GB2350206A

GB2350206A - Switching between lean and rich fuel/air mixtures in an internal combustion engine

Info

Publication number: GB2350206A
Application number: GB9910780A
Authority: GB
Inventors: Trevor Charles Taylor
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1999-05-11
Filing date: 1999-05-11
Publication date: 2000-11-22
Also published as: GB9910780D0

Abstract

The present invention relates to a spark ignition reciprocating piston internal combustion engine 1 having fuel injection directly into a cylinder 11-14, and to a method of operating such an engine, in which the engine is operated lean at relatively low engine demands, and repeatedly switched between lean and relatively rich operation on subsequent cycles of engine cylinders 11-14 at higher engine demands in order to reduce engine emissions.

Description

1.

2350206 Internal Combustion Engine The present invention relates to a spark ignition, reciprocating piston internal combustion engine having fuel injection directly into a. cylinder, and to a method of operating such an engine, in which the engine is operated lean at relatively low engine demands, and repeatedly switched between lean and relatively rich operation at higher engine demands.

It is necessary to balance a number of factors in setting optimal engine. operation. For example, in a motor car it is generally desirable to maximise fuel economy, minimise emissions of harmful exhaust gasses, and maximise engine power. It is, of course, not possible to achieve best operation in all areas at once, and so a compromise must be made according to various criteria.

In normal operation, when the driver or engine demand is relatively low, engine operation is set to relatively lean at an air/fuel ratio of about 20:1 (or more in the case of stratified charge) for a gasoline engine, for good fuel economy and low emissions of NOx, CO and unburned hydrocarbons HC. At higher air fuel ratios, the amount of NOx is reduced, but the level of HC rises sharply. Normally, for a motor car a catalytic converter is provided downstream from a catalytic converter.

As the air/fuel mixture is made less lean, the emissions of HC remain relatively constant whilst those of NOx rise to a peak before falling again towards a relatively rich air/fuel ratio, which would be about 14:1 for a gasoline engine. Therefore, although engine power increases steadily as the air/fuel ratio becomes less lean, emissions reach a peak at an intermediate lean air/fuel ratio. It is undesirable to run such an engine in the 98072OGB 06/05/99 2 - region of the intermediate peak, therefore, if engine or driver demand exceeds that available at the relatively lean point, engine operation switches immediately to a relatively rich level for increased power albeit at 5 increased emissions.

It is an object of the present invention to provide an engine and a method of operating such an engine with improved emissions at times when engine or driver demand exceeds that available at lean operating conditions.

Accordingly, the invention provides a spark ignition reciprocating piston internal combustion engine, comprising at least one cylinder, an air inlet port is leading to the cylinder, an exhaust outlet port leading from the cylinder, a fuel injector for direct fuel injection into the cylinder, a throttle for controlling airflow in the inlet port, an engine management system for controlling the air/fuel mixture for the cylinder according to engine demand, an engine demand signal supplied to the engine management system, in which the cylinder in operation: generates exhaust emmissions including NOx; has a first operating region at a relatively rich air/fuel mixture capable of meeting a first level of engine demand; and has a second operating region at a relatively lean air/fuel mixture capable of meeting a second level of engine demand, the first level of engine demand being greater than the second level of engine demand and the first and second operating regions being separated by a third operating region in which the exhaust emmisions of NOx are elevated relative to such emissions in the first operating region and the second operating region; wherein the engine management system:

98072OGB 06/05/99 a) operates the cylinder in the second region as long as engine demand does not exceed the second level of engine demand; and b) when the engine demand exceeds the second level of engine demand, switches cylinder operation on subsequent cycles of the engine repeatedly between the second region and the first region in order to meet the engine demand.

Direct injection of fuel permits the engine management system to control and set different air/fuel mixtures for each firing event of the cylinder. Therefore, when the engine demand exceeds the second level by a relatively small amount, only a small proportion, for example one in ten, of the engine cycles may be relatively rich, with the rest remaining relatively lean. This will result in a small engine power increase, as averaged out over ten engine cycles. The averaging of power is facilitated by the inertia of the pistons, and any crank shaft, propeller shaft or flywheel. If the engine is in a motor car or other vehicle having an automatic transmiss-ion, then this transmission can have a significant beneficial effect in smoothing the minor power fluctuations that would result from having at least some cylinders fire with increased power at least some of the time.

