Question

Draw a free body diagram of a piston, connecting rod and crank shaft. Indicate on the
diagram the effective forces acting on the piston and the crank shaft during the expansion
stroke. Now let’s do some sample calculations assume 2500 RPM and neglecting friction.

a. For a spark ignition engine with a 150mm bore, a compression ratio ofr c = 10 and a
maximum cylinder pressure of 20b abr at TDC, calculate the vertical and horizontal
forces on the piston and the torque on the crank shaft when the crank shaft is at θ = 0°,
45° and 90°. You can assume that the S/B = 1 and l/a = 3, that the piston has a mass of
0.75kg, and for simplicity that the expansion of theas hforot mg TDC to BDC is
isentropic with k =1.4.

Answer 1

When a piston reciprocates inside an engine, there are three differentr types of forces acting on it.

1. Inertia force due to mass of piston
2. Frictional force acting along the wall of cylinder.
3. gas force due to pressure of gas exerted on top of the piston