GB2546115A

GB2546115A - Internal combustion engine

Info

Publication number: GB2546115A
Application number: GB1600482.2A
Authority: GB
Inventors: Zhao Hua; Tsoi Hei Ma Thomas; Hans Ruhlan Helmut
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2016-01-11
Filing date: 2016-01-11
Publication date: 2017-07-12
Also published as: GB201600482D0

Abstract

Disclosed is a spark ignition internal combustion engine having at least one combustion chamber, a spark plug 12 for igniting a charge in the combustion chamber, a fuel injection system including an injector 10 positioned close to the spark plug for introducing fuel directly into the engine combustion, an intake air pressure boosting system and an Exhaust Gas Recirculation, EGR, system. The engine further comprises a control system that, during one mode of operation, causes the intake air to be pressurised by the boosting system, and causes recirculation of a high proportion of EGR gases by the EGR system. During such a mode of operation, the control system activates the fuel injection system twice during each engine operating cycle. The first activation occurs during an intake period of the engine to produce a homogeneous fuel lean mixture. The second occurs toward the end of a compression period of the engine so as to create a stratified charge in which the overall mixture strength is stoichiometric and a fuel-rich mixture region is present in the vicinity of the spark plug.

Description

INTERNAL COMBUSTION ENGINE

Field of the invention

The present invention relates to a method of operating an internal combustion engine, having at least one combustion chamber, a fuel injection system for introducing fuel directly into the engine combustion chamber, an intake air pressure boosting system and an EGR system.

The term "EGR system" is used herein to refer to an exhaust gas recirculation system, in which exhaust gases from an operating cycle are retained within, or recirculated to, the combustion chamber for inclusion in the charge of a later combustion cycle. Similarly, "EGR gases" refers to exhaust gases from a previous combustion cycle.

Background of the invention

It is known that high dilution burn with air and/or EGR gases could improve the fuel economy and lower the exhaust emissions of a spark ignition internal combustion engine. It is also known that boosting the intake pressure, by means of a turbocharger or a supercharger, could extend the power range of high dilution burn. A known obstacle to the above strategy is the lack of ignition stability when the engine is running in a high dilution mode.

Object of the invention

The present invention seeks therefore to improve the ignition stability of a spark ignited engine when operating in a high boost and high EGR dilution mode.

Summary of the invention

According to the present invention, there is provided in accordance with a first aspect a method of operating a spark ignition internal combustion engine having at least one combustion chamber, a spark plug for igniting a charge in the combustion chamber, a fuel injection system including an injector positioned close to the spark plug for introducing fuel directly into the engine combustion, an intake air pressure boosting system and an EGR system, wherein the method comprises operating the engine in a mode in which the combustion charge is pressurised by the boosting system and contains a high proportion of EGR gases, in which mode fuel is introduced into the combustion chamber by two or more activations of the fuel injection system during each engine operating cycle, a first quantity of fuel being injected during an intake period of the engine to produce a homogeneous fuel-lean mixture, and a second quantity of fuel being injected toward the end of a compression period of the engine to produce a stratified charge that is locally fuel-rich mixture in the vicinity of the spark plug, the ratio of the total quantity of fuel injected into the combustion chamber to the mass of air trapped in the combustion chamber during each operating cycle being substantially stoichiometric.

The invention is applicable to both two-stroke and four stroke engines. In the case of a four-stroke engine, the term "period" is used herein in a manner synonymous with "stroke" but in the case of a two-stroke engine the "period" refers to the crankshaft phase angles during which intake, compression, expansion and exhaust occur.

The EGR gases may be a combination of internal (retained) EGR gases or external (recirculated) EGR gases. Internal EGR gases may be produced by a known variety of valve timing means while external EGR gases may be derived by passing gases drawn from the exhaust system back to the intake system of the engine. Internal EGR is hotter than external EGR hence it is preferred when a higher combustion charge temperature is required for ignition stability. On the other hand, external EGR can be easily cooled with an EGR cooler and is preferred when a lower combustion charge temperature is required for reduced emissions.

