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In recent years, legislative and market requirements have driven the need to reduce fuel consumption while meeting increasingly stringent exhaust emissions. This trend has dictated increasing complexity in automotive engines and new approaches to engine design. A key research objective for the automotive engineering community has been the potential combination of gasoline-engine specific power with diesel-like engine efficiency in a cost-competitive, production-feasible power train. One promising engine development route for achieving these goals is the potential application of lean burn direct injection (DI) for gasoline engines. In carburetors the fuel is sucked due to the pressure difference caused by the incoming air. This will affect the functioning of the carburetor when density changes in air are appreciable. There was a brief period of electronically controlled carburetor, but it was abandoned due to its complex nature. On the other hand in fuel injection the fuel is injected into the air.
TRANSITION OF FUEL SUPPLY SYSTEM
The transition of the fuel supply system used in automobiles is graphically shown below. In carburetor the fuel from the fuel chamber is sucked in by the pressure variation caused due to the incoming air. The fuel then mixes with the air and reaches the cylinder through the inlet manifold. Where as in a port injection system the fuel to the cylinder is supplied by a separate fuel injector placed near the inlet valve of the cylinder. And in a direct injection system the fuel to the cylinder is supplied by a fuel injector placed inside the cylinder.
OPERATING DIFFICULTIES FOR A CARBURETOR.
Some problems associated with comfortable running of the carburetor are discussed here.
1. Ice formation: The vaporisation of the fuel injected in the current of the air requires latent heat and the taken mainly from the incoming air. As a result of this, the temperature of the air drops below the dew point of the water vapour in the air and it condenses and many times freeze into ice if the temperature falls below dew point temperature.
2. Vapour Lock: The improved volatility of modern fuels and the necessity of providing heat to prevent the ice formation, has created carburetion difficulties due to vaporisation of fuel in pipes and float chamber. The heating may also occur due to petrol pipes being near the engine. If the fuel supply is large and supply is small, a high velocity will result causing high vacuum. This causes considerable drop which may also cause the formation of vapour bubbles. If these bubbles formed accumulate at the tube bend, then they may interrupt the fuel flow from the tank or the fuel pump and engine will stop because of lack of fuel. Vapour lock is formed because of rapid bubbling of fuel and usually happens in hot summer.
3. Back Firing: During the starting of an engine under cold working conditions, the usual manipulation of the choke varies the mixture from too lean to too rich. A very lean mixture will burn very slowly and the flame may still exist in cylinder when the exhaust valve is about to open. The fresh charge in the intake manifold is about to open. The fresh charge in the intake manifold is not so diluted as when inducted into the cylinder and mixed with the clearance gases and consequently burn more rapidly than the charge in the cylinder. If lean charge comes in contact with flames existing in the cylinder, there will be flash of flame back through the intake manifold, burning the charge therein and causing the customary back firing in the carburetor.
ADVANTAGES OF FUEL INJECTION OVER CARBURETOR
The fuel injection eliminates several intake manifold distribution problems. One of the most difficult problems in a carbureted system is to get the same amount and richness of air-fuel mixture to each cylinder. The problem is that the intake manifold acts as a storing device, sending a richer air fuel mixture to the end cylinders. The air flows readily around the corners and through various shaped passages. However the fuel, because it is heavier is unable to travel as easily around the bends in the intake manifold. As a result, some of fuel particles continue to move to the end of the intake manifold, accumulating there. This enriches the mixture going the end cylinder. The center cylinder closest to the carburetor gets the leanest mixtureÃ‚Â¬Ã‚Â¬Ã‚Â¬Ã‚Â¬Ã‚Â¬. The port injection solves this problem because the same amount of fuel is injected at each intake valve port. Each cylinder gets the same amount of air-fuel mixture of the same mixture richness.
Another advantage of the fuel injection system is that the intake manifold can be designed for the most efficient flow of air only. It does not have to handle fuel. Also, because only a throttle body is used, instead of a complete carburetor, the hood height of the car can be lowered.