As engine or driver demand increased towards the maximum available from the engine at a relatively rich operating point, then the proportion of cycles in which the cylinder was fuelled with a relatively rich air/fuel ratio would also rise until all cylinders were fuelled with a relatively rich air/fuel ratio.

Each cylinder is therefore fuelled and fired in the 35 relatively rich region, where emissions are not as low as 98072OGB 06/05/99 in the relatively lean region, only as often as is required to meet engine or driver demand.

Generally, the engine will have a plurality of cylinders.

The, it will often be the case that not all of the cylinders will have their operation switched from the second region to the first region, or vice versa on any one cycle of the engine. An engine cycle is the number of revolutions of the engine (two for a four stroke engine) for a piston to complete a fuelling and firing cycle. For example, in a four-stroke, four cylinder engine in which it is necessary to have two cylinders operated in the first region (i.e. at a relatively rich level) in every ten engine spark events, in the first engine cycle only cylinder I will be operated in the first region, in the second engine duty cycle only cylinder II will be operated, and so on.

In addition to controlling the air/fuel ratio by 20 controlling the amount of fuel injected directly into the or each cylinder, it may also be possible to control at least to some degree the air fuel ratio by controlling the amount of air allowed into each cylinder, for example by controlling an air inlet throttle.

Also according to the invention, there is provided a method of operating a spark ignition reciprocating piston internal combustion engine, the engine comprising at least one cylinder, an air inlet port leading to the cylinder, an exhaust outlet port leading from the cylinder for carrying exhaust emissions including NOx from the cylinder, a fuel injector for direct fuel injection into the cylinder, a throttle for controlling airflow in the inlet port, and an engine management system, wherein the method comprises the steps of:

980720GB 06/05/99 a) supplying the engine demand signal to the engine management system; b) using the engine management system to controlling the 5 air/fuel mixture for the cylinder according to the engine demand signal; c) operating the cylinder in a second operating region at a relatively lean air/fuel mixture capable of meeting a second level of engine demand; wherein if the engine demand exceeds the second level, then d) operating the cylinder in a first operating region at a relatively rich air/fuel mixture capable of meeting a first level of engine demand in excess of the second level of engine demand, the first and second operating regions being separated by a third operating region in which the exhaust emmisions of NOx are elevated relative to such emissions in the first operating region and the second operating region; e) alternating steps c) and d) repeatedly in subsequent engine cycles in order to meet the engine demand.

The invention will now be described in further detail by way of example, with reference to the accompanying drawings, in which:

Figures 1A and 1B are a schematic drawings of a f our cylinder fuel injection internal combustion engine according to the invention, with an engine management system that receives an exhaust gas condition signal and an engine speed signal from a sensor that detects the passage of teeth on a crankshaft f lywheel; 98072OGB 06/05/99 Figure 2 is a plot of engine emissions and maximum power output against air/fuel mixture; Figure 3 is a flow diagram describing the control of the engine by the engine management system; and Figure 4 is a chart showing the number of cylinders set to run relatively lean and rich for averaged duty cycle of rich to lean running.

Figure 1A shows schematically a four-cylinder, four-stroke internal combustion engine 1 suitable for use in a motor car (not shown), having a direct injection device by which each of four cylinders 11,12,13,14 is supplied with fuel by an electro-injector 2. The engine I is a gasoline engine, and so is also equipped with spark plugs 4. The invention is, however, equally engines having a lesser or greater number of cylinders.

The opening sequence and timing of each electro-injector 2 and spark plug 4 is controlled by an electronic engine management system 10, which determines the amount of fuel and timing of fuel and spark events depending on engine operating conditions.

This engine control system 10 receives input signals, performs operations and generates output control signals, particularly for the fuel injectors 2 and spark plugs 4.

The electronic engine management system 10 conventionally comprises a micro-processor (AP) 12, a random access memory (RAM) 45, a read only memory (ROM) 16, an electrically programmable ROM (EPROM) 44, an analogtodigital converter (A/D) 18 and various input and output interfaces, including a spark plug driver 20, a throttle control 21, and an injector driver 22.

98072OGB 06/05/99 The throttle control 21 controls a butterfly throttle valve 15 in an air inlet line 17 leading to at least one inlet port 19 for each cylinder 11- 14.