It is known that spark ignited engines can operate in an "auto-ignition" mode. By this it is meant that the temperature and pressure within the combustion chamber may reach levels that result in spontaneous combustion of the fuel without its being initiated by a spark. Auto-ignition is what causes "dieseling" when a spark ignited engine continues to run after the ignition system has been switched off .

When using hot EGR gases, the temperature of the combustible mixture of the first fuel injection quantity with air and EGR may be controlled to be sufficiently hot to reach, towards the end of the compression period of the engine, a temperature just below the auto-ignition threshold of the mixture. In this case, when the second quantity of injected fuel is ignited by the spark plug, combustion of the fuel-rich mixture lying in the immediate vicinity of the spark plug could raise the overall mixture temperature and pressure past the auto-ignition threshold thereby initiating controlled auto-ignition of the whole mixture after occurrence of the spark.

The invention is thus capable, under high EGR and high boost conditions, of enabling reliable spark ignition followed by either smooth combustion by flame propagation or controlled auto-ignition.

Furthermore, on account of the stoichiometric exhaust gas composition, the invention enables effective exhaust gas after-treatment to minimise emission of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx).

In accordance with a second aspect, the invention provides a spark ignition internal combustion engine having at least one combustion chamber, a spark plug for igniting a charge in the combustion chamber, a fuel injection system including an injector positioned close to the spark plug for introducing fuel directly into the engine combustion, an intake air pressure boosting system and an EGR system, the engine further comprising a control system that is operative during one mode of operation to cause the intake air to be pressurised by the boosting system, to cause recirculation of a high proportion of EGR gases by the EGR system and to activate the fuel injection system two ore more times during each engine operating cycle, the first activation occurring during an intake period of the engine to produce a homogeneous fuel-lean mixture and the second toward the end of a compression period of the engine so as to create a stratified charge in which the overall mixture strength is stoichiometric and a fuel-rich mixture region is present in the vicinity of the spark plug.

Brief description of the drawings

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

Figure 1 is a perspective view of a cylinder head of gasoline direct injection (GDI) engine having four valves per cylinder,

Figure 2 is a section through a cylinder of the engine of Figure 1, the section plane passing through one intake and one exhaust valve,

Figure 3 is a schematic representation of the engine of Figures 1 and 2 when equipped with a turbocharger and an external EGR circuit, and Figure 4 is a timing diagram showing an implementation of a method in accordance with the invention.

Detailed description of the drawings

Figure 1 shows a section of an engine cylinder head 10 for a GDI engine, wherein each cylinder has both a spark plug 12 and an in-cylinder fuel injector 14 in close proximity to the spark plug 12. The illustrated cylinder head has four valves per cylinder controlling two intake ports 16 and two exhaust ports 18 but the number of valves is not relevant in the present context as the engine may alternatively have two or three valves per cylinder.

The cylinder head 10 of Figure 1 is shown in Figure 2 mounted to an engine block 30 having a cylinder bore 32 that receives a sliding piston 34. The piston 34 is connected by a connecting rod 36 to an eccentric bearing on the crankshaft (not shown) so that it reciprocates in the cylinder bore 32 as the crankshaft rotates. Figure 2 also shows one of the inlet valves 20 and one of the exhaust valves 22 and an overhead cam 24 operating the inlet valve 20. The engine construction shown in Figure 2 is entirely conventional and its operation will be well understood by the person skilled in the art without the need for further explanation.

The engine of Figures 1 and 2 is shown schematically in Figure 3 where like parts have been allocated the same reference numerals. The engine has an intake system that is pressurised in the case of the embodiment shown in Figure 3 by means of an exhaust drive turbocharger generally designated 40. The turbocharger 40, as is well known, comprises an exhaust gas driven turbine 40a driving an impeller 40b compressing the intake air by way of a shaft 40c. As an alternative, the intake system may be pressurised by a supercharger. Both turbochargers and superchargers are well known per se and need not therefore be described further in the present context.

