GB2296786A

GB2296786A - Lean burn ic engine

Info

Publication number: GB2296786A
Application number: GB9507171A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1994-12-30
Filing date: 1995-04-06
Publication date: 1996-07-10
Also published as: GB9426399D0; GB9507171D0

Abstract

In an ic engine which is to operate with a lean air-fuel ratio at normal loads and with a stoichiometric air-fuel ratio at high loads, a rapid, imperceptible change between the two conditions is effected by the provision of a supply of diluting air to the intake manifold 20 via a bypass pipe 30, 32 in addition to the metered quantity supplied through the main throttle 28. During such a change the output power of the engine is substantially unchanged since the quantity of fuel supplied to the engine is determined in accordance with the metered quantity of air passing the main throttle valve 28 which is itself substantially unchanged by the addition of the bypass air. The bypass valve 38 is mechanically linked to the main throttle valve 28 so as to move in synchronism therewith. EGR gases may be used to provide a dilution of up to 20%. Beyond this additional air is provided through the bypass pipe in accordance with a two way valve 34. <IMAGE>

Description

Engine Management System Field of the invention The present invention relates to an engine management system for an engine that is operated with a stoichiometric air to fuel ratio (AFR) under high load conditions and with a lean AFR under lower load conditions.

Background of the invention Because of the limitations placed by the current design of catalytic converters, it is not possible to meet emissions regulations, in particular regarding NOx, when operating the engine under high load with a lean mixture. The three-way catalytic converter that can reduce the NOx emissions, requires a stoichiometric exhaust gas composition to operate correctly. For these reasons, it has been proposed to operate an engine with a lean mixture under low load and speed conditions to improve efficiency and to switch to a stoichiometric mixture under high load and speed conditions in order to comply with emissions regulations, thereby achieving the best compromise between fuel economy and environmental pollution.

In the common practice of calibrating an engine to operate at one of two discrete AFR levels, namely at stoichiometry or lean, it is important to switch rapidly between the two levels, because during the intervening values of AFR (around 16:1), the proportion of NOx emissions is at its highest.

A problem encountered in the above calibration strategy is that such rapid AFR perturbation in the mixture flow would cause a sudden change in the fuel flow supplied to the engine. This results in a sudden change in the engine.

output power that could be disconcerting to the driver.

A moderately rapid torque increase through richer fuel calibration during acceleration of the vehicle is desirable to give the driver a favourable performance feel. This fuelling strategy is commonly known as power enrichment.

However the magnitude of this torque increase and the rate of change should be kept within acceptable limits and should only be introduced with caution.

If during acceleration of the vehicle the fuel calibration of the engine is moved from a lean region (22:1 AFR) of the calibration map through a rapid transition into a stoichiometric region (14.5:1 AFR), there will be a sudden 508 increase in the engine torque causing the vehicle to surge forward. The driver would react to this unexpected behaviour and lift the accelerator pedal. This could cause the fuel calibration to move from the stoichiometric region back into the lean region and produce an opposite effect where the engine torque would suddenly decrease and the vehicle would stall backwards.The driver would react again and press the accelerator pedal in an attempt to maintain the desired acceleration, and in doing so trigger the fuel calibration to move once again into the stoichiometric region. This sequence of events could cause a series of severe positive and negative torque excursions which is very disconcerting to the driver.

In order to make the rapid AFR transit ion imperceptible to the driver, the engine throttle must be quickly changed independently of the driver's accelerator pedal position to introduce a correction in the mixture flow at the new AFR such that the actual fuel flow supplied to the engine remains substantially unchanged.

The control of this correction procedure requires sensors, actuators and complex algorithms incorporated into what is termed as Electronic Throttle Control. Each of these components has unavoidable hysteresis and proqessing delays, and there are further difficulties caused by the fuel perturbation which disturb the equilibrium amount of fuel wetting the walls of the intake port. All these effects tend to cause other drivability problems and add to the cost and complexity of the vehicle.

ObJect of the invention The present invention seeks to provide a lean burn engine in which the AFR may be rapidly switched between lean and stoichiometry without causing a sudden change in the engine output power.

