Fuel metering for SI engines The task of fuel metering is to control the mass and physical conditions of fuel, added to the new air that is induced in the cylinder at the beginning of each working cycle. An IC engine requires a certain air/fuel ratio to work properly at any operating condition. Air/fuel mixing in IC engines is influenced by: Injection system Fuel spray evolution Fuel properties Charge motions

Introduction Spark-ignition engines use fuels which are sufficiently volatile to be easily vaporized and mixed with air, before combustion is started with a spark-plug (premixed charge).

As a consequence, the task of fuel metering is mainly reduced to the control of the fuel mass, necessary to obtain an air fuel/ratio required by the engine in each point of its operating map. Spark-ignition engines use fuels which are sufficiently volatile to be easily vaporized and mixed with air, before combustion is started with a spark-plug (premixed charge). As a consequence, the task of fuel metering is mainly reduced to the control of the fuel mass, necessary to obtain an air fuel ratio α required by the engine in each point of its operating map. Mixture Requirements The optimum air/fuel ratio for SI engines depends on the engine performance which has to be optimized. At full load conditions, with a given filling coefficient: 1) The maximum power is obtained with a slightly rich mixture: φ= αstoich/α≈1.1 2) The maximum efficiency (or minimum specific fuel consumption) requires a lean mixture:φ= αstoich/α≈0.9 3) To reduce the pollutant emissions and to allow a correct operation of the three way catalyst, the engine should run with stoichiometric mixtures (φ= 1). In this way, conversion efficiencies for CO, NOx and HC are very high. For each operating regime, the best trade-off between these different requirements has to be found, choosing the air/fuel ratio αwhich gives a good value for the most important performance in that specific operating conditions.

In passenger car SI engines, the fuel metering system has to provide a value close to stoichiometric mixture, so that the three-way catalyst converter, used to clean up the exhaust gases from pollutants, can operate with high simultaneous conversion efficiencies both in reduction and oxidation of chemical reactions. Only in the high power area, which is not involved in the city use of the car, richer mixture are required to approximated the value of maximum power and to improve drivability and sport behaviour of the car. In new stratified-charge, SI engines, a more complex set of A/F ratio is required over their operating map. In any case, the fuel metering systems of SI engines have to control the fuel mass and mix it with air, to satisfy the complex mixture requirements of the engine. This result can be achieved using one of the following physical principles: 1) Carburetion: the pressure drop, produced by the air flow rate through a converging-diverging nozzle, to meter an appropriate fuel flow 2) Injection: the pressure increase produced in the fuel by a suitable pump, to inject a fuel spray in the induced air. Carburettors have been widely used in the past because of their operating simplicity. However, during the last decades, the increasingly severe legislations on pollutant emissions imposed the injection systems in passenger car SI engines. Today, carburettors are only used on SI engines for minor applications: chainsaws, for example.

Gasoline Injection Several gasoline injection systems have been developed for SI engines. They are based on different solutions for the main components of the system. The injector can be placed: 1) Outside the combustion chamber, so that the fuel is injected in the intake system: indirect-injection, port-fuel injection (PFI). 2) On the head of each cylinder, so that the fuel is directly injected into the combustion chambers (direct-injection, GDI).

Indirect Injection Single-point injection systems: they replaced carburettors. The single injector was placed upstream the throttle valve, in the position of the traditional carburettor. This solution is simple and cheap, however it does not allow a very precise control of the fuel distribution among the cylinders and the air/fuel ratio. Multi-point injection systems: they are now widely used. Gasoline is injected in the air stream near the intake port of each cylinder. The injection occurs during the first part of the intake stroke, so that the liquid spray has

enough time to mix with the air and to evaporate, forming a homogeneous mixture before the combustion starts (with uniform air/fuel ratio). Advantages of indirect-injection: 1) A low pressure is required upstream the injectors (0.3-0.4 MPa), because the droplet size in the liquid spray has not major importance for the air-fuel mixture formation. 2) The plant cost is reduced because of the low injection pressure and the position of the injectors, which do not have to face the high temperatures and pressures of the combustion chamber. 3) The typical high velocities, reached by the fluids during their passage through the valve throat, are used to improve the homogenization of the cylinder charge.

The fuel is injected near the valve and a fuel film could be form on the wall of the intake duct. This is a negative aspect for mixture preparation if evaporation is not completed and could be also critical in terms of soot formation in gasoline engine. If a single injection point is considered the velocity of incoming air and high volatility of fuel guarantee the completion of the process.

