WO2012019261A1

WO2012019261A1 - Fuel injection system for a spark ignited internal combustion engine

Info

Publication number: WO2012019261A1
Application number: PCT/BR2011/000270
Authority: WO
Inventors: Marcos Langeani
Original assignee: Sygma Motors - Engenharia, Indústria E Comércio De Motores Ltda
Priority date: 2010-08-11
Filing date: 2011-08-11
Publication date: 2012-02-16
Also published as: BRPI1002727A2; BRPI1002727C1

Abstract

Fuel injection system for optimum control of in-cylinder fluid properties around spark plug at combustion ignition time. The system features a Primary Subsystem, which injects a fuel, or a fuel mixture, in liquid, vapor or gas form, and a Secondary Subsystem, which injects a mixture containing at least one fuel and one no-fuel in, or mainly, vapor or gas form. In one invention variant fuel, or fuel mixture, from the Secondary Subsystem is injected directly into the combustion chamber, whereas fuel, or fuel mixture, from the Primary Subsystem is injected into the intake system. In a second invention variant fuel, or fuel mixture, from the Secondary Subsystem is injected directly into the combustion chamber, and fuel, or fuel mixture, from the Primary Subsystem is also injected into the combustion chamber. In a third variant fuels and fuel mixtures from both the Primary and Secondary Subsystems are injected directly into the combustion chamber through the same injector.

Description

FUEL INJECTION SYSTEM FOR A SPARK IGNITED INTERNAL

COMBUSTION ENGINE FIELD OF THE INVENTION

The invention relates to internal combustion engines and to fuel injection systems applied to internal combustion engines.

BACKGROUND OF THE INVENTION

Usually, dilution of intake charge is understood as the addition of a gas that, although not effectively participating in the combustion process, increases the thermal capacity of the resulting mixture. Both air, ingested in quantities higher than those strictly required to oxidize all in-cylinder fuel - situation referenced as lean mixture - and burned gases are typical diluents. Burned gases may be either in-cylinder residues from previous cycle - situation referenced as Internal Exhaust Gases Recirculation - or collected from the exhaust system, then usually cooled, and added to the fresh charge upstream of the intake valve(s) - situation referenced simply as Exhaust Gases Recirculation.

Ideal cycle analysis reveals that lower working fluid temperatures, caused by addition of thermal diluents, are beneficial to the fluid thermal properties, allowing more energy to be extracted along the expansion stroke. Additionally, temperatures reduction alone diminishes thermal energy transfer to components. At low loads, dilution can allow operation with lower pump losses, a factor especially critical in automobile urban use. At very high loads, dilution slows-down pre-flame reactions that lead to engine knocking, thus allowing increase of either compression ratio or supercharging level. Experimental evidence, as shown in paper SAE 2009-01-0694, suggest that dilution inhibits pre/post-ignition, abnormal combustion regimes detrimental to engine operation and integrity.

Dilution of intake charge can also occur with substances in the liquid state; however, cycle temperatures decrease is likely to be more a result from the liquid vaporization than higher thermal capacity. Fuel in excess relative to that strictly required to consume all oxygen - situation referenced as rich mixture - is a normal practice to avoid engine knock at high loads, but both efficiency and emissions suffer. A less usual practice is addition of water in liquid state.

On the other hand, dilution difficults both flame initiation and propagation. As the mixture is diluted, the combustion period increases. As a consequence, substantial spark timing advance is required and the combustion end delays excessively. Here, excessively means an environment which is not well appropriate for fast combustion reactions. Thus, there is usually a practical limit from which further dilution causes efficiency losses instead of additional gains. Also, combustion may become so irregular that engine operation is not acceptable even if efficiency is still reasonable. Finally, very high dilutions may cause either misfiring or incomplete combustion, increasing emissions to an intolerable level.

Dilution by Exhaust Gases Recirculation is extremely effective in inhibiting abnormal combustion, as shown in paper SAE 2009-01-0694, and, in the of case engine operation with stoichiometric mixture

(chemically correct proportion of fuel and air), allows efficient operation of three-way catalytic convertors, making feasible ultra-low emissions at no excessive manufacturing and maintenance costs.

Dilution by air is not so effective regarding abnormal combustion avoidance and makes more challenging to comply with the emission legislation, especially for oxides of nitrogen. On the other hand, dilution by air allows better flame initiation and propagation and working fluid properties, opening the opportunity for higher efficiencies if associated drawbacks are not relevant or could be circumvented.