With fuel injection, fuel mixture requires no extra heating during warm up. No manifold heat control valve or heated air system is required. Throttle response is faster because the fuel is under pressure at the injection valves at all times. An electric fuel pump supplies the pressure. The carburetor will depend on differences in air pressure as the force that causes the fuel to feed into the air passing through.
Fuel injection has no choke, but sprays atomized fuel directly into the engine. This eliminates most of the cold start problems associated with carburetors.
Electronic fuel injection also integrates more easily with computerized engine control systems because the injectors are more easily controlled than a mechanical carburetor with electronic add-ons.
Multi port fuel injection (where each cylinder has its own injector) delivers a more evenly distributed mixture of air and fuel to each of the engine's cylinders, which improves power and performance.
Sequential fuel injection (where the firing of each individual injector is controlled separately by the computer and timed to the engine's firing sequence) improves power and reduces emissions.
ELECTRONIC FUEL INJECTION
The main components of electronic fuel injection are described below.
1. Engine Control Unit (ECU)
3. Fuel Injectors
Engine Control Unit (ECU): This unit is the heart of electronic injection system which is responsible for metering the quantity of fuel supplied to each cylinder. The unit contains a number of printed circuits boards on which, a series of transistors, diodes and other electronic components are mounted. This makes the vital data analysing circuits responding to various input signals. After processing the input data, the power output circuits in the control unit generates current pulses which are transmitted to the solenoid injectors to operate the injector for the required period.
For example, when the pedal of the vehicle is stepped on, the throttle valve (this is the valve that regulates how much air enters the engine) opens up more, letting in more air. The engine control unit (ECU) "sees" the throttle valve open with the help of sensors and increases the fuel rate in anticipation of more air entering the engine. It is important to increase the fuel rate as soon as the throttle valve opens; otherwise, when the gas pedal is first pressed, there may be a hesitation as some air reaches the cylinders without enough fuel in it. Sensors monitor the mass of air entering the engine, as well as the amount of oxygen in the exhaust. The ECU uses this information to fine-tune the fuel delivery so that the air-to-fuel ratio is just right.
The ECU generally works in two operating modes, namely open loop and closed loop. In closed loop Oxygen sensor is used to sense the quantity of excess Oxygen in the smoke and this information is used for the next cycle of injection. This is also called feedback mode. On the other hand in open loop system the Oxygen sensor is not used.
Engine Sensors: In order to provide the correct amount of fuel for every operating condition, the engine control unit (ECU) has to monitor a huge number of input sensors. Here are just a few:
Â¢ Mass airflow sensor - Tells the ECU the mass of air entering the engine
Â¢ Oxygen sensor - The device measures the amount of oxygen in the exhaust gas and sends this information to the electronic control unit. If there is too much oxygen, the mixture is too lean. If there is too little, the mixture is too rich. In either case, the electronic control unit adjusts the air fuel ratio by changing the fuel injected. It is usually used with closed loop mode of the ECU.
Â¢ Throttle position sensor - Monitors the throttle valve position (which determines how much air goes into the engine) so the ECU can respond quickly to changes, increasing or decreasing the fuel rate as necessary
Â¢ Coolant temperature sensor - Allows the ECU to determine when the engine has reached its proper operating temperature
Â¢ Voltage sensor - Monitors the system voltage in the car so the ECU can raise the idle speed if voltage is dropping (which would indicate a high electrical load)
Â¢ Manifold absolute pressure sensor - Monitors the pressure of the air in the intake manifold. The amount of air being drawn into the engine is a good indication of how much power it is producing; and the more air that goes into the engine, the lower the manifold pressure, so this reading is used to gauge how much power is being produced.
Â¢ Engine speed sensor - Monitors engine speed, which is one of the factors used to calculate the pulse width.
Â¢ Crank Angle sensor - Monitors the position of the piston and gives the information to the ECU. Accordingly the ECU adjusts the valve timing.
