Compound engines expand steam through a high-pressure stage and then a larger low-pressure stage. Hiscox figure 233 shows a paired arrangement for twin screws, with dotted upper cylinders indicating a tandem version.
The animation shows two tandem compound engines, each driving its own shaft. The calculator compares bores, geometric swept volumes and absolute pressure ratios. Those pressure ratios are not automatically volume-expansion ratios or efficiency predictions.
Compound Engines Interactive Calculator
Compare two tandem compound engines for twin screws. Enter cylinder bores, stroke and absolute pressures; inspect the separate shafts, rigid rods and pressure ratios.
Equation Used
- Two identical tandem compound engines with separate shafts.
- Common stroke and continuous piston rod for each HP/LP pair.
- Connecting-rod length4r; exact slider-crank closure.
- Absolute reservoir pressures only; no valve or thermodynamic cycle model.
Hiscox233 tandem option. Separate shafts; equal display speed and phase are illustrative. Pressure ratios are not volume expansion, steam consumption or efficiency.
Two tandem engines for two shafts
Each engine has an upper HP piston and lower LP piston on a continuous piston rod. The crosshead constrains the rod to vertical motion, while a connecting rod drives a crank on that engine’s shaft. The left and right shafts are separate: the frame between them does not gear or lock their rotations together.
The illustrated shafts counter-rotate at equal display speed with a chosen quarter-turn phase relationship. That is an animation convention, not a required phase or speed relationship for twin-screw machinery. The lower end-view diagrams identify the corresponding propeller shafts; blade number and shape are illustrative.
Steam flows from the boiler through the HP stage, then via a receiver to the LP stage and exhaust. The pressure strip displays the entered absolute reservoir pressures. Individual valve timing, chamber pressure and steam transfer rates are not simulated.
Compare dimensions and pressure levels
Changing HP or LP bore changes the corresponding cylinder widths, piston-area ratio and full-bore swept volume. Common stroke changes both tandem pistons’ travel. Each engine uses the same entered dimensions; the outputs are per cylinder on each engine, not totals for the pair.
Use absolute pressures in psia. A gauge-pressure value must first be converted using the appropriate atmospheric reference. If the chosen pressures do not decrease from boiler through receiver to exhaust, the calculator flags the ordering while retaining the diagram and numerical ratios.
Geometry and pressure ratios
LP/HP area=(DLP/DHP)²; swept volume V=πD²S/4
These are full-bore head-side volumes per stroke. They do not deduct piston-rod area or include clearance. With equal piston strokes the swept-volume ratio equals the area ratio.
HP pressure ratio=P1/P2; LP pressure ratio=P2/P3; overall pressure ratio=P1/P3
All pressures are absolute. The two stage pressure ratios multiply to the overall pressure ratio. Volume expansion also depends on the thermodynamic path, cutoff and clearance, so pressure ratios alone are not an expansion or efficiency calculation.
The crank radius is r=S/2 and the connecting rod is L=4r. For crank angle θ, crosshead height relative to the shaft is z=r sinθ+√(L²−r²cos²θ). The two pistons on its common rod have fixed vertical offsets 3r and 7r. Both therefore travel exactly S.
Default comparison
With 18 inch HP and 36 inch LP bores, the LP/HP area ratio is four. A 24 inch stroke gives approximately 6,107.26 in³ HP and 24,429.02 in³ LP full-bore swept volume per stroke on each engine.
At 180 psia boiler, 50 psia receiver and 5 psia exhaust, the pressure ratios are 3.6, 10 and 36. These describe the specified pressure levels; they do not establish a steam-consumption rate, expansion volume or shaft power.
What the illustration shows
The construction reference is Hiscox figure 233, including its dotted tandem-cylinder option. The cutaway cylinder spacing, four-to-one rod ratio, frame and paired propeller end views are explicit explanatory choices. They do not reproduce a particular ship’s dimensions or performance.
Steam routing and pressure readings are a static operating overview, separate from the rigid linkage animation. No valve sequence, chamber pressure law, torque, balancing, propeller thrust or efficiency is inferred.
Use the pressure ratios to compare the boiler, receiver and exhaust pressure levels. Predicting expansion through each cylinder or engine efficiency also requires valve timing and steam-cycle data.
Compound-engine questions
Are the engines on one shaft?
No. This twin-screw arrangement has one shaft per engine. Each tandem HP/LP pair shares a piston rod, but the two engines are mechanically separate.
Why are the pressure units psia?
Pressure ratios require an absolute reference. Dividing gauge pressures would give a different and generally unsuitable ratio.
Does pressure ratio equal volume expansion?
Not by itself. The thermodynamic process and valve events must also be specified.
What happens with a smaller LP bore?
The diagram and volume outputs remain available, with a note that the selected bores reverse the usual larger-LP arrangement.
Can these inputs predict power?
No. Steam-cycle data, speed and losses would be needed. These outputs are geometry and pressure comparisons.
Construction reference
Hiscox, Mechanical Movements, Powers, Devices and Appliances, printed page 70, figure 233: compound engines for twin screws, with dotted cylinders for the tandem model. Figure 231 on the same page illustrates a continuous piston rod in a tandem vertical engine.
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