Summary of the invention According to the present invention, there is provided a spark ignition internal combustion engine for a motor vehicle having an intake system comprising an intake manifold, a main throttle for regulating the rate of flow of ambient air into the intake manifold, a bypass pipe having a flow regulating valve and arranged in parallel with the main throttle to dilute the charge reaching the engine with additional air, an air flow meter for measuring directly or indirectly the flow rate of air through the main throttle only and a fuel metering system for supplying to the engine a quantity of fuel substantially stoichiometrically related to the measured air flow through the main throttle only, the bypass pipe being sized to allow the total amount of the air in the charge reaching the engine combustion chambers to be increased owing to the additional air dilution by at least 20%, characterised in that the flow regulating valve in the bypass pipe is mechanically linked for movement in synchronism with the main throttle and in that means are provided for selectively rendering the bypass pipe operative and inoperative so as to switch rapidly between two discrete values of air to fuel ratio in the total charge reaching the engine combustion chambers.

Prior art systems using electronic throttle control to drive the bypass throttle must be busy continuously and must respond rapidly in order for the bypass throttle to follow the movement of the main throttle at all times. In the present invention, there is no intervening actuator and no risk of response delay since the flow regulator valve in the bypass pipe would always shadow the movement of the main throttle, being directly connected to the main throttle.

This has the advantage of simplicity and low cost, yet capable of providing the same flexibility in calibration and refinement in drivability as electronic throttle control.

Introducing air dilution by suddenly rendering the bypass pipe operative causes a reduction in the manifold vacuum.

While the density of the air charge is increased, there is a small decrease in flow through the main throttle because of reduced pressure difference across the throttle. This in turn causes a corresponding decrease in the amount of fuel metered to the engine and a decrease in the engine power.

Stopping the air dilution by suddenly rendering the bypass pipe inoperative causes an increase in the manifold vacuum.

While the density of the air charge is decreased, there is a small increase in flow through the main throttle because of increased pressure difference across the throttle. This in turn causes a corresponding increase in the amount of fuel metered to the engine and an increase in the engine power.

Another consequence of the change in manifold vacuum from rendering the bypass pipe operative or inoperative is that the pumping losses of the engine are also reduced or increased correspondingly. This compensates to a large extent for the change in engine power caused by the change in fuel flow described above, such that the nett change in output power is in most cases imperceptible to the driver.

As long as the above changes in output power are moderate, rendering the bypass pipe operative or inoperative during driving could be advantageous for improving the performance feel of the vehicle. If the AFR is changed from stoichiometric to lean at the same time as the driver is slowing down, the small reduction in engine power is usually not perceptible to the driver. If the AFR is changed from lean to stoichiometric at the same time as the driver is accelerating the vehicle, the small increase in engine power could sharpen the response of the vehicle as perceived by the driver.

Preferably, the means of rendering the bypass pipe operative and inoperative comprises a two-position valve connected in series with the flow regulating valve in the bypass pipe.

Preferably, an EGR pipe is merged with the bypass pipe at a point downstream of the two-position valve to supply exhaust gases into the intake manifold through the bypass pipe, the EGR having its own flow regulating valve or cut-off valve which can be switched in co-ordination with the two-position valve in the bypass pipe.

During operation of the engine at full load, no dilution is required to allow the maximum engine output power to be achieved. The resultant mixture supplied to the engine will be stoichiometric and though NOX will be present in the feedgas to the catalytic converter, a three-way catalyst can be used to reduce the level of NOX in the gases discharged to atmosphere.

At moderately high engine load, dilution with EGR gases can be used in preference to dilution with air as this results in lower feedgas emissions of NOX and does not interfere with the operation of the three-way catalyst because the overall AFR is still stoichiometric.

EGR dilution is only practicable up to a limit of around 20k whereas dilution with air allows stable engine operation at as much as 75%, this corresponding to an AFR of 25:1 as against 14.5:1 at stoichiometry. At lower engine load, therefore, increasing amount of air dilution can be used to give better engine efficiency and even lower NOX feedgas emissions. Of course, during lean operation, the three-way catalytic converter will be acting only as an oxidation catalyst and will not be able to reduce the NOX content of the feedgas. The switching between EGR and air dilution should therefore take place near the maximum limit of EGR in order to take full advantage of the NOX conversion efficiency of the three-way catalytic converter.