Last, the engine is fitted with an external EGR circuit comprising a passage or conduit 50a, 50b leading from the exhaust manifold, which contains a lambda-1 sensor, to the intake manifold through an EGR valve 52. The direction of gas flow in the manifolds and in the conduit 50a, 50b is represented by arrows. The EGR circuit allows exhaust gases from a previous operating cycle to be mixed with the charge of a subsequent cycle thereby having the same effect as reducing the displacement volume of the cylinder. As the EGR gases contain no oxygen, they reduce the mass of oxygen that can be contained in the cylinder at the commencement of the subsequent operating cycle, thereby allowing the engine to burn a lesser quantity of fuel without reducing the mixture strength.

As an alternative to recycling exhaust gases after they have left the combustion chamber, it is possible to retain exhaust gases in the cylinder by altering the valve timing, this being referred to as internal EGR. For example, if the opening of the inlet valve is advanced to occur before top dead centre (TDC) at the end of the exhaust period, exhaust gases will be expelled into the intake manifold during the exhaust period then readmitted into the cylinder during the intake period. Likewise, if the exhaust event is retarded so that the exhaust valve remains open at the start of the intake period, exhaust gases will be drawn in from the exhaust ports during the intake period. In another example, if the exhaust valve is closed normally during the exhaust period and then re-opened briefly during the intake period, exhaust gases will be drawn in from the exhaust ports during the intake period.

In the present invention it is required for the engine to have an EGR system by means of which, under certain conditions, the intake charge can contain a proportion of EGR gases but it is not material to the invention whether the EGR system relies on internal or external exhaust gas recirculation.

The method of the invention will now be explained by reference to Figure 4 where the exhaust valve event is represented by the line 60, the inlet valve event by the line 62 and the fuel injection events by the periods 64a and 64b. The timing diagram of Figure 4 is for a four stroke engine where the intake-compression-power-exhaust events occur over two revolutions of the crankshaft but the invention could equally be implemented in a two-stroke which fires once per revolution of the crankshaft.

In Figure 4, the engine is assumed to operate with internal EGR and the inlet valve event has been shown as having been advanced and the exhaust event retarded to achieve internal EGR. Two fuel injection events occur, the bulk of the fuel being injected during the period 64a that occurs towards the start of the intake period and a smaller period 64b occurring towards the end of the intake period.

Under high load conditions, it is desired to burn a maximum quantity of fuel during each combustion cycle. The engine is therefore operated with minimum EGR and high boost.

However, with a decrease in load it is desirable to include increasing proportion of EGR while still operating with high boost, the EGR reducing the quantity of trapped oxygen in each charge and therefore requiring less fuel to be injected to maintain a stoichiometric ratio. However, such a mode of operation, where there is a high dilution ratio of EGR gases, results in combustion instability.

The purpose of the two stage fuel injection that is herein proposed, and shown in Figure 4, is to mitigate this problem. The first injection 64a occurs at the commencement of the intake period and results in the fuel being mixed with the intake air and EGR gases by the turbulent or swirling motion that takes place within the combustion chamber during intake and compression. This thus creates a homogeneous charge of a slightly fuel-lean mixture but this mixture is diluted with EGR gases and cannot be ignited reliably. The second injection 64b occurs towards the end of the compression period creates a region with higher fuel concentration, that is to say a richer mixture, in the immediate vicinity of the spark plug. Thus at the time of ignition, the charge is stratified and the region of the charge exposed to the spark is relatively rich and easy to ignite. The spark creates a flame kernel which propagates through the remainder of the charge, but being a flame rather than just a spark, it ensures that the fuel throughout the charge is fully burnt.

