FUEL INJECTION PROCESS
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FUEL INJECTION PROCESS
The internal combustion engine is an engine in which the combustion of a fuel (normally a fossil fuel) occurs with an oxidizer (usually air) in a combustion chamber. In an internal combustion engine the expansion of the high-temperature and -pressure gases produced by combustion applies direct force to some component of the engine, such as pistons, turbine blades, or a nozzle. This force moves the component over a distance, generating useful mechanical energy.
The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as the more familiar four-stroke and two-stroke piston engines, along with variants, such as the Wankel rotary engine. A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines, each of which are internal combustion engines on the same principle as previously described.
The internal combustion engine (or ICE) is quite different from external combustion engines, such as steam or Stirling engines, in which the energy is delivered to a working fluid not consisting of, mixed with, or contaminated by combustion products. Working fluids can be air, hot water, pressurized water or even liquid sodium, heated in some kind of boiler.
A large number of different designs for ICEs have been developed and built, with a variety of different strengths and weaknesses. Powered by an energy-dense fuel (which is very frequently petrol, a liquid derived from fossil fuels), the ICE delivers an excellent power-to-weight ratio with few disadvantages
Four-stroke cycle (or Otto cycle)
As their name implies, operation of four stroke internal combustion engines have four basic steps that repeat with every two revolutions of the engine:
o Combustible mixtures are emplaced in the combustion chamber
o The mixtures are placed under pressure
3. Combustion (Power)
o The mixture is burnt, almost invariably a deflagration, although a few systems involve detonation. The hot mixture is expanded, pressing on and moving parts of the engine and performing useful work.
o The cooled combustion products are exhausted into the atmosphere
Many engines overlap these steps in time; jet engines do all steps simultaneously at different parts of the engines.
All internal combustion engines depend on the exothermic chemical process of combustion: the reaction of a fuel, typically with oxygen from the air
Functional requirement of an injection system
For a proper running and good performance from the engine, the following requirement must be met the injection system
1. Timing the injection of the fuel correctly in the cycle so that maximum power is obtain ensuring fuel economy and clean burning.
2. Proper control of fuel injection so that the proper heat release pattern is achieved during combustion.
3. Proper atomization of the fuel is obtained.
4. Proper spray pattern to ensure rapid mixing of fuel and air.
5. Uniform distribution of the fuel droplet throughout the combustion chamber.
6. To supply equal quantities of the metered fuel to the entire cylinder in case of multi cylinder engine.
7. No tag during beginning and end of injection.
8. Accurate metering of the fuel injection per cycle. This is very critical due to the fact that very small quantity of fuel being handled.
9. Metering errors may cause drastic variation from the desired output. The quantity of the fuel metered should vary to meet changing speed and load requirements of the engine.
10. The rate of injection should be such that it the desired heat release patterns.
Types of injection system
Diesel injection system can be divided into two basic types.
1. air injection
2. solid injection
The fuel is metered and pumped to the fuel valve by a camshaft driven fuel pump. The fuel valve is opened by means of a mechanicals linkage operated by the camshaft which controls the time of injection. the fuel valve is also connected to a high pressure air line fed by a multi-stage compressor which supplies air at a pressure of about 60 to 70bar.
When the fuel valve is opened the blast air sweeps the fuel along with it and a well atomized fuel spray is sent to the combustion chamber.
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Fuel rail connected to the injectors that are mounted just above the intake manifold on a four cylinder engine.
Fuel injection is a system for mixing fuel with air in an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s.
A fuel injection system is designed and calibrated specifically for the type(s) of fuel it will handle. Most fuel injection systems are for gasoline or diesel applications. With the advent of electronic fuel injection (EFI), the diesel and gasoline hardware has become similar. EFI's programmable firmware has permitted common hardware to be used with different fuels.
Carburetors were the predominant method used to meter fuel on gasoline engines before the widespread use of fuel injection. A variety of injection systems have existed since the earliest usage of the internal combustion engine.
The primary difference between carburetors and fuel injection is that fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, while a carburetor relies on low pressure created by intake air rushing through it to add the fuel to the airstream.
The fuel injector is only a nozzle and a valve: the power to inject the fuel comes from a pump or a pressure container farther back in the fuel supply.