During the switching between EGR and air dilution, the rapid transition from, for example, 20% EGR (0W air dilution) to 50k air dilution (0k EGR) is almost instantaneous. The resultant mixture would jump rapidly from stoichiometric to 50k lean (22:1 AFR), and skip over the region of 10t lean (16:1 AFR) where the NOX concentration in the feedgas is at its highest.

The two-position valve is preferably electrically operated allowing it to be controlled directly by the electronic management system of the engine, the switching point being preferably the point where the EGR dilution rate reaches the combustion instability threshold. The EGR dilution rate is of course a known parameter in the engine management system which includes a conventional EGR regulating system.

The flow regulating valve in the present invention is preferably linked mechanically for movement in unison with the main throttle and has a similar geometry to the main throttle. This maintains a fixed area ratio of the flow control cross-sections so that the flow rate of the air dilution would match the air flow rate through the main throttle, thereby providing a constant percentage dilution and a predetermined lean AFR at any time that the bypass pipe is rendered operative.

Because the flow in the main throttle is either equal to the total air flow to the engine or is in a fixed relation to the total flow if the bypass pipe is in operation, the flow through the main throttle may be either be measured directly, or estimated indirectly by calculating the fraction of the total air flow passing through the main throttle.

While the present invention is effective in preventing any large change in output power during rapid switching of the engine AFR, there may remain, under certain switching conditions, some lesser transient effect which could be adversely perceptible. In order to soften such effect, it is possible to match the engine calibration dynamically during switching of the two-position valve in order to make the change imperceptible. One method is to adjust the ignition timing of the engine to compensate for the transient torque change. Another method is to fine-tune the fuel flow by applying a varying fuel correction factor during the switching.

The latter method may of course also be used to fine-tune the final value of the resultant AFR after switching of the two-position valve. Alternatively, it is possible to finetune the AFR by providing a small second bypass pipe in parallel with the first bypass pipe and varying the air flow through the second bypass pipe. By fine-tuning, the resultant AFR may be shifted nearer to the lean instability limit by closed-loop control using an in-cylinder combustion sensor or a flywheel acceleration sensor to provide a feedback signal to reduce the AFR when the instability threshold is reached. In another application, the resultant AFR may be adjusted to match a target value of AFR by first switching the two-position valve to achieve a step change in AFR in the desired direction, followed by fine-tuning the AFR using a signal from an exhaust oxygen sensor to measure the AFR in the exhaust system of the engine.

In the prior art systems, the complex control required to smooth out the torque excursions caused by rapid AFR changes is a consequence of operating to regulate the overall mixture flow when the mixture strength of the flow is variable. These systems failed to appreciate the advantages of this invention of providing two separate flows with their relative proportion predetermined by the mechanical linkage of the respective flow throttles, one flow being metered conventionally to a predetermined AFR and the other flow adding only to the air dilution. The two flows, when combined, determine the resultant AFR of the mixture but do not significantly affect the actual fuel flow supplied to the engine.

The outlet of the bypass pipe may be connected to the plenum of the intake manifold where the dilution air flow is uniformly mixed with the air flow from the main throttle before the combined flow is delivered to the engine.

Alternatively, the outlet of the bypass pipe may be separately connected downstream of the plenum to a plurality of outlets at the branches of the intake manifold leading to the intake ports of the engine cylinders. The intake charge flowing through each branch may in this case be stratified as it enters the combustion chamber.

If a series of incremental AFR steps is required, it is possible to provide a plurality of bypass pipes in parallel with one another and with the main throttle, each bypass pipe having its own flow regulating valve all ganged to operate in unison with the main throttle, and each having its own two-position valve for rendering each respective bypass pipe operative and inoperative. Such a system is still relatively compact and offers the advantage of digital AFR control by selecting each two-position valve individually for rapidly switching through a series of incremental AFR steps.

The invention is not to be confused with engines that have air-assisted fuel injectors with a supply of air that bypasses the throttle and sometimes the air flow meter.