Injector types for PFI engines Standard injection valves for today’s modern fuel-injection systems are characterized by its small external dimensions and its low weight. They demonstrates excellent hot-fuel performance (little tendency for vapour-bubble formation when the fuel is hot). For better fuel atomization, the injection orifice plates with four orifices usually used are replaced by multi-orifice plates with up to 12 injection orifices. There are a wide variety of injectors available for different areas of application. These feature different lengths, flow classes, and electrical properties. This makes it possible to adapt individually to the engine’s intake-manifold geometry. An injector’s spray formation, i.e., its spray shape, spray angle, and fuel-droplet size, influences the formation of the air/fuel mixture. A: Tapered-spray: Individual jets of fuel are discharged through the holes in the injection-orifice plate. The tapered spray cone results from the combination of these fuel jets. Tapered-spray injectors are typically used in engines which feature only one intake valve per cylinder. B: Dual spray: Dual-spray formation is often used in engines which feature two intake valves per cylinder. Engines with three intake valves per cylinder must be equipped with dual spray injectors. The holes in the injection-orifice plate are arranged in such a way that two fuel sprays–which can be formed from a number of individual sprays (two tapered sprays)–leave the injector and impact against the respective intake valve or against the web between the intake valves. C: Gamma angle: Referred to the injector’s principle axis, the fuel spray in this case (single spray and dual spray) is at an angle, the offset spray angle. Injectors with this spray shape are mostly used when installation conditions are difficult

Fuel delivery with PFI An electric fuel pump delivers the fuel and generates the injection pressure, which for manifold injection is typically about 0.3-0.4MPa (3-4 bar). The built-up fuel pressure to a large extent prevents vapour bubbles from forming in the fuel system. The fuel filter and a pressure sensor are generally present and placed inside the tank and so the return duct is avoided. There can be a pump that is continuously working with the fuel excess or a demand-controlled system in which the fuel-supply pump delivers only that amount of fuel that is really used by the engine and that is required to set up the desired pressure. Pressure control is effected by means of a closed control loop in the engine ECU. To adjust the delivery volume of the fuel-supply pump, its operating voltage is altered by means of a clock module that is triggered by the engine ECU. The system is equipped with a pressure relief valve to prevent the build up of excessive pressure even during overrun fuel cut off or after the engine has been switched off.

Evolution of fuel metering systems 1) In 1967 Bosch with D-Jetronic introduced for the first time an electronic system in which

fuel was injected via electromagnetically actuated fuel injectors intermittently onto the intake valve of each cylinder (multi-point injection). The D-Jetronic comprises a gasoline-injection system that is controlled by intake-manifold pressure and engine speed. The electronic control unit receives signals for intake-manifold pressure, intake-air temperature, cooling-water (coolant) and/or cylinder-head temperature, throttle-valve position and movement, and starting, engine speed, and start of injection. The ECU processes these data, and sends electrical pulses to the fuel injectors. The ECU is interconnected with the electrical components via a multiple connector and wiring harness. The fuel injectors spray the fuel into the intake manifolds of the cylinders. The pressure sensor sends engine-load data to the ECU. The temperature sensors communicate the temperatures of air and coolant to the ECU. The thermo-time switch switches the electric start valve, which injects additional fuel into the intake manifold during low-temperature starts. The electric fuel pump continuously delivers fuel to the fuel injectors. The fuel filter is integrated in the fuel line to remove contaminants. The fuel-pressure regulator maintains a constant fuel pressure in the fuel lines. The temperature-dependent function of the auxiliary-air device provides additional air during engine warm-up. The throttle-valve switch sends engine idle and full-load states to the ECU.

2) K-Jetronic (1973) is a mechanically-hydraulically controlled fuel-injection system which needs no form of drive and which meters the fuel as a function of the intake air quantity and injects it continuously onto the engine intake valves. An electrically driven roller-cell pump pumps the fuel from the fuel tank at a pressure of

over 5 bar to a fuel accumulator and through a filter to the fuel distributor. The pressure regulator integrated in the fuel distributor holds the delivery pressure in the fuel system (system pressure) at about 5 bar. From the fuel distributor, the fuel flows to the fuel injectors. The fuel injectors inject the fuel continuously into the engine’s intake ports engine. When the intake valve is opened, the air drawn in by the engine carries the waiting “cloud” of fuel with it into the cylinder. An ignitable air-fuel mixture is formed during the induction stroke due to the swirl effect. The amount of air, corresponding to the position of the throttle valve, drawn in by the engine serves as the criterion for metering of the fuel to the individual cylinders. The amount of air drawn in by the engine is measured by the air-flow sensor, which, in turn, controls the fuel distributor. Injection occurs continuously, i.e., without regard to the position of the intake valve. When the intake valve is closed, the mixture is stored.

