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.