This topic is very large and can be brain smoker. Since this is just the 101 intro course I'll keep it fairly dumbed down .

Here's a excellent source for more detailed explainations/history. http://en.wikipedia.org/wiki/Mechanical ... Mechanical

Overview- Three things make a engine run: compression, spark, and fuel. The latter is all we will be discussing. If your thinking about fuel in the mindset of carbs, vacuum drawing(leaking, hehe ) fuel out at a constant rate, you'll get lost...fast. A computer controls injector's pulse width and when it fires by using information from various sensors. That is as simple as I can describe it, and basically all that happens.

injectors- they are basically a solenoid with fuel pressure on one side and cylinder vacuum/pressure on the other side. Older injectors are usually low impedance (3ohms) and the newer ones are usually high impedance(~10ohms). Injectors are typically rated by how many cc/min or ftlb/hr at 43.5psi (3bar) of fuel pressure. Different injectors have different lag times or latency times (there are probably 5 other names for the same thing, lol), that is the time it takes from the injector getting energized to till it is fully open and is measure in ms (milliseconds). Voltage levels affect it as well as fuel pressure, but I digress. One other key note about injectors is their spray patterns, some are pencil beams, others are wide misting patterns and everything inbetween. Ideally a fine atomized spray pattern is prefered, but not nessacary on all applications.

MAPS and AFM- just about all FI systems have a Manifold Absolute Pressure sensor (MAPS), and some have a Air Flow meter or a Mass Air Flow sensor (MAF), they are different but basically do the same thing. Under WOT (this discussion is only deallying with naturally asperated engines, FI will be save for the advanced classes) manifold pressure will be equal to atmosphere (14.7psi @ sea level). At idle there will be a strong vacuum, no need to post numbers because lots of things affect the amount of vacuum, biggest being the cam. For the air meters (bosch Ljet uses this), there is either a mechanical flapper or a sensor that detects air density flowing past it in the intake. Its generally refered to as a speed density setup if your using the MAP for the ECU's reference and a Mass Air if your using a MAF, there are vaiants Alpha N, yada yada yada.

Temp senders- another critical input to the ECU is the coolant temp and the intake air temp. The coolant temp is critical for warm up, generally cold starts require more fuel because the air is denser. It is a well know fact that a hot intake charge will lower the knock threshold of the engine. A fi system can add fuel on a hot day to help compensate for that, thus allowing a little more timing to be run safely. CLT sensors can be placed anywhere that gets a good represenstation of the engines temp, I got mine on the cylinder head, but guys have mounted theirs in the valve covers and to the oil sump. IAT sensors just need to be anywhere that will have accurate sampling of the intake charge. In my case anywhere in the engine will work, but i mounted mine on the end of the air filter.

TPS- throttle position sensor, its exactly what the name inferes. useful for when you flooded your motor, tells the computer to not spray any fuel during cranking so you can clear it out. Also useful for accel/decel enrichments. Some ECU's use it for their fueling maps, my honda does that, really complicated stuff.

Misc- fuel pump and regulator just supply a steady fuel pressure, as long as the flow rate is high enough and you can run at the correct pressure for the injectors then anything will pretty much work. One neat way to get more flow out of your injectors is to run more fuel pressure. My OG bosch injectors flow 168cc at 30psi(what my regulator is rated at), but at 43.5 they flow 202cc's thats a 15% increase in flow. The ECU also needs a trigger to tell it when to fire injectors, the simplest being from the coil (-), and the more advanced a trigger wheel and either a hall sensor or a VR sensor. Other optional inputs include a oxygen sensor and a barometric pressure sensor. Both are optional, but an O2 sensor allows closed loop mode. A baro sensor is helpful while driving up and down mountains(not useful around here).

last but not least the most important part, the ECU- The ecu is the brains for the whole operation, takes all that info from all the various sensors, and computes when and how long the injectors pulse width is. Seems simple but there is a lot going on. There are enrichenment tables for cranking, after start, voltage compensation, baro compensation, temp compensation, acceleration/deceleration...ect, ect, ect.

Once you get everything sorted with an o2 sensor and have it in closed loop then your setup should have no problem starting in the cold/hot, elevation changes should not make any difference, temp changes should not affect the performance (all cars run faster in the cold then the hot, that has to do with air density), and most of all it runs efficent all the time. There is no maintance aside from a fuel filter every 30,000 miles or so and maybe injectors cleaned every 15 years or so. This is the future, auto manufactures quit using carbs 20 years ago. Also mine makes a really cool sound when i get on it .

I'll post some pics of the different stuff later.

Fuel enrichment in a cold motor is needed more because of fuel condensation on the intake tract and cylinder walls (some of the fuel condenses out of suspension on those surfaces, making it un-burnable, thus more needs to be added to compensate) than air density per se.

derrr, your absolutely right. a warmed up motor helps to evaporate that fuel.