The total amount of fuel injected during each cycle is metered in known manner, using feedback from the lambda-1 sensor 70 in the exhaust system, to ensure that the overall or average mixture strength of the charge is stoichiometric. The mixture strength in the homogeneously mixed part of the charge is set slightly lean and the second injection tops up the charge with the small amount of remaining fuel needed to achieve an overall stoichiometric calibration. A consequence of stoichiometric calibration for a spark ignited gasoline engine is that the EGR gases contain no oxygen. For this reason, unlike the case of diesel engines where the exhaust gases do contain oxygen, dilution with EGR gases in spark ignited gasoline engines increases combustion instability. However, the injection of fuel close to the spark plug helps to counteract the problem.

It has been described above how the fuel in homogeneous regions of the combustion chamber can be ignited by flame propagation but it is alternatively possible for the burning of the fuel in the vicinity of the spark plug to trigger auto-ignition of the remainder of the charge. It is known that gasoline will auto-ignite, in a similar manner to diesel oil, if the temperature and pressure are sufficiently high. If the charge contains a high proportion of hot EGR gases, the temperature and pressure in the combustion chamber at the end of the compression period can be just below the auto-ignition point of the mixture. In this case, the burning of rich charge region in the vicinity of the spark plug can raise the temperature and pressure of the whole charge beyond the auto-ignition point causing the remainder of the charge to auto-ignite instead of being consumed by flame propagation.

Claims

1. A method of operating a spark ignition internal combustion engine having at least one combustion chamber, a spark plug for igniting a charge in the combustion chamber, a fuel injection system including an injector positioned close to the spark plug for introducing fuel directly into the engine combustion, an intake air pressure boosting system and an EGR system, wherein the method comprises operating the engine in a mode in which the combustion charge is pressurised by the boosting system and contains a high proportion of EGR gases, in which mode fuel is introduced into the combustion chamber by two or more activations of the fuel injection system during each engine operating cycle, a first quantity of fuel being injected during an intake period of the engine to produce a homogeneous fuel-lean mixture, and a second quantity of fuel being injected toward the end of a compression period of the engine to produce a stratified charge that is locally fuel-rich mixture in the vicinity of the spark plug, the ratio of the total quantity of fuel injected into the combustion chamber to the mass of air trapped in the combustion chamber during each operating cycle being substantially stoichiometric.

2. A method as claimed in claim 1, wherein, in a mode of operation of the engine, spark ignition of the fuel-rich mixture in the region of the spark plug creates a flame that propagates through the remainder of the charge to cause combustion of the regions of the charge having a fuel-lean mixture .

3. A method as claimed in claim 1 or 2, wherein, in a mode of operation of the engine, the quantity and temperature of the EGR gases result in the temperature and pressure of the charge, after compression of the charge within the combustion chamber, being below a threshold at which auto-ignition can occur, and wherein spark ignition of the fuel-rich mixture in the region of the spark plug raises the temperature and pressure in the combustion chamber above the latter threshold to cause the remainder of the charge to auto-ignite.

4. A spark ignition internal combustion engine having at least one combustion chamber, a spark plug for igniting a charge in the combustion chamber, a fuel injection system including an injector positioned close to the spark plug for introducing fuel directly into the engine combustion, an intake air pressure boosting system and an EGR system, the engine further comprising a control system that is operative during one mode of operation to cause the intake air to be pressurised by the boosting system, to cause recirculation of a high proportion of EGR gases by the EGR system and to activate the fuel injection system two ore more times during each engine operating cycle, the first activation occurring during an intake period of the engine to produce a homogeneous fuel-lean mixture and the second toward the end of a compression period of the engine so as to create a stratified charge in which the overall mixture strength is stoichiometric and a fuel-rich mixture region is present in the vicinity of the spark plug.

5. An engine as claimed in claim 4, wherein the engine is a four-stroke engine.

6. An engine as claimed in claim 4, wherein the engine is a two-stroke engine.