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For Gasoline engines, Carburetor s were the predominant method to meter fuel prior to the widespread use of fuel injection, however various fuel injection schemes have existed since the earliest usage of the internal combustion engine. Prior to 1980, nearly all gasoline engines used carburetors. Since 1990, almost all gasoline passenger cars sold in the United States use electronic fuel injection ( EFI ). The functional objectives for fuel injection systems can vary. All share the central task of supplying fuel to the combustion process, but it is a design decision how a particular system will be optimized. There are several competing objectives such as:
Certain combinations of these goals are conflicting, and it is impractical for a single engine control system to fully optimize all criteria simultaneously. In practice, automotive engineers strive to best satisfy a customer's needs in a competitive manner. The modern digital EFI system is far more capable at optimizing these competing objectives than a carburetor. BENEFITS An engine's air/fuel ratio must be Accurately controlled under all operating conditions to achieve the desired engine performance, emissions, driveability and fuel economy. Modern EFI systems meter fuel with great precision, and when used in conjunction with an Exhaust Gas Oxygen Sensor (EGO sensor), they are also very accurate. The advent of digital Closed Loop fuel control, based on feedback from an EGO sensor, permit EFI to significantly out perform a carburetor. The two fundamental improvements are: #Reduced response time to rapidly changing inputs, e.g., rapid throttle movements. #Deliver an accurate and equal mass of fuel to each cylinder of the engine, dramatically improving the '''cylinder-to-cylinder''' distribution of the engine. These two features result in the following performance benefits:
Injection systems have evolved significantly since the mid 1980s. Current EFI systems provide an accurate and cost effective method of metering fuel. The emission and subjective performance characteristics have steadily improved with the advent of modern digital controls, which is why EFI systems have replaced carburetors in the marketplace. EFI is becoming more reliable and less expensive through widespread usage. At the same time, carburetors are becoming less available, and more expensive. Even marine applications are adopting EFI as reliability improves. If this trend continues, it is conceivable that virtually all internal combustion engines, including garden equipment and snow throwers, will eventually use EFI. It should be noted that a carburetor's fuel metering system is a less expensive alternative when strict emission regulations are not a requirement, as is the case in developing countries. EFI will undoubtedly replace carburetors in these nations too as they adopt emission regulations similar to Europe, Japan and North America. REGULATORY MOTIVATION Throughout the 1950s and 1960s, various federal, state and local governments conducted studies into the numerous sources of air pollution. These studies ultimately attributed a significant portion of air pollution to the automobile, and concluded air pollution is not bounded by local political boundaries. At that time, the primary source of emission regulations was legislated at the local level. The ineffective scope of local regulations was gradually superseded with more strategically comprehensive state and federal regulations. By 1967 the state of California (Governor Reagan), created the California Air Resources Board , and in 1970, the U.S. Environmental Protection Agency was formed. Both agencies now create and enforce emission regulations from automobiles, as well as many other sources. Additionally, similar studies and regulations were simultaneously developed in Europe and Japan. The primary source of internal combustion engine emissions is the incomplete combustion of a miniscule fraction of the total fuel consumed. The unburned portion of fuel is so small, the lost energy is trivial to fuel efficiency, and therefore commercially insignificant to the final customer. Auto manufacturers were eventually motivated by emission regulations to address this issue. The modern EFI system evolved to gain deliberate control of the small fraction of unburned fuel. The ultimate combustion goal is to match each molecule(s) of fuel with a corresponding molecule(s) of oxygen so that neither has any molecules remaining after combustion - (see Stoichiometry ). This is a gross oversimplification of complex combustion chemistry that occurs in a difficult to manage environment. However, it accurately describes the magnitude of the fuel metering task, as well as the precision of a modern EFI system. BASIC FUNCTION The process of determining the amount of fuel, and its delivery into the engine, are known as fuel metering. Early injection systems used mechanical methods to meter fuel (non electronic, or mechanical fuel injection). Modern systems are nearly all electronic, and use an electronic solenoid (the injector) to inject the fuel. An electronic Engine Control Unit calculates the mass of fuel to inject. The Fuel Injector acts as the fuel-dispensing nozzle. It injects liquid fuel directly into the engine's air stream. In almost all cases this requires an external pump. The pump and injector