A standard approach to minimize the impact of dilution on the combustion process is to intensify in-cylinder turbulence. This

intensification could be obtained from the establishment of a more coherent in-cylinder flow along the intake process, so preserving part of the intake-generated kinetic energy to be dissipated into turbulence at combustion time, as described in chapter 8 of Internal Combustion Engine Fundamentals book, by John B. Heywood, 1988. Piston movement can promote generation of turbulence at combustion time too.

The positive impact of intensifying turbulence is not unbounded. A very intense flow or turbulence intensifies heat losses to engine

components. They can delay, or even interrupt, flame propagation, due to both inner flame stretching and charge cooling. Additionally, generation of very intense coherent flows may cost too much in terms of pump losses. Finally, turbulence has a limited impact during ignition and very early development and a very intense flow is not necessarily beneficial to flame initiation.

As dilution is increased, the minimum volume affected by spark required to produce a subsequent self-sustained flame front increases, as reported in chapter 9 of Internal Combustion Engine Fundamentals book, by John B. Heywood, 1988. This demands wider spark plug electrodes gap as well as higher energy from the ignition system, as described in paper SAE 2004-32-0086. Eventually, a point is reached where either it is not possible to provide still more energy or if higher energy is delivered the spark plug life is compromised.

An approach to minimize part of those conflicts is to create, in the vicinity of the spark plug, a zone better suited for ignition. One class of solution is to provide an almost closed small space, an auxiliary chamber, connected to the main one, where either fuel-air mixture, or turbulence, or spark discharge are better conditioned to improve ignition and early flame propagation, as explained in chapter 8 of Internal Combustion Engine

Fundamentals book, by John B. Heywood, 1988. Another class of solution, without any auxiliary volume, is to produce, by adequate control of the mixture preparation, a richer zone in the vicinity of the spark plug (as explained in Automotive Gasoline Direct-Injection Engines, F. Zhao, M.-C. Lao e D. L. Harrington, 2002), the rest of the combustion chamber containing a leaner mixture or no fuel at all. The first class of solution, usually named jet or torch ignition, received some attention in the past, and survives nowadays essentially in big engines, as described, for instance, in paper ASME n° JRC/ICE2007- 40095. In bigger engines the drawbacks associated with pressure and thermal losses produced by the high speeds in the connecting nozzles, in addition to the higher complexity, are more than offset by the gains. Such a system improves not only ignition, but also leverages flame development - jets spread fastly towards combustion chamber periphery, not only carrying flame front but also increasing turbulence.

The second class of solution, usually known as fuel stratification, became an industrial solution only in the last fifteen years, despite being described in the Nicolaus Otto's original four-stroke patent - it requires a control sophistication made available only recently. Stratification is effectively produced by means of late fuel direct injection into combustion chamber, being conceptually recognized the following possible methods: Air Guided, Wall Guided and Spray Guided. In general, this class of solution is not particularly effective to deal with dilution by burnt gases alone.

To those two classes of solution it can be added the dual-fuel engine. Typically, a relatively small amount of diesel acts as a pilot flame, natural gas usually being the main fuel. This configuration does not need spark plug, but the flame propagation mechanism through the natural gas is intended to be the same of spark ignited engines. The main drive for its initial development was the need for an engine that could consume natural gas, but did not loose its capability to run on diesel only. Being a relatively inexpensive adaptation of diesel units, dual-fuel engine became later a solution on its own, as described in the chapter 25 of Diesel Engine

Reference Book, Bernard Challen e Rodica Baranescu.

Efficiency gains, expected to be realized with fuel stratification, are partially denied by, among others, a less favorable combustion rate shape, lower combustion efficiency, and high pressure fuel pump power. A factor even more critical for stratification success is the difficulty in controlling emission of oxides of nitrogen with traditional aftertreatment devices in an overall lean burned gas. As a result, more and more attention was diverted to the use of direct injection as knock inhibitor.

Using indirect injection, heat transfer from engine components to droplets or fuel film laying on the walls is a very relevant source of the thermal energy required for fuel evaporation. Direct injection, on the other hand, can reduce substantially fuel collision with engine components, thus making in-cylinder gases the main source of energy for vaporization. As a consequence, the final temperature of the mixture containing vaporized fuel is reduced. This effect, together with high levels of supercharging, allows the use of either much smaller engines or lower speeds, without peak power and indicated efficiency losses, thus providing much lower fuel consumption in typical engine operational conditions, as described in Automotive Gasoline Direct-Injection Engines, F. Zhao, M.-C. Lao e D. L. Harrington, 2002. However, operating with undiluted mixture, the fuel efficient potential of spark ignited engine is still not fully realized. It should be noted that the typical dual-fuel engine, because of indirect injection of the main fuel, cannot take advantage of this in-cylinder massive charge cooling.

