A type of fuel delivery system on vehicles consisting of pressurized fuel lines and fuel injectors. The fuel/air mixture is precisely metered as it enters the combustion chamber to obtain greater efficiency.
At some point in engine design history, some genius thought to himself, "Why are we still using this complicated and inefficient carburetion system when we could just have a little spray bottle that spritzes the right amount of fuel into the air?" That year was 1966, when electronic fuel injection, or EFI, was invented.

Why is fuel injection so fantastic? In order for a body of fuel and air to combust properly, there are two critical factors:

If there is too little fuel, the mixture is lean, and much more explosive. This can cause carbon deposits on the piston head and valve faces, which degrade performance and smoothness of operation. Repeated detonation can lead to massive engine trauma, such as failed head gaskets or destroyed valves.

If the mixture contains too much fuel, or is rich, combustion may be weak, or not occur at all due to over saturation of fuel. After the air is then expended from the cylinders and into the exhaust manifold, it may thin out and ignite when it hits the hot catalytic converter, causing a hundreds of dollars in damage.

Finally, if the fuel is not properly atomized, the combustion may progress jerkily, as it hits one pocket of properly mixed fuel, slows down, then finds another pocket, resulting in uneven combustion and inefficient power conversion.

Proper fuel ignition affects not only efficiency of the engine, but how much power is produced. That's right, it is possible to both increase gas mileage while simultaneously increasing horsepower.

In the past several decades, fuel injection has been refined time and time again. This will give a run down of the major types of fuel injection.

Throttle Body Fuel Injection

With throttle body fuel injection, or TBI, one or two fuel injectors are located between the intake manifold system and the throttle body. The computer looks at how much air is flowing based on readings from the mass air sensor, then determines how much fuel to spray into the air as before it is distributed to the cylinders.

TBI was a nice step beyond carburetors, but it is not that efficient. Changes in the airflow amount, coupled with changes in the load of engine duty, require precise distribution of fuel in order to ensure proper combustion. Also, the fuel is injected early in the air distribution stage, and has plenty of time to de-atomize. Which, as said above, is not a good thing.

It's nearly impossible to find a TBI engine in a modern automobile, though they were very common up until around 1990.

For more information on TBI, there's already an excellent node dedicated to it.

Multi-port Fuel Injection / Sequential Fuel Injection

Multi-port fuel injection (MPFI) was a huge leap beyond TBI. In an MPFI engine, there is one fuel injector for every cylinder. The fuel injectors are located late in the air distribution stage, generally, on top of the lower manifold near the end of the runners.

Putting the injectors closer to the cylinders allows atomization to be kept much better than in a TBI system. Most MPFI systems worked on two alternating pulses per crankshaft rotation; in other words, for every full rotation of the crankshaft, half of the injectors fired once, then the other half fired.

However, this system was quickly realized to have its own downfalls. By having half of the injectors fire at the same time, you had excellent atomization in the first cylinder, decent atomization for the next cylinder, so on down the line, until the other half fire, and the cycle then repeated..

Manufacturers quickly realized this system would be perfect if each injector fired right before its cylinder opened. MPFI then gave way to Sequential Fuel Injection (SFI), which did just that.

SFI is the basis for nearly every EFI consumer automobile currently being manufactured. A less expensive, more efficient option has yet to be found.

Direct Fuel Injection

Ahhhhh. This is the stuff that dreams are made of.

Imagine, if you will, that you throw out the injectors sitting on the lower manifold. You instead place injectors pointing directly into each cylinder of the engine. By placing it here, you can perfectly time when the fuel fires, allowing uncompromisingly precise atomization necessary for a perfect combustion process.

That's Direct Fuel Injection (DFI). Even better atomization possible than through SFI, not to mention the ability to meter the precise amount of fuel necessary for that moment in the engine's life, not a drop wasted. Higher mileage, lower emissions, while getting more power.

DFI has yet to be seen in any commercially-available automobiles. It was used to great success in Cadillac's 2002 Concept Car, the Cien, to squeeze 750 horsepower out of a 7.5-liter V12, which gets similar gas mileage to that of your average modern V8 engine. That's not a small feat, that's groundbreaking. It probably cost a few hundred thousand dollars to make, but who's counting?

The Future

There isn't much more that can possibly be done to improve fuel injector efficiency, aside from mainstreaming DFI. At that point, the injectors will be as close to the combustion chamber as possible, and injector technology is basically at its peak. Where will fuel mileage improvements come from in the next few years? Some in the industry believe it will come from micro-piezo inkjet nozzles. The R&D departments at both Hewlett-Packard and Seiko Epson are researching ways to make auto-grade ink nozzles capable of dispersing perfectly atomized fuel in place of traditional solenoid fuel injectors. At this point, ink can be dispersed by the picoliter instead of the microliter, and fuel effciency can make yet another leap.

Automotive enthusiasts around the world are no doubt keeping their eyes open for that next big thing.

fuel injection

Fuel injection is used on almost all compression ignition reciprocating engines and on most spark ignition engines of more than a few cubic inches displacement manufactured since strict emissions laws were introduced.

Fuel injection has only recently eliminated carburetors from liquid fuel spark ignition engines, but it is not new. In various forms it dates back to the 1920s.