are only two of several components in a complete fuel injection system. In contrast to an EFI system, a Carburetor directs the induction air through a Venturi , which generates a minute difference in air pressure. The minute air pressure differences both Emulsify (premix fuel with air) the fuel, and then acts as the force to push the mixture from the carburetor nozzle into the induction air stream. As more air enters the engine, a greater pressure difference is generated, and more fuel is metered into the engine. A carburetor is a self-contained fuel metering system, and is cost competitive when compared to a complete EFI system. An EFI system requires several peripheral components in addition to the injector(s), in order to duplicate all the functions of a carburetor. A point worth noting during times of fuel metering repair is that EFI systems are prone to diagnostic ambiguity. A single carburetor replacement can accomplish what might require numerous repair attempts to identify which one of the several EFI system components is malfunctioning. On the other hand, EFI systems require little regular maintenance; a carburetor typically require seasonal and/or altitude adjustments. TYPE OF FUEL The calibration, and often the design, of a fuel injection system differs depending on the type of fuel: Propane ( LPG ), Gasoline , Ethanol , Methanol , Methane (natural gas), Hydrogen or Diesel . The vast majority of fuel injection systems are for gasoline or diesel applications, and in the past, their components and designs were quite different. With the advent of "electronic" fuel injection, the diesel and gasoline hardware have grown quite similar. EFI's programmable software has permitted common hardware to be used across some of the fuels.
DETAILED FUNCTION
Typical EFI components
Functional description A contemporary EFI system requires a number of Sensor s to measure the engine's operating conditions. A CPU interprets these conditions in order to calculate the amount of fuel, among numerous other tasks. The desired "fuel flow rate" depends on several conditions, with the engine's "air flow rate" being the fundamental factor. The electronic fuel injector is normally closed and opens to flow fuel as long as an electric pulse is applied to the injector. The pulse's duration (pulsewidth) is proportional to the amount of fuel desired. The pulse is applied once per engine cycle, which permits pressurized fuel to flow from the fuel supply line, through the open injector, into the engine's air intake, usually just ahead of the intake valve. Since the nature of fuel injection dispenses fuel in discrete amounts, and since the nature of the 4-stroke -cycle engine has discrete induction (air-intake) events, the CPU calculates fuel in discrete amounts. The injected fuel mass is tailored for each individual induction event. In other words, every induction event, of every cylinder, of the entire engine, is a separate fuel mass calculation, and each injector receives a unique pulsewidth based on that cylinder's fuel requirements. It is necessary to know the mass of air the engine "breathes" during each induction event. This is proportional to the intake manifold's air pressure/temperature, which is proportional to throttle position. The amount of air inducted in each intake event is known as "air-charge", and this can be determined using one of several methods, but this is beyond the scope of this topic. (See MAF Sensor , or MAP Sensor .)
The three elemental ingredients for combustion are fuel, air and . Deviations from stoichiometry are required during non-standard operating conditions such as heavy load, or cold operation, in which case, the mixture ratio can range from 10:1 to 18:1 (for gasoline).
Additionally, final pulsewidth is inversely related to pressure difference across the injector inlet and outlet. For example, if the fuel line pressure increases (injector inlet), or the manifold pressure decreases (injector outlet), a smaller pulsewidth will meter the same fuel. Fuel injectors are available in various sizes and spray characteristics as well. Compensation for these and many other factors are programmed into the CPU's software. In summary, the vehicle operator opens the engine's throttle (right pedal), atmospheric pressure forces air into the engine past sensors that indicate air mass flow. The CPU interprets these signals from the sensors, calculates the desired air/fuel ratio, and then outputs a pulsewidth providing the exact mass of fuel for optimal combustion. This process is repeated every time an intake valve opens. The modern EFI system treats each injection as a discrete event, which when all strung together, perform one, smooth, seamless experience. An oversimplified analogy is that it is not unlike a motion picture that appears to move from a series of individual images. Sample pulsewidth calculations
Calculate injector pulsewidth from airflow :First the CPU determines the air mass flow rate from the sensors - lb-air/min. ''(The various methods to determine airflow are beyond the scope of this topic. See MAF Sensor , or MAP Sensor .)''
::::''- min/rev is the reciprocal of engine speed (RPM) – minutes cancel.'' ::::''- rev/4-intake-stroke for an 8 cylinder 4-stroke -cycle engine.''
::::''- fuel/air is the desired mixture ratio, usually stoichiometric, but often different depending on operating conditions.''