3) L-Jetronic (1973) is an electronically controlled fuel-injection system which injects fuel

intermittently into the intake manifolds. It does not require any form of drive. The electric fuel pump supplies the fuel to the engine and generates the pressure necessary for injection. Fuel injectors inject the fuel into the individual intake manifolds. An electronic control unit controls the fuel injectors. The ECU evaluates the signals delivered by the sensors and generates the appropriate control pulses for the fuel injectors. The amount of fuel to be injected is defined by the opening time of the fuel injectors.

4) Mono-Jetronic (1987) is an electronically controlled low-pressure central injection system for four-cylinder engines with a centrally situated electromagnetic fuel injector(single-point injection). The heart of Mono-Jetronic is the central injection unit with an electromagnetic fuel injector for intermittent fuel injection above the throttle valve. The lower section of the central injection unit comprises the throttle valve together with the throttle-valve potentiometer. The upper section accommodates the fuel system with the fuel injector, the pressure regulator and the fuel passages. In addition, the air-temperature sensor is located on the upper-section cap. The fuel flows to the injector via the lower passage. The upper passage is connected to the lower chamber of the pressure regulator, from which point excess fuel enters the fuel-return line via the plate valve.

Direct-injection (GDI) Nowadays, different Gasoline Direct-Injection Engines (GDI) are available on the market. The main advantages of direct-injection for SI engines are: 1) Better control of the air-fuel distribution in each cylinder. 2) Reduced knock risks (very important in turbocharged, downsized engines) 3) Possibility to run the engine with stratified charge combustion, where the mixture is no longer homogeneous within the combustion chamber, but stratified.

4) The mixture is rich close to the spark-plug, while is lean far from it. The overall equivalence ratio is lean. In this way, both fast flame propagation and reduced fuel consumption can be achieved. 5) Furthermore, it is also possible to regulate the engine load by varying the air/fuel ratio since very lean mixtures (α> 30) can be used.

In case of GDI engines, the pressure inside the cylinder is higher than in the intake manifold and the time available for the injection of the total mass of gasoline is shorter. Because of both these reasons, higher injection pressures have to be used (10-12 MPa). More robust and expensive injectors are required, because they have to stand the increased injection pressures and the high cylinder temperatures and pressures. Gasoline direct-injection represents the most convenient way to obtain a stratified injection, in those points of the engine operating map where the combustion of a stratified charge is convenient. At the same time, this injection system allows to return to the homogeneous mixture, just advancing the injection timing (i.e. 270°BTDC, during the first part of the induction stroke), to leave a longer time for the mixture homogenization inside the cylinder.

Charge Motion Control

1) Spray-guided: it is dominated by spray characteristics.

Advantages: - Potential for highly stratified operation Disadvantages: - Current spray are not ideal (therefore requires deep bowl in piston for containment) - Require spraying directly onto spark-plug electrodes (therefore reduced plug durability)

2) Wall-guided: it is dominated by the interaction between spray and wall. Advantages: - Light-load stratification easier to achieve - Combustion is less sensitive to spray characteristics Disadvantages: - Wall-wetting during cold and heavy-load operation causes HC emissions and smoke - Combustion modes are different in light-load and heavy-load regimes

3) Air-guided: it is dominated by the interaction of bulk air flow with spray. Advantages: - Combustion rate scales better with engine speed - Combustion control over load-range is easier Disadvantages: - Optimization over speeds and loads range is challenging, since spray is real time event - Potential for highly stratified operation may be limited

Direct Fuel-Injection Benefits 1) Increase volumetric efficiency: Direct-Fuel-Injection can result in an increase in air-flow due to spray-cooling of the intake air, when injection occurs during the intake stroke. The resulting increased performance can be converted to 1-2%increase of fuel economy. 2) Increased compression ratio: Direct-Fuel-Injection permits an increase of compression ratio resulting in about 2% increased efficiency. The increase in compression ratio results from an higher knock-tolerance due to:

- Spray cooling of the intake air when injection occurs during the intake stroke - Reduced eng-gas temperature when injection occurs during compression stroke

3) Decreased throttling losses: throttling losses are reduced by diluting the mixture with excess air or with EGR. But in a conventional homogeneous-charge system , the extent of dilution is limited due to the flame initiation and propagation limits. By stratifying the fuel-air mixture within the combustion chamber, the engine can be operated with extended dilution, at air-fuel ratios of 50:1 or greater.

