Required force to rotate the Engine piston

  1. Dears, please help me:

    How to Calculate the Required Force to Push the Car Engine Piston Down if I Want to Rotate my shaft in 1500rpm, My Piston Dia Is 50mm, Piston Hight Is 100mm, Piston Mass Is 1kg, My Engine Is 4cylender and 4stroke?

  2. jcsd
  3. Simon Bridge

    Simon Bridge 15,474
    Science Advisor
    Homework Helper
    Gold Member

    Welcome to PF;
    Unless the piston is right at the top - it should fall all by itself.
    You need to find out what is stopping this.

    In a 4-cylinder engine, it is probably attached to three other pistons, via the crank, so pushing one down means lifting another one and variously moving the other two ... this usually means that the pistons won't move by themselves as they are arranged to be balanced. You will have to overcome the friction (and drag) between the components - it's the sort of thing you measure rather than calculate.

    If the car is in gear - then the engine is attached to a load ... the additional force depends on the load.

    What do you need to know this force for?
  4. The easier way to do this is to use these formulas:

    BMEP (Four-Stroke Piston Engine) in PSI = [(150.8 x Ft-Lbs Torque) / (CID)]

    CID = Cubic Inch Displacement of entire piston engine

    BMEP (Two-Stroke Piston Engine) in PSI = [(75.4 x Ft-Lbs Torque) / (CID)]

    CID = Cubic Inch Displacement of entire piston engine

    Brake Horsepower (BHP) = [(BMEP x L x A x N x K) / (33,000)]

    BMEP = Average Gas Pressure Exerted On Piston Head Area in PSI per powerstroke
    L = Piston Stroke Length in Feet.
    A = Piston Head Area in Square Inches = [(Piston Diameter / 2)² x (Π)]
    N = Number of Powerstrokes Per Minute ==> Four-Stroke = RPM / 2
    ==> Two Stroke = RPM
    K = Number of Cylinders

    Ft-Lbs Torque at Crankshaft (4-Stroke Piston Engine) = [(BMEP x CID) / (150.8)]

    Ft-Lbs Torque at Crankshaft (2-Stroke Piston Engine) = [(BMEP x CID) / (75.4)]

    Force (pa) = [(Pressure) x (Area)]

    Force in Lbs
    Pressure in PSI (Lbs/Sq In)
    Area in Square Inches

    Force (ma) = [(Mass) x (Acceleration)]

    Force in Newtons
    Mass in kilograms
    Acceleration in meter/sec²

    1 kilogram = 2.2 Lbs
    1 meter = 3.28 feet
    1 Lbs = 4.45 Newtons

    Piston Acceleration (G-Max) =

    [(N² x L) / (2,189) x [(1) + (1 / (2A))]
    N = crankshaft speed (RPM)
    L = stroke in inches
    A = the ratio of the connecting rod length, (length between the center of crankpin to the center of the wristpin) to the piston stroke length.

    Total Force = [Force (pa)] + [Force (ma)]

    Use Lbs values for both Force types and add together for Total Force

    Work = Force x Distance

    Work in Ft-Lbs
    Force in Lbs
    Distance in Feet

    Power = [(Force x Distance) / (Time)]

    Power in Ft-Lbs/Sec ==> 550 Ft-Lbs/Sec = 1 Horsepower = 746 Watts = 746 Joule/Sec.
    Force in Lbs
    Distance in Feet
    Time in Seconds

    Instead of figuring out what linear force on piston head is required to develop a certain torque and RPM at the crankshaft the better way to do this is to use the formulas up above. They translate the linear force on the piston head (based on BMEP & Piston Head Area) into Torque at the crankshaft.


    - MisterDynamics -

    January 08, 2014
    lonely_nucleus likes this.
  5. Mister Dynamics - What does 150.8 and 75.4 represent in your equations for the two and four stroke engines. Thanks.
  6. Never mind. Understood.
  7. Help plz
    My engine specification
    Whether calculated force is right or wrong....
  8. jack action

    jack action 892
    Science Advisor
    Gold Member

    Actually, the radius is equal to half the stroke:

  9. Thnk u
  10. If engine having 8 Nm torque,whenever crank shaft rotates torque acting on each point or angle(means at angle theta of 30,45,90,135....)is sam or different ????
    Is it ri8????abov eqn
  11. jack action

    jack action 892
    Science Advisor
    Gold Member

    The 8 N.m is an average torque for an entire cycle. So, yes, the torque will vary depending on the crank position:

    [​IMG] torque.jpg
    green: single cylinder;
    purple: 4-cylinder;
    black: 8-cylinder.

    More info in this thread.
  12. How can i calculate movement of piston with rotation of crankshaft????
  14. jack action

    jack action 892
    Science Advisor
    Gold Member

    Click on (more) next to «Connecting rod length» a the bottom of this page to find the relation between the two.
  15. Plz help...
    electromagnet having iron core with length of "L" thn for calculating force exerted by electromagnetic on any material given below is right formula...plz help
    N=no. Of turn
    I=current in A
    G=Distance upto which force has to cal
    L=length of core
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