APPARATUS FOR DETECTING COMBUSTION-SUPPORTING CONSTITUENTS IN EXHAUST GAS

Abstract: Apparatus for detecting combustion-supporting con­ stituents in exhaust gas comprises a heated platinum wire 16 over which exhaust gas from inlet 12 and butane from gas supply 25 are fed together. The heating effect on the wire due to any combustion of the butane in residual oxygen in the exhaust gas gives a measure of uncombined oxygen in the exhaust gas. A convergent-divergent nozzle 15 near the outlet controls the flow rate. A damping chamber 19 in association with orifice 18 damps out pressure pulses in the exhaust.

Description: Apparatus for detecting combustion-supporting constituents in exhaust gas The invention relates to apparatus for detecting combustion-supporting constituents in exhaust gas from an internal combustion engine To achieve efficient operation with such engines, the optimum air/fuel ratio depends on various factors such as load, speed, water temperature etc, and may be lean (where the fuel proportion is low) or rich (where the fuel proportion is high in comparison with a stoichiometric mixture. Various proposals have been put forward for detecting combustible or combustion-supporting constituents in the exhaust gas in order to measure the air/fuel ratio of the mixture entering the cylinders with the object of controlling the air/fuel ratio in a feed-back mode. The present invention is concerned with detecting when the air/fuel ratio is lean and in a typical installation may be used in conjunction with apparatus for detecting when the air/fuel ratio is rich but it could be used independently. United Kingdom Patent application number 2 013 892 proposes an arrangement where exhaust gas is divided into two streams, one of which is saturated with whichever of a combustible or combustion-supporting material it is deficient in. Each of the two streams imp;pges on a hot wire sensor, whose resistance changes with temperature and the surface of which acts as a catalyst. The temperature difference measured between the streams is used to estimate the quantity of combustible or combustion-supporting constituents in the exhaust gas. It has also been proposed in the Applicants UK Patent application no * 82 16444 to add the combustible or combustion-supporting material in such a way that its quantity increases gradually and then to obtain a value corresponding to the maximum heat release at the hot wire sensor. The present invention is particularly concerned with an apparatus for obtaining a measure of the combustion-supporting constituents in exhaust gas using the maximum heat release measuring technique of application number 82 16444 referred to above. According to the present invention there is provided apparatus for detecting combustion-supporting constituents in exhaust gas from an internal combustion engine comprising: a testing chamber containing an electrical conductor which acts as a catalyst, an exhaust flow path to provide flow of exhaust gas through the testing chamber, a fuel supply for intermittently supplying fuel in addition to exhaust gas into the testing chamber so that fuel is combusted at the conductor by combustionsupporting constituents in the exhaust gas and the effect of this combustion can be detected by the response of the conductor to the heating effect of the combustion, wherein a damping chamber is provided in the exhaust gas flow path, upstream of the testing chamber, to attenuate flow fluctuations through the testing chamber due to fluctuations in the pressure in the exhaust system. It has been found that flow fluctuations resulting from the intermittent opening of engine exhaust valves can interfere with accurate measurement but by providing a damping chamber and thus controlling these fluctuations to within about plus or minus 20% of the nominal mean flow rate, sufficiently accurate readings are obtainable. Preferably a flow-constricting orifice is provided upstream of the damping chamber. Preferably the outlet from the exhaust gas flow path leads to an engine inlet manifold through a convergent-divergent nozzle which tends to equalize flow through the flow path over a wide range of manifold depression. Preferably an air by-pass passage (which may be provided with a constricting orifice) is provided which conneets the outlet from the exhaust gas flow path upstream of the convergentdivergent nozzle with the atmosphere. This allows the convergent-divergent nozzle to be of a practical size which is unlikely to become blocked or constricted, without requiring a correspondingly high flow rate through the testing chamber. As an alternative a bypass passage may be provided in parallel with the part of the flow path through the testing chamber. The apparatus may incorporate heating means to discourage formation of liquid condensate in the apparatus, and also to evaporate any liquid condensate which forms, particularly in the bottom of the damping chamber. The apparatus may include flow constricting orifices in the part of the exhaust gas flow path leading from the damping chamber to the testing chamber, both upstream and downstream of the connection to the fuel supply and a flow constricting orifice in the fuel supply. This arrangement results in effect in a mixing chamber bounded by the orifices and in this mixing chamber, the proportion of fuel gradually increases when the fuel supply is turned on. This is a useful feature when using the peak value maximum heat release technique referred to above. Preferably a dilution passage incorporating a flow constricting orifice is provided, leading from the damping chamber to a part of the exhaust gas flow path between the fuel supply and the testing chamber. The mixing together of exhaust gas from the dilution passage with the mixture of fuel and exhaust gas assists mixing of the exhaust gas with the fuel and assists the gradual increase in fuel concentration in the testing chamber. Two embodiments of the invention will be described with reference to the accompanying drawings, in which Figure 1 is a diagrammatic cross-section through a first form of apparatus in accordance with the invention, and Figure 2 is a diagrammatic