In direct injection spark ignition engines, the engine operates at or near wide open throttle during stratifie air/fuel operation in which the combustion chambers contain stratifie layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during its intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.

Direct injection engines are also coupled to conventional three way catalytic converters to reduce CO, HC, and NOx. When operating at air/fuel mixtures lean of stoichiometry, a NOx trap or catalyst is typically coupled downstream of the three way catalytic converter to further reduce NOx.

The inventors herein have recognised that lean air/fuel operation may cause the catalytic converter and NOx trap to operate inefficiently. The inventors herein have also recognised numerus avantages in prior approached to heating catalytic converters. For example, retarding ignition timing is inefficient because engine surfaces such as cylinder walls will be heated in addition to heating exhaust gases. Further, the amount of heat which may be generated by retarding ignition timing is limite. And, retarding ignition timing is not feasible during stratifie mode operation. The inventors also recognise that use of a variable length exhaust pipes to control NOx trap

temperature is limited i8n the range of temperature change and not desirable because of cost and packaging.

According to the present invention, there is provided a control method for a spark ignited four stroke engine having multiple combustion chambers coupled to an emission control device, comprising: injecting a first quantity of fuel directly into each of the combustion chambers to operate the combustion chambers at an average air/fuel mixture lean of stoichiometry; providing an indication of temperature of the emission control device; indicating when said emission control device temperature is below a preselected temperature; and injecting an additional second quantity of fuel directly into each of the combustion chambers during an exhaust stroke of each of the combustion chambers to provide an exotherm for heating the emission control device in response to said low temperature indication.

An avantage of the present invention is that temperature of the emission control device is maintained at a desired temperature for effective operation without creating wasted energy to heat the engine such as combustion. chamber walls. A further avantage is that temperature of the emission control device is maintained at a desired temperature for effective operation without adding additional hardware.

A control method embodying the invention is provided for a spark ignited four stroke engine having multiple combustion chambers with fuel injecte directly into each combustion chamber and at least one of the combustion chambers being coupled to an emission control device. The combustion chamber has a homogeneous mode of operation with a homogeneous mixture of air and fuel and a stratifie mode of operation with a stratifie mixture of air and fuel. The method embodying the invention comprises injecting fuel into the combustion chamber during a compression stroke to operate the combustion chamber in the stratifie mode; indicating when temperature of the emission control device is below a preselected temperature; injecting additional

fuel into the combustion chamber during an exhaust stroke while the combustion chamber is operating in the stratifie mode to provide an exotherm for heating the emission device in response to said low temperature indication; and switching from the stratifie mode to the homogeneous mode and discontinuing the additional injection of fuel when total fuel injecte into the combustion chamber exceeds a predetermined value. Preferably, the predetermined value of fuel occurs when total fuel injecte during stratifie operation, including the additional amount, is approximately equal to the fuel which would be expended during the homogeneous mode to produce substantially the same power.

Stated another way, when the stratifie mode no longer provides a fuel savings because of the additional fuel injecte to maintain temperature of the emission control device, the engine air/fuel operating mode is switched to the homogeneous mode and injection of additional fuel is discontinued thereby maximising fuel economy.

The invention will now be described, by way of example,- with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an embodiment in which the invention is used to avantage; and Figure 2 is a high level flowchart which a portion of operation of the embodiment shown in Figure 1.

Direct injection spark ignited internal combustion engine 10, comprising a plurality of combustion chambers, is controlled by electronic engine controller 12. Combustion chamber 30 of engine 10 is shown in Figure 1 including combustion chamber walls 32 with piston 36 positioned therein and connecte to crankshaft 40. In this particular example piston 30 inclues a recess or bowl (not shown) to help in forming stratifie charges of air and fuel.

Combustion chamber 30 is shown communicating with intake manifold 44 and exhaust manifold 48 via respective intake valves 52a and 52b (not shown), and exhaust valves 54a and

54b (not shown). Fuel injector 66 is shown directly coupled to combustion chamber 30 for delivering liquid fuel directly therein in proportion to the pulse width of signal fpw received from controller 12 via conventional electronic driver 68. Fuel is delivered to fuel injector 66 by a conventional high pressure fuel system (not shown) including a fuel tank, fuel pumps, and a fuel rail.

Intake manifold 44 is shown communicating with throttle body 58 via throttle plate 62. In this particular example, throttle plate 62 is coupled to electric motor 94 so that the position of throttle plate 62 is controlled by controller 12 via electric motor 94. This configuration is commonly referred to as electronic throttle control (ETC) which is also utilise during idle speed control. In an alternative embodiment (not shown), which is well known to those skilled in the art, a bypass air passageway is arrange in parallel with throttle plate 62 to control inducted airflow during idle speed control via a throttle control valve positioned within the air passageway.

Exhaust gas oxygen sensor 76 is shown coupled to exhaust manifold 48 upstream of catalytic converter 70. In this particular example, sensor 76 provides signal EGO to controller 12 which converts signal EGO into two-state signal EGOS. A high voltage state of signal EGOS indicates exhaust gases are rich of stoichiometry and a low voltage state of signal EGOS indicates exhaust gases are lean of stoichiometry. Signal EGOS is used to avantage during feedback air/fuel control in a conventional manner to maintain average air/fuel at stoichiometry during the stoichiometric homogeneous mode of operation.

Conventional distributorless ignition system 88 provides ignition spark to combustion chamber 30 via spark plug 92 in response to spark advance signal SA from controller 12.