::::''- injector-size is the flow capacity of the injector, which in this example is 24-lbs/hour if the fuel pressure across the injector is 40 psi.'' :Combining the above three terms . . .
:Substituting real variables for the 5.0L engine at idle.
:Substituting real variables for the 5.0 L engine at maximum power.
: ''Injector pulsewidth typically ranges from 2 ms/engine-cycle at idle, to 20 ms/engine-cycle at wide-open throttle. The pulsewidth accuracy is approximately 0.01 ms; injectors are very precise devices.'' Calculate fuel-flow rate from pulsewidth
::::''Looking at it another way:''
::::''Looking at it another way:''
:Substituting real variables for the 5.0 L engine at idle.
:Substituting real variables for the 5.0L engine at maximum power.
: ''The fuel consumption rate is 68 times greater at maximum engine output than at idle. This dynamic range of fuel flow is typical of a Naturally Aspirated passenger car engine. The dynamic range is greater on a Supercharged or Turbocharged engine. It is interesting to note that 15 Gallon s of gasoline will be consumed in 37 minutes if maximum output is sustained. On the other hand, this engine could continuously idle for almost 42 hours on the same 15 gallons.'' VARIOUS INJECTION SCHEMES Throttle body injection (TBI or CFI) Throttle-body injection (called '''TBI''' by General Motors and '''CFI''' by Ford ) was introduced in the mid 1980's as a transition technology toward individual port injection. The TBI system injects fuel at the Throttle Body (the same location where a carburetor introduced fuel). The induction mixture passes through the intake runners like a carburetor system. The justification for the TBI/CFI phase was low cost. Many of the carburetor's supporting components could be reused such as the air cleaner, intake manifold and fuel line routing. This postponed the redesign and tooling costs of these components. Most of these components were later redesigned for the next phase of fuel injection's evolution, which is individual port injection, commonly known as EFI. TBI was used briefly on passenger cars during the mid 80's, and by GM on heavy duty trucks all the way through OBD-I (ending in 1995). Continuous injection Bosch's K-Jetronic or CIS used a continuous injection method. Gasoline was pumped through a large control valve called a ''fuel distributor'', which sat atop a control vane mounted in the air intake pathway. The fuel went from there to the injectors on each cylinder's intake port (which were simply nozzles with no valves in them). The system worked by varying fuel mixture based on the amount of air flowing past the control vane. This system was used for many years by Mercedes Benz , Volkswagen and Volvo . There was also a variant of the system called KE-Jetronic that used an Oxygen Sensor to fine-tune the mixture. Central port injection (CPI) General Motors developed an "in-between" technique called "central port injection" (CPI) or "central port fuel injection" ('''CPFI'''). It uses tubes from a central injector to spray fuel at each intake port rather than the central throttle-body. However, fuel is continuously injected to all ports simultaneously, which is less than optimal. Multi-port fuel injection (PFI or EFI or SEFI) This system uses a single injector per cylinder, and traditionally injects fuel immediately ahead of the intake Valve s. Direct injection Many Diesel Engine s feature direct injection ('''DI'''). The injection nozzle is placed inside the Combustion Chamber and the Piston incorporates a depression (often Toroid al) where initial combustion takes place. Direct injection diesel engines are generally more efficient and cleaner than Indirect Injection engines. Some recent Petrol Engine s utilize direct injection as well. This is the next step in evolution from multi port fuel injection and offers another magnitude of emission control by eliminating the "wet" portion of the induction system. ''See also: Gasoline Direct Injection '' EVOLUTION Pre-emission era Frederick William Lanchester joined the Forward Gas Engine Company Birmingham , England in 1889. He carried out what were possibly the earliest experiments with fuel injection. Fuel injection has been used commercially in Diesel Engine s since the mid 1920s. The concept was adapted for use in petrol-powered aircraft during World War II , and direct injection was employed in some notable designs like the Daimler-Benz DB 603 and later versions of the Wright R-3350 used in the B-29 Superfortress . One of the first commercial gasoline injection systems was a mechanical system developed by Bosch and introduced in 1955 on the Mercedes-Benz 300SL . An early electronic fuel injection system was developed by the Bendix Corporation , but a commercial application was impractical at the time; there did not yet exist solid-state sensors or mass-produced transistors. This did not stop them from being introduced in the 1958 DeSoto Adventurer , arguably the