In a conventional spark ignition engine, the or each cylinder has one or more inlet ports usually connected to a common plenum chamber via which air is supplied to the cylinder(s). Gas is exhausted via exhaust ports from the cylinders. The inlet and exhaust ports are closed by intake and exhaust valves respectively.

The load of a spark ignition engine is generally controlled by throttling the flow of air entering the engine. In a conventional plenum chamber intake system this causes a pressure difference to exist between the intake and exhaust ports of the cylinder. As a result, exhaust gas re-enters the cylinder and flows into the intake ports during the period when both intake and exhaust valves are open (the overlap period) . Under these part load conditions, the pressure difference between inlet and exhaust ports causes dilution of the air/fuel charge with residual exhaust gases arising from the combustion process.

This effect is most pronounced at idle and increases with increasing valve overlap.

The presence of exhaust gases tends to lower the peak combustion temperature which can be desirable as a means of lowering NO _x emissions. However, excess dilution causes instability and increases hydrocarbon (HC) emissions. The optimum amount of dilution varies with engine operating conditions. Conventionally these requirements are resolved by recirculating etered quantities of exhaust gas to the cylinder. This is accomplished conventionally by means of an external pipe and valve system. A conventional system will only recycle a mass average of hydrocarbons.

In accordance with the present invention, an exhaust gas control system for connection to an inlet port of a

cylinder of a spark ignition engine comprises a plenum chamber having an air input port and an air output port connected to the cylinder inlet port via a primary throttle; and a secondary throttle to control air flow into the plenum chamber.

The invention modifies the conventional system by providing throttles both upstream and downstream of the plenum chamber. This enables much better control to be achieved of the plenum vacuum or depression. This is advantageous since the amount of exhaust gas dilution of the fuel/air charge is dependent upon the plenum chamber depression. In systems according to the invention, N0 _X emissions are reduced to a similar extent to that in the conventional recirculation systems but in addition HC emissions are reduced. This is because the recirculated exhaust gas, which would normally be the last out of the cylinder, has a much higher concentration of unburnt HC than the mass averaged concentration of exhaust gas recirculated by the conventional external recirculation system.

Other advantages of the invention are that existing exhaust gas recirculation systems can be replaced thus reducing the space required for the engine, pumping losses are reduced and hence fuel consumption is improved, there is an improved transient response, and since increased valve overlap is achievable, improved full load performance characteristics are possible.

Although in some applications both primary and secondary throttles could be manually controlled, preferably the control system further comprises control means for controlling the position of the secondary throttle automatically in accordance with a predetermined algorithm. For example, the control means may be responsive to the position of the primary throttle to control the position of the secondary throttle. The control means could also be responsive to one or a

combination of the load of the engine, the speed of the engine, and the plenum depression.

The predetermined algorithm will be devised so as to set the primary and secondary throttle positions so as to optimise performance and in particular to reduce N0 _X and HC emissions. This could be done by conventional calibration techniques.

In other examples, the secondary throttle could be manually controlled and the primary throttle(s) automatically controlled.

Preferably, the or each primary throttle comprises a barrel type valve. This has the advantage that where more than one primary throttle is required, these can be linked together by a common shaft, and can be produced and operated such that closely controlled amounts of charge are fed to each inlet.

In a preferred example, a fuel injector is positioned upstream of and adjacent to the primary throttle so as to inject fuel past the primary throttle into the cylinder. The advantage of this is that use can be made of the pressure difference which exists across the primary throttle to assist in the fuel injection, this pressure difference aiding atomisation of the fuel.

The invention is applicable for use with multiple inlets to separate cylinders and multiple inlets to one cylinder.

An example of a control system according to the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic cross-section through the control system, also showing the electronic control unit; Figure 2 is a plan of the system shown in Figure 1; and

Figure 3 illustrates the intake system of Figure 1 in more detail.

The system shown in the drawings comprises a set of three cylinders mounted in a conventional cylinder block 1,

one of the cylinders 2 being shown in Figure 1. Each cylinder 2 has an exhaust gas outlet 3 and an air inlet 4 which open into the cylinder 2 and which are selectively closed via conventional valves 5, 6. The air inlet 4 contains a primary throttle 7 positioned between the opening of the air inlet into the cylinder 2 and a plenum chamber 8. The plenum chamber 8 is common to all the air inlets 4 as shown in Figure 2 and has a single air intake 9 which contains a secondary throttle 10. As explained above, the amount of dilution of the air/fuel charge entering the cylinder is dependent inter alia on the plenum depression or vacuum. Thus, by controlling this depression by using the throttles 7, 10 it is possible to optimise the amount of dilution and hence reduce N0 _X emissions. HC emissions are also reduced since the recirculated exhaust gas has a higher percentage of HC than the mass average.

This control is achieved in the following manner. An electronic control unit (ECU) 11 is provided, such as a microprocessor, which is connected to a throttle control member 12 which controls the position of the throttle 7. Typically, the control member 12 will be manually operated by the vehicle user. A pressure sensor 13 is positioned within the plenum chamber 8, the output of the sensor being fed to the ECU 11. In addition, signals representing the load on the engine and the speed of the engine are fed to the ECU 11. The ECU 11 responds to these various inputs to generate a control signal to a throttle actuator (not shown) for actuating the secondary throttle 10 in accordance with a predetermined algorithm.

In one (open loop) example, the ECU 11 contains a table which is addressed by the four inputs and at each address stores a suitable control value for use in setting the position of the secondary throttle 10. These control values will have been set up initially by calibration so that the secondary throttle setting is at its optimum relative to the primary throttle setting.

In a closed loop example, the ECU 11 could monitor plenum pressure and adjust the secondary throttle position until a desired plenum pressure is obtained corresponding to the load, speed and primary throttle position parameters.

Although the throttles 7, 10 could take any suitable form, for example butterfly valves, the preferred construction for the throttles 7 is as barrel type valves as shown in Figure 3. This is particularly suitable since the primary throttles 7 could be linked together by a common shaft 14 (Figure 2) so that rotation of this common shaft will cause a similar actuation of all the primary throttles. Also, the barrel valves can be integrally formed to close tolerances allowing close control of air/fuel charge to each cylinder.

In the preferred arrangement shown, a fuel injector 15 (Figure 3) is positioned just upstream of each primary throttle 7. For part of the operating cycle a pressure difference will exist across the primary throttle 7 where P, > P ₂ , and this can be utilised to aid in atomisation of fuel injected into the cylinder 2.

In operation, this system will allow significantly greater valve overlap than in conventional systems while maintaining stability and achieve significant reduction in N0 _X and HC exhaust gas emissions.