Fuel injection

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 airstream.

* 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 airstream. 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.

The carburetor modifications and complexities needed to comply with increasingly-strict US exhaust emission regulations of the 1970s and 1980s gradually eroded and then reversed the simplicity, cost, and packaging advantages carburetors had traditionally offered. Fuel injection appeared first on American-made cars in the late 1950s (the Rochester Fuel Injected Chevrolet Corvette, manufactured from 1957 through 1965), and later in European-made cars in the late 1960s. It was phased in through the latter '70s and '80s at an accelerating rate, with the US and German markets leading and the UK and Commonwealth markets lagging somewhat, and since the early 1990s, almost all gasoline passenger cars sold in first world markets like the United States, Europe, Japan, and Australia have come equipped with electronic fuel injection (EFI).

The fuel injector is only a nozzle and a valve: the power to inject the fuel comes from further back in the fuel supply, from a pump or a pressure container.


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:

* power output
* fuel efficiency
* emissions performance
* ability to accommodate alternative fuels
* durability
* reliability
* driveability and smooth operation
* initial cost
* maintenance cost
* diagnostic capability
* 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 electronic fuel injection 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:

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.


Those two features result in these performance benefits:

* Exhaust Emissions
o Significantly reduced "engine out" or "feedgas" emissions (the chemical products of engine combustion).

o A reduction in the final tailpipe emissions (≈ 99.9%) resulting from the ability to accurately condition the "feedgas" to make the catalytic converter as effective as possible.


* 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 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.


o 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.


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 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.


o 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.

It should be noted that a carburetor's fuel metering system is a less expensive alternative when there are not strict emission regulations, as in developing countries. EFI will undoubtedly replace carburetors in these nations too as they adopt emission regulations similar to Europe, Japan, and North America.