Each cylinder has at least one outlet port 29 leading to an exhaust line 35 in which there is a conventional threeway catalytic converter 47 and optionally a lean NOx trap (not shown).

The input signals to the engine management system 10 comprise a driver demand signal (DD) 24, an engine temperature signal (T) 25 from an engine temperature sensor 23, an exhaust gas temperature signal (EGT) 26 from an exhaust temperature sensor 27, a universal exhaust gas oxygen signal WEGO) 28 from an exhaust gas oxygen sensor 29 that produces a linear output, and a signal 30 from a variable reluctance sensor (VRS) 32, all of which are digitized by the A/D converter 18 prior to being passed to the microprocessor 12.

Figures 1A and 1B show how the variable reluctance sensor 32 senses the passage of teeth 33 spaced circumferentially around the periphery of a flywheel 34 on an engine crankshaft 36. The flywheel 34 has a conventional arrangement of teeth referred to herein as 36-1 teeth, wherein thirty-five identical teeth 33 are equally spaced by thirty-four gaps 37 between teeth, and with one pair of teeth being spaced by a larger gap 38 three times as large as the other gaps 37. The larger gap 38 corresponds to one missing tooth. The VRS signal 30 therefore comprises a series of essentially sinusoidal pulses for each revolution of the crankshaft, with one missing pulse. Digitization of the raw VRS signal 30 by the A/D converter 18 yields a digitized VRS signal, comprising a series of essentially square waves, with one missing pulse 98072OGB 06105199 corresponding to the missing pulse 38 in the raw VRS signal 30. The existence of the missing tooth allows the identification of a Top Dead Centre (TDC) position for the engine 1.

Figure 2 is a schematic plot of the main engine exhaust components, carbon dioxide CO, hydrocarbons HC and nitrogen oxides NOx. In normal operation, when the driver or engine demand is relatively low, engine operation is set to relatively lean at an air/fuel ratio of about 20:1 (about X=1.43 where X is defined as (actual air/fuel ratio divided by stoichiometric air/fuel ratio), for good fuel economy and low emissions of NOx, CO and unburned hydrocarbons HC. Figure 2 shows this operation region, referred to herein as the 'first" operation region 41. At higher air fuel ratios in, in an operating region referred to herein as a "thire operation region 43, the amount of NOx is reduced, but the level of HC rises sharply. Towards a 'second" operation 42 near to a relatively rich of about 14:1 (X=1.0) at which NOx emissions are lower than in the third region 43, but higher than in the first region 41.

Figure 2 also shows schematically how the maximum available engine torque T (or equivalently power P) varies with the air/fuel mixture, with an increase from a level of TI in the f irst region 41 to a level of T2 in the second region 42. The actual value of maximum torque T (or power P) will of course vary on a number of factors, particular engine temperature and engine speed, and any exhaust gas recirculation.

Reference is now made also to Figure 3, which shows a flow diagram 50 describing the control of the engine by the engine control system 10 and engine control software running in the microprocessor 12. Normally, the engine 98072OGB 06/05/99 will be running lean in the first region 41. The microprocessor 12 calculates engine rpm 51, driver demand 52, engine temperature 53 and exhaust gas temperature 54, and from this retrieves 55 from a look-up table 56 in the EPROM 44 to calculate 57 a required engine torque R. In general, it will also be necessary for the engine control system to calculate a desired spark advance so that each spark plug is fired at the optimum time depending on engine operating parameters.

The microprocessor then retrieves 58 from a second look-up table 59 in the EPROM 44 to calculate 60 an available torque A (or equivalently an available engine power). Next, the microprocessor 12 tests 61 if the required torque R is less than the maximum available torque A. If so 62, then the engine continues to run lean 63. If not 64, the microprocessor calculates 65 an 'air/fuel ratio" (AFR) duty cycle.

