GB2194586A - Fuel-injected i c engine exhaust gas recirculation control - Google Patents

Abstract

In a mechanically governed fuel-injected internal combustion engine (most probably a diesel engine), to vary the amount of EGR (exhaust gas recirculation) flow at different throttle positions, an air throttle valve (28) is located in the engine air inlet (12). The valve position is related to the setting of the injection pump (22) through a mechanical linkage and in a non linear manner, and another valve (18) controlling the flow in the recirculation passage (16) is moved in an opening or closing direction as a result of signals representing the pressure drop across the throttle valve.

Description

SPECIFICATION Exhaust gas recirculation This invention relates to exhaust gas recirculation, and in particular to an internal combustion engine with fuel injection in which a proportion of the exhaust gas is recirculated to the engine intake.

The invention is most suitable for diesel engines, although it could be arranged for use on a petrol engine.

The purpose of exhaust gas recirculation (which is a well-known technique and is also referred to in this specification as EGR) is to reduce undesirable engine exhaust emissions, in particular the nitrogen oxides (referred to as NOx) emissions. By recirculating a proportion of the exhaust gas, a substantial reduction in NOx emissions is possible.

Some exhaust gas recirculation systems are known where the recirculation passage from the exhaust to the engine intake has only two positions; fully open and fully closed. This is not satisfactory because the proportion of recirculated gas needs to be variable over a wide range in order to obtain the maximum benefits in reduction of undesirable emissions.

There are two parameters which are important in that they have an effect on the exhaust emissions at any particular moment from a particular engine. These parameters are engine speed and engine load. For each position of the accelerator pedal there is a repeatable relationship between load and speed. In practical embodiments, the accelerator pedal will be connected to an engine control device which may differ from engine to engine. In this specification, the parameter corresponding to the accelerator pedal position will therefore be referred to as the "input lever".

According to the invention, there is provided an internal combustion engine which incorporates an exhaust gas recirculation system and which has an injection system for supplying fuel to the engine, the engine having an air intake passage, an exhaust gas passage, a recirculation passage leading from the exhaust passage to the intake passage and means for controlling flow through the recirculation passage, wherein the setting of the flow controlling means is determined in accordance with the pressure drop across an air throttle valve located in the air intake passage upstream of the position where the recirculation passage enters the intake passage, and wherein the control for the fuel injection system is indirectly connected to the air throttle valve through a cam which makes the air throttle valve setting follow the fuel injection system setting in a non-linear manner.

EGR produces a substantial reduction of the NOx emissions of a diesel engine and a noticeable, but less marked reduction in HC emissions. There is however an associated tendency for the smoke levels to increase with increasing EGR. Emission standards, which have to be met by all vehicle manufacturers, set maximum levels for each of these parameters. In the absence of any EGR, the highest smoke levels are at maximum engine load. Any use or increase of EGR to reduce NOx must be controlled to ensure that it does not result in an impermissibly high smoke level. The highest EGR rate will occur at low loads.

The cam shape will need to be determined for each engine/fuel pump combination in order to obtain the best possible characteristics in each case.

The system described is most suitable for application to a diesel engine.

The air throttle valve can conveniently be a butterfly valve. It is necessary to accurately determine the air flow rate at idle, and the cam can be designed so that the butterfly plate does not completely close the air flow passage at idle. Alternatively, the butterfly plate could have a hole through it of a predetermined size, or a flat on one side, to accurately determine the air flow rate at idle.

Pitot tubes either side of the butterfly valve can be used to sense the pressure drop across the butterfly and to provide a signal to an EGR valve which controls flow through the recirculation passage, to position the valve.

The valve may be controlled by a diaphragm which moves in response to the pressure difference across the butterfly valve.

A pressure amplifier could be used to amplify the pressure differential in order to ensure proper operation of the EGR valve.

It would be possible to design an electronically controlled system where engine load, engine speed and input lever position could be mapped on a three-dimensional model to determine an optimum EGR valve setting for any engine state. Such controls are complex and expensive. The system provided by the present invention enables a result only slightly inferior to the ideal solution to be obtained, at a very much lesser cost and with components that are easy to adjust and service.

The invention will now be further descibed, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing of an EGR system according to the invention; Figure 2 is another schematic view of the system of Figure 1, showing the hardware in more detail; Figure 3 is an installation drawing of the system; and Figure 4 is a graph where the axes represent engine load and engine speed and where the characteristic gradients are shown for various input lever positions.