Advantages of adopting direct injection is more prominent with alcohol fuels due to their higher heat of evaporation and more fuel mass required to produce stoichiometric mixtures. Hydrous ethanol intensifies still more this effect. However, depending on the level of water content, combustion may deteriorate as described in SAE paper 2007-01-2648.

Aiming at profiting from this ethanol superiority over gasoline, US 7314033 patent describes a system which combines direct injection of ethanol with indirect injection of gasoline, for a supercharged engine. At low and medium loads, engine operates solely on gasoline. As load is increased above a certain level, more and more ethanol is directly injected into combustion chamber to inhibit knock. Expectations are a substantial reduction of engine size without power loss, resulting in a 20% fuel economy in typical drive cycles. However, gains to be provided by dilution are still denied.

The article Ethanol Boosted Direct Injection from Ricardo Quartely Magazine, Q1/2009 edition, shows the use of direct injection combined with dilution by EGR and turbocharging control for the Ethanol Boost Direct Injection (EBDI) engine. Actually, this technology can be applied to gasoline as well. Break mean effective pressure up to 3500 kPa have been reported.

Fuel stratification can be realized by injection of fuel-air mixture instead of fuel alone. Such systems were developed and can be found in WO 9702425 patent, SAE papers 910664 and 890415, but only the former reached industrial status.

SAE paper 890415 describes a system featuring an auxiliary chamber connected to the combustion chamber through a poppet valve smaller than intake and exhaust ones. Towards the end of the

compression stroke, this auxiliary chamber receives air from the main chamber, closing afterwards. After valve is closed part of fuel to be burned is injected into this auxiliary chamber. In the following cycle, the valve opens and lets the air-fuel mixture escape to the combustion chamber, fuel-air mixture being convected to the spark plug where it is ignited.

Orbital system, described in WO 9702425 patent, is fed by segregated fuel and previously compressed air lines. A main injector features a chamber receiving the pressurized air, where, at appropriate time, fuel is injected into from a secondary standard fuel injector. When the main injector needles lifts, the fuel-air mixture rushes into the combustion chamber and a very fine mist is produced. This system can make engine operation very tolerant to exhaust gas recirculation by means of the replacement of burned gas by air in the vicinity of the spark plug.

Conceptually, the system described in SAE paper 910664 is similar to the one described in SAE paper 890415, but compressed air is obtained from another cylinder in the compression stroke end instead of using an independent air compressor.

JP 2004257258, JP 2004028024, EP 1098074, US 5553579 and US 4066046 patents describe different forms and different objectives to stratify fuel in the combustion chamber, but none of them alludes to either stratification by means of vapor injection or stratification of the properties of the fuel-air mixture.

US 6725828 patent describes an increase of fuel vapor around spark plug, but does not allude to the generation and subsequent injection of this vapor. Instead, fuel is injected as liquid and, by means of structured flow inside the combustion chamber, vapor resulting from posterior fuel evaporation is directed towards the spark plug.

This invention relates to an innovative way of operating with diluted charge by adopting stratification of the properties in general of the in- cylinder charge instead of fuel stratification only. This invention allows a better and more independent and ample control of the fuel-air mixture properties at and around the spark plug in comparison with the rest of the combustion chamber, a control not possible to be obtained with existing systems.

SUMMARY OF THE INVENTION

This invention relates to a fuel injection system for optimum control of in-cylinder fluid properties around spark plug at combustion ignition time. The system features a Primary Subsystem, which injects a fuel, or a fuel mixture, in liquid, vapor or gas form, and a Secondary Subsystem, which injects a mixture containing at least one fuel and one no-fuel in, or mainly, vapor or gas form. In the context of this invention, a fuel mixture must be understood as a mixture of substances of which, at least one of them alone is a fuel. When air or plain oxygen is added to such a mixture, if proportion among reagents is adequate, combustion becomes possible if enough energy is provided for its initiation. In one invention variant fuel, or fuel mixture, from the Secondary Subsystem is injected directly into the combustion chamber, whereas fuel, or fuel mixture, from the Primary Subsystem is injected into the intake system. In a second invention variant fuel, or fuel mixture, from the

Secondary Subsystem is injected directly into the combustion chamber, and fuel, or fuel mixture, from the Primary Subsystem is also injected into the combustion chamber. In a third variant fuels and fuel mixtures from both the Primary and Secondary Subsystems are injected directly into the combustion chamber through the same injector.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a sketch of one invention variant where a fuel mixture from the Secondary Subsystem is injected directly into the combustion chamber, whereas fuel, or fuel mixture, from the Primary Subsystem is injected into the intake system.

