Why fuel injection? Well, let's begin by reviewing how a carburetor works. Air rushing through the carburetor on its way into the engine passes through a calibrated space called the carburetor's venturi. A low pressure pocket is formed in this space and from it a tube is suspended into a cavity of fuel that is vented to atmosphere. The difference between the fuel's atmospheric pressure and the venturi's less-than atmospheric pressure coaxes the fuel upward into the intake airstream. Thus we have "fuel discharge." From that point on, airflow changes generated by throttle operation bring proportional changes in venturi pressure, resulting in proportionally-different fuel discharges into the intake. This is the classic carburetor. It all works rather well, too. Up to a point.
That point is reached when adaptability is needed. Any irregular condition, even one as simple yet as frequent as cold starting, throws a monkey wrench into the works, requiring as it does the consistent manual override of an enrichening circuit. Massive enrichening is necessary because the carburetor cannot sense the engine's reduced ability to vaporize its fuel due to its low temperature -- fuel collects in wet splotches in the cylinder instead of spreading all over -- so much more is needed before the cylinder's air charge is sufficiently spiked to be combustible. Add a more significant variation such as a change in altitude and the carburetor's capabilities are overwhelmed altogether. A condition such as that must simply be endured because the carburetor has no method of detecting it.
This is in fact the carburetor's main problem -- that it operates solely on signals created by its own airflow to produce fuel discharge, and thus has no means of compensating (other than disassembly and recalibration) for any change from baseline conditions. The carburetor's weak airflow-induced fueling signals are the crux of the problem.
Not incidentally these same meager signals also result in mediocre atomization, the other of the carburetor's two main shortcomings. Atomization, the breaking of fuel into a fine mist, is spotty in a carburetor due to the fuel being under merely atmospheric pressure. Atomization is important because it is the first step in the combustion process. The better the fuel is atomized, the more evenly it is distributed in the cylinder, the more readily it vaporizes (turns into a gas), and the easier it burns. The mist that results from only a 2-3 psi pressure difference could, if under more pressure, be finer and lighter, and thus would more easily vaporize when encountering the engine's heat. The result would be improved throttle response, increased torque, and fewer exhaust emissions.
This then is the beginning of what is so great about fuel injection from an operational standpoint. One, it adjusts to all those small yet important things that the carburetor cannot sense: the engine's temperature and load, for example, resulting in improved engine operation under varying conditions. And two, through external fuel pressurization, fuel injection breaks its fuel into much finer particles upon discharge, resulting in improved combustion and power.
These are great advantages but there is a whole lot more, because fuel injection offers benefits far beyond smoother operation, enhanced power and lowered emissions. Plusses such as minimal maintenance, especially in regard to vehicle storage; less susceptibility to environmental conditions, especially vehicle motion and attitude (think air-getting motocross bikes); and, better compatibility with low-energy ignition systems, should give you the idea. But there is also better starting in cold conditions, faster warmup, and many can easy tuning using a laptop computer. And, there's still more. Fault override is programmed into fuel injection systems, allowing them to keep operating even with one or mire failed sensors. Modern fuel injection also integrates the fuel and ignition systems together, permitting independent ignition timing in each cylinder, for example, which nearly all fuel injected vehicles employ today. Also rpm limiting, speed control, variable engine power settings, traction control, cruise control, exhaust system valve operation, cam timing variation -- it's all part of modern fuel injection and it's all great!
Go on to Part 2: Fuel Injection System Parts