cross-section through a second form of apparatus in accordance with the invention. Figure 1 of the drawings shows a corrosion resistant metal body 11 having an inlet 12 from the exhaust system of an internal combustion engine and an outlet 13 leading to the inlet manifold of the internal combustion engine. A solenoid valve 14 is arranged to open the inlet 12 when the apparatus is to be used and to close the inlet when the apparatus is not in use. Exhaust gas is drawn through the apparatus by a pressure differential between the inlet and outlet, caused primarily by the inlet manifold depression. A convergent-divergent nozzle 15 controls the mean flow rate through the apparatus and holds it substantially constant over a wide range of inlet manifold depression, e.g. for absolute pressures between 0.85 and 0.3 Bar. An electrically heated platinum or platinumcoated wire 16 is provided within a testing chamber 17. The electrical supply to the hot wire is controlled in such away as to maintain it at a constant temperature, with the result that when the environment surrounding the wire is such as to generate heat by combustion, a lower current is supplied than when the whole of the heating is performed electrically. The current supplied to the wire then gives a measure of the heating effect caused by combustion. The electrical circuit for providing a constant wire temperature and deriving a signal representative of the heating effect on the wire forms no part of the present invention and will not be described. The primary exhaust gas flow path from the inlet 12 to outlet 13 through the testing chamber 17, passes through a flow constricting orifice 18, a damping chamber 19, a passage 21 incorporating flow constricting orifices 22 and 23, the testing chamber 17, an outlet passage 24, and the convergent-divergent nozzle 15. A fuel supply 25 is connected to a source of gaseous fuel, typically butane or propane, and is connected through a solenoid valve 26 and a flow constricting orifice 27 to the passage 21 between the orifices 22 and 23. A dilution passage 28 incorporating a flow constricting orifice 29 leads from the damping chamber 19 to the passage 21 at a position downstream of orifice 23 but before the testing chamber 17. Passages 21 and 28 both communicate with the damping chamber 19 near the top thereof. Passage 24 connects with passage 34 upstream of convergent-divergent nozzle 15. The passage 34 has a flow constricting orifice 35 and communicates with atmosphere. Block 11 incorporates water passages 33 through which water is circulated from the cooling system of the internal combustion engine. When the engine has reached normal operating conditions, the water in these passages serves to maintain the temperature of the apparatus at approximately 80 C. In use, solenoid valve 14 is opened to provide a supply of exhaust gas through the apparatus from inlet 12 to outlet 13. The size of orifices 22, 23 and 29, is such as to give a desired flow rate through the testing chamber 17. In this condition, with the fuel supply off, the current required in the electrically heated wire 16 to maintain it at a required temperature of for example 10000C is measured and recorded for comparison purposes. This setting and measuring is carried out automatically. Fuel is then introduced from inlet 25 by opening solenoid valve 26. The arrangement and sizes of the orifices 22, 23 and 27 is such that the fuel gas concentration in what is in effect a small reservoir between orifices 22, 23 and 27 builds up gradually over a period of perhaps 150my. The fuel gas content of the exhaust gas supplied to the testing chamber 17 builds up more slowly, due to the effect of exhaust gas in dilution passage 28 over a period of perhaps 300 ms. The fuel carried by the exhaust gases impinges on the conductor 16 and if the air/fuel of the original mixture in the internal combustion engine has been lean, the residual oxygen is used up in combusting some of the added fuel gas due to the catalytic effect of the heated platinum conductor. This combustion tends to heat the conductor so that it requires less current to maintain its constant temperature. The current reduction takes place gradually over a period corresponding to that during which the fuel supply is built up gradually. The current flowing in the conductor 16, or some other measurement of the heat dissipated in the conductor is derived from the electrical control circuit (not shown). This measurement is sampled at frequent intervals, for example every 1.44ms so that the peak value can be detected and recorded to give a measure of the proportion of free oxygen or other combustionsupporting meaterial in the exhaust gas. This in turn gives a measure of the weakness of the internal combustion engine mixture supply. Means is provided for effecting a correction to the air/fuel mixture but this is not described as it forms no part of the present invention. A suitable flow rate through the testing chamber 17 is 100 grammes per hour. A large flow rate is undesirable because there would be an associated increased volume flow of contaminants which could coat the conduc tor and cause it to lose its effectiveness as a catalyst. However, it is not convenient to produce a convergent/divergent nozzle sized to produce 50 g/hr, partly because such a small nozzle could easily become blocked. For this reason, the air bypass passage 34 is provided and its orifice 35 is such as to provide an air by-pass of approximately 450 gr/hr. Thus the nozzle 15 is sized to produce flow rate of approximately 500 g/hr. An important reason for holding the flow rate through the testing chamber to about 50 g/hr is that this keeps the butane or propane supply within acceptable limits. the intention is that a small gas canister should have a life of at least 1 year or 20,000 kilometres. Without the damping chamber 19, pressure pulses in the internal combustion engine exhaust system, associated with the firing cycle of the engine, would tend to produce surges of flow and pressure within the apparatus and these would interfere with the accurate steady operation of the system. In particular, the signal to