Figure 2 shows a sketch of another invention variant where a fuel mixture from the Secondary Subsystem is injected directly into the combustion chamber, and fuel, or fuel mixture, from the Primary

Subsystem is also injected into the combustion chamber.

Figure 3 shows a sketch of a third invention variant where fuels and fuel mixtures, both from the Primary and Secondary Subsystems are injected directly into the combustion chamber through the same injector.

Figure 4 shows a simplified example of one possible fuel line.

Figure 5 shows a simplified example of another possible fuel line

Figure 6 shows a simplified example of third possible fuel line.

Figure 7 shows a simplified example of a secondary fuel

subsystem, which features two line branches connected to the fuel injector.

DETAILED DESCRIPTION OF THIS INVENTION

Figure 1 shows engine elements - piston (1 ), cylinder (2), intake port (3), exhaust port (4), direct injector (5) and indirect injector (6).

In the variant shown in figure 1 , direct injector (5) feeds the fuel, or the fuel mixture, from the Secondary Fuel Line (7) into the combustion chamber, whereas the indirect injector (6) feeds the fuel mixture from the Primary Fuel Line(8) into the intake system. Port fuel injection is only one possibility for indirect injection: injectors can be located further upstream.

The direct injector (5) and the Secondary Fuel Line (7) form the

Secondary Subsystem (9). Analogously, the indirect injector (6) and the Primary Fuel Line (8) form the Primary Subsystem (10).

In the variant shown in figure 2, both the injector (5A), part of the Secondary Subsystem (9), and the injector (11 ), part of the Primary Subsystem (10), perform their respective injections directly into the combustion chamber.

The variant shown in figure 3 features a single injector (12) to inject both the fuel, or the fuel mixture, from the Primary Fuel Line (8), and the fuel mixture, from the Secondary Fuel Line (7). Injector (12) complements both the Primary Subsystem(10) and the Secondary Subsystem(9).

In what follows, the above mentioned fuel lines will be discussed further, including some, but not only, possible configurations.

Figure 4 shows a simplified example of one fuel system

configuration. The following devices are illustrated: a Vapor Generator (13), a Gases&Vapor Mixer (14), the engine - excluding the fuel system - (15), and the Reservoir (16). Optionally, as in figure 6, a Cooler (17) may be included.

In the fuel system represented in figure 4, the liquid fuel (A) is either a single substance or a mixture of single substances, where each of these single substances may feature a singular behavior regarding the different processes that occur, or can occur, in the fuel and engine systems, for instance, vaporization, ignition, flame propagation, pre/post-ignition and autoignition. Gasoline is an example of such a mixture. Additionally, the fuel mixture may contain non flammable substances, but that affects fuel behavior and engine processes anyway, for instance, the water contained in hydrous ethanol. In figure 4, part of the liquid fuel (A) is diverted to the inferior branch, the Primary Fuel Line (8), then flowing to correspondent injectors. The rest of the liquid fuel is diverted to the superior branch, the Secondary Fuel Line (7), where it suffers some processes before being sent to correspondent injectors

The Vapor Generator (13) allows that the fuel (A), initially liquid, sent to the Secondary Subsystem, be injected as vapor. In order to do so, a controlled heating of the liquid fuel (A) occurs inside the Vapor

Generator (13). Possible sources of heat, but not limited to, are engine cooling water, exhaust gases or electrical resistor. In the configuration illustrated in figure 4 the source, or sources, must provide all heat required to fully vaporize the liquid fuel.

From the Vapor Generator (13), vapor flows to Gases& Vapor Mixer (14), a device aiming at mixing the diluent, or a combination of diluents (B and/or C), with the vaporized fuel. The diluent (B and/or C), or diluent mixture (B and/or C), can have more than one source and may contain, but is not restricted to, air, recirculated burned gases, or even vaporized water. The burned gases can contain hydrogen coming from reforming part of burned gases. The Gases&Vapor Mixer (14) must be effective enough in order to produce a mixture homogenization that does not result in significant combustion process and pollutant emissions differences from cylinder to cylinder and from cycle to cycle.

