Fuel injection is superior to carburetion, sure. But how? In a word, atomization. Constant, high pressure fuel forced through a tiny hole results in fuel atomization vastly superior to that of a carburetor. And since atomization is the first step in combustion, the better the fuel is atomized, the more evenly it is distributed in the cylinder, the more readily it vaporizes, and the easier it burns -- all of which translate into increased cylinder filling and better combustion. The bottom line is improved throttle response and increased torque.
But superior atomization isn’t the whole story. The other big deal about fuel injection is that it is able to account for all those little things affecting combustion that the carburetor’s pressure differences were blind to -- the engine’s temperature and load, for example. As a system able to deliver what the engine really needs, fuel injection far exceeds the carburetor.
Not long after the diesel engine industry switched from coal dust to light fuel oil, one man supplied all its fuel injection technology. That man was Robert Bosch, a developer of the technology since 1912 and a name now synonymous with it. By the end of WWII, fuel injection’s broad application inspired a young hot-rodder named Stuart Hilborn to adapt the technology to his Ford racer, and the American fascination with fuel injection began.
The earliest fuel injection systems were entirely mechanical, meaning that timing, pressure and distribution were all handled by mechanical means. German WWII fighter planes for example used fourteen fuel pumps -- one for each cylinder--mounted in a common housing containing a rotating valve. This engine-driven system’s output varied with rpm, and a spill valve rerouted fuel back to the tank in inverse proportion to throttle opening; i.e. more throttle, less return to the tank, and visa-versa. Systems managed by throttle opening and rpm became known as "Alpha-N" type, the phrase derived from the engineering symbols for angle (the throttle’s) and speed (the engine’s). Today they are more commonly called "Speed / Density" (the engine’s speed, the intake’s density). As for the injectors, as in all mechanical fuel injection systems, the warplane’s were spring-loaded, open full-time, and oscillated rapidly to maximize atomization. With parts and controls like these, early fuel injection systems were little more than controlled leaks.
Mechanical fuel injection had one big drawback: With only throttle opening and rpm for controls, output was very linear. This is a problem because the fuel needs of an internal combustion engine are anything but linear, as a look at any torque curve will show. The resulting mismatch created wide mixture variations. When American carburetor manufacturer Rochester put fuel injection in Chevrolet’s Corvette, it addressed linearity by including a venturi linked to a fuel metering valve, thereby adding flow information to the standard Alpha-N control format, resulting in an output more closely matched to the combustion characteristics of the engine. The Bosch company went a step further by making the venturi hourglass-shaped, to more closely approximate most engine’s rich-lean-rich fuel curves. Each represents the best in mechanical fuel injection systems.
Rochester eventually sold its technology to Bendix, who improved it by replacing many of
the mechanical parts with electronic ones, creating the first mass-produced electronic fuel injection system. Detroit failed to sell more than a handful of fuel injected cars however, prompting Bendix to turn its rights over to Bosch, who persistently refined the system until the early 1970's, when emissions and fuel economy concerns coincided with advances in electronics technology to make fuel injection more acceptable and affordable. A few years later, microprocessors turned electronic fuel injection into "smart" systems--i.e. ones that could interpret data and makes decisions--and before long, other engine functions came under the control of the same electronic boxes, to give us what are now called "Engine Management Systems."
Fuel Injection in a Nutshell
All modern electronic fuel injection systems work in essentially the same way. Many times a second, a handful of sensors translate various physical conditions both inside and outside the engine into electrical voltages which supply the system’s computer with the same mixture requirement information that in a carburetor came merely from the pressure differences.
The computer "sees" this data as cylinder air volume and density, and matches this information with any number of pre-programmed "maps", three-dimensional records of that engine’s dyno-determined fuel requirements, and before making a final decision, tweaks the outcome slightly using information supplied by one or more of the most critical sensors (such as engine temperature).
The computer then converts the final calculation into a timed voltage pulse which prompts the injector to open at a specific time and for a specific duration. The injector electromagnetically pulls back its internal tapered plug ("pintle"), and high pressure from an electric pump upstream in the system forces fuel through one or more holes whose incredibly small size causes the fuel to "fog", making it more thoroughly vaporized and therefore more readily combustible.
Indirect vs. Direct Injection
There are two basic kinds of fuel injection, indirect and direct. Indirect fuel injection is the familiar system wherein the injectors are placed in the intake ports. This is the historical system. Direct injection on the other hand is the diesel-style system, now increasingly found in two-stroke application, which has the injectors in the combustion chamber. Direct injection's advantage is that with injection so vigorous, the two-stroke engine's exhaust port can be allowed to close before the mixture is injected, greatly reducing exhaust emissions.
Speed/Density vs. Mass-Air
Fuel injection systems also have just two ways in which they monitor the quantity and quality of the incoming air. The previously-mentioned Speed/Density system uses a combination of engine rpm and throttle opening, whereas the other, the Mass-Air system (also known as Flow Control), uses a flow sensing device (a pivoting vane or heated wire) in the intake tract.
Closed Loop vs. Open Loop
Finally, there are two forms of internal control for fuel injection systems. The most common method uses a special component called a Lambda (or O2) sensor, which permits the system to compare the amount of oxygen in the exhaust with the amount in the outside air, and then signal the computer telling it to adjust the fuel mixture. This is called "Closed Loop," and most fuel injection systems are this type. There are however still many motorcycles and other recreational vehicles that are of the Open Loop design, which is simply a system which does not have an O2 sensor.