This invention relates to exhaust gas recirculation, and in particular to a mechanically governed diesel engine in which a proportion of the exhaust gas is recirculated to the engine intake.

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.

In the past, diesel engine fuel injection pumps have commonly been governed by air governors. A throttle valve is placed in the air intake pipe, and the angular position of the valve is determined exclusively by the position of the driver's foot oh an accelerator pedal. A Pitot tube passes through the wall of the air intake pipe adjacent to the throttle valve, and a vacuum signal is produced in this pitot tube, the magnitude of which depends on the throttle position and engine speed. The vacuum signal is then used to control the fuel injection pump.

In a mechanically governed system however, the governing of the pump is done by springs and weights in the fuel pump. There is no requirement for a vacuum signal from the air intake pipe, and in conventional mechanically governed systems, no throttle is fitted in the air intake pipe.

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 entirely satisfactory because the

/01685 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 the rest of 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 a driver operated input lever connected directly to a mechanically governed fuel pump, 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 an air throttle valve is located in the air intake passage upstream of the position where the recirculation passage enters the intake passage and ^* the setting of the flow controlling means is determined in accordance with the pressure drop resulting from the air throttle, and wherein the driver operated input lever is 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 and particulate 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, inter alia, 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/injector combination in order to obtain the best possible characteristics in each case.

The invention therefore also provides a method of setting up a mechanically governed diesel engine which has a driver operated input lever connected directly to the fuel pump, an air intake passage, an exhaust gas passage, a recirculation passage leading from the exhaust passage to the intake passage, means for controlling flow through the recirculation passage, and an air throttle valve located in the air intake passage upstream of the position where the recirculation passage enters the intake passage, the setting of the flow controlling means being determined in accordance with the pressure drop resulting from the air throttle, characterised in that optimum settings of the flow controlling means are determined for a range of different input lever positions, and a cam is designed from data thus obtained, the cam being used to relate the position of the input lever over a range of positions to the air throttle valve setting and thus to the flow controlling means.

The cam may comprise a plate mounted on the axis of the butterfly valve with a non-linear cam track formed in the plate and having parallel side walls, and the fuel pump control then has a follower which runs in the track so that the follower can be moved.

One or both ends of the track may be formed in an auxiliary plate articulated to the main cam plate, so that the track shape can be adjusted to conform to the follower position at the end of the follower travel and the auxiliary plate can then be locked in position.

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.

Pressure tappings 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 speed and input lever position could be mapped on a 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 described, 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;

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; and

Figures 5 and 6 are respectively diagrammatic and assembled views of a cam in accordance with the invention.

Figure 1 shows a fuel-injected diesel 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 which has internal mechanical governing, 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 relationship can be determined experimentally from a particular engine system.

In order to relate the position 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 biasing 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 ^' with a * closed cam track 39, 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 inlet 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. As an alternative to a Bowden cable, a rod could be used, and this rod could be of adjustable

length so that the system can be accurately set up. 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 an 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. It will be seen both from this Figure and from Figures 5 and 6 that the cam track 39 is not linear, neither is it a single smooth curve. It is necessary for the track to be a closed track, ie to have two opposite parallel side walls 39a and 39b to ensure that the follower 40 does actually faithfully follow the designed track shape. A cable 56 operates the lever 50. The pressure sensitive control unit is connected to the intake pipe 12 on the downstream side ,qf 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 plot- ting points obtained experimentally during test operation of the engine to which the cam is to be fitted, or by making use of computer-generated operation simulations. This operation will now be described in more detail.

Figure 4 shows a typical engine characteristic curve where 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 rp , 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 while monitoring the engine smoke characteristic. In practical terms, the EGR valve position can only be altered by altering the butterfly valve position. 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. This represents a first supposition as to the correct setting of the EGR valve 18, 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. Finally, the point 62 must be revisited to ensure that no EGR flows at this point. The position of the butterfly valve corresponding to the thus determined setting of the EGR valve is then noted and this fixes one point on the cam.

A particular setting of the butterfly 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. There will be no EGR 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 butterfly 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 butterfly 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.

Because all mechanically governed fuel injection pumps have minor variations which may arise, for example, from very small variations in spring rate between the springs which operate the governor mechanism and from other cumulative tolerances in the pump mechanism, it is desirable to have some means of fine tuning each individual pump/engine/injector assembly before sale.

This is achieved by using a cam plate as shown in Figures 5 and 6. In this cam plate 70, a track 39 is provided as already described. At one end however (preferably the low speed/low load end) the track 39 is widened. An auxiliary cam 72 (Figure 6) is fitted over this widened part of the track and is pivotable about a centre 74. In the Figures,

this is achieved by forming a part-circular periphery 76

- on part of the auxiliary cam plate and providing a guide for this periphery in four guide pegs 78. The guide positions could be formed by punching out of the material of the cam plate 70, or by other means. A position securing screw 80 is present to lock the auxiliary and main cam plates permanently together once the correct final position of the auxiliary plate has been found, and once this has been done, the plate will act as a single, unitary cam plate with a single fixed cam track.

In use, the cam will be fitted to the appropriate pump/engine/injector combination with the auxiliary plate free to pivot about the centre 74. On hot test of the engine, the input lever will be set to idle and the screw 80 will be finally tightened to lock together the two cam plates. It is intended that this should be a one-time operation for the life of the engine and thus instead of a screw, a shear pin or rivet could secure the two plates together.