Compression Stroke Calculator
This calculator isolates the "Compression Stroke" phase of the Otto Cycle. It computes
values from BDC to TDC at 10 crankshaft degrees increments. Select an engine and
camshaft intake profile or enter your own values. Then click on the Calculate button.
The results table has the following meanings:
- Piston Pressure is the NET pressure on the piston top.
- Rod Angle is the angle in degrees between the bore centerline and the
line between the small & big rod end centers.
Side Thrust is the sidewards force on the piston due to Rod Angle.
Rod Force is the downwards force on the connecting rod due to NET
Piston Pressure over the area of the piston top through the Rod Angle.
It is coincident with the connecting rod
centerline acting on the crank pin journal centerline.
Effect Lever is the effective lever arm in inches. Its maxiumum value is
half of stroke which occurs at the optimum crank angle.
Effect Torque is the Rod Force times the Effect Lever.
Avg TQ is the average Area Under Curve of Effect Torque from BDC to TDC.
- At BDC the Piston Pressure is zero (0 PSI) with respect to
atmospheric pressure (14.7psi), i.e., no NET pressure effect on the piston. This is
because the intake valve is open greater than 0.050" lobe lift (see IVC ABDC
under Valve Events in camshaft specs). Effectively, compression pressure is all
leaking out the intake valve.
- Between IVC ABDC "Adv" and "050", the compression pressure on
the piston is only partly leaking out the intake valve.
- After IVC ABDC "Adv" event, there is no further leaking out the intake valve.
The compresson piston pressure is entirely due to piston motion up the cylinder, i.e., it
increases as the cylinder volume between the piston top and the combustion chamber decreases.
- After the spark ignition point (arbitrarily 30deg BTDC in this calculator), the compression
pressure additionally increases due to the air/fuel mixture starting to burn (combust).
NOTE 2: Avg TQ is "anti-torque" in the sense that it is subtracted from the Avg TQ
calculated in the "Power Stroke" calculator as a component of the net torque in the Otto Cycle.
See "Friction Calculator" for the other "anti-torque" component.
NOTE 3: Effects due to intake tract resonance, Volumetric Efficiency, spark timing changes,
piston ring leakage, etc. are ignored by this calculator.