Figure 1 shows a fuel-injected engine 10 with an air intake pipe 12 and an exhaust pipe 14. A recirculation passage 16 leads between the pipes 12 and 14 and contains a valve 18 which controls flow through the passage. Air is introduced into the pipe 12 through an air filter 20. Fuel is supplied to the engine by a fuel pump 22, and an input lever 24 on the pump is connected to the accelerator pedal through a link 26.

Experimental tests show that the degree of exhaust gas recirculation can be related to the movement of the input lever, but that this is not a linear relationship. The actual relation ship can be determined experimentally from a particular engine system.

In order to relate the positon of the EGR valve 18 to the position of the input lever 24, a variable restriction in the form of a butterfly flap 28 is placed in the air intake pipe 12. A pressure drop will occur across the restriction, and the magnitude of this drop will be sensed as the pressure in a pipe 32 downstream of the restriction. This pressure is applied to one side of a piston 34 in a pressure sensitive control unit 35 which is connected to the recirculation passage valve 18, so that the valve is moved in the opening direction by the pressure, against the biassing force of a light spring 36, and is moved in-the closing direction by the spring. As an alternative to a piston, a diaphragm could be used.An additional pipe 30 upstream of the flap 28 and downstream of the air filter 20 is provided so as to compensate for a change in the absolute pressure level as the air filter becomes partially blocked during service.

The setting of the flap 28 thus determines the position of the valve 18 at a particular flow rate. The flap itself is set by the input lever through a cam 38, the flap having a cam follower 40 which follows the cam shape as the cam itself is moved linearly by a link 42 connected to the input lever 24.

Figure 2 relates this hardware more nearly to an actual engine. The air intake 12 leads into an iniet manifold 44. The recirculation passage 16 leads from an exhaust manifold 46 to the intake manifold and contains the pressure operated valve 18. The exhaust pipe 14 leads off from the manifold 46.

A shaped cam track 48 is secured on the axis of rotation of the flap 28. A lever 50 is pivoted at 52 to a fixed point on the engine, carries a cam follower 54 at one end and is attached to the inner cable of a Bowden cable 56 at the other end. The Bowden cable 56 is connected to the input lever 24 so that as the driver's foot 58 presses down on the accelerator, the input lever is moved, firstly to operate the pump 22 but also to pivot the lever 50 and to move the flap 28.

Figure 3 shows arl actual air intake pipe 12 with a butterfly flap 28 controlled by a cam track 48, a typical detail shape of which can now be seen, and a lever 50 pivoted at point 52 and with a cam follower 54 which runs in the track. A cable 56 operates the lever. The pressure sensitive control unit is connected to the intake pipe 12 on the downstream side of the flap 28 as before. A pressure amplifier 37 of a conventional type is also provided to boost the pressure appearing at the downstream side of the flap before the pressure is used to operate the control 35.

The shape of the cam track 48 can be determined by plotting points obtained experimentally during test operation of the engine to which the cam is to be fitted. This operation will now be described in more detail.

Figure 4 shows a typical engine characteristic curve were engine speed is plotted along the x-axis and engine load along the y-axis.

The engine can operate at any point below the full load line 60. In the absence of any EGR, maximum smoke will occur at the full load line, and as already mentioned, it is required that this smoke level should not exceed certain specified emission limits.

The engine is then run on a dynamometer with no EGR and with a throttle position approximately corresponding to the lowest speed at which the full load line 60 can be reached (this is likely to be around 1000 rpm, and 1000 rpm may be a convenient point to choose). Full load is applied so that the engine operates at the point 62. The throttle or input lever is then fixed, and the applied load is reduced in steps. The load reduction leads to an increase of speed, and a curve such as that at 64 is generated.

Next these steps are repeated, but at a point 66 which represents say a 20% reduction below full load, the EGR valve is gradually opened (ie the butterfly valve is gradually closed) while monitoring the engine smoke characteristic. The 20% reduction is suggested because this is an informed guess at the point at which the smoke reading will peak when EGR is introduced. Without EGR the smoke characteristic at this point would be substantially below the maximum smoke level. The EGR valve is opened as far as possible, until the smoke level rises to the specified maximum emission limit. The setting of the EGR valve 18 is then fixed, and smoke readings are taken at various positions along the curve 64 using a conventional smoke measuring instrument. The object is to ensure that at no point along the curve does the smoke reading exceed the permitted value. If a peak reading is found at a point other than at the (20% reduction) first chosen, then the process should be repeated after resetting of the EGR valve (ie closing it a bit) to give a maximum allowable smoke reading at the newly determined peak point.