Controller 12 causes combustion chamber 30 to operate in either a homogeneous air/fuel mode or a stratifie air/fuel mode by controlling injection timing. In the

stratifie mode, controller 12 activates fuel injector 66 during the engine compression stroke so that fuel is sprayed directly into the bowl of piston 36. Stratifie air/fuel layers are thereby formed. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. During the homogeneous mode, controller 12 activates fuel injector 66 during the intake stroke so that a substantially homogeneous air/fuel mixture is formed when ignition power is supplie to spark plug 92 by ignition system 88. Controller 12 controls the amount of fuel delivered by fuel injector 66 so that the homogeneous air/fuel mixture in chamber 30 can be selected to be at stoichiometry, a value rich of stoichiometry, or a value lean of stoichiometry. The stratifie air/fuel mixture will always be at a value lean of stoichiometry, the exact air/fuel being a function of the amount of fuel delivered to combustion chamber 30. An additional split mode of operation wherein additional fuel is injecte during the exhaust stroke while operating in the stratifie mode is described later herein with particular reference to Figure 2. The split mode is used to create an exotherm for heating catalytic converter 70 and NOx trap 72.

Nitrogen oxide (NOx) absorbent or trap 72 is shown positioned downstream of catalytic converter 70. NOx trap 72 absorbs NOx when engine 10 is operating lean of stoichiometry. The absorbe NOx is subsequently reacted with HC and catalyse during a NOx purge cycle when controller 12 causes engine 10 to operate in either a rich homogeneous mode or a stoichiometric homogeneous mode.

Controller 12 is shown in Figure 1 as a conventional microcomputer including: microprocessor unit 102, input/output ports 104, an electronic storage medium for executable programs and calibration values shown as read only memory chip 106 in this particular example, random access memory 108, keep alive memory 110, and a conventional data bus. Controller 12 is shown receiving various signals

from sensors coupled to engine 10, in addition to those signals previously discussed, including: measurement of inducted mass air flow (MAF) from mass air flow sensor 100 coupled to throttle body 58; engine coolant temperature (ECT) from temperature sensor 112 coupled to cooling sleeve 114; a profile ignition pickup signal (PIP) from Hall effect sensor 118 coupled to crankshaft 40; and throttle position TP from throttle position sensor 120; and absolut Manifold Pressure Signal MAP from sensor 122. Engine speed signal RPM is generated by controller 12 from signal PIP in a conventional manner and manifold pressure signal MAP provides an indication of engine load.

In this particular example, temperature Tcat of catalytic converter 70 and temperature Ttrap of NOx trap 72 are inferred from engine operation as disclosed in U. S.

Patent No. 5,414,994 the specification of which is incorporated herein by reference. In an alternate embodiment, temperature Tcat is provided by temperature sensor 124 and temperature Ttrap is provided by temperature sensor 126.

Referring now to Figure 2, a process is described for maintaining the temperature of NOx trap 72, and if desired, catalytic converter 70, by use of the split mode injection timing previously referred to herein. When engine 10 is not operating in the stratifie charge mode (SC) as indicated in step 202, fuel injection and ignition timing are scheduled for the homogeneous lean mode or homogeneous stoichiometric mode (step 206). Selection of homogeneous lean or stoichiometric mode is determined by other operating parameters such as desired power, fuel vapour purge, and NOx trap 72 purge.

When engine 10 is operating in the stratifie mode (step 202), temperature of NOx trap 72 (Ttrp) is determined from the Temperature estimator. As previously described herein, the Temperature estimator is described in U. S.

Patent No. 5,414,994, the specification of which is

incorporated herein. Alternatively, the temperature of NOx trap 72 may be provided by temperature sensor 126.

When NOx trap temperature Ttrp is greater than critical temperature (Tcrit), which is associated with lowest temperature for efficient trapping effectiveness of NOx trap 72 and HC oxidation of catalytic converter 70, engine operation continues with fuel injection and ignition timing scheduled for stratifie charge mode (steps 214 and 218).

On the other hand, when NOx trap temperature Ttrap is less than critical temperature Tcrit (step 214), difference in temperature AT is determined in step 218. Additional quantity of fuel Fadd, which is proportional to AT is determined in step 222. Additional quantity of fuel Fadd is injecte during the exhaust stroke of each combustion chamber to introduce unburned HC into the exhaust stream.

The HC will create an exotherm with the oxygen occurring during lean stratifie operation to heat catalytic converter 70 and NOx trap 72. In this manner, the temperature of NOx trap 72 is maintained above critical temperature Tcrit.

When additional fuel Fadd needed to raise NOx trap 72 temperature to the desired temperature is greater than the fuel savings associated with stratifie engine operation, engine 10 is switched from the stratifie mode to the homogeneous lean mode (steps 228 and 230). Stated another way, when additional fuel Fadd is greater than maximum fuel Faddmax (step 228), engine 10 is switched to the homogeneous lean mode (step 230). Fuel injection and ignition timing are then set for the homogeneous lean mode in step 206.

On the other hand, when additional fuel Fadd is less than Faddmax (step 228), stratifie operation continues with additional fuel Fadd added as shown in step 232.

Those skilled in the art may also practice the invention by using the above described process to more rapidly heat catalytic converter 70 during engine cold start. And, the invention may be used to heat NOx trap 72 to a temperature associated with purging sulphur oxides.