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What is the definition of Engine Load

  1. Jul 27, 2013 #1
    What is the definition of "Engine Load"

    A lot of things I have read make reference to engine load but it is not clear if there are different meanings.

    One definition seems to be that the engine load is the amount of air flowing through the engine as a percentage of the theoretical maximum.

    So if I had a vehicle running in neutral with the throttle wide open I would be very close to the maximum of that type of "engine load". But the engine wouldn't really be doing any work. So if work was the type of load that we are talking about then it's different.

    Like towing a trailer up a grade is another type of engine load, and it has more to do with how much resistance there is to turning the crankshaft.

    I think perhaps the actual power output is what's different, the amount of WORK being performed.

    And from what I can tell running an engine under a heavy work load produces different conditions like higher cylinder temperatures and peak pressures.

    Engine Load is an OBD-II standard parameter but I still don't understand how to interpret it properly.

    I almost forgot to mention it but the amount of vacuum in the intake manifold is often said to indicate the engine loading. I'm not really sure why. From what I know manifold vacuum is affected by the engine speed, the atmospheric pressure, and the throttle position mainly (in naturally aspirated engines).

    High vacuum may indicate a high rate of air consumption by the engine, or high restriction from the throttle. Low vacuum could indicate lower consumption by the engine or highly open throttle conditions.

    Vacuum is complicated by the engine characteristics that affect cylinder filling like valve timing lift and duration, engine speed, tuned ports etc.

    I see that these things are all correlated to the concept of engine load but dont fully understand the rationale. Fuel pressure regulators are controlled using manifold vacuum for example.

    One other thing, engine load seems closely tied to how the engine utilized, in a passenger car the engine probably operates at very light loads the vast majority of the time, putting out only enough power to maintain the current speed. Load would be higher when accelerating the vehicle quickly such as driving onto a highway via an on ramp, but would generally be low, with a correspondingly low throttle opening. Heavier vehicles towing heavy loads may operate at higher sustained engine loading. Lastly from what I've heard aircraft engines typically operate at a high load with little fluctuation.

    It strikes me that the transmission gearing is as significant the throttle position and other items when considering engine load and total power output.

    Thanks,
    -Andrew
     
    Last edited: Jul 27, 2013
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  3. Jul 27, 2013 #2

    SteamKing

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    "One definition seems to be that the engine load is the amount of air flowing through the engine as a percentage of the theoretical maximum."

    I don't know where you got that definition of engine load. The statement is, however, the definition of the volumetric efficiency of an engine.

    When running, an engine is always producing torque, and power is proportional to the amount of torque produced times the rotational speed of the engine. Whether this power is actually used to move the vehicle or tow something, for instance, is another matter entirely.

    "It strikes me that the transmission gearing is as significant the throttle position and other items when considering engine load and total power output."

    All that transmission gearing does is reduce the output RPM from the engine to the wheels while also multiplying the torque from the engine. Except for losses in the transmission, the engine power output remains roughly the same, for manual gearboxes. Automatic transmissions take some of the engine power to operate the hydraulics, separate from whatever gear ratios are sued.

    Aircraft engines see their heaviest loading on takeoff. You are trying to accelerate a fully laden aircraft from stop to takeoff speed over a limited distance. Once the aircraft has taken off and climbed to its cruising altitude, then the engine is operating at a somewhat reduced load.
     
  4. Jul 27, 2013 #3
    "I don't know where you got that definition of engine load. The statement is, however, the definition of the volumetric efficiency of an engine."

    From http://obdcon.sourceforge.net/2010/06/about-pid-calculated-load-value/

    The OBD regulations previously defined CLV as:
    (current airflow / peak airflow @sea level) * (BARO @ sea level / BARO) * 100%

    LOAD_PCT = [current airflow] / [(peak airflow at WOT@STP as a function of rpm) * (BARO/29.92) * SQRT(298/(AAT+273))]

    And from the EPA, page six:
    http://www.epa.gov/ttnchie1/conference/ei20/session8/aalessandrini_pres.pdf

    and others.

    "All that transmission gearing does is reduce the output RPM from the engine to the wheels while also multiplying the torque from the engine."

    The volumetric efficiency of the engine is closely related to the engine speed, so wouldn't changing the engine speed have an impact on the airflow through the engine just like changing throttle position?
     
  5. Jul 30, 2013 #4

    jack action

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    The engine load is the torque output of the engine.

    An internal combustion engine is - approximately - a constant torque motor, meaning it can produce the same maximum torque at any rpm. The fact that it happens at a rpm or another, dictates how much power it produces.

    Torque is proportional to the amount of force put on the piston, which is proportional to the amount of air being burnt in the combustion chamber, which is why you can relate the engine load to the quantity of air going into the engine, knowing the maximum amount of air that go in at a particular rpm.

    With the equation found on page 6 of your EPA document, it is clearly written about the OBD parameter:
    It also says:
    If you measure vacuum in the intake manifold, you will have high values when the throttle is closed. As the throttle opens, the vacuum will lower (meaning the pressure comes closer to the atmospheric pressure) as the pressure from the intake and the outside atmosphere tends to equalized. When this condition is reached, then there is no restriction to the airflow and maximum torque is achieved at that particular rpm (No matter how wide the throttle is opened, so the throttle angle is not as good of a measurement).

    Of course, ODB parameter and vacuum does not give you the torque output of the engine, but knowing the peak torque is practically constant throughout the rpm range, you can find a good approximation of the torque output of an engine with these parameters. Multiply this torque with the rpm and you get the power output as well.
     
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