A particular setting of the EGR valve does not result in constant EGR flows along the curve 64. The actual amount of EGR flowing will also depend on the pressures in the inlet and exhaust manifolds which drive the EGR flow. EGR will have a negligible effect on smoke at maximum load because the settings ensure that the intake manifold depression will always be less than the threshold setting of the EGR valve when at this condition.

Once an optimum setting for the EGR valve at a certain input lever position has been determined, the relationship between the lever position and the valve position is noted. The input lever, or throttle, is then moved to a different setting and another set of readings is obtained, and another EGR valve setting determined relative to a corresponding input lever position. This process is repeated until a comprehensive range of input lever positions have been mapped. The shape of the cam track 48 can then be determined. The track is unlikely to be a regular shape. If it is very irregular, it may be necessary to compromise the calculated shape to obtain a cam shape which can be followed in use by follower 54.

Claims (16)

1. An internal combustion engine which incorporates an exhaust gas recirculation system and which has an injection system for supplying fuel to the engine, the engine having an air intake passage, an exhaust gas passage, a recirculation passage leading from the exhaust passage to the intake passage and means for controlling flow through the recirculation passage, wherein the setting of the flow controlling means is determined in accordance with the pressure drop across an air throttle valve located in the air intake passage upstream of the position where the recirculation passage enters the intake passage, and wherein the control for the fuel injection system is indirectly connected to the air throttle valve through a cam which makes the air throttle valve setting follow the fuel injection system setting in a non-linear manner.

2. An engine as claimed in Claim 1, wherein the cam shape is separately determined for each engine/fuel pump combination in order to obtain the best possible characteristics in each case.

3. An engine as claimed in Claim 1 or Claim 2, wherein the air throttle valve is a butterfly valve.

4. An engine as claimed in Claim 3, wherein the cam is designed so that the butterfly plate does not completely close the air flow passage at idle.

5. An engine as claimed in Claim 3, wherein the butterfly plate has a hole through it of a predetermined size to accurately determine the air flow rate at idle.

6. An engine as claimed in any preceding claim, wherein a pitot tube downstream of the air throttle valve is used to sense the pressure drop across the butterfly and to provide a signal to an EGR valve which controls flow through the recirculation passage, to position the valve.

7. An engine as claimed in Claim 6, wherein the pressure sensed by the pitot tube is applied to one side of a piston moving in a cylinder, the piston being biassed in one direction by pitot tube pressure and in the other direction by a spring.

8. An engine as claimed in Claim 6, wherein the pressure sensed by the pitot tube is applied to one side of a diaphragm fitted in a chamber, the diaphragm being biassed in one direction by pitot tube pressure and in the other direction by a spring.

9. An engine as claimed in Claim 7 or Claim 8, wherein the pitot tube pressure acts in the direction of opening the flow controlling means in the exhaust gas recirculation passage and the spring acts in the closing direction.

10. An engine as claimed in any one of Claims 7,8 or 9, wherein the piston or the diaphragm, as the case may be, is mechanically linked to the flow controlling means in the recirculation passage.

11. An engine as claimed in Claim 10, wherein the flow controlling means is a valve with a linearly movable valve member, and the valve member is connected to the piston or the diaphragm, as the case may be.

12. An engine as claimed in Claim 11, wherein the valve is a poppet valve.

13. An engine as claimed in any one of Claims 6 to 12, wherein a second pitot tube is provided upstream of the air throttle valve and downstream of an air filter positioned in the air intake passage, the pressure sensed by the second pitot tube being applied in opposition to that of the first-mentioned pitot tube.

14. An engine as claimed in any preceding claim, wherein a pressure amplifier is provided to amplify the pressures sensed in order to ensure proper operation of the flow controlling means in the recirculation passage.

15. A diesel engine having the features claimed in any preceding claim.

16. An internal combustion engine substantially as herein described with reference to the accompanying drawings.