Indicated horsepower is the rate of work delivered to a piston, computed from mean effective pressure, stroke, piston area and working strokes per minute. This calculator applies that identity to two identical assumed double-acting cylinders.
Twin-screw Vertical Cylinder Engine Interactive Calculator
Enter assumed mean effective pressure, cylinder bore, piston stroke and crank speed. Results are ideal indicated arithmetic for one cylinder and two identical cylinders—not brake power, propeller power or the operating performance of a particular marine engine.
Equation Used
- Each represented unit is one ideal double-acting cylinder.
- Both units use identical entered bore, stroke, speed and mean effective pressure.
- The entered pressure is already mean effective pressure over a cycle.
- One mechanical horsepower is 33,000 ft-lbf/min.
- Valve timing, cutoff, expansion, condensation, clearance volume, rod area, friction, leakage, inertia, coupling, crank phase, compound staging, shaft efficiency, propeller load and structural limits are omitted.
Vertical Cylinder, Connecting Rod and Shaft
The Smithsonian describes a vertical steam engine as a cylinder above a frame, with its piston rod and connecting rod turning a crank on the shaft. Its collection also documents twin-screw arrangements using parallel shafts for marine propellers.
The animation shows two equal schematic cylinder/shaft units. Their phase is illustrative only.
What This Calculator Represents
The tool evaluates the classic PLAN identity for one entered double-acting cylinder and doubles that result for two identical assumed units. It also reports mean piston speed and the simple product of entered MEP and piston area.
It does not predict an actual engine's indicator diagram, coupling, shaft output or propeller performance.
Ideal Indicated-Power Arithmetic
working strokes per minute N = 2 × RPM
one-cylinder indicated hp = p × L × A × N / 33,000
two-cylinder indicated hp = 2 × one-cylinder result
mean piston speed = 2 × L × RPM
mean pressure force = p × A
Worked Arithmetic Example
At entered MEP 80 psi, 6 in bore, 7.2 in stroke and 350 rpm, piston area is about 28.27 in² and N is 700 working strokes per minute. The result is about 28.8 indicated hp for one assumed cylinder/shaft unit and 57.6 indicated hp for two identical units. Mean piston speed is 420 ft/min and pA is about 2,262 lbf.
Model Boundary
- Calculated: ideal mean-value PLAN arithmetic from entered values.
- Illustrated: two equal vertical cylinder/shaft units.
- Not calculated: indicator-card shape, valve timing, cutoff, compound expansion, condensation, clearance, piston-rod area, friction, leakage, mechanical efficiency, brake power, crank phase, coupling, shaft torsion, propeller thrust, fuel or steam rate, strength or safety.
Frequently Asked Questions
Is the pressure boiler pressure?
No. It is entered mean effective pressure over the cycle.
Why is N twice RPM?
The model assumes one working stroke on each side of a double-acting piston per revolution.
Is IHP per shaft measured shaft power?
No. It is the ideal indicated result for one assumed cylinder/shaft unit.
Why is total IHP doubled?
The model assumes two identical units at the same entered operating point.
Is the illustrated phase a design claim?
No.
Can this size an engine or propeller?
No.
References
- Steam and Steam Engines, University of Chicago Library scan, sec. 1269: PLAN/33,000, double-acting N=2 RPM example and piston-travel relation.
- Smithsonian National Museum of American History, Model of a Bottle-Frame Steam Engine: vertical cylinder, piston rod, connecting rod and crankshaft arrangement.
- Smithsonian National Museum of American History, Loper's Patent Model of a Steam Engine: parallel crankshafts and twin marine propellers.
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