The Gases&Vapor Mixer (14) can, besides diluents (B and/or C), ad to the vaporized fuel another fuel, or fuel mixture, that is in the gaseous state in the operating temperature of the Secondary Fuel Line, for instance, methane, carbon monoxide, hydrogen or even a mixture of these and other gases. Hydrogen or mixture of gases containing hydrogen, methane, carbon monoxide can be provided by, but not restricted to, reforming either the same fuel being consumed by the engine without any reforming or another fuel. The Gases&Vapor Mixer (14) can receive additives or catalysts that modify mixture performance along evaporation, combustion and/or pollutant formation processes. Adding these and other substances to vaporized fuel may require more than two inlets from the Gases&Vapor Mixer (14).

The later in the compression stroke is the injection from the

Secondary Subsystem, the higher will be the required injection pressure. It might happen that the pressure is so high that the temperature required to fully vaporize the fuel in the Vapor Generator (13) renders this method not feasible, both on technical and economic grounds. One likely cause for a technical unfeasibility would be fuel chemical change. Aiming at avoiding such an excessive temperature, heated, but still, liquid fuel can be directly injected into the Gases&Vapor (14), where it will be then vaporized at a partial pressure lower than the prevailing inside the Mixer. In case that maximum fuel temperature allowed still precludes full fuel vaporization, diluents (B and/or C) can be pre-heated by means of a heater. If fuel vaporization effectively occurs only in the Gases&Vapor Mixer (13), the Vapor Generator (13) works just as a liquid fuel (A) heater.

From the Gases&Vapor Mixer (14), mixture is sent to the injector along the rest of the fuel line. A thermal insulation, or even heating, can be necessary to keep the mixture temperature above a specified level in order to, for instance, avoid vapor condensation. To have this line incorporated into the cylinder head is one possible alternative. Another alternative is to use a diluent pre-heater (B and/or C) to reach a

temperature above that strictly needed to vaporize the liquid fuel (A) or the placement of a post-heater after the Gases&Vapor Mixer (14).

In case the fuel, or fuels, supplied by the Secondary Subsystem is gas in the subsystem environment, natural gas for instance, the Vapor Generator is unnecessary, allowing adoption of configuration illustrated in figure 5.

As depicted in figure 5, while secondary fuel (D) is supplied, diluents and/or additives (B and/or C) can be provided through one or more inlets. Even using gaseous fuel, both pre and post-heater may be useful herein, the former being applied for the different inlets (B and/or C), either to vaporize diluents or to improve in-cylinder fluid properties. In the inferior branch, the Primary Fuel Line (8), liquid fuel (A) is sent to correspondent injectors as shown in figure 5.

Figure 6 introduces a simplified sketch of a more complex configuration than the one shown in figure 4, even without the occasional inclusion of pre and post-heaters. In this illustration a first split between the Primary and Secondary Fuel Line already occurs before the Vapor Generator (13), but it could occur only after.

In figure 6, the Vapor Generator features two outlets, one driving the produced vapor to the Gases&Vapor Mixer (14) and the other returning the remaining hot liquid to the Primary Subsystem. This sketch also shows a cooler (17) used to reduce the temperature of the returning hot liquid fuel.

Depending on the nature of the liquid fuel (A) supplied to the Vapor

Generator (13), the chemical composition of fuel vapor can be different from the one of the remaining liquid fuel. Gasoline is one example and azeotropic hydrous ethanol is a counter-example. One way of controlling vapor composition is by temperature of the vaporizing liquid fuel (A) inside the Vapor Generator. The liquid fuel (A) and heat fluxes should keep the specified temperature long enough to allow liquid-vapor equilibrium or, at least, close, before vapor is driven to the Vapor Generator outlet (13).

It may happen that the pressure needed for an appropriate vapor injection into the Gases&Vapor Mixer (14) implies in an excessive fuel temperature. One solution is to introduce a gas, for instance, air or burned gases, into the Vapor Generator (13), thus allowing fuel injection with a (partial) vapor pressure lower than the one prevailing in the Gases&Vapor Mixer (14).

As engine regime changes, the required fuel vapor rate changes. If liquid fuel (A) flow through the Vapor Generator (13) is high enough, the necessary abrupt increase on fuel vapor production can be provided by a fast flow increase in other gases present in the Generator. If a tight control of fuel vapor composition is needed for correct engine operation, positive managing of the amount of liquid fuel (A) that is transferred to Secondary Subsystem may be critical, thus requiring adoption of valves.

When engine stops all fuel system will be cooled, what will eventually condense the vaporized fuel. During engine restart, this fuel, in the liquid state, could be driven to Secondary Subsystem injectors in an irregular way, deteriorating engine operation and emissions. To avoid this situation may be necessary to purge vapor from the Secondary

Subsystem when engine stops. One alternative is to blow diluents (B or C) into the Secondary Subsystem, pushing away all vapors to a reservoir (16), through a canister. This procedure would not be needed in cases the fuel in the Secondary Subsystem is in gas state in the prevailing

environmental temperature, as natural gas.

