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Fuel consumption,throttle position, engine speed

  1. Sep 12, 2008 #1
    hey dudes
    whats been puzzling me is what determines fuel consumption in a cars engine.

    i suppose i can reduce my question to this
    what determines how much feul is injected into the cylinder in any one stroke?

    scenario 1: if i'm towing a heavy trailer uphill, in say 4th gear [manual transmission]
    and i'm going as fast as i can, i floor the accelerator, and nothing happens, the engine speed does not increase so i'm assuming no more fuel is going to the engine
    so what prevents more fuel being injected?

    scenario 2
    i'm now on the flat going at 100km/h in 4th gear my engine speed is 3000rpm
    i shift to 5th maintaining 100km/h and my engine speed drops to 2500rpm.
    my wheels are putting out the same amount of power to keep that speed but my engine speed has dropped so....
    am i using the same feul in 4th gear at 3000rpm and 100km/h
    as i would in 5th gear at 2500rpmm and 100km/h

    or am i using more fuel in 4th gear by virtue of higher engine speed the extra energy of which is going to overheating my engine?

    i know this question is a little hazy but any input is always appreciated!!
  2. jcsd
  3. Sep 12, 2008 #2


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    Gold Member

    Fuel injection quantity in a modern engine is basically determined by engine speed and inlet manifold pressure. Throttle position, air flow and exhaust lambda are sometimes taken into account too, but these are secondary.

    Scenario 1.
    If you've got your foot to the floor, the engine is developing as much power as it can at that speed. If you're at 80% throttle, towing your trailer up a hill at constant speed, and you go to 100% throttle, more power will be developed and you will accelerate. It just might not seem like you do if it's very slow.

    Scenario 2.
    No, you're not necessarily using the same fuel in 4th as you are in 5th, even though your power and vehicle speed is the same. All engines have a point on the torque curve where they operate with their greatest fuel efficiency. It's hard to say whether you're using less fuel in 4th or 5th in that situation; I'd put my money on 5th (but you'd have to test or look at engine data to say for sure).
  4. Sep 13, 2008 #3


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    Science Advisor

    In old carburetors, Bernoulli's Principle essentially drove the thing. There was basically a straw dipped into fuel that was inserted into the airstream. As air flows over the straw, the increase in velocity causes a decrease in pressure which "sucks" the fuel into the air stream.

    Nowadays, as brewnog said, the manifold pressure is used to determine the engine load, or how hard the engine is trying to work. After that, it's all done electronically.
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