On spark ignition engines, there is a great deal of variety in fuel injection systems. They may be of the direct injection or indirect injection type, and if indirect injection may be continuous flow, semi-timed or timed. They may also be fully mechanical, hydromechanical, or electronic and may utilize a number of inputs to control fuel metering.

Compression ignition engines may be said to use direct or indirect injection as well, but these terms have a different meaning in that context. All CI engines have precisely timed (relatively) high pressure injectors which ejaculate directly into the combustion space. Indirect injection, in this case, means there is a segregated prechamber or energy cell connected to or partially cordoned off from the main combustion chamber space. The injector has to cope with combustion pressures, temperatures, and thermal cycling at least at its tip. CI engines are, however, peripheral to the main purpose of this document, which is SI (gasoline, alcohol, LPG) engine fuel injection technology.

Early SI fuel injection technologies were mechanical and of two primary types: indirect continuous flow, such as Hilborn, Bendix, and Continental, and direct high-pressure timed injection adapted from compression-ignition practice, primarily Bosch inline "jerk pump" and pressure nozzles modified to handle unlubricious fuels and connected to the air throttle on the intake manifold.

Bosch direct timed fuel injection systems were used primarily by German engines, starting with the DB 601 and similar V12 aeroengines during WWII and continuing with the Mercedes-Benz W196 Grand Prix and 300SLR straight-eight DOHC and 300SL SOHC six racing and sports car engines.

American aircraft during and after WWII and drag and circle-track racing cars in the 1950s and 1960s used simpler, less expensive, and often more reliable indirect continuous flow systems. The simplest was (and still is) the Hilborn system, which has a simple centrifugal engine driven pump,a metering block hooked to the air throttle, and a return line to the tank which is restricted by a pill or a needle valve which is changed to adjust mixture. Teledyne Continental aircraft engines use essentially this exact system. Lycoming aircraft engines (with one exception,the Simmonds injected GSO-480) and turbocharged circle track racing engines such as the Offfenheuser and DFX Cosworth, usually running on methanol, used a Bendix system relying on air flow density and speed, similar in concept to the much later Bosch L-Jetronic or CIS.

By the 1960's, no auto manufacturer was content to not have a fuel injected offering in its lineup on its more performance or luxury oriented models. In the US, Chrysler pioneered an electronic system from Bendix called the Electrojector in the late 1950's, but due to lack of experience in building sufficiently ruggedized electronics, the systems failed and Chrysler retrofitted every single car so equipped with carburetors. (Bendix found that automobiles were actually a more hostile and difficult environment than aircraft, or at least light ones, to design for.) General Motors introduced a continuous flow system by Rochester on some Chevrolet and Pontiac models. It lasted considerably longer but had some design flaws which GM never sufficiently addressed, particularly a small flexible driveshaft from the distributor that failed frequently.

European carmakers became enamored of fuel injection slightly later. Mercedes stuck with Bosch, adapting the diesel style pumps to indirect injection with the nozzle moved to the intake manifold. Alfa Romeo used a similar system using the Italian Spica, and BMW used a Kugelfischer system. But it was Volkswagen who, with Robert Bosch, re-introduced electronic FI to the world with the D-Jetronic system, a solid state version of the old Bendix system. With the somewhat later adoption of emissions control in Europe, Bosch systems-D-Jet, K-Jet, and L-Jet-became de rigeur for most European cars, while Asian manufacturers used similar systems under license.

These systems-except for Bosch CIS, a more physically awkward but more detailed variant of Bendix/RSA speed density systems-all relied on an air throttle, mass air flow(MAF) and manifold absolute pressure (MAP)sensors, a constant pressure fuel rail and electromechanical solenoid injectors located between air throttle and intake valve, and an analog or digital controller which increasingly controlled not only the 'electronic' injectors but also the ignition timing, turbo wastegate (if fitted), and other parameters and which had exhaust oxygen sensors, outside air and coolant temperature sensors, vehicle road speed and gear position sensors, and other inputs as desired for emissions and other regulation.

American automotive practice closely followed European norms, with the use by GM, Ford and Chrysler of primarily in-house built systems instead of Bosch, ND, or other outside vendors' being the only major difference. Today, there is no significant difference between US and world practice in automotive engine control systems in terms of core technology, and engine controllers are becoming ever more critical to engine operation as functions like coolant temperature regulation, camshaft degree positioning, and even electronically controlled direct actuation of intake and exhaust valves become commonplace. In 1992, one still could (emissions regulations notwithstanding) convert the most modern electronically controlled engines to contact-breaker or even magneto ignition and a float-bowl carburetor. On many of the current production engines this would be impossible without major structural modification, as no place exists to mount a distributor, and the variable camshafts would have to be locked in position. However, except for a very few individuals gearing up for a nuclear war, the trend today among modifiers is not to get rid of the modern technology, but to fit older engines with the most modern engine management systems. Because performance, driveability, and fuel mileage are all substantially improved with properly designed engine management, one can see such unlikely sights as a 1953 Hudson with its original flathead engine sporting the latest Bosch Digifant system or 1950s "shoebox' Chevy sedans with the original Rochester injection air box but modern electronic control, injectors, and a Ford MAF sensor. Somehow, one gets the feeling that Zora Arkus-Duntov would approve despite corporate management-after all, his first design was a Hemi head set for the venerable flathead Ford V8 block.

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