The solenoid-operated fuel injector is shown in the figure above. It consists of a valve body and needle valve to which the solenoid plunger is rigidly attached. The fuel is supplied to the injector under pressure from the electric fuel pump passing through the filter. The needle valve is pressed against a seat in the valve body by a helical spring to keep it closed until the solenoid winding is energized. When the current pulse is received from the electronic control unit, a magnetic field builds up in the solenoid which attracts a plunger and lifts the needle valve from its seat. This opens the path to pressurised fuel to emerge as a finely atomised spray.
The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open. This is called the pulse width, and it is controlled by the ECU. The injectors are mounted in the intake manifold so that they spray fuel directly at the intake valves. A pipe called the fuel rail supplies pressurized fuel to all of the injectors.
ELECTRONIC FUEL INJECTION
There are two types of electronic fuel injection. They are,
1. Multipoint Fuel Injection (MPFI)
2. Gasoline Direct Injection (GDI)
MULTI POINT FUEL INJECTION (MPFI)
Engines with multi port injection have a separate fuel injector for each cylinder, mounted in the intake manifold or head just above the intake port.
Thus, a four-cylinder engine would have four injectors, a V6 would have six injectors and a V8 would have eight injectors. Multi port injection systems are more expensive because of the added number of injectors. But having a separate injector for each cylinder makes a big difference in performance. The same engine with multi port injection will typically produce 10 to 40 more horsepower than one with carburetor because of better cylinder-to-cylinder fuel distribution.
Injecting fuel directly into the intake ports also eliminates the need to preheat the intake manifold since only air flows through the manifold. This, in turn, provides more freedom for tuning the intake plumbing to produce maximum torque.
GASOLINE DIRECT INJECTION (GDI)
Fig7: A GDI System
In conventional engines, fuel and air are mixed outside the cylinder. This ensures waste between the mixing point and the cylinder, as well as imperfect injection timing. But in the GDI engine, petrol is injected directly into the cylinder with precise timing, eliminating waste and inefficiency. By operating in two modes, Ultra-Lean Combustion Mode and Superior Output Mode, the GDI engine delivers both unsurpassed fuel efficiency and superior power and torque. The GDI engine switches automatically between modes with no noticeable shift in performance. All the driver notices is a powerful driving experience, and much lower fuel bills. It's the best engine on the market. A Gasoline direct injection system consist various components as shown in the figure below.
MAJOR OBJECTIVES OF THE GDI ENGINE
Ultra-low fuel consumption, which betters that of diesel engines.
Superior power to conventional MPI engines
THE DIFFERENCE BETWEEN NEW GDI AND CURRENT MPI
For fuel supply, conventional engines use a fuel injection system, which replaced the carburetion system. MPI or Multi-Point Injection, where the fuel is injected to each intake port, is currently the one of the most widely used systems. However, even in MPI engines there are limits to fuel supply response and the combustion control because the fuel mixes with air before entering the cylinder. Mitsubishi set out to push those limits by developing an engine where gasoline is directly injected into the cylinder as in a diesel engine, and moreover, where injection timings are precisely controlled to match load conditions. The GDI engine achieved the following outstanding characteristics.
Â¢ Extremely precise control of fuel supply to achieve fuel efficiency that exceeds that of diesel engines by enabling combustion of an ultra-lean mixture supply.
Â¢ Very efficient intake and relatively high compression ratio unique to the GDI engine deliver both high performance and response that surpasses those of conventional MPI engines.
OUTLINE: Major Specifications (Comparison with MPFI)
Item GDI Conventional MPFI
Bore x Stroke (mm) 81.0 x 89.0
Number of Cylinders IL-4
Number of Valves Intake: 2, Exaust: 2
Compression Ratio 12.0 10.5
Combustion Chamber Curved Top Piston Flat top Piston
Intake Port Upright Straight Standard
Fuel System In-Cylinder Direct Injection Port Injection
Fuel Pressure (MPa) 50 3.3
The GDI engines foundation technologies
The GDI engines foundation technologies
There are four technical features that make up the foundation technologies are described below.
