Fuel injection is a means of metering fuel into an internal combustion engine. In modern automotive applications, fuel metering is one of several functions performed by an "engine management system".
For gasoline engines, carburetors were the predominant method to meter fuel before the widespread use of electronic fuel injection (EFI). However, a wide variety of injection systems have existed since the earliest usage of the internal combustion engine.
Differences between carburetors and fuel injection include:
Fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, but a carburetor relies on the vacuum created by intake air rushing through it to add the fuel to the air stream.
A carburetor performs several important functions in one single component: it measures engine load, calculates the amount of fuel needed, and adds the required fuel to the air stream. With fuel injection, these functions are performed by separate subsystems and components. This means that each subsystem can be specialized and optimized for its particular role, which brings a number of important performance benefits compared to the compromise solution offered by carburetors.
Objectives
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:
1. power output
2. fuel efficiency
3. emissions performance
4. ability to accommodate alternative fuels
5. durability
6. reliability
7. driveability and smooth operation
8. initial cost
9. maintenance cost
10. diagnostic capability
11. range of environmental operation
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 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 very precisely, and when used together 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, let EFI significantly outperform 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.
Those two features result in these performance benefits:
1. Exhaust Emissions
2. Significantly reduced "engine out" or "feedgas" emissions (the chemical products of engine combustion).
3. A reduction in the final tailpipe emissions resulting from the ability to accurately condition the "feedgas" to make the catalytic converter as effective as possible.
4. General Engine Operation
5. Smoother function during quick throttle transitions.
6. Engine starting.
7. Extreme weather operation.
8. Reduced maintenance interval.
9. A slight increase in fuel economy.
10. Power Output
Fuel injection often produces more power than an equivalent carbureted engine. However, fuel injection alone does not increase maximum engine output. Increased airflow is needed to burn more fuel to generate more heat to generate more output. The combustion process converts the fuel's chemical energy into heat energy, whether the fuel arrived via EFI or via a carburetor. Airflow is often improved with fuel injectors, which are much smaller than a carburetor.
Their smaller size 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 about as far from each of the engine's cylinders. These design constraints generally compromise airflow into the engine.
A carburetor relies on a drag-inducing venturi to create a local air pressure difference, which forces the fuel into the air stream. The flow loss caused by the venturi is small compared to other flow losses in the induction system. In a well-designed carburettor induction system, the venturi is not a significant airflow restriction.
Fuel injection is more likely to increase efficiency than power. When cylinder-to-cylinder fuel distribution is improved (common with EFI), less fuel is needed for the same power output. Engine efficiency is known as the BSFC (brake specific fuel consumption). When cylinder-to-cylinder distribution is less than ideal (and it always is under one condition or another, and worse on carburetor systems), more fuel than necessary is metered to the rich cylinders to provide enough fuel to the lean cylinders. Power output is asymmetrical with respect to air/fuel ratio. In other words, burning extra fuel in the rich cylinders does not reduce power nearly as quickly as burning too little fuel in the lean cylinders. The standard fuel metering compromise is to run the rich cylinders "even richer" than the best air/fuel ratio, to provide enough fuel to the leaner cylinders. The net power output improves with all the cylinders making maximum power. An analogy is painting a wall: one coat of paint may not cover the wall properly; a second coat dramatically improves the appearance of the poorly covered areas, but some paint is wasted on areas that were already well covered.
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.
There are other benefits associated with fuel injection, such as better atomization of the fuel in the intake (constant-choke carburetors have poor atomization at low air speeds, needing modifications such as sequential twin-barrel designs) and better breathing due to eliminating the carburetor's venturi.
Injection systems have evolved significantly since the mid 1980s. Current EFI systems provide an accurate and cost effective method of metering fuel. Emission and subjective performance have steadily improved as modern digital controls came, 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.
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 requires seasonal and/or altitude adjustments.
Detailed function
Typical EFI components
1. Injectors
2. Fuel Pump
3. Fuel Pressure Regulator
4. ECM - Engine Control Module; includes a digital computer and circuitry to communicate with sensors and control outputs.