Such an air supply is continuous and the flow rate, supplied by the small size of the air jets, is very small in comparison with the air flow through the main throttle during driving conditions. It is not possible in such a system for the air supplied to the injectors to have a significant effect on the AFR of the mixture supplied to the engine under anything but near idling conditions. At idle, the air supplied to the injectors is also the major source of air supplied to the engine with the main throttle practically closed. Disrupting this air supply would risk stalling the engine.

A lean NOX trap may be provided in the catalytic converter in the exhaust system of the engine to store the NOX during lean operation and to convert the NOx, that is to say to regenerate the trap, during stoichiometric operation. In order to regenerate the trap from time to time during prolonged cruising condition at lean calibration, it is necessary to introduce to the engine a stoichiometric mixture briefly at regular intervals to deliver pulses of stoichiometric exhaust gases through the trap. In the prior art systems, the rich spikes are provided by perturbing the fuel calibration and simultaneously retarding the ignition timing so that there is no perceptible change in the engine output power.In this invention, the rich spikes in the mixture may readily be introduced by briefly rendering the bypass pipe inoperative, while setting the calculated AFR to a rich value.

Brief description of the drawing The invention will now be described further, by way of example, with reference to the accompanying drawing which is a schematic diagram showing an engine intake system embodying the invention.

Detailed description of the preferred embodiment The drawing shows an engine cylinder 10 having an intake port 12, an exhaust port 14 and a spark plug 16. The air supply to the intake port 12 from the ambient atmosphere is regulated by a main throttle 28 arranged downstream of an air flow meter 22 in the air intake manifold 20. A fuel management computer 24 receives an input signal from the air flow meter 22 and controls the opening times of fuel injectors 26 to vary the amount of fuel supplied to the cylinders 10 for the combustion cycle. The amount of fuel injected is at all times related substantially stoichiometrically to the measured rate of air flow through the main throttle 28.

Gas for diluting the main charge in the intake manifold 20 can be introduced through a branch 32 that is connected to a point downstream of the main throttle 28 thus bypassing both the main throttle and the air flow meter 22. The dilution gas can be drawn either from ambient air or from the exhaust system depending upon the positions of a two-position valve 34 operated by a solenoid 35 and an EGR regulating valve 48, both valves being controlled by an electronic management system of the engine. In the position illustrated, air is drawn from the ambient through a bypass pipe 30 containing the two-position valve 34 and a second throttle 38 which is geometrically similar in design to the main throttle 28 and is mechanically linked by a spindle 36b, 36a to the main throttle 28 so that the two throttles are movable in unison.

EGR gases are drawn from the exhaust manifold through an EGR pipe 40 which merges with the bypass pipe 32, the EGR gases flowing through the regulating valve 48 and a flow meter 42.

Under full load operation, both the two-position valve 34 and the EGR valve 48 are completely closed. Thus the intake charge consists only of the air entering past the main throttle 28 and the corresponding amount of fuel set by the fuel management computer 24 to achieve a stoichiometric mixture in the combustion chamber of the cylinder 10. This undiluted charge permits the engine to operate at maximum power with a stoichiometric mixture. The NOX gases that are given off during such full load operation are neutralised within the three-way catalytic converter which functions correctly under stoichiometric conditions.

As the engine load is reduced from full load, the throttle 28 is progressively closed to reduce the air charge quantity and the regulating valve 48 is opened to increase the EGR.

The mixture reaching the catalytic converter is still stoichiometric under these conditions as dilution with EGR does not affect the AFR. The effect of the EGR is to reduce the feedgas NOX and it also improves engine efficiency by reducing air pumping losses. There is however an upper limit on the proportion of EGR gases that can be added, this corresponding to a dilution of approximately 20%. Exceeding this limit results in combustion instability.

At a given load value within the combustion stability limit of dilution by EGR gases, the two-position valve 34 is opened at the same time as the EGR valve 48 is closed. The intake charge is now diluted with ambient air drawn in through the bypass pipe 30 and the throttle 38. The effect of ganging the throttles 38 and 28, which are geometrically similar, is to set a predetermined ratio between the metered air and the bypass air so that at all positions of the main throttle 28, a fixed proportion of dilution air will be drawn in through the bypass pipe 30. The preferred ratio is 2:1, that is to say that twice as much air should be drawn in through the main intake throttle 28 as through the bypass throttle 38 giving a resultant AFR of around 22:1.