The Upright Straight Intake Port supplies optimal airflow into the cylinder.
The Curved-top Piston controls combustion by helping shape the air-fuel mixture.
The High Pressure Fuel Pump supplies the high pressure needed for direct in-cylinder injection.
The High Pressure Swirl Injector controls the vaporization and dispersion of the fuel spray.
MAJOR CHARACTERISTICS OF THE GDI ENGINE
Lower fuel consumption and higher output: Using methods and technologies, the GDI engine provides both lower fuel consumption and higher output. This seemingly contradictory and difficult feat is achieved with the use of two combustion modes. Put another way, injection timings change to match engine load.
For load conditions required of average urban driving, fuel is injected late in the compression stroke as in a diesel engine. By doing so, an ultra-lean combustion is achieved due to an ideal formation of a stratified air-fuel mixture. During high performance driving conditions, fuel is injected during the intake stroke. This enables a homogeneous air-fuel mixture like that of in conventional MPI engines to deliver higher output.
Two Combustion Modes: In response to driving conditions, the GDI engine changes the timing of the fuel spray injection, alternating between two distinctive combustion modes- stratified charge (Ultra-Lean combustion), and homogenous charge (Superior Output combustion).
Under normal driving conditions, when speed is stable and there is no need for sudden acceleration, the GDI engine operates in Ultra-Lean Mode. A spray of fuel is injected over the piston crown during the latter stages of the compression stroke, resulting in an optimally stratified air-fuel mixture immediately beneath the spark plug. This mode thus facilitates lean combustion and a level of fuel efficiency comparable to that of a diesel engine.
The GDI engine switches automatically to Superior Output Mode when the driver accelerates, indicating a need for greater power. Fuel is injected into the cylinder during the piston's intake stroke, where it mixes with air to form a homogenous mixture. The homogenous mixture is similar to that of a conventional MPI engine, but by utilising the unique features of the GDI, an even higher level of power than conventional petrol engines can be achieved.
In-cylinder Airflow: The GDI engine has upright straight intake ports rather than horizontal intake ports used in conventional engines. The upright straight intake ports efficiently direct the airflow down at the curved-top piston, which redirects the airflow into a strong reverse tumble for optimal fuel injection.
Precise Control over the Air/Fuel Mixture: The GDI engine's ability to precisely control the mixing of the air and fuel is due to a new concept called wide spacing," whereby injection of the fuel spray occurs further away from the spark plug than in a conventional petrol engine, creating a wide space that enables optimum mixing of gaseous fuel and air.
In stratified combustion (Ultra-Lean Mode), fuel is injected towards the curved top of the piston crown rather than towards the spark plug, during the latter stage of the compression stroke. The movement of the fuel spray, the piston head's deflection of the spray and the flow of air within the cylinder cause the spray to vapourise and disperse. The resulting mixture of gaseous fuel and air is then carried up to the spark plug for ignition. The biggest advantage of this system is that it enables precise control over the air-to-fuel ratio at the spark plug at the point of ignition.
The GDI engine's intake ports have been made straight and upright to create a strong flow that facilitates mixing of the air and fuel. Air is drawn smoothly and directly down through the intake ports toward the cylinder, where the piston head redirects it, forcing it into a reverse vertical tumble flow, the most effective flow pattern for mixing the air and fuel and carrying the mixture up to the spark plug. The GDI engine's pistons boast unique curved tops-forming a rounded combustion chamber-the most effective shape for carrying the gaseous fuel up to the spark plug.
In addition to its ability to mix thoroughly with the surrounding air, the fuel spray does not easily wet the cylinder wall or the piston head. In homogeneous combustion (Superior Output Mode), fuel is injected during the intake stroke, when the piston is descending towards the bottom of the cylinder, vapourising into the air flow and following the piston down. Again, it's all in the timing. By selecting the optimum timing for the injection, the fuel spray follows the movement of the piston, but cannot catch up. In this case, as the piston moves downward and the inside of the cylinder become larger in volume, the fuel spray disperses widely, ensuring a homogenous mixture.