noise ratio in the electrical measuring circuit decreases to such an extent that accurate readings cannot be obtained. The damping chamber 19, particularly when taken in association with the flow control orifice 18, tends to damp out these surges so that they are of the order of plus or minus 20% of the nominal flow rate. With variations in slow of this order, the apparatus operates effectively. A suitable volume for the damping chamber is approximately l00mls. The air by pass passage 34 to atmosphere with orifice 35 also tends to reduce pulsation effects in the testing chamber 17. Hot exhaust gases carry substantial quantities of water, both from the atmosphere and from the combustion process and this water often contains undesirable contaminants. Any tendency for this water with its contaminants to condense on the conductor 16 would tend to render it ineffective over a period of time. The heating of the apparatus by water from the internal combustion engine through passages 33 restricts this condensation. Also, any condensation tend to occur in the damping chamber 19 and the liquid of course falls to the bottom of this chamber. Small quantities of condensate which occur if the apparatus is not fully up to temperature, can be evaporated again and pass through the remainder of the apparatus in a gaseous state. It is intended that the apparatus should be brought into operation only occasionally in order to monitor the air/fuel ratio in the supply to the engine and that the engine should then be set to a correct air/fuel ratio following this measurement. Thus soleniod valve 14 is closed for most of the period while the engine is operating and the apparatus remains inoperative. When the apparatus is brought into operation, a typical cycling period is a 2.5 second period with exhaust gas only impinging on coated wire 16 (valve 14 open, valve 26 closed) followed by a 1.5 second period with exhaust gas and ruel impinging on coated wire 16 (valve 14 open, valve 26 open). This would be repeated five times and the measurements thus obtained of free oxygen (or other combustion-supporting material) in the exhaust gas averaged. The air fuel ratio is then corrected if necessary. Referring to Figure 2, in which like reference numerals have been given to like parts, the second form of apparatus differs from the first only in that the air bypass 311 is not present and a by-pass passage 31 is provided leading from damping chamber 19 to outlet passage 24 upstream of convergentdivergent nozzle 15. The by-pass passage 31 has an orifice 32 sized in relation to orifices 22, 23 and 29 so that there is a greater flow through the bypass 31 than through the testing chamber by a ratio of about tl:l. There is a small volume of the damping chamber below the outlet to passage 31 and this acts as a reservoir for water and any other liquid which may condense within the damping chamber. If there is an excess of condensation, it fills the reservoir and overflows through the bypass passage 31. The comparatively large size of orifice 32 and of the nozzle 15 through which the liquid has to flow, enables the liquid to escape without causing a blockage. A suitable flow rate through the apparatus is 500 g/hr, of which 400 g/hr would pass through the bypass 31 and 100 g/hr would pass through the testing chamber 17.

Claims: CLAIMS 1 Apparatus for detecting combustion-supporting constituents in exhaust gas from an internal combustion engine comprising: a testing chamber containing an electrical conductor which acts as a catalyst, an exhaust gas flow path to provide flow of exhaust gas through the testing chamber, a fuel supply for intermittently supplying fuel in addition to exhaust gas into the testing chamber so that fuel is combusted at the conductor by combustion-supporting constituents in the exhaust gas and the effect of this combustion can be detected by the response of the conductor to the heating effect of the combustion, wherein a damping chamber is provided in the exhaust gas flow path, upstream of the testing chamber, to attenuate flow fluctuations through the testing chamber due to fluctuations in the pressure in the exhaust system. 2 Apparatus as claimed in claim 1, including a flow constricting orifice upstream of the damping chamber. 3 Apparatus as claimed in claim 1 or claim 2, wherein the outlet from the exhaust gas flow path leads to an engine inlet manifold through a convergent-divergent nozzle which tends to equalise flow through the flow path over a wide range of manifold depression. 4 Apparatus as claimed in claim 3 wherein an air by-pass passage connects the outlet from the exhaust gas flow path upstream of the convergent-divergent nozzle with the atmosphere. 5 Apparatus as claimed in claim 4 wherein the air by-pass passage includes a constricting orifice. 6 Apparatus as claimed in claim 3 including a by-pass passage in parallel with the part of the flow path through the testing chamber to provide more flow through the convergent-divergent nozzle than through the testing chamber. 7 Apparatus as claimed in claim 6 including a constricting orifice in the by-pass passage. 8 Apparatus as claimed in any one of the preceding claims including heating means to discourage formation of liquid condensate in the apparatus. 9 Apparatus as claimed in any of the preceding claims including flow constricting orifices in the part of the exhaust gas flow path leading from the damping chamber to the testing chamber, both upstream and downstream of the connection to the fuel supply and a flow constricting orifice in the fuel supply. 10 Apparatus as claimed in any of the preceding claims including a dilution passage incorporating a flow constricting orifice and leading from the damping chamber to the part of the exhaust gas flow path between the fuel supply and the testing chamber. 11 Apparatus for detecting combustion supporting constituents in exhaust gas from an internal combustion engine, substantially as described with reference to and as illustrated in the accompanying drawings.

Inventors: PARKER, PETER HARRY
Applicants: GAYDON TECHNOLOGY LIMITED
Application year: 1984
Main classification: G01N27
IPC codes: G01N27