first production (throttle-body) EFI system. The patents were subsequently sold to Bosch. In 1957, Chevrolet introduced a mechanical fuel injection option, made by General Motors ' Rochester division, for its 283 V8 Engine . This system directed the inducted engine air across a "spoon shaped" plunger, which moved in proportion to the air volume. The plunger connected to the fuel metering system which mechanically dispensed fuel to the cylinders via distribution tubes. This engine produced 283 hp (211 kW) from 283 in³ (4.6 L), making it the first production engine in history to exceed 1 hp/in³ (45.5 kW/L). In another approach, Mercedes' used six individual plungers to feed fuel to each of the six cylinders. During the 1960's, other mechanical injection systems such as Hilborn were occasionally used on modified American V8 engines in various racing applications such as Drag Racing , Oval Racing , and Road Racing . These racing-derived systems were not suitable for everyday street use. Post emission era Bosch developed the second production electronic fuel injection system, called D-Jetronic (D for Druck, the German word for pressure), which was first used on the Volkswagen 411 in 1967. This was a speed/density system, using engine speed and intake manifold air density to calculate "air mass" flow rate and thus fuel requirements. The system used all analog, discrete electronics, and an electro-mechanical pressure sensor. The sensor was susceptible to vibration and dirt. This system was adopted by VW , Mercedes-Benz , Porsche , Citroën , Saab and Volvo . Lucas licensed the system for production with Jaguar . Bosch replaced the D-Jetronic system with the L-Jetronic system. L-Jetronic uses a mechanical airflow meter (L for Luft, German for air) which produces a signal that is porportional to "air volume". This approach required additional sensors to measure the barometer and temperature, to utlitmately calculate "air mass". This system first appeared on the 1974 Porsche 914. L-Jetronic was widely adopted on European cars of that period, and a few Japanese models a short time later. In 1975, California's emissions regulations (the most stringent in the world) required manufacturers to dramatically reduce tailpipe emissions. The only feasible technology of that era that enabled auto manufacturers to meet the new regulations was the Catalytic Converter . GM had only recenlty invented the automotive exhaust catalyst, and automakers rushed the new technology into production. A Catalyst promotes a reaction without itself becoming consumed in the reaction. In this case, an Oxidation catalyst was designed into the vehicle's exhaust system to promote reactions of the exhaust constituents in the presence of heat. When hot products of combustion, such as unburned Hydrocarbon s and Carbon Monoxide , are exposed to the catalyst material ( Platinum and/or Palladium ), the exhaust compounds are nearly all oxidized into water and Carbon Dioxide . Stricter legislation to further limit a family of compounds called oxides of Nitrogen occurred in 1980. This required a Reduction catalyst ( Rhodium ) to Reduce the various Nitrogen Oxides into free nitrogen and oxygen. The addition of a "reducing" catalyst, along with the oxidation catalyst, is an approach called a "3-way" catalyst system. The "3" comes from the ability to dramatically reduce all three families of regulated compounds addressed in the EPA "Clean Air Act. The reduction catalyst is placed upstream of the oxidation catalyst, usually in the same housing. The reduction process liberates oxygen from the NOx compounds, and this oxygen is then used in the downstream catalyst to oxidize unburned hydrocarbons and carbon monoxide. In order to take maximum advantage of a 3-way catalyst, excellent air/fuel ratio control is essential. EFI systems improved fuel control in two major stages.
Combining all three features,
current exhaust emissions are now less than 0.1% of their pre-regulated level. In 1982, Bosch introduced a sensor that directly measures the air mass flow into the engine, on their L-Jetronic system. Bosch called this LH-Jetronic (L for Luft, or air, and H for Heiße-leitung, or hot-wire). The mass air sensor utilizes a heated platinum wire placed in the incoming air flow. The rate of the wire's cooling is proportional to the "air mass" flowing across the wire. Since the "hot wire" sensor directly measures air mass, the need for additional temperature and pressure sensors is eliminated. The LH-Jetronic system was also the first "all digital" EFI system, which is now the standard approach. The advent of the digital microprocessor permitted the integration of all powertrain sub-systems into a single control module. Full exploitation of the digital revolution has further improved EFI air/fuel ratio control, as well as many other automotive control systems unrelated to the engine. EXTERNAL LINKS |
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