Fuel Spray: Newly developed high-pressure swirl injectors provide the ideal spray pattern to match each engine operational modes. And at the same time by applying highly swirling motion to the entire fuel spray, they enable sufficient fuel atomization that is mandatory for the GDI even with a relatively low fuel pressure of 50kg/cm2.
Optimized Configuration of the Combustion Chamber: The curved-top piston controls the shape of the air-fuel mixture as well as the airflow inside the combustion chamber, and has an important role in maintaining a compact air fuel mixture. The mixture, which is injected late in the compression stroke, is carried toward the spark plug before it can disperse.
Realization of lower fuel consumption
In conventional gasoline engines, dispersion of an air-fuel mixture with the ideal density around the spark plug was very difficult. However, this is possible in the GDI engine. Furthermore, extremely low fuel consumption is achieved because ideal stratification enables fuel injected late in the compression stroke to maintain an ultra-lean air-fuel mixture.
An engine for analysis purpose has proved that the air-fuel mixture with the optimum density gathers around the spark plug in a stratified charge. This is also borne out by analyzing the behavior of the fuel spray immediately before ignition and the air-fuel mixture itself.
As a result, extremely stable combustion of ultra-lean mixture with an air-fuel ratio of 40 is achieved as shown below.
Combustion of Ultra-lean Mixture
In conventional MPI engines, there were limits to the mixtures leanness due to large changes in combustion characteristics. However, the stratified mixture of the GDI enabled greatly decreasing the air-fuel ratio without leading to poorer combustion. For example, during idling when combustion is most inactive and unstable, the GDI engine maintains a stable and fast combustion even with an extremely lean mixture of 40 to 1 air-fuel ratio.
VEHICLE FUEL CONSUMPTION
Fuel Consumption during Idling: The GDI engine maintains stable combustion even at low idle speeds. Moreover, it offers greater flexibility in setting the idle speed.
Compared to conventional engines, its fuel consumption during idling is 40% less.
Fuel Consumption during Cruising Drive: At 40km/h, for example, the GDI engine uses 35% less fuel than a comparably sized conventional engine.
Better Fuel Efficiency: The concept of wide spacing makes it possible to achieve a stratified mixture, enabling the GDI engine to offer stable, ultra-lean combustion, allowing a significant improvement in fuel efficiency. In addition to ultra-lean combustion, the GDI engine achieves a higher compression ratio because of its anti-knocking characteristic and precise control of injection timing. These features contribute to drastically lower fuel consumption. The GDI engine improves fuel economy by 33% in the Japanese 10-15 mode driving cycle which represents typical urban driving conditions.
Emission Control: Previous efforts to burn a lean air-fuel mixture have resulted in difficulty to control NOx emission. However, in the case of GDI engine, 97% NOx reduction is achieved by utilizing high-rate EGR (Exhaust Gas Ratio) such as 30% that is allowed by the stable combustion unique to the GDI as well as a use of a newly developed lean-NOx catalyst.
REALIZATION OF SUPERIOR OUTPUT
To achieve power superior to conventional MPI engines, the GDI engine has a high compression ratio and a highly efficient air intake system, which result in improved volumetric efficiency. In high-load operation, a homogeneous mixture is formed. (When extra power is needed, the GDI engine switches automatically to Superior Output Mode.) Because it burns a homogenous mixture in this mode, the GDI engine functions like any other MPI engine. However, by maximising its technical features, the GDI engine achieves substantially higher power than a conventional engine.
One of the principal reasons for this is that a fine spray of fuel is injected in a wide shower directly into the cylinder, where it vapourises instantly into the air flow. This causes the air to cool and contract, allowing additional air to be drawn in and improving volumetric efficiency. The cooling of the intake air prevents knocking, and results in higher power output.