Along in-cylinder intake and compression processes, first occurs the injection, or injections, of all, or most of, substances injected from the Primary Subsystem. Depending on fuel system architecture, this injection can occur either directly or indirectly into the combustion chamber. Later, occurs the injection, or injections, of all, or most of, substances injected from the Secondary Subsystem, directly into the combustion chamber. The mixture injected from the Secondary Subsystem affects mainly, but not necessarily only, the working fluid in the vicinity of the spark plug. Working fluid not so close to the spark plug can be also affected, aiming at, for instance, inducing faster flame propagation in specific directions or zones, for instance, to a zone more likely to induce knock.

Usual stratification produces a non uniform fuel distribution at ignition and flame propagation time. The Secondary Subsystem extends this very same concept to several properties that characterize the working fluid and are relevant for engine operation, for instance, fuel composition. Therefore, the adoption of the term Extended Stratification.

Among the alternative methods to produce extended stratification, the spray-guided, or more properly saying, the jet-guided, resulting from the Secondary Subsystem injection tends to be the more effective option. While others methods require some stabilization of the stratified mixture, the jet-guided stratification allows jet produced convection and turbulence, first, to spread rapidly the flame front and, second, disperse the

occasionally richer and hotter zone formed by secondary injections in the combustion chamber. On the other hand, mixture injection at ignition time at high loads can pose a challenge harder to be overcome.

Secondary injection is made of, essentially, a homogeneous mixture of vaporized and gaseous substances in specific proportions - convenction and turbulence rather than diffusion are the dominant transport mechanism. Thus, the level of change imposed on the working fluid by secondary injection can be directly controlled by Secondary Subsystem pressure, duration and timing. Not having any atomization requirement for secondary injection, jet penetration can be freely modified by adjusting pressure difference between the subsystem and the combustion chamber. However, it should be noted that this difference is likely to change substantially along injection, even if the subsystem pressure is kept constant. Therefore, provided that an effective

stratification process is developed, although not necessarily the mixture properties in the Secondary Subsystem are reproduced in the mixture-to- be-burned around the spark plug, how much these properties can be modified is positively and predictably controlled by secondary injection parameters.

The relative freedom to define the properties of the mixture to be injected by the Secondary Subsystem, together with the high level of control on how much the working fluid previously prevailing around the spark plug is replaced by the former, provides a dominium on the flame initiation phase not possible to be obtained with more conventional systems. Some examples on how to benefit from this invention follows.

In an engine operating with stoichiometric mixture and a high level of dilution by recirculated burned gases, the injection, by the secondary subsystem, of a mixture containing vaporized fuel and air at the

stoichiometric ratio will increase remarkably engine tolerance to overall dilution without precluding an efficient use of a three-way catalyst.

Injection timing and duration, as well multiple injections, control the composition, the volume and depth of the region affected by secondary injection. Whereas the mixture formed uniquely by the primary injection aims at optimizing engine efficiency by best balancing fluid properties, flame propagation, and heat and flow losses, avoiding abnormal combustion, secondary injection assures a fast and stable combustion initiation and early development.

Two concerns may arise for this kind of application. The first is connected to the amount of mass to be injected by the Secondary

Subsystem and the associated power. However, the injected mass being essentially gas, no atomization required, it is possible to limit the

necessary jet penetration by an appropriate design; both the amount of gas and pressure difference between the Secondary Subsystem and the combustion chamber can be reduced to a tolerable level. A second concern is backfiring, somewhat similar to what can occur in engines with central mixture preparation. This may represent a substantial risk depending on the reliability of the injector. Protection routines can be incorporated in the system control, deactivating ignition when any abnormality is detected, or reverse flow can simply be blocked by an appropriate device. Another possible way is to make the mixture in the Secondary Subsystem rich enough to not propagate a flame, leaning around the spark plug at ignition time being provided by a lean main charge. Mainly convection and turbulence would be responsible for diluting the richer mixture in the main charge.

In engines operating with lean mixtures, both emissions of oxides of nitrogen and, if highly supercharged, abnormal combustion tend to be a tougher challenge than in engine with stoichiometric mixtures diluted by burned gases. Abnormal combustion likelihood can be reduced by a further overall enleanment, while keeping a relatively richer mixture around spark plug by means of secondary injection. In case this richer zone is minute and it is diluted by nearby leaner gases just after it is burned, production of oxides of nitrogen can be reasonably controlled. Mixture enrichment can be reduced if it is possible to take full advantage from jet produced turbulence. Control of the temperature of the mixture injected from the Secondary Subsystem can be instrumental in reducing emissions of oxides of nitrogen. In this respect use of lighter fractions of gasoline or gasohol can be favorable due to vapor production at lower temperatures.

