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Internal Combustion Engines

  1. Jun 13, 2009 #1
    I am sure someone wrote with great detail on the subject, but i just can't seem to find it in the forums...

    I want to know about the parameters influencing the performance of an IC engine...obviously, not all parameters, but those we can conviently control

    specifically...how is bore/stroke (or the combination of both as displacement volume) related to the power output of an engine
    and what would be the other factors affecting the power?

    even more specifically, i want to make a small vehicle...something like 150Kgs in weight, and make it as fuel efficent as possible....i am looking to use single-cylinder motorcycle engine (i think it would be 70cc)

    thanks in advance
  2. jcsd
  3. Jun 13, 2009 #2
    The power out of an IC engine is P = T x (2 pi RPM/60) in watts
    where T is torque in Newton meters
    RPM = revolutions per minute
    Power = torque x (2 pi RPM/60)
    torque = BMEP V /4 pi (for 4-stroke engine)
    So power = (BMEP V /4 pi) x (2 pi RPM/60)

    where BMEP = brake mean effective pressure (in Pascals)
    V = displacement (in cubic meters) (= liters/1000)
    torque is in Newton-meters
    power is in watts
    Example for 2.7 liter engine
    BMEP=10 atm = 106 Pascals
    V=2.7 liters = 2.7 x 10-3 m3
    RPM = 4000
    so torque = 215 N m
    Power = 90,000 watts = 121 HP

    The engine typically is most efficient at 80% of max torque, and 35% of redline.
    The torque curve is flat, and drops off at high RPM because of intake valve and manifold restrictions, and at low RPM because the compressed air at the top of the compression stroke (and the ignited mixture) will cool by convection to the cylinder walls.
  4. Jun 14, 2009 #3
    Well in a broad sense practical things that affect performance of a given engine are:

    volumetric efficiency
    fuel air ratio
    spark advance

    There are many many others but those are the main ones that we can control most easily. The problem is without specific analysis on the engine you cent really tell what they will do to power as you'll find that as you positively change 1 thing is an engine it adversly affects something else. The key is striking he best balance.

    Obviously a bigger engine will produce more power but at a cost of increased fuel consumption.
  5. Jun 14, 2009 #4

    Ranger Mike

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    Science Advisor
    Gold Member

    see the form subject
    Horsepower - Please help - Confused! posted 14 Feb 2009
  6. Apr 17, 2011 #5
    can you go from bmep to net work?
  7. Apr 17, 2011 #6
  8. Apr 17, 2011 #7
  9. Apr 17, 2011 #8
    From url:

    Brake Mean Effective Pressure or bmep is, as usual, calculated from measured dynamometer torque. Indicated mean effective pressure or imep is calculated using the indicated power; i.e., the pressure volume integral in the work per cycle equation. Sometimes the term fmep (friction mean effective pressure) is used as an indicator of the mean effective pressure lost to friction (or friction torque) and is just the difference between imep and bmep.

    So basically bmep is mep minus friction.

    Bob S
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