GB2362842A

GB2362842A - Internal combustion engine fuel control system

Info

Publication number: GB2362842A
Application number: GB0110558A
Authority: GB
Inventors: Ilya V Kolmanovsky; Jessy W Grizzle; Jing Sun
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2000-05-11
Filing date: 2001-04-30
Publication date: 2001-12-05
Also published as: GB0110558D0; DE10122061A1

Abstract

A fuel control system for an internal combustion engine in which a three-way catalytic converter and a lean NO<SB>x</SB> trap are located in series on the exhaust side of the engine, the engine including a fuel vapour storage canister located between the engine intake manifold and the engine fuel tank. A fuel injector distributes fuel directly to the combustion chamber, and a throttle-controlled air intake communicates with the engine intake manifold whereby stratified combustion of the air/fuel mixture in the intake manifold occurs. Air/fuel vapour delivered from the canister to the intake manifold results in the development of excess hydrocarbons in the exhaust gases when needed to raise the temperature in the lean NO<SB>x</SB> trap, which results in improved operating efficiency of the lean NO<SB>x</SB> trap.

Description

2362842 1 INTERNAL COMBUSTION ENGINE FUEL SYSTEM AND METHOD FOR

COORDINATING EXHAUST GAS AFTER-TREATMENT TEMPERATURE CONTROL AND VAPOUR PURGE MANAGEMENT The invention relates to internal combustion engines and to engine exhaust gas temperature control to optimise the performance of an exhaust emission control system.

Internal combustion engines for contemporary automobile vehicle drivelines typically use three-way catalytic converters in the exhaust gas flow circuit to reduce undesirable exhaust gas emissions; i.e., carbon monoxide, unburned hydrocarbons and oxides of nitrogen. To supplement the function of the three-way catalytic converter, an additional three-way catalytic converter, commonly referred to as a lean NOx trap (LNT), typically is included in the exhaust gas circuit on the flow outlet side of the catalytic converter for engines with lean burn capability.

The lean NOx trap operates at its best efficiency when the exhaust gas temperature is within a predetermined range.

In a typical automobile engine, the desired temperature window for best LNT efficiency would be in the appropriate range of 2500C to 4500C. If the engine operates with an air/fuel mixture leaner than stoichiometric, exhaust gas temperature decreases. This would result in a reduction in the efficiency of the lean NOx trap in removing undesirable oxides of nitrogen from the exhaust gases.

In copending pending patent application Serial No.

09/525,803, filed March 15, 2000, entitled "FUEL INJECTION METHOD FOR AN INTERNAL COMBUSTION ENGINE", we have described a solution to the problem of removing oxides of nitrogen from exhaust gases while maintaining a relatively high engine operating efficiency. That copending patent application is assigned to the assignee of the present invention.

Unlike some known engine exhaust control systems in which fuel is injected directly into the combustion chamber during the exhaust stroke of the combustion cycle, the 1 2 system described in our copending application describes a method that includes injecting fuel directly into the combustion chamber during the intake stroke. That injection is followed by a second injection of an appropriate amount of fuel during the compression stroke of the combustion cycle. When operating in the stratified mode, the fuel injected during the intake stroke mixes with air inducted from an air intake manifold to create a relatively rich air/fuel mixture near the ignition point. The air/fuel ratio becomes progressively more lean as the stratified air/ fuel charge travels through the combustion chamber. At regions of the combustion chamber remote from the ignition point, the mixture is too lean to support combustion.

In the system of our copending application, the lean portion of the stratified charge that does not burn is expelled through the exhaust valve. This portion of the charge contains part of the fuel injected during the intake stroke. This adds unburned hydrocarbons to the exhaust gases so that sufficient oxidation or burning of the hydrocarbons may take place in the catalytic converter to increase the temperature in the lean NOx trap.

Our copending application describes also an alternative method for increasing unburned hydrocarbons in the exhaust gas flow circuit. This involves advancing the fuel injection timing slightly (i.e., about 10'). This allows additional time for the fuel and air to become mixed, which results in a richer air/fuel ratio at outer regions of the combustion zone, although the air/fuel ratio in that zone is too lean to burn. Excess hydrocarbons in the combustion chamber then are discharged to the exhaust gas flow circuit, thereby maintaining a higher operating temperature of the exhaust gases in the three-way catalytic converter and raising the temperature in the lean Nox trap where exhaust gas is oxidised further in the presence of a catalyst to reduce NOx.

