Dissolving the piston force

Subject:

  • Dissolving the piston force

Dissolving the piston force:
A pressure (p) is created above the piston as a result of combustion during the power stroke. The resulting force is transmitted by the connecting rod to the crank pin. The resulting moment (force * arm) causes the crankshaft to rotate. This is where it originates torque (torque) of the engine.

The five images below show the forces created in the crank-connecting rod mechanism when the combustion pressure pushes the piston down. An explanation of the interplay of forces is then described for each enlarged image.

1. The combustion pressure (p) creates a force on the piston (Fz) and the main bearing (Fh). We can determine this force on the basis of the indicator diagram.

The power Fz is passed on to the connecting rod. In this situation, the connecting rod is perpendicular above the crank pin and main bearing (Fh). The actual combustion, at which the pressure p is maximum, takes place approximately around 8 crankshaft degrees after TDC. The power Fz is equal to the force applied to the connecting rod and main bearings of the crankshaft. So we can write: Fz = Fh.

In this image we see a yellow dashed line running from the center of the crankshaft to the center of the crankpin. The connecting rod circles around this.

In the following pictures, the forces that arise when the connecting rod tilts are dissolved.

Overview of the abbreviations used:

  • p: combustion pressure;
  • Fz: piston force;
  • Fh: force on the main bearing
1. The piston is at TDC.

2. The force from the piston is transferred through the connecting rod to the crank pin. In doing so, we dissolve the force on the piston (Fz) and on connecting rod force (Fd) running in the direction of the connecting rod.

Due to the connecting rod tilt and force Fz, the piston is pressed against the cylinder wall. This force is denoted by Fl (slide force). This is where the piston and cylinder wear the hardest. 

The connecting rod force Fd acts on the crank pin and decomposes as a result of the twist on the crank pin in the tangential circumferential force (Ft) and the radial force on the main bearing (Fh). The radial force transmits the force through the upper connecting rod bearing to the crankshaft.

The tangential force (Ft) depends on both the connecting rod force and the position of the crank connecting rod mechanism. Because the tangential force determines the torque (torque) of the motor, the torque is constantly changing magnitude. The mass of the flywheel ensures that these changes in torque do not directly affect the crankshaft rotational speed.

Overview of the abbreviations used:

  • p: combustion pressure;
  • Fz: piston force;
  • Fd: connecting rod force;
  • Fl: slide force;
  • Fh: force on the main bearing;
  • Ft: tangential force.
2. The crankshaft has been turned 45 degrees.

3. The center lines of the connecting rod and crank pin are at a 90 degree angle to each other. The tangential force (Ft) is the same as the connecting rod force (Fd) and, like the obtained torque, highest at this point. So we can write: Fd = Ft.

The main bearing is now not loaded. There is now no question of the force Fh. In its stead, gives "r” the distance between the center of the crankshaft and the center of the crank pin.

Overview of the abbreviations used:

  • p: combustion pressure;
  • Fz: piston force;
  • Fd: connecting rod force;
  • Fl: slide force;
  • Fh: force on the main bearing;
  • r: distance between crankshaft center and connecting rod center.
3. The crankshaft has been turned nearly 90 degrees.

4. As the crankshaft rotates further, the tangential force decreases (Ft). The tangential force is now no longer in line with the connecting rod force.

The guideway force (Fl) has now increased, because the angle at which the connecting rod now stands is maximum.

Overview of the abbreviations used:

  • p: combustion pressure;
  • Fz: piston force;
  • Fd: connecting rod force;
  • Fl: slide force;
  • Fh: force on the main bearing;
  • Ft: tangential force.
4. The crankshaft has been turned 90 degrees.

5. The piston continues to move towards the ODP. The force on the main bearing (Fh) increases and is maximum when the piston has reached the ODP all the way.

Also, the guideway force (Fl) decreased; this force becomes 0 N the moment the piston reaches the ODP.

Overview of the abbreviations used:

  • p: combustion pressure;
  • Fz: piston force;
  • Fd: connecting rod force;
  • Fl: slide force;
  • Fh: force on the main bearing;
  • Ft: tangential force.
5. The piston is 45 crankshaft degrees in front of the ODP.
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