5. Wiring Harness
6. Various Sensors (Some of the sensors required are listed here.)
Crank/Cam Position: Hall effect sensor
Airflow: MAF sensor, sometimes this is inferred with a MAP sensor
Exhaust Gas Oxygen: O2 Sensor, Oxygen sensor, EGO sensor, UEGO sensor
Functional description
A contemporary EFI system comprises a digital computer "engine control module" (ECM) and a number of sensors to measure the engine's operating conditions. The ECM 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 ECM 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.
Note: The right pedal is not the gas pedal; it is the air pedal. The throttle pedal determines the air, and in turn, the air mass determines the fuel mass. The same is true for carburetors, only carburetors were volume, not mass based devices. With some recent systems, the right pedal isn't even an "air pedal"... it has evolved to a "power demand pedal" - it isn't connected to the throttle at all, it signals the CPU how far the driver has depressed the pedal, and the CPU determines how far to open the throttle using an electric motor. This has many benefits some of which include: controlling emissions during transients, cruise control, traction control, engine start/cranking, driveline clunk, idle speed control, air conditioning load compensation, etc.
The three elemental ingredients for combustion are fuel, air and ignition. However; complete combustion can only occur if the air and fuel is present in the exact stoichiometric ratio, which allows all the carbon and hydrogen from the fuel to combine with all the oxygen in the air, with no undesirable polluting leftovers.
To achieve stoichiometry, the air mass flow into the engine is measured and multiplied by the stoichiometric air/fuel ratio 14.64:1 (by weight) for gasoline. The required fuel mass that must be injected into the engine is then translated to the required pulse width for the fuel injector.
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).
Note: The stoichiometric ratio changes as a function of the fuel; diesel, gasoline, ethanol, methanol, propane, methane (natural gas), or hydrogen.
Also, 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 ECM'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 ECM 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 like a motion picture that appears to move, made from a series of individual images.
Various injection schemes
Throttle body injection
Throttle-body injection (called TBI by General Motors and CFI by Ford) was introduced in the mid 1980s 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 '80s, and by GM on heavy duty trucks all the way through OBD-I.
Continuous injection
Bosch's K-Jetronic (K stands for kontinuierlich, or continuous) was introduced in 1974. In this system, fuel sprays constantly from the injectors, rather than being pulsed in time with the engine's intake strokes. Gasoline is pumped from the fuel tank to a large control valve called a fuel distributor, which separates the single fuel supply pipe from the tank into smaller pipes, one for each injector. The fuel distributor is mounted atop a control vane through which all intake air must pass, and the system works by varying fuel volume supplied to the injectors based on the angle of the air vane, which in turn is determined by the volume flowrate of air past the vane. The injectors are simple spring-loaded check valves with nozzles; once fuel system pressure becomes high enough to overcome the counterspring, the injectors begin spraying. K-Jetronic was used for many years between 1974 and the mid 1990s by Lamborghini, Ferrari, Mercedes-Benz, Volkswagen, Ford, Porsche, Audi, Saab, and Volvo. There was also a variant of the system called KE-Jetronic with electronic trim, able to use a catalytic converter.
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-point fuel injection
Multi-point fuel injection injects fuel into the intake port just upstream of the cylinder's intake valve, rather than at a central point within an intake manifold. MPFI systems can be sequential, in which injection is timed to coincide with each cylinder's intake stroke, batched, in which fuel is injected to the cylinders in groups, without precise synchronization to any particular cylinder's intake stroke, or Simultaneous, in which fuel is injected at the same time to all the cylinders.
Direct injection
Many diesel engines feature direct injection (DI). The injection nozzle is placed inside the combustion chamber and the piston incorporates a depression (often toroidal) where initial combustion takes place. Direct injection diesel engines are generally more efficient and cleaner than indirect injection engines. See also High-pressure Direct Injection (HDi) .
Some recent petrol engines utilize direct injection as well. Volkswagen and Audi (FSI), Mitsubishi(GDI), Mazda(DISI), Ford(DISI),BMW, Saab, Saturn and GM. 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.
Article courtesy Wikipedia