3 - The present invention provides an alternative system and method for reducing exhaust emissions as the temperature of the exhaust gases is raised by adding unburned hydrocarbons to the engine exhaust gas flow circuit. Unlike the doubleinjection of fuel associated with the system of the copending application, fuel vapours are added to the intake manifold of the engine system of the present invention by purging a fuel vapour storage canister during stratified engine operation. The fuel vapours provide unburned hydrocarbons or reductants to generate exothermal activities in the oxygen-rich environment of the catalytic converter to increase the feed gas temperature for the lean NOx trap.

Rather than over-mixing fuel during a double injection technique advancing injection timing, as described in the copending application, excess unburned hydrocarbons are generated by the system of the present invention by activating a canister fuel vapour purge. The purged fuel vapour then is mixed with air in the intake manifold. The fuel vapour and intake air then are mixed with the fuel injected during the compression stroke to form a stratified charge.

A large portion of the fuel vapour, under normal stratified operating conditions, is in a region of the combustion chamber remote from the ignition point. It will not support combustion because it is too lean. The hydrocarbons in the vapour will be exhausted from the combustion chamber with the combustion gases and will be oxidised in the presence of the oxidation agents in the catalytic converter to raise the feed gas temperature of the LNT.

By practising the present invention, fuel vapour from the canister is used rather than liquid fuel from the tank as reductants are added to the exhaust gas flow circuit. The invention furthermore achieves a canister purge during stratified operation without having adverse emission consequences.

- 4 Unlike the systems that use injection timing or spark timing to develop excess hydrocarbons, the use of fuel vapour from the canister fuel vapour source does not cause undesirable torque variations. There is no need, therefore, for the engine controller to be burdened with the task of compensating for torque changes as the exhaust gas temperature is controlled.

In practising the invention, the fuel is injected in timed fashion into the combustion chamber during the compression stroke. A fuel storage tank is connected to the fuel injector through a fuel line. The fuel vapour canister communicates through a vapour purge line with the fuel tank and with the intake manifold.

The catalytic converter in the exhaust gas manifold system converts pollutants in the exhaust gases, including oxides of nitrogen, to develop nitrogen, carbon dioxide and water vapour. A vapour purge valve in the vapour purge line controls delivery of fuel vapours from the canister to the air intake manifold to create a non-combustible air/fuel charge, including hydrocarbons in the fuel vapours, in the combustion chamber. The non-combustible charge is delivered to the catalytic converter, thereby increasing the effective exhaust gas temperature as unburned hydrocarbons in the exhaust gas are oxidised.

The canister is purged, using the improved exhaust gas temperature control method of the invention, by reason of a pressure differential between the intake manifold and the canister. It is rare that the manifold pressure will be too high to create a pressure differential. If the engine, for some reason, operates for a substantial part of the total operating time with a manifold pressure at which the pressure differential is not great enough to effect purging, the method of the copending application could be used to complement the control of exhaust gas temperature using the present inventiort.

is Figure 1 is a schematic block diagram of an automotive vehicle engine and exhaust system capable of embodying the present invention; Figure 2 is a schematic cross-sectional representation of a cylinder head and a combustion chamber for a stratified charge internal combustion engine; Figure 3 is a plot showing the relationship between air/fuel ratio and the efficiency of a three-way catalyst in removing hydrocarbons, carbon monoxide and NOx from the lo exhaust gases of an internal combustion engine; Figure 4 is a plot showing lean NOx trap efficiency for a range of lean NOx trap temperatures; Figure 5 is a plot showing the engine exhaust gas temperature variation as the air/fuel ratio changes from rich to lean; and Figure 6 is a plot showing the effect of hydrocarbon content in the engine exhaust gases as the injection timing is advanced an additional 100 before top dead centre.

In Figure 1, the numeral 10 designates four cylinders of an internal combustion engine. Each cylinder delivers exhaust gas to an exhaust manifold 12 through exhaust runners 14.

At the exhaust gas flow output side of the manifold 12 is located a threeway catalytic converter 16, which may be of well-known design. The catalytic converter feed gas enters the converter through flow path 18 and is discharged through flow path 20, where it enters lean NOx trap 22. The exhaust gas flow output side of the lean NOx trap (LNT) communicates with the vehicle tailpipe through exhaust flow path 24.