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:
• power output
• fuel efficiency
• emissions performance
• ability to accommodate alternative fuels
• driveability and smooth operation
• initial cost
• maintenance cost
• diagnostic capability
• range of environmental operation
• Engine tuning
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 competitively. The modern digital electronic fuel injection system is far more capable at optimizing these competing objectives consistently than a carburetor. Carburetors have the potential to atomize fuel better (see Pogue and Allen Caggiano patents).
Operational benefits to the driver of a fuel-injected car include smoother and more dependable engine response during quick throttle transitions, easier and more dependable engine starting, better operation at extremely high or low ambient temperatures, increased maintenance intervals, and increased fuel efficiency. On a more basic level, fuel injection does away with the choke which on carburetor-equipped vehicles must be operated when starting the engine from cold and then adjusted as the engine warms up.
An engine's air/fuel ratio must be precisely controlled under all operating conditions to achieve the desired engine performance, emissions, driveability, and fuel economy. Modern electronic fuel-injection systems meter fuel very accurately, and use closed loop fuel-injection quantity-control based on a variety of feedback signals from an oxygen sensor, a mass airflow (MAF) or manifold absolute pressure (MAP) sensor, a throttle position (TPS), and at least one sensor on the crankshaft and/or camshaft(s) to monitor the engine's rotational position. Fuel injection systems can react rapidly to changing inputs such as sudden throttle movements, and control the amount of fuel injected to match the engine's dynamic needs across a wide range of operating conditions such as engine load, ambient air temperature, engine temperature, fuel octane level, and atmospheric pressure.
A multipoint fuel injection system generally delivers a more accurate and equal mass of fuel to each cylinder than can a carburetor, thus improving the cylinder-to-cylinder distribution. Exhaust emissions are cleaner because the more precise and accurate fuel metering reduces the concentration of toxic combustion byproducts leaving the engine, and because exhaust cleanup devices such as the catalytic converter can be optimized to operate more efficiently since the exhaust is of consistent and predictable composition.
Fuel injection generally increases engine fuel efficiency. With the improved cylinder-to-cylinder fuel distribution, less fuel is needed for the same power output. When cylinder-to-cylinder distribution is less than ideal, as is always the case to some degree with a carburetor or throttle body fuel injection, some cylinders receive excess fuel as a side effect of ensuring that all cylinders receive sufficient fuel. Power output is asymmetrical with respect to air/fuel ratio; burning extra fuel in the rich cylinders does not reduce power nearly as quickly as burning too little fuel in the lean cylinders. However, rich-running cylinders are undesirable from the standpoint of exhaust emissions, fuel efficiency, engine wear, and engine oil contamination. Deviations from perfect air/fuel distribution, however subtle, affect the emissions, by not letting the combustion events be at the chemically ideal (stoichiometric) air/fuel ratio. Grosser distribution problems eventually begin to reduce efficiency, and the grossest distribution issues finally affect power. Increasingly poorer air/fuel distribution affects emissions, efficiency, and power, in that order. By optimizing the homogeneity of cylinder-to-cylinder mixture distribution, all the cylinders approach their maximum power potential and the engine's overall power output improves.
A fuel-injected engine often produces more power than an equivalent carbureted engine. Fuel injection alone does not necessarily increase an engine's maximum potential output. Increased airflow is needed to burn more fuel, which in turn releases more energy and produces more power. The combustion process converts the fuel's chemical energy into heat energy, whether the fuel is supplied by fuel injectors or a carburetor. However, airflow is often improved with fuel injection, the components of which allow more design freedom to improve the air's path into the engine. In contrast, a carburetor's mounting options are limited because it is larger, it must be carefully oriented with respect to gravity, and it must be equidistant from each of the engine's cylinders to the maximum practicable degree. These design constraints generally compromise airflow into the engine. Furthermore, a carburetor relies on a restrictive venturi to create a local air pressure difference, which forces the fuel into the air stream. The flow loss caused by the venturi, however, is small compared to other flow losses in the induction system. In a well-designed carburetor induction system, the venturi is not a significant airflow restriction.
Fuel is saved while the car is coasting because the car's movement is helping to keep the engine rotating, so less fuel is used for this purpose. Control units on modern cars react to this and reduce or stop fuel flow to the engine reducing wear on the brakes.
History and development
Herbert Akroyd Stuart developed the first system laid out on modern lines (with a highly-accurate 'jerk pump' to meter out fuel oil at high pressure to an injector. This system was used on the hot bulb engine and was adapted and improved by Robert Bosch for use on diesel engines — Rudolf Diesel's original system employed a cumbersome 'air-blast' system using highly compressed air[clarification needed].
