can they vary the amount of fuel they inject or is the engine at 4k rpm consistantly putting out twice as much fuel compared to 2k rpm?

 

a fuel injector is basically an open and shut device. it gets and electrical charge and it opens. the electrical charge goes away and it shuts.

you can control how much fuel comes out by controlling how long the injector stays open and by controlling the fuel pressure that feed into the injector.

think of it like this:

you have a field full of cows. you have an electric gate that opens and stays open as long as you push the button down. once you release the button, the gate snaps back closed.

you also have a dog to scare the cows toward the gate.

if your goal is to have a steady rate of cows exiting the field then you need to have a constant pressure behind them (the barking dog) and you need to have a steady open and shut action on the gate.

you open the gate (fuel injector)for five seconds and let two cows (x amount of fuel) out, then you close the gate. you open the gate for five more seconds and let two more cows out. you could do this at this rate(cows per minute would be the same as lbs per hour or what ever unit of measure you choose for fuel flow) all day long provided the dog (fuel pressure regulator) does his job and keeps them flowing your way at a steady rate. (fuel pressure.)

if you decide you want more cows to come out per minute then you could make the dog more vicious (raise the fuel pressure) or keep the gate open longer (longer duty cycle of the injector)

that pretty much sums it up. the exception ot the rule would be a rising rate fuel pressure regulator. it would be the equivalent of every few seconds throwing an extra few dogs in the field. the idea is that you cant control larger injectors due to limitations in engine management or simply the lack of larger injectors/ you dont want they pressure to constantly be higher so you use a vacuum sensitive device that causes the fuel [ressure to increase as the engine rpms increase. you see this set up on a lot of ghetto turbo kits. its a shitty idea.

hope that lame ass analogy helps.

 

wow, thnx.


i was specifically wondering, if you are travelling at 60 mph and are steady at 3k rpms. then, a big truck gets in front of you and you are in the slip stream behind it, if you try to stay at 60mph, will your revs stay at 3k and if so, will they be using the same or less gas than before?

 

I may be totally wrong, but here is how I see it:

Your engine is hard-connected to the wheels. The wheels don't change in size, so whether you're following a truck or not, if your speed doesn't change, your rpms won't change (obviously) unless you switch gears.

However, being a behind a truck will significantly lower the amount of resistance your car encounters at highway speeds. So as a result, you will wind up pressing down on the gas pedal less in order to maintain your 3k rpms (and speed). With less pedal pressing, you get less gas in the engine and better mileage.

 

so the injectors inject less gas while staying at 3k rpms?

 

Yes.

To also add: in colder weather, the car is capable of spraying more fuel (at the same RPM) into the engine because the air is more dense, giving you more power. However, this doesn't translate into using more gas at lower temperatures because the process is more efficient (assuming you're driving the similarly.