Still looking at reduction of oxides of nitrogen, a substance with higher molar specific heat than burned gases can be injected, for instance, carbon dioxide or water, thus allowing a higher thermal dilution with a lower reduction of oxygen concentration. Another improvement for engines operating with overall lean mixtures is addition, through the Secondary Subsystem, of other fuel more tolerant to dilution. This fuel may be originated from separation of components of the externally supplied fuel, or from a second external fuel supply, or even from on-board generation as in a reformer. Additives or catalyst can be used, either to promote combustion or to inhibit formation of oxides of nitrogen. Additives are understood as substances that although are consumed in the combustion chamber have as main purpose modifying the chemical processes rather than supplying thermal energy. Therefore, additives are not regarded as fuel in this context. Secondary injection can render additives and catalysts economically feasible due to the possibility of obtaining their full effect on combustion initiation and early development consuming only a very little amount of them.

One of the negative consequences arising from the use of dilution, either by air or burned gases, is the need for supercharging, or a higher level of supercharging, in order to recover power. A point can be reached where inefficiencies associated to supercharging are so high that a reduction rather than an increase of engine efficiency results. Also, peak pressures can reach a level not tolerable by engine components. Addition of water in liquid state, be as thermal diluent, be as a heat sink during evaporation, can act in a similar way to air in excess and recirculation of cooled burned gases with lower penalty on power and peak pressures. In engines operating with ethanol, higher water content has an equivalent effect. However, more water, as other diluent, has a negative impact on combustion, especially at ignition time and earlier flame development. Secondary injection can be applied to produce a mixture where water concentration is lower around the spark plug compared with overall charge water dilution.

Although secondary injection is more effective earlier in

combustion, benefits can be somewhat extended to later phases. A well developed stratification process will allow a smooth transition from properties prevailing around spark plug to those prevailing in more remote zones. This transition can be obtained by means of an additional secondary injection before the secondary injection that controls more closely the properties around the spark plug at ignition time or,

alternatively, a longer secondary injection. Later combustion phases can benefit, either directly or indirectly, from secondary injection. Direct benefit can be attained by an enough advanced secondary injection to disperse partially mixture in the entire combustion chamber. The indirect benefit may arise from the improvement caused by secondary injection on flame initiation and earliest combustion phases, so that other factors influencing combustion performance, such as intake port and combustion chamber shape, can be designed to privilege later combustion phases.

For illustrating direct benefits of secondary injection on later combustion phases, consider the use of hydrous ethanol with high water content. As load is reduced flame propagation becomes more and more difficult. As compensation, more and more anhydrous ethanol from the secondary injection can be earlier injected to sustain an appropriate flame speed towards combustion end.

Contrarily to examples shown, subsystems can, in specific engine regimes, operate alone. Secondary Subsystem deactivation could be implemented to save more expensive consumables. Primary Subsystem deactivation could be implemented to reduce engine throttling at partial loads. Compared with a conventional stratified direct injection system, a better stratification control and mixture properties would result from its stand-alone operation.

Two subsystems creating, inside the combustion chamber, flammable mixtures with different properties, increases the effective control over the combustion process, from cycle to cycle, from cylinder to cylinder as, just for instance, in an engine transient. Appropriate routines can be implemented to take full advantage of this level of control. Energy input split between both subsystems can be related to operational and environmental parameters, such as, but not limited to, engine load and speed, intake pressure, temperature and humidity, ignition timing and water temperature.

It must be obvious to those skilled in the art that this injection system can be applied to the combustion process known as, besides others, Controlled Autolgnition, where fuel already mixed with air burns inside the combustion chamber spontaneously, i.e., without direct assistance of any specific device, be a spark plug, a glow plug, or something similar. This injection system can be equally applied to a hybrid combustion process, where ignition is triggered by a specific device, but a substantial part of the charge burns without a discernible flame front propagation.

Not necessarily the mixture injected by the Secondary Subsystem must be prepared far upstream of in-cylinder injector nozzle. Mixing substances from Secondary Subsystem just before in-cylinder injection may be advantageous. For instance, final ratios among substances from Secondary Subsystem can be reached only short before injection to avoid accidental flame travelling upstream the fuel line. This separation can also aim at avoiding or reducing specific chemical reactions, for instance, oxidation of fuel. Additionally, properties of mixture injected from

Secondary Subsystem can be rapidly adjusted to suit changes in engine operational conditions. Properties can even be changed along the secondary injection period to improve still more control of in-cylinder properties.