The first use of direct gasoline injection was on the Hesselman engine invented by Swedish engineer Jonas Hesselman in 1925. Hesselman engines use the ultra lean burn principle; fuel is injected toward the end of the compression stroke, then ignited with a spark plug. They are often started on gasoline and then switched to diesel or kerosene. Fuel injection was in widespread commercial use in diesel engines by the mid-1920s. Because of its greater immunity to wildly changing g-forces on the engine, the concept was adapted for use in gasoline-powered aircraft during World War II, and direct injection was employed in some notable designs like the Junkers Jumo 210, the Daimler-Benz DB 601, the BMW 801, the Shvetsov ASh-82FN (M-82FN) and later versions of the Wright R-3350 used in the B-29 Superfortress.
Alfa Romeo tested one of the very first electric injection systems (Caproni-Fuscaldo) in Alfa Romeo 6C2500 with "Ala spessa" body in 1940 Mille Miglia. The engine had six electrically operated injectors and were fed by a semi-high pressure circulating fuel pump system.
The term Mechanical when applied to fuel injection is used to indicate that metering functions of the fuel injection (how the correct amount of fuel for any given situation is determined and delivered) is not achieved electronically but rather through mechanical means alone.
In the 1940s, hot rodder Stuart Hilborn offered mechanical injection for racers, salt cars, and midgets.
One of the first commercial gasoline injection systems was a mechanical system developed by Bosch and introduced in 1952 on the Goliath GP700 and Gutbrod Superior 600. This was basically a high pressure diesel direct-injection pump with an intake throttle valve set up. (Diesels only change amount of fuel injected to vary output; there is no throttle.) This system used a normal gasoline fuel pump, to provide fuel to a mechanically driven injection pump, which had separate plungers per injector to deliver a very high injection pressure directly into the combustion chamber.
Another mechanical system, also by Bosch, but injecting the fuel into the port above the intake valve was later used by Porsche from 1969 until 1973 for the 911 production range and until 1975 on the Carrera 3.0 in Europe. Porsche continued using it on its racing cars into the late seventies and early eighties. Porsche racing variants such as the 911 RSR 2.7 & 3.0, 904/6, 906, 907, 908, 910, 917 (in its regular normally aspirated or 5.5 Liter/1500 HP Turbocharged form), and 935 all used Bosch or Kugelfischer built variants of injection. The Kugelfischer system was also used by the BMW 2000/2002 Tii and some versions of the Peugeot 404/504 and Lancia Flavia. Lucas also offered a mechanical system which was used by some Maserati, Aston Martin and Triumph models between ca. 1963 and 1973.
A system similar to the Bosch inline mechanical pump was built by SPICA for Alfa Romeo, used on the Alfa Romeo Montreal and on US market 1750 and 2000 models from 1969-1981. This was specifically designed to meet the US emission requirements, and allowed Alfa to meet these requirements with no loss in performance and a reduction in fuel consumption.
Chevrolet introduced a mechanical fuel injection option, made by General Motors' Rochester Products division, for its 283 V8 engine in 1956 (1957 US model year). This system directed the inducted engine air across a "spoon shaped" plunger that 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 system was not a "pulse" or intermittent injection, but rather a constant flow system, metering fuel to all cylinders simultaneously from a central "spider" of injection lines. The fuel meter adjusted the amount of flow according to engine speed and load, and included a fuel reservoir, which was similar to a carburetor's float chamber. With its own high-pressure fuel pump driven by a cable from the distributor to the fuel meter, the system supplied the necessary pressure for injection. This was "port" injection, however, in which the injectors are located in the intake manifold, very near the intake valve. (Direct fuel injection is a fairly recent innovation for automobile engines. As recent as 1954 in the aforementioned Mercedes-Benz 300SL or the Gutbrod in 1953.) The highest performance version of the fuel injected engine was rated at 283 bhp (211.0 kW) from 283 cubic inches (4.6 L). This made it among the early production engines in history to exceed 1 hp/in³ (45.5 kW/L), after Chrysler's Hemi engine and a number of others. General Motors' fuel injected engine — usually referred to as the "fuelie" — was optional on the Corvette for the 1957 model year.
During the 1960s, 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, having no provisions for low speed metering or even starting (fuel had to be squirted into the injector tubes while cranking the engine in order to start it). However they were a favorite in the aforementioned competition trials in which essentially wide-open throttle operation was prevalent.