As the engine speed is reduced to idling speed, the bypass pipe may be isolated by switching the two-position valve 34 back to its closed position so that the system goes back to stoichiometry at idle.

During the switching of the two-position valve 34, there will be no major change in the quantity of fuel in the combustion charge and consequently no significant change in the engine output power. There will however be small secondary effects caused by the change in manifold pressure in the intake manifold 20. These secondary effects may cause a small decrease in power when decelerating and a small increase in power when accelerating which may be perceived as enhancements in performance feel. However if these secondary effects are perceived to be excessive, they can be attenuated by varying the ignition timing of the engine or by applying a varying correction factor to the fuel flow to compensate for the torque change during the switching.

The latter method may also be used to fine-tune the final value of the resultant AFR after switching of the twoposition valve 34. Alternatively, it is possible to finetune the AFR by providing a small second bypass pipe (not shown) in parallel with the first bypass pipe 30,32 and varying the air flow through the second bypass pipe. By fine-tuning, the resultant AFR may be shifted nearer to the lean instability limit by closed-loop control using an incylinder combustion sensor or a flywheel acceleration sensor to provide a feedback signal to reduce the AFR when the instability threshold is reached.

As a further alternative, the resultant AFR may be adjusted to match a target value of AFR by first switching the twoposition valve 34 to achieve a step change in AFR in the desired direction, followed by fine-tuning the AFR using a signal from an exhaust oxygen sensor to measure the AFR in the exhaust system of the engine.

If a series of incremental AFR steps is required, it is possible to provide a plurality of bypass pipes (not shown) in parallel with one another and with the main throttle 28, each bypass pipe having its own flow regulating valve all ganged to operate in unison with the main throttle, and each having its own two-position valve for rendering each respective bypass pipe operative and inoperative. Such a system is still relatively compact and offers the advantage of digital AFR control by selecting each two-position valve individually for rapidly switching through a series of incremental AFR steps.

The outlet of the bypass pipe 32 may be connected to the plenum of the intake manifold 20 where the dilution air flow is uniformly mixed with the air flow from the main throttle before the combined flow is delivered to the engine.

Alternatively, the outlet of the bypass pipe 32 may be separately connected downstream of the plenum to a plurality of outlets at the branches of the intake manifold leading to the intake ports 12 of the engine cylinders. The intake charge flowing through each branch may in this case be stratified as it enters the combustion chamber.

The design of the connections between the bypass pipe 32 and the branches of the intake manifold leading to the intake ports can be designed to ensure that the flow from the bypass pipe 32 enters into the combustion chambers separately from the air flowing through the main throttle 28, so as to promote charge stratification within the combustion chambers.

A lean NOX trap may be provided in the catalytic converter in the exhaust system of the engine to store the NOX during lean operation and to convert the NOx, that is to say to regenerate the trap, during stoichiometric operation.

In order to regenerate the trap from time to time during prolonged cruising condition at lean calibration, it is necessary to introduce to the engine a stoichiometric mixture briefly at regular intervals to deliver pulses of stoichiometric exhaust gases through the trap. In the present invention, rich spikes in the mixture may readily be introduced by briefly rendering the bypass pipe inoperative using the two-position valve 34, while setting the calculated AFR to a rich value.

Claims

1. A spark ignition internal combustion engine for a motor vehicle having an intake system comprising an intake manifold, a main throttle for regulating the rate of flow of ambient air into the intake manifold, a bypass pipe having a flow regulating valve and arranged in parallel with the main throttle to dilute the charge reaching the engine with additional air, an air flow meter for measuring directly or indirectly the flow rate of air through the main throttle only and a fuel metering system for supplying to the engine a quantity of fuel substantially stoichiometrically related to the measured air flow through the main throttle only, the bypass pipe being sized to allow the total amount of the air in the charge reaching the engine combustion chambers to be increased owing to the additional air dilution by at least 20%, characterised in that the flow regulating valve in the bypass pipe is mechanically linked for movement in synchronism with the main throttle and in that means are provided for selectively rendering the bypass pipe operative and inoperative so as to switch rapidly between two discrete values of air to fuel ratio in the total charge reaching the engine combustion chambers.