Direct-injection system for SI engines The system consists of a low pressure stage, where gasoline is kept at a low constant value (0.35 MPa) by an electric pump placed in the fuel tank. This stage supplies gasoline to the high pressure pump, which is driven by the engine and increases the fuel pressure to values ranging from 2 to 20-30 MPa. The compressed gasoline is collected in a common-rail, which supplies each electro-injector for the different cylinders. The injection timing and fuel mass are settled by the electronic control unit through voltage pulses sent to the electro-injectors, on the basis of information taken from several sensors: air mass flow rate, engine speed and load, oxygen sensors in the exhaust gases, pressure in the common-rail, cooling and lubricant temperature, amount of EGR, … The pressure in the common-rail is measured by the sensor and kept to desired value by the pressure control valve. The air mass flow rate is controlled by the driver, as a function of both the required load and the fuel/air mixture characteristics (homogeneous or stratified charge), by means of an electronically controlled throttle valve. The actual value of the air mass flow rate is measured by the meter, based on the principle of a hot film kept at constant temperature (by electric Joule effect), while is cooled by the air flow. Two oxygen sensors, placed upstream and downstream the catalytic converters, control the desired air/fuel ratio and the efficiency of catalysts. The valve and the pressure sensor in the intake manifold control the amount of recycled exhaust gases (EGR) to the intake system to reduce the nitric oxide emissions.

Oxygen Sensor •Oxygen sensors are used to precisely control the mixture air/fuel ratio, which is supplied to the engine under various speed, loads and temperature operating conditions. •An oxygen sensor in the exhaust system, detects whether the engine is operating with lean or rich mixture, providing a signal to the ECU of the injection system to keep the actual air/fuel ratio as its set value. •The transducer is called oxygen sensor or lambda (λ) sensor. •The sensor consists in a cell where a solid state electrolyte (made of ZrO2) separates two electrodes of gas-permeable micro porous platinum. •The atmospheric air gets to the inner electrode surface through suitable holes in the sensor body. •Similar openings in the steel protection housing expose the outer electrode surface to the exhaust gases. •The outer electrode, exposed to exhaust gases, is covered by a thin layer of porous ceramic, to protect it from the thermal and chemical actions of burned gases. •The cell separates two gas mixtures (atmospheric air and burned gases), where the oxygen presents strongly different partial pressures. •Once a cell proper temperature is reached (above 300°C), at the platinum electrodes electrochemical oxidation-reduction reactions of oxygen occurs, while oxygen ions conducts the electric charge through the solid electrolyte. •The voltage available between the two electrodes depends only on the difference in the oxygen partial pressures on both sides of the element. •When the engine is running with rich mixtures (λ< 1), practically O2is not present in the burned gases. The very high O2partial pressure difference cause a large flow of oxygen ions to pass from the inner to the outer electrode, producing a relatively high voltage signal from the λ sensor. •When the mixture entering the cylinders shifts into a slightly lean fuel/air ratio, the O2 partial pressure in the exhaust gases abruptly increases of many orders of magnitude (about 10^6). This change is sufficient to suddenly cut down the oxygen ions migration through the solid-state electrolyte and consequently drops down the sensor voltage signal. •The λ sensor measures the oxygen content in the exhaust gases, which directly relates to the fuel/air ratio of the mixture entering the cylinders. •The sensor signal shows a relatively large stepped voltage variation at the stoichiometric mixture (λ= 1). •This makes a λ-sensor particularly proper for a closed-loop control system, which has to keep the composition of the induction mixture in a very narrow range around λ= 1, to maximize the global conversion efficiency of a three-way catalytic converter.

Injector types for GDI engines •In a SI engine, there are different sensors sending to the engine ECU the information about the changes of parameters influencing the required optimum mixture composition. On the basis of these values, the control unit states the mass of fuel mf that has to be injected per cycle in each