The air intake for the engine cylinders is shown at 26. A throttle body 28, located in the air intake flow path, includes an adjustable throttle valve 30. The air inducted into the engine system enters intake manifold 32, where it is distributed to the combustion chambers of the engine cylinders through intake runners 34. The air is distributed to the combustion chambers through intake valves in known 6 - fashion. Exhaust valves in the cylinders distribute exhaust gases to the exhaust manifold 12 in known fashion.

A fuel injector (one of four) is indicated schematically at 36. It is under the control of an electronic engine controller 38. An output driver circuit of the controller 38 supplies an injector signal for the injector 36, as indicated at 40. An ignitor or spark plug 42 is provided for each cylinder. It is supplied with an ignitor output from the controller 38.

The controller 38 also provides a purge valve signal, as shown at 44. That signal is distributed through a signal flow path 46 to a vapour purge valve, schematically shown at 48. The purge valve controls the delivery of fuel vapours from a fuel vapour storage canister 50. The fuel tank 52 communicates with the canister 50, as shown. It communicates also with the fuel injector 36 through fuel line 54.

Vapour purge valve 48 may be a valve having a variable setting, such as the throttle valve 30, so it is effective to continuously control the flow of fuel vapour from the canister 50 to the intake manifold 32. It is under the control of the processor 38, which responds to powertrain variables developed by various powertrain sensors, as shown at 56. These sensors include a mass air flow sensor (MAF), a tank pressure sensor (TANK PRESS), an intake air temperature sensor (IAT), a manifold vacuum pressure sensor (MANVAC), a heated exhaust gas oxygen sensor (HEGO), a vehicle speed sensor (VS), a throttle position sensor (TP), a fuel level sensor (FUEL LEVEL), an engine coolant temperature sensor (ECT), an engine load sensor (LOAD), and an engine speed sensor (ENGSP). A soak timer (SOAK TIMER) for the controller 38 measures the time the fuel vapours are being collected and how long the canister has been purging.

The powertrain inputs are received by an input conditioning circuit 58 of the processor 38. The processor, which may be of a conventional digital type commonly used in the automotive industry, would include a central processor 7 unit (CPU). The CPU fetches and acts upon information and data in ROM memory where control algorithms are stored. Input data from the sensors is received in the RAM memory and accessed by the CPU.

Figure 2 shows the combustion chamber of one of the engine cylinders. The combustion chamber is defined by reciprocating piston 60 and the walls of cylinder 62. A cam-operated intake valve 64 controls communication between the combustion chamber and air intake passage 66, which communicates with the throttle body 28. An exhaust valve 68 controls communication between the combustion chamber and exhaust port 70, which communicates with exhaust manifold 12. The ignition point is the igniter or spark plug, shown at 72.

is During operation of the engine, a stratified charge will be developed in the combustion chamber, including the combustible region 74. This charge is developed as fuel is injected by injector into the combustion chamber during the compression stroke. At the instant of the fuel injection by the injector, the combustion chamber is filled with air and with the fuel vapour purged from the canister 50.

The fuel vapours developed by the canister comprise a lean homogeneous mixture, which is shown in Figure 2 at 76. The stratified charge, shown at 74 becomes progressively more lean as the burning begins at the ignition point 72 and progresses toward the unburnable lean mixture at 76. The outer extremity of the stratified charge is too lean to burn.

In a conventional combustion cycle, the space occupied by the mix of air and vapour at 76 would be filled with air, assuming that the injection timing is normal (i.e., the timing desired for.maximum engine operating efficiency), as indicated by the dotted line in the plot of Figure 6, which would be the timing desired for maximum engine operating efficiency. Unlike the injection timing modification described in the copending patent application, which would require an injection timing characteristic as shown by the 8 - full line in Figure 6, unburned hydrocarbons are added to the exhaust gases in the exhaust port 70 using the design of the present invention without departing from the desired injection timing shown by the dotted line in Figure 6.

Figure 3 shows a plot of the efficiency of conversion of hydrocarbons, carbon monoxide and NOx during operation of the engine at various air/fuel ratios. The plot of Figure 5 shows the relationship between the exhaust gas temperature and air/fuel ratio. It is indicated in Figure 5 that the exhaust temperature during operation with lean air/fuel ratios is much lower than the temperature of the exhaust gases when the air/ fuel ratio is stoichiometric or close to stoichiometric. A lean air/fuel ratio, as demonstrated in Figure 3, would reduce the conversion efficiency for NOx.