Figure 7 shows one conceptual design for varying fuel mixture along secondary injection. Two branches, SLA and SLB, connected to the injector 12B, contain, at least, one different substance from each other and, in general, at different temperatures. Branch SLA has a slight lower pressure than branch SLB, but flow is limited at branch SLB by the restriction, or valve, 18. When main injector needle closes, stopping any injection into cylinder, a reduced flow from branch SLB to injector 12B and to branch SLA is established due to branch pressures difference. Any mixing chamber upstream main injector needle and branch SLA end have an enrichment of substances coming from branch SLB. After main injector needle lifts off again, injection starts with a ratio between substances coming from SLA and SLB lower than the average ratio. As injection evolves, fraction of substances originally coming from SLB become smaller, the opposite occurring for substances originally coming from branch SLA, provided pressure loss at restriction 18 becomes large enough.

While the invention was described in terms of some illustrative embodiments, those skilled in the art can replace or modify its architecture and elements without changing its essentials as covered by the claims.

Claims

1. Fuel injection system for a spark ignition engine,

characterized by two subsystems, a Primary Subsystem (10), which supplies fuel, or a fuel mixture, in liquid, vapor or gas forms, and a

Secondary Subsystem (9), which supplies fuel, or a fuel mixture, mostly or totally in the vapor or gas forms.

2. Fuel injection system according to claim 1 , wherein the Secondary Subsystem (9) supplies to the engine a mixture containing at least one fuel and, additionally, at least either one additive, or one catalyst, or one diluent.

3. Fuel injection system according to claims 1 and 2, wherein injections performed by the Primary Subsystem (10) occur in the intake system, through the injector (6), and wherein injections performed by the Secondary Subsystem (9) occur directly in the combustion chamber, through the injector (5).

4. Fuel injection system according to claims 1 and 2, wherein injections performed by the Primary Subsystem (10) occur directly in the combustion chamber, through the injector (11), and wherein injections performed by the Secondary Subsystem (9) occur directly in the

combustion chamber, through the injector (5A).

5. Fuel injection system according to claims 1 and 2, wherein injections performed both by the Primary Subsystem (10) and by the Secondary Subsystem (9) occur directly in the combustion chamber, through the same injector (12).

6. Fuel injection system according to claims 2 or 3 or 4 or 5, wherein Secondary Subsystem features more than one fuel branch (SLA and SLB) connected to the secondary fuel injector (12) or injector (12B), allowing properties of fuel mixture being injected from the Secondary Subsystem to vary from cycle to cycle and along injection, or injections, occurring in the same thermodynamic cycle.

7. Fuel injection system according to claims 2 or 3 or 4 or 5 or 6, wherein fuel, or fuel mixture, supplied by the Secondary Subsystem (9) results from vaporization, partial or total, of the fuel, or fuel mixture, provided to this Subsystem in the liquid state.

8. Fuel injection system according to claims 2 or 3 or 4 or 5 or 6, wherein fuel, or fuel mixture, supplied by each Subsystem (9 and 10) features different chemical composition one from another.

9. Fuel injection system according to claim 8, wherein the said different chemical compositions between Subsystems (9 and 10) results from the addition of, at least, one diluent (B and/or C) to the Secondary Subsystem (9).

10. Fuel injection system according to claim 9, wherein the said diluents can be air, recirculated burned gases, water or any combination of them.

11. Fuel injection system according to claim 8, wherein the said different chemical compositions between Subsystems (9 and 10) result from partial vaporization in the Secondary Subsystem (9) of one fuel, or a fuel mixture, provided to this Subsystem in the liquid state.

12. Fuel injection system according to claim 8, wherein the said different chemical compositions between Subsystems (9 and 10) result from water addition to the Primary Subsystem (10).

13. Fuel injection system according to claim 8, wherein the said different chemical compositions between Subsystems (9 and 10) result from supplying different fuels to each subsystems (9 and 10).

14. Fuel injection system according to claim 13, wherein at least one diluent (B or C) is added to the Secondary Subsystem (9).

15. Fuel injection system according to claim 8, wherein the said different chemical compositions between Subsystems (9 and 10) result from adding at least one additive or at least one catalyst to at least one of the Subsystems (9 and 10) in a proportion not exceeding 15%.