cylinder, under those specific operating conditions. The final action of the process is then completed by electromagnetically operating injectors. •With a proper timing, an electric pulse (for a time length Δt) is sent to the solenoid valve to rise (≈0.1 mm) the jet needle from its seat. The fuel is so forced by its upstream pressure to exit through the injector calibrated nozzle. When the electric pulse drops down, the spring again compresses the jet needle on its seat, cutting down the fuel injection. •Because the pressure difference Δp is acting across the injector is kept constant by the pressure regulator of the injection system, the mass mf of injected fuel can be related to the pressure difference Δp and the time interval Δt by the following expression: •For a desired mixture composition (α= ma/mf), on the basis of the measured total air mass flow rate, the control unit can predict by the previous relation the value of the required Δt: Therefore, on the basis of the influence produced by the engine operating conditions, the following main parameters have to be continuously measured in an injection system, to properly control the injected fuel mass: 1) The inducted air mass flow rate, as the main information about the air mass trapped in the cylinders, 2) The engine speed, to find the required fuel mass that must be injected per cycle in each cylinder, 3) The engine load, to select (together with the engine speed) the optimum air/fuel ratio for each operating condition 4) The pressure difference across the injector nozzle, to control the fuel mass injected during its operating interval, 5) The crank angle, to set the correct timing of the injection process within the engine power cycle, 6) The oxygen content in the exhaust gases, to precisely control the actual air/fuel ratio, 7) The temperature and pressure of the ambient air, to consider the density of the air supplied to the cylinders, 8) The cooling and lubricant temperatures, to take into account the engine thermal conditions (cold start, warm-up, etc.) 9) The amount of exhaust gas recycled to the intake, to control the propagation speed of the flame front 10) The signal of a knock sensor, to use mixture composition suitable to avoid abnormal combustions.

Fuel delivery in GDI Compared with injecting fuel into the intake manifold, there is only a limited time window available for injecting fuel directly into the combustion chamber. The fuel system is divided into: •Low-pressure circuits Low-pressure circuits for gasoline direct injection essentially use the fuel systems and components known in manifold-injection systems. Demand-controlled low-pressure are particularly well suited here in that the optimum admission pressure in each case can be set for every engine operating

state. •The high-pressure circuit consists of - High-pressure pump - High-pressure fuel rail - High-pressure sensor and, depending on the system,

- Pressure-control valve, or - Pressure-limiting valve

Depending on the operating point, a system pressure of between 5 and 12 MPa and in 2nd-generation systems of up to 20 MPa is set by means of high-pressure control in the engine ECU. The high-pressure injectors injecting the fuel directly into the engine’s combustion chamber are mounted on the fuel rail. In a demand-controlled system, the high pressure pump is driven by the engine camshaft. The delivery quantity is adjusted by a fuel-supply control valve. The engine ECU actuates the pump’s fuel supply control valve in such a way as to obtain in the rail the necessary system pressure for a given operating point.

Mixture formation for GDI engines The injected fuel leaves the nozzle as a jet with high speed, which breaks up in small droplets because of its high velocity, relative to the surrounding air and of the turbulence in the jet itself and in the air. The aerodynamic interactions produce a continuous division in droplets of smaller and smaller diameter and their progressive evaporation. Finally, the fuel vapour mixes with the air, forming a burnable mixture. The mixture formation process is not important in PFI engines. •In case of direct-injection, the characteristics of the spray produced by the injector and its aerodynamic interactions are more crucial, especially when a stratified charge is sought. •The three main physical characteristics of the spray are given by: 1) The mean diameter of the component droplets (atomization) 2) The mean space travelled by the droplets (penetration) 3) The angular opening of the spray (diffusion) •A correct mixture distribution is usually reached by controlling the spray evolution in time and space through the values of: injection pressure, injector geometry, turbulent air motion and combustion chamber geometry.

Cavitations: Formation of vapour cavities in a

liquid due to the decrease of static pressure below the vapour pressure. Starts from small nuclei/ micro bubbles.

Flash-Boiling: Flash boiling is the transient phenomenon that occurs when a liquid is subjected

to an environment where the ambient pressure is lower than the saturation vapour pressure of the liquid. Flash boiling can be obtained either by increasing the fluid temperature or by decreasing the ambient pressure. Flash-boiling atomization exploits this thermodynamic instability to break up a liquid jet through a three stage mechanism. First bubble nucleation is induced in the fluid followed by a rapid bubble growth leading to a boiling two-phase flow. As regards fuel injectors, as the heated liquid is accelerated through the injection nozzle its pressure decreases and, if the pressure falls sufficiently below the saturation vapour pressure, flash boiling of the liquid can result. Benefits of flash boiling are reduced drop sizes (smaller mean droplet diameter for larger surface area and improved vaporisation), increased cone angles (for better volume repartition) and reduced drop velocities (for reduced risk of piston crown impact). The combination of these benefits results in reduced spray-impingement and gives lower engine-out HC emissions. However, a potential strong change in spray shape can affect the mixture formation with significant deviation from the initial targeting and spray collapse. It is of primary importance to keep the spray spatial development under control. The heat transfer phenomena occurring in a GDI engine cylinder head cause the injected fuel to be heated before injection. During the injection process, when the heated fuel reaches a vapour pressure higher than the cylinder pressure, part of it is going to evaporate quickly through flash boiling. Flash boiling occurs when a cooled liquid is rapidly depressurized to a pressure sufficiently below the saturation pressure of the liquid.