 25. These each comprise plates 28 which are mutually loaded by a draw spring 26 and pivotable around centre lines 27 and which in the position shown here co-act substantially sealingly with side elements 29. Through-flowing air can press the plates 28 aside as according to arrows 30, whereby the non-return valve 25 opens. Under the influence of the draw spring 26 the plates 28 are urged back to their rest position shown in figure 3.

The non-return valves 25 also close in the case of a pressure pulse supplied to the device 17 via the air discharges 22. This prevents the combustion gases coming from discharge pipes 7 flowing through the device 17 in the direction opposite to the nominal direction.

Between the outlet of the pipes 24 and the inlet of the non-return valves 25 an adjustment plate 33 is ar¬ ranged between two partitions 31, 32. This is supported by the output shaft 34 of a stepping motor 35 which is controlled by the electronic control unit 15 accommodated in the housing 23. This stepping motor determines the angular position of the adjustment plate 33 and thereby the effective passage of the device 17.

Throttle plates 37 adjustable by means of external operating means 36 are arranged in the pipes 24. A rough pre-adjustment of the effective passage can take place herewith.

Figure 4 shows the effectiveness of the auxiliary device according to the invention. This figure shows a graph in which the number of grammes of emission per kilometre for a particular type of motorcycle is set out vertically, while a comparison per component, CO, HC, NO _χ , HC + N0 _χ respectively is made between a standard engine without catalytic converter and auxiliary device, an engine with catalytic converter 200, an engine with catalytic converter 200 and an auxiliary device according to the invention and an engine with catalytic converter 400 and an auxiliary device according to the invention. It will be apparent from this comparison that the auxil- iary device according to the invention makes a very good contribution to cleaning combustion gases prior to emis¬ sion.

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