The AFR duty cycle is set out in Figure 4 in a table 80. In the top row 81 of the table, f or every spark event in which of a cylinder is fuelled and fired in the relatively rich second region 42 (1=1.0), there are eight spark events in which cylinders are fuelled and f ired in the relatively lean f irst region 41. This gives an AFR duty cycle of about 13%. On average, an engine run according to the f irst row 81 in the table 80 will have a maximum available torque higher than that available f rom engine operation continuously in the first region 41 by an amount 30 of about 11-0k the difference T2-T1 in available engine torque between the second and first regions 42,41. In the last displayed row 82 of the table 80, there are seven rich engine ignition events for every eight lean ignition events, which implies an AFR duty cycle of 47%. 35 980720GB 06/05/99 The ratio of rich to lean engine ignition events can therefore be varied to essentially to achieve any desired available engine maximum torque T between T1 and T2.

The microprocessor can then calculate 66 the needed AFR duty cycle. Relatively rich and relatively lean spark events are then scheduled depending on whether the current cylinder is near stoichiometric or relatively lean 68 according to duty cycle logic 70 that spreads relatively rich spark events amongst the relatively lean spark events. The microprocessor then calculates 72 a needed fuel pulse width for each injector and also a particular spark advance.

is Emissions will scale according to same ratio, as driver demand approaches the maximum available in the second region 42. Engine emissions are therefore improved up until the point at which the engine is running continuously in the second region 42.

98072OGB 06/05/99

Claims

1. A spark ignition reciprocating piston internal combustion engine, comprising at least one cylinder, an air inlet port leading to the cylinder, an exhaust outlet port leading from the cylinder, a fuel injector for direct fuel injection into the cylinder, a throttle for controlling airf low in the inlet port, an engine management system for controlling the air/fuel mixture for the cylinder according to engine demand, an engine demand signal supplied to the engine management system, in which the cylinder in operation: generates exhaust emmissions including Nox; has a first operating region at a relatively rich air/fuel mixture capable of meeting a first level of engine demand; and has a second operating region at a relatively lean air/fuel mixture capable of meeting a second level of engine demand, the first level of engine demand being greater than the second level of engine demand and the first and second operating regions being separated by a third operating region in which the exhaust enunisions of NOx are elevated relative to such emissions in the first operating region and the second operating region; wherein the engine management system:

a) operates the cylinder in the second region as long as engine demand does not exceed the second level of engine demand; and b) when the engine demand exceeds the second level of engine demand, switches cylinder operation on subsequent cycles of the engine repeatedly between the second region and the first region in order to meet the engine demand.

2. An internal combustion engine as claimed in Claim 1, in which the engine has a plurality of cylinders and not 980720GB 06/05/99 all of the cylinders are switched from the second to the first region or vice versa on any one cycle of the engine.

3. An internal combustion engine as claimed in Claim 1 or Claim 2, in which the engine management system controls the air/fuel ratio by controlling the amount of fuel injected directly into the or each cylinder.

4. An internal combustion engine as claimed in Claim 3, in which the engine management system controls the air fuel ratio by controlling the air inlet throttle.

5. A motor car with an internal combustion engine, the internal combustion engine is as claimed in any preceding is claim.

6. A motor car as claimed in Claim 5, comprising a catalytic converter and/or lean NOx trap f or NOx downstream of the exhaust port.

7. A method of operating a spark ignition reciprocating piston internal combustion engine, the engine comprising at least one cylinder, an air inlet port leading to the cylinder, an exhaust outlet port leading from the cylinder for carrying exhaust emissions including NOx from the cylinder, a fuel injector for direct fuel injection into the cylinder, a throttle for controlling airflow in the inlet port, and an engine management system, wherein the method comprises the steps of:

a) supplying the engine demand signal to the engine management system; b) using the engine management system to controlling the air/fuel mixture for the cylinder according to the engine demand signal; 98072OGB 06/05/99 c) operating the cylinder in a second operating region at a relatively lean air/fuel mixture capable of meeting a second level of engine demand; wherein if the engine demand exceeds the second level, then d) operating the cylinder in a first operating region at 10 a relatively rich air/fuel mixture capable of meeting a first level of engine demand in excess of the second level of engine demand, the first and second operating regions being separated by a third operating region in which the exhaust emmisions of NOx are elevated relative to such is emissions in the first operating region and the second operating region; e) alternating steps c) and d) repeatedly in subsequent engine cycles in order to meet the engine demand.

8. A spark ignition reciprocating piston internal combustion engine substantially as herein described, with reference to or as shown in the accompanying drawings.

9. A method of operating a spark ignition reciprocating piston internal combustion engine substantially as herein described, with reference to the accompanying drawings.

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