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Fuel System

  1. Dec 29, 2009 #1
    i am interested to know the major components of the fuel system of a car. I know, these are the major ones:
    Fuel Tank, Fuel Pump, Fuel Filter, Injector Valve, Return Lines

    but what i am interested is in the fuel system for (1) a single cylinder engine, (2) no carburetor (instead, i am looking for the injector valve to be electronically controlled), and (3) with the fuel tank above the engine

    it must be kept in mind that my aim is to make the engine consume as little fuel as possible for a given (required) output
    my specific questions:
    1. If we have the tank above the engine (allowing the fuel to be gravity-fed), would the system not need the fuel pump?
    2. I have the option of pressurizing the fuel tank to up to 5 bars. How can pressurized fuel (in tank) help me? Is the combination of this pressure + gravity-feeding sufficient reason not to have a fuel pump?
    3. Can i expect the system's fuel consumption to be better (meaner) than with a carburetor?
    4. Does the system (jpeg attached) look ok?

    Attached Files:

  2. jcsd
  3. Dec 29, 2009 #2
    1) Kinda depends on the engine itself, as well as the injector valve. In automobiles, the injectors are somewhere around the 15-25 lbs/hr (the standard measure for injectors). This requires a fuel pump, and also a fuel pressure regulator (which you didn't list). If you were to use a small enough injector with a small enough flow rate, then you might be able to get by, but I personally do not think this is possible.

    2) Pressurizing the fuel tank would in essence allow you to ignore the pump. Considering a single cylinder engine (what is the displacement?) with a small enough valve, you could conceivably do this, though as the fuel level in the tank reached the bottom, your pressure will be lower and you will also begin to get air in the fuel line, which kills performance. You would definitely need a regulator of some sort to ensure that the fuel pressure remained consistent.

    3) Honestly, no, unless you intend to add the various different sensors that a normal fuel injection system on an automobile would have, such as an exhaust sensor that detects how much unburnt fuel there is in the exhaust, as well as a positioning sensor that tells the valve when to open. Essentially, your idea at the moment is even more basic than a carburetor, as you're pretty much squirting fuel in at a constant rate with no control other than "on" or "off".

    Think of a carburetor as an analog computer. They're far more advanced than most give them credit. A carburetor has metering blocks, jets, etc., all of which work together in order to control the rate that the fuel is injected into the intake airflow.

    In short, no, unless you are putting sensors in and haven't told us, I would assume the carburetor will perform better.

    4) It looks fine, except no regulator or pump, which I addressed above.
  4. Dec 29, 2009 #3
    how about a mechanical system similar to how cummins "big cam diesels" work.
    a "3rd rocker arm" mechanically fires the injector. It could be both pump and injector, drive off of the cam lobes

  5. Dec 30, 2009 #4
    thanks guys for the replies
    dr dodge, hadn't heard of this concept...looking into it

    anyway, if i have the same configuration but instead of the injector nozzle, i put a carburetor, would i still need the pump and the regulator?
    because my understanding on how the carburetor works is that sucks the fuel according to the intake of the air...so isn't the 'pumping' action met by this 'sucking' of the fuel?

    PS: the engine is going to be a small one. Something like 70 cc
  6. Dec 30, 2009 #5
    in most light duty engines (briggs, etc) the fuel pump is in the carb. The intake pulses move a diaphram thru 2 check valves. this fills the fuel bowl which then allows the suction to pull in the fuel.
    in a carb, float level is critical. to low, fuel leans out, too high too rich. Air bleeds allow incoming air to mix with the fuel to pull the atomized, metered fuel into the intake.
    this diagram is not entirely correct, but pretty close

    still trying to find a good diag for the "big cam" but basicly there is a 3rd lobe, pushrod, and rocker. It mechanically fires the injector. timing is set with the thickness of the gasket on the cam cover which carries the roller lifters.

  7. Dec 30, 2009 #6


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    Good diagram of the unit pump injector here:

    http://www.langleyvw.ca/tech/pd.html [Broken]

    Can be overhead cam or pushrod, and can be mechanically or electronically controlled, but the pumping principle is the same.

    OP: What are you actually trying to achieve?
    Last edited by a moderator: May 4, 2017
  8. Dec 30, 2009 #7
    If you elevate the fuel source enough, no, you won't need the pump or regulator. I've actually gotten a 400 cubic inch V-8 to run off a milk jug full of gas with a rubber hose connected to the carburetor.
  9. Dec 30, 2009 #8


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    But you'd need a huge head to achieve this with an injection system as proposed.
  10. Dec 30, 2009 #9
    thats correct, but the varying pressure in the jug varies the float level

    not to mention the fact that somneone has to sit on the roof and hold the jug

  11. Dec 30, 2009 #10
    I had assumed we were looking at a carburetor concept and had ditched the injector idea, should have clarified, I suppose.
  12. Jan 3, 2010 #11


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    This is merely a historical side-note prompted by your post. My grandfather was the foreman of the CPR road crew that built the first road between my town and Calgary. There was a huge valley between (which is now full of houses). Everyone had to stop at the bottom of the upward part of the journey (going either way) and turn around. They had to back up the hill because model T's had gravity-feed carbs and they had to keep the gas tank higher than the engine.
  13. Jan 3, 2010 #12
    That's hilarious! I had a Toyota pickup (early '80s vintage) that would stall on steep up-hills if there was less than 1/4 tank of fuel. (It was a camper conversion, so that may not have been a fault of Toyota.)
  14. Jan 3, 2010 #13
    In the Battle of Britain the Spitfires were carb fed, and the ME109's were injection. For a spitfire to follow an ME109 in a steep dive it had to flip over first or the fuel mix blew out of the carb the wrong way.

    Not exactly relevent, but it just reminded me :)
  15. Jan 3, 2010 #14
    And to make things even more off topic, this answers why in the movie "Battle of Britain," you always saw the Spits rolling over before diving into attack. Before, I always thought that they just did that to show the gracefulness of the airplane.
  16. Jan 4, 2010 #15


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    Up until the introduction of "Miss Shilling's Orifice". Google it; it's a nice story.
  17. Jan 4, 2010 #16
    "Miss Shilling's Orifice"
    do you think her fellow engineers ever let her live that one down?
    I swear, you brits can make the simplest think sound dirty
    (I guess that why I like ya'll...lol)

  18. Jan 4, 2010 #17


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    I always supposed it was her fellow engineers who coined the term!
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