Maximus
12-18-05, 01:20 AM
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 the feedback from an EGO sensor, permitted EFI to significantly out perform a carburetor. The two fundamental improvements are:
1. Reduced response time to rapidly changing inputs, e.g., rapid throttle movements.
2. 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:
* Exhaust Emissions
o Significantly reduced "engine out" or "feedgas" emissions (the chemical products of engine combustion).
o A reduction in final tailpipe emissions (≈ 0.99%) resulting from the ability to accurately condition the "feedgas" in a manner that maximizes the function of the catalytic converter.
* General Engine Operation
o Smoother function during quick throttle transitions.
o Engine starting.
o Extreme weather operation.
o Reduced maintenance interval.
o A slight increase in fuel economy.
* Power Output
o Fuel injection often produces more power than an equivalent carbureted engine. However, fuel injection alone does not increase maximum engine output. Increased airflow is necessary to permit oxidizing more fuel, which generates more heat, which in turn generates more output. The combustion process converts the fuel's chemical energy into heat energy; whether the fuel arrived via EFI or a carburetor is not significant. Air flow is often improved with fuel injectors, which are much smaller than a carburetor. Their smaller size permits 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 approximately equal distance from each of the engine's cylinders. These design constraints generally compromise the air's induction path.
o A carburetor relies on a drag-inducing venturi to create a local air pressure difference to force fuel into the airstream. The flow loss caused by the venturi is small in comparison to other flow losses in the induction system. In a well designed carbureted induction system, the venturi in and of itself is not a significant airflow restriction.
o Fuel injection is more likely to increase efficiency than power. When cylinder-to-cylinder fuel distribution is improved (common with EFI), less fuel is required to generate the same power output. This increases efficiency - (BSFC, brake specific fuel consumption). When distribution is less than ideal (and it always is under one condition or another), more fuel than necessary is metered to the rich cylinders in order to provide enough fuel to the leaner cylinders. Power output is asymmetrical with respect to air/fuel ratio. In other words, burning extra fuel in the rich cylinders does not reduce their 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" of the optimal air/fuel ratio, in order to provide enough fuel to the leaner cylinders. The net power output improves with all the cylinders making maximum power. An analogy is that of painting a wall. One coat of paint may not cover very well. The second coat dramatically improves the appearance of the poorly coverd areas, but some extra paint is consumed on areas that were already well-covered.
o Deviations from perfect air/fuel distribution, however subtle, dramatically impact emissions, by forfeiting combustion events at the chemically-ideal, stoichiometric air/fuel ratio. Grosser distribution problems eventually begin to negatively impact efficiency, and the grossest distribution issues finally affect power. The hierarchy of negative functional impact with regard to inceasingly poorer air/fuel distribution is: emissions, efficiency, and power.
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 rapidly adopting EFI as the electronics' reliability improve. If this trend continues, it is conceivable that virtually all internal combustion engines, including garden equipment and snow throwers, will eventually use EFI. The 1990 Subaru Justy was the last carbureted passenger car sold in the U.S.
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.
Basic Function
The fuel injector, which acts as the dispensing nozzle, injects liquid fuel directly into the engine's air stream. This usually requires an external pump. The pump and injector are only two of many components in a complete fuel injection system.
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 EFI). Modern systems, nearly all of which are electronic, use an electronic injector to inject the fuel, and a CPU to calculate the mass of fuel.
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), and then push the mixture into the engine’s air intake. 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.
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 the feedback from an EGO sensor, permitted EFI to significantly out perform a carburetor. The two fundamental improvements are:
1. Reduced response time to rapidly changing inputs, e.g., rapid throttle movements.
2. 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:
* Exhaust Emissions
o Significantly reduced "engine out" or "feedgas" emissions (the chemical products of engine combustion).
o A reduction in final tailpipe emissions (≈ 0.99%) resulting from the ability to accurately condition the "feedgas" in a manner that maximizes the function of the catalytic converter.
* General Engine Operation
o Smoother function during quick throttle transitions.
o Engine starting.
o Extreme weather operation.
o Reduced maintenance interval.
o A slight increase in fuel economy.
* Power Output
o Fuel injection often produces more power than an equivalent carbureted engine. However, fuel injection alone does not increase maximum engine output. Increased airflow is necessary to permit oxidizing more fuel, which generates more heat, which in turn generates more output. The combustion process converts the fuel's chemical energy into heat energy; whether the fuel arrived via EFI or a carburetor is not significant. Air flow is often improved with fuel injectors, which are much smaller than a carburetor. Their smaller size permits 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 approximately equal distance from each of the engine's cylinders. These design constraints generally compromise the air's induction path.
o A carburetor relies on a drag-inducing venturi to create a local air pressure difference to force fuel into the airstream. The flow loss caused by the venturi is small in comparison to other flow losses in the induction system. In a well designed carbureted induction system, the venturi in and of itself is not a significant airflow restriction.
o Fuel injection is more likely to increase efficiency than power. When cylinder-to-cylinder fuel distribution is improved (common with EFI), less fuel is required to generate the same power output. This increases efficiency - (BSFC, brake specific fuel consumption). When distribution is less than ideal (and it always is under one condition or another), more fuel than necessary is metered to the rich cylinders in order to provide enough fuel to the leaner cylinders. Power output is asymmetrical with respect to air/fuel ratio. In other words, burning extra fuel in the rich cylinders does not reduce their 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" of the optimal air/fuel ratio, in order to provide enough fuel to the leaner cylinders. The net power output improves with all the cylinders making maximum power. An analogy is that of painting a wall. One coat of paint may not cover very well. The second coat dramatically improves the appearance of the poorly coverd areas, but some extra paint is consumed on areas that were already well-covered.
o Deviations from perfect air/fuel distribution, however subtle, dramatically impact emissions, by forfeiting combustion events at the chemically-ideal, stoichiometric air/fuel ratio. Grosser distribution problems eventually begin to negatively impact efficiency, and the grossest distribution issues finally affect power. The hierarchy of negative functional impact with regard to inceasingly poorer air/fuel distribution is: emissions, efficiency, and power.
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 rapidly adopting EFI as the electronics' reliability improve. If this trend continues, it is conceivable that virtually all internal combustion engines, including garden equipment and snow throwers, will eventually use EFI. The 1990 Subaru Justy was the last carbureted passenger car sold in the U.S.
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.
Basic Function
The fuel injector, which acts as the dispensing nozzle, injects liquid fuel directly into the engine's air stream. This usually requires an external pump. The pump and injector are only two of many components in a complete fuel injection system.
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 EFI). Modern systems, nearly all of which are electronic, use an electronic injector to inject the fuel, and a CPU to calculate the mass of fuel.
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), and then push the mixture into the engine’s air intake. 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.