Another reason for the GDI engine's ability to offer such superb power is that it prevents knocks. With conventional MPI engines, strong knocking occurs during acceleration. This is caused by petrol adhering to the intake ports. The low-octane elements of the fuel are forced into the cylinder immediately after accelerating, where they mix with air and ignite, causing knocking. With the GDI engine, fuel is injected directly into the cylinder and burned completely, meaning that transient knocking is suppressed. This in turn, allows higher output in the early stages of acceleration, when power is most needed. The most significant feature of petrol direct-injection is the fact that engine technology has finally achieved precise control over formation of the air/fuel mixture. We have capitalised on this achievement to develop an innovative anti-knock technology called Two-Stage Mixing. In high load, when it is necessary to supply large amounts of fuel, a homogenous air/fuel mix is used to prevent partially dense mixtures that cause soot to form. In contrast, the new Two-Stage Mixing technology prevents soot even during stratified mix, when a dense mixture forms. This is how knocking can be prevented.
In Two-Stage Mixing, about 1/4 of the total volume of fuel is injected during the intake stroke. This forms an ultra-lean fuel mixture which is too lean to burn under normal conditions. The remaining fuel is injected during the latter stages of the compression stroke. The key is that the air/fuel mixture is divided into a very lean air/fuel mixture and a rich air/fuel mixture. Knocking occurs most frequently in a stochiometric mixture, but is less likely to occur when the mixture becomes leaner or richer. Because the rich mixture is formed immediately before ignition, there is no time for the chemical reaction that causes knocking to take place. This is another of the factors that prevent knocking.
More important to note, is that the emission of soot is prevented, even when a dense air/fuel mixture is formed, and excess air is not sufficient. If air were the only gas present in the combustion chamber-as is the case with an ordinary diesel engine-the enriched charge would cool, causing soot to form. With Two-Stage Mixing, the enriched charge, created in the part of the chamber where the dense air/fuel mixture exists, shifts toward the other side of the chamber, where the mixture is leaner, as it burns. At this point, the enriched charge causes the ultra-lean mixture, which is too lean to burn under ordinary circumstances, to ignite. The combustion of the ultra-lean mixture, in turn, causes the enriched charge to re-ignite. It is this process that suppresses the formation of soot. This is the first time in the long history of petrol engines that direct control of combustion has been used to suppress knocking, and it further underscores the importance of achieving precise control over the air/fuel mixture.
Improved Volumetric Efficiency: Compared to conventional engines, the Mitsubishi GDI engine provides better volumetric efficiency. The upright straight intake ports enable smoother air intake. And the vaporization of fuel, which occurs in the cylinder at a late stage of the compression stroke, cools the air for better volumetric efficiency.
Increased Compression Ratio: The cooling of air inside the cylinder by the vaporization of fuel has another benefit, to minimize engine knocking. This allows a high compression ratio of 12, and thus improved combustion efficiency.
Engine performance: Compared to conventional MPI engines of a comparable size, the GDI engine provides approximately 10% greater outputs and torque at all speeds.
Vehicle Acceleration: In high-output mode, the GDI engine provides outstanding acceleration. The following chart compares the performance of the GDI engine with a conventional MPI engine.
Frequent operation in stratified mode.
Reduction of CO2 production by nearly 20 percent.
Provides improved torque.
Fulfills future emissions requirements.
97% NOx reduction is achieved.
Improve the brake specific fuel consumption.
Smooth transition between operation modes.
Reduced fuel consumption 15-20%
Higher torque 5-10%
Up to 5% more power
Spontaneous response behavior
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2) YONG-JIN KIM, Effect of Motion on Fuel Spray Characteristics in A GDI Engine, Institute for Advanced Engineering, 1999-01-0177.
3) DOMKUNDWAR, A course in IC engines.
4) CROUSE/ANGLIN, Automotive mechanics.