Register to reply

Required force to rotate the Engine piston

by ibrahim almalk
Tags: engine, force, piston, piston force, required, rotate
Share this thread:
ibrahim almalk
Dec14-13, 06:43 AM
P: 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?

Phys.Org News Partner Engineering news on
DIY glove-based tutor indicates muscle-memory potential
Tricorder XPRIZE: 10 teams advance in global competition to develop consumer-focused diagnostic device
Study shows local seismic isolation and damping methods provide optimal protection for essential computing equipment
Simon Bridge
Dec14-13, 11:40 PM
Sci Advisor
HW Helper
Simon Bridge's Avatar
P: 12,961
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?
Jan7-14, 11:22 PM
P: 16
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) 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/sec

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

Piston Acceleration (G-Max) =

[(N 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

Register to reply

Related Discussions
Statics, determine the force required to rotate a body Introductory Physics Homework 2
Force required to rotate spindle. Mechanical Engineering 1
Force pushing against piston in car engine General Engineering 7
What force is required to rotate a space ship? General Physics 8
Force must be applied to the small piston to lift the engine? Introductory Physics Homework 1