2. An intake system as claimed in claim 1, wherein the main throttle and the regulating valve in the bypass pipe have similar geometry and are ganged in movement in unison whereby when the bypass pipe is operative the effective flow cross-section of the bypass valve is in a fixed ratio to the effective flow cross-section of the main throttle.

3. An internal combustion engine as claimed in claim 1 or 2, wherein the air flow through the main throttle is measured directly by an air flow meter in series with only the main throttle.

4. An internal combustion engine as claimed in 2, wherein the total air flow delivered to the engine is measured and the air flow through the main throttle is measured indirectly as a calculated fraction of the total air flow.

5. An internal combustion engine as claimed in any preceding claim, wherein the means of rendering the bypass pipe operative and inoperative comprises a two-position valve connected in series with the flow regulating valve in the bypass pipe.

6. An internal combustion engine as claimed in claim 5, wherein the two-position valve is electrically operated and is controlled directly by an electronic management system of the engine.

7. An internal combustion engine as claimed in any preceding claim, wherein an exhaust gas recirculation (EGR) pipe is merged with the bypass pipe at a point downstream of the two-position valve to supply exhaust gases into the intake manifold through the bypass pipe.

8. An internal combustion engine as claimed in any preceding claim, wherein the bypass pipe is connected to a plenum of the intake manifold common to all engine cylinders.

9. An internal combustion engine as claimed in any one of claims 1 to 7, wherein the bypass pipe is connected separately to a plurality of outlets downstream of a plenum common to all engine cylinders, the outlets opening into respective branches of the intake manifold leading to the intake port of each cylinder.

10. An internal combustion engine as claim in any preceding claim, wherein the ignition timing of the engine is varied dynamically by the electronic engine management system during switching of the two-position valve in order to improve the transient drive quality of the vehicle.

11. An internal combustion engine as claim in any preceding claim, wherein a fuel correction factor is applied dynamically by the electronic management system to fine-tune the value of the resultant AFR during switching of the twoposition valve in order to improve the transient drive quality of the vehicle.

12. An internal combustion engine as claim in any one of claims 1 to 9, wherein a fuel correction factor is applied by the electronic engine management system after switching of the two-position valve to fine-tune the switched value of the resultant AFR.

13. An internal combustion engine as claim in any one of claims 1 to 9, wherein a small second bypass pipe is connected in parallel with the first bypass pipe and an electronically controlled flow regulating valve is provided in the second bypass pipe to fine-tune the switched value of the resultant AFR.

14. An internal combustion engine as claimed in claim 12 or 13, wherein the resultant AFR is fine-tuned by closed-loop control to the maximum possible AFR under any given engine operating condition without causing combustion instability.

15. An internal combustion engine as claimed in claim 12 or 13, wherein the resultant AFR is fine-tuned to match a target value of AFR by making use of a signal from an exhaust oxygen sensor to measure the AFR in the exhaust system of the engine.

16. An internal combustion engine as claimed in any preceding claim, wherein a lean NOX trap is provided in the catalytic converter in the exhaust system of the engine to store NOX gases during lean operation and to convert the NOX gases and regenerate the trap during stoichiometric operation.

17. An internal combustion engine as claimed in claim 16, wherein in order to regenerate the trap from time to time during prolonged cruising condition at lean calibration, fuel rich spikes are provided by briefly rendering the bypass pipe inoperative.

18. An internal combustion engine having an intake system comprising a multiple assembly of the intake system as claimed in claim 1 or 2, wherein a plurality of bypass pipes is provided in parallel with one another and with the main throttle, each bypass pipe having its own flow regulating valve all ganged to operate in unison with the main throttle, and each having its own two-position valve for rendering each respective bypass pipe operative and inoperative, the plurality of two-position valves being individually selectable for rapidly switching through a series of incremental AFR steps.

19. An internal combustion engine constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompany drawing.