It is indicated in Figure 4 that maximum efficiency of the lean NOx trap is at a maximum when the LNT gas temperatures are within the desired efficiency window (e.g., 2500C to 450OC). By practising the teachings of the present invention, the amount of unburned hydrocarbons can be increased appreciably because of the presence of the fuel vapours purged from the canister, thereby maintaining the LNT gas temperature within the desired efficiency window. Those fuel vapours, together with intake air, would occupy the area shown at 76 in Figure 2. In the absence of fuel vapours from the canister, the space shown at 76 in Figure 2 would be occupied only by air inducted through the intake manifold.

Although a preferred embodiment of the invention has been disclosed, persons skilled in the art may make modifications without departing from the scope of the invention. All such modifications and improvements thereof are intended to be covered by the following claims.

1 - 9

Claims

1. A method for controlling delivery of a combustible mixture of fuel and air to a combustion chamber of a fuel 5 injected,- throttle-controlled, internal combustion engine sy stem, including an engine characterised by an intake stroke, a compression stroke, an expansion stroke and an exhaust stroke, the system including further a fuel vapour canister between a fuel storage tank and an intake manifold, the combustion chamber communicating with the intake manifold through an intake valve, the engine system having an exhaust system including an exhaust gas manifold communicating with the combustion chamber through an exhaust valve, and a catalytic converter in communication with the is exhaust gas manifold for oxidising carbon monoxide, unburned hydrocarbons and oxides of nitrogen; the method comprising the steps of: injecting fuel into the combustion chamber during the compression stroke to create a stratified air/fuel charge; igniting the air/fuel charge near the end of the compression stroke to create combustion gases during the subsequent expansion stroke; purging the fuel vapour canister and delivering the purged vapour to the intake manifold whereby unburned 2s hydrocarbon in the purged vapour are added to the air/ fuel mixture in the stratified charge; and exhausting the combustion gases through the exhaust valve into the exhaust valve system.

2. The method set forth in claim 1 including the step of oxidising unburned hydrocarbons from the fuel vapour canister in the catalytic converter whereby the effective operating temperature of the exhaust gases in the catalytic converter is maintained at an optimum level for increased 3s efficiency in reducing oxides of nitrogen.

1

3. An internal combustion, spark-ignited, fuelinjected, engine system for an automotive vehicle comprising an engine with at least one cylinder and piston assembly defining an air/fuel combustion chamber, the engine being characterised by an intake stroke, a compression stroke, an expansion stroke and an exhaust stroke, the engine having a throttle-controlled air intake manifold and an exhaust gas manifold system; a fuel injection means for delivering timed fuel lo injections to the combustion chamber during the compression stroke; a fuel storage tank connected through a fuel delivery line to the fuel injection means; a fuel vapour canister communicating through a vapour purge line with the fuel storage tank and with the air intake manifold; a catalytic converter system including a catalytic converter with oxidisation catalysts in the exhaust gas manifold system for oxidising exhaust emission gases including oxides of nitrogen; and a vapour purge valve means in the vapour purge line for controlling delivery of fuel vapours from the vapour storage canister to the air intake manifold whereby a non combustible air/fuel charge including hydrocarbons in the fuel vapours is created in the combustion chamber; the non-combustible charge being delivered to the catalytic converter system, thereby increasing the effective exhaust gas temperature of the exhaust gases as unburned hydrocarbons in the exhaust gases are oxidised.

4. The engine system set forth in claim 3 wherein the catalytic converter system comprises a lean NOx trap with reductants in communication with the catalytic converter whereby the effective operating temperature of the NOx trap is increased by the presence of unburned hydrocarbons delivered from the vapour canister to the intake manifold.

1 - 11 The engine system set forth in claim 3 or claim 4 wherein the purge valve means comprises a variable flow valve means providing variable communication between the vapour canister and the intake manifold; and electronic controller means for varying the rate of delivery of fuel vapours to the intake manifold in response to changes in engine operating variables thereby maintaining an optimum air/fuel ratio in the combustion chamber.

6. The engine system set forth in claim 3 or claim 4 wherein the purge valve means comprises a variable flow valve means providing variable communication between the vapour canister and the intake manifold; and electronic controller means for varying the rate of is delivery of fuel vapours to the intake manifold in response to changes in engine operating variables, including exhaust system temperature, thereby maintaining an optimum air/fuel ratio in the combustion chamber.