For selected mean effective pressure pme and swept volume per revolution Vrev, indicated work is Wrev = pmeVrev, mean indicated torque is Wrev/(2Ï€), and indicated power is WrevN/60. The animation shows the rotating-cylinder arrangement described by Hiscox, with illustrative dimensions.
Ruth's Engine: Motion and Indicated Output
Explore three rotating cylinders and calculate indicated work, mean torque and power from selected pressure and swept volume.
Equation Used
- Mean effective pressure is a valid net cycle-average value, not supply pressure.
- Swept volume is total active volume displaced per revolution.
- Selected speed is steady and nonnegative.
- Leakage, friction, auxiliaries, transients and ratings are excluded. The animation uses illustrative dimensions.
The animation shows three pistons following a circular guide around an offset shaft. The entered swept volume is independent of the illustration.
Three cylinders and an eccentric guide ring
Three cylinders radiate from a rotating shaft that is offset from the center of a fixed outer ring. Each piston is connected to a follower at its outer end. As the cylinder assembly turns, the followers travel around the ring and the pistons slide in and out of their bores.
Hiscox describes steam admission just beyond the shortest radius, followed by outward piston travel. Exhaust opens for the return, when the guide ring pushes the piston inward. The cutaway illustrates that geometry; it does not simulate detailed port timing or pressure.
What the calculator determines
Select mean effective pressure, total swept volume per revolution and speed. The calculator returns indicated work per revolution, mean indicated torque, indicated power in kilowatts and the same power in horsepower.
It does not predict mean effective pressure from supply pressure, geometry, porting, expansion, leakage or speed and does not calculate brake output, steam consumption, efficiency, shaft load or safe operation.
Selected indicated-output identities
Wrev = pmeVrev. Ti,mean = Wrev/(2Ï€). Pi = WrevN/60, with N in revolutions per minute and the division by 60 converting to revolutions per second.
Mean effective pressure is a selected cycle-average value, not inlet or boiler pressure. Swept volume is the selected total active volume displaced per shaft revolution.
Selected-input example
For 5 bar mean effective pressure and 1 L/rev selected swept volume, indicated work is 500 J/rev and mean indicated torque is about 79.58 N·m. At 500 rpm, indicated power is about 4.167 kW or 5.587 hp.
These are indicated identities from selected inputs, not predicted brake output for a historical engine.
Using the indicated-output results
Mean effective pressure summarizes net indicated work over a cycle. Use a measured or otherwise justified value for the engine and operating condition; inlet pressure alone is not enough to determine it.
The drawing uses illustrative proportions and does not set cylinder bore, stroke or total swept volume. Mechanical friction and auxiliary loads reduce the shaft power below the indicated power calculated here.
Questions
Is mean effective pressure the selected supply pressure?
No. It is a net cycle-average pressure that must already account for the actual pressure history.
Is mean indicated torque instantaneous torque?
No. It is work per revolution divided by 2Ï€.
Does the animation define swept volume?
No. The drawing is normalized and the entered swept volume is independent.
Does indicated power equal shaft output?
No. Mechanical and auxiliary losses must be subtracted to obtain brake output.
Technical references
Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances (1901), page 95, item 347.
- OpenStax University Physics, Work — work as force through displacement and energy units.
- OpenStax Physics, Angular Velocity — rotational speed relationships.
- NIST Special Publication 811 — SI units and conversions.
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