A compound oscillating engine combines staged steam expansion with cylinders that rock about fixed trunnions. The illustrated V arrangement has two straight piston rods connected to one crankpin. Each cylinder body remains rigid while its piston slides inside. The calculator compares swept volume, a selected receiver volume, LP bore and gross HP pressure-area force.
How actuator stroke affects Compound Oscillating Engine Mechanism
With compound oscillating engine mechanism, the motion profile matters. A linear actuator gives direct extension and retraction, but the output linkage may still create non-linear speed, changing mechanical advantage, and end-of-travel loads.
For oscillating motion, check the reversal points. The actuator sees a change in load direction at each end of the sweep, and that is where loose joints, flexible brackets, or marginal controls usually show up first.
The mechanism may also change speed as the linkage angle changes. A constant actuator extension speed does not always create constant output speed, so timing should be checked at the actual output point.
- Map actuator stroke to the actual output travel before choosing a model.
- Check for binding, changing leverage, and peak load near the ends of travel.
- Decide whether timed motion is acceptable or position feedback is required.
For a FIRGELLI design review, calculate the load case with the FIRGELLI actuator force calculator and plan the switching or controller arrangement with the linear actuator wiring diagram generator. If an actuator or cylinder is already installed, the linear actuator replacement finder helps keep the comparison tied to real dimensions.
Compound Oscillating Engine Interactive Calculator
Change bore, crank throw and selected volume ratios. The cylinders rock without stretching while their pistons drive a common crankpin. The pressure-area result is a gross full-face comparison; receiver back pressure and other losses are not supplied.
Equation Used
- Rigid cylinder bodies rotate about fixed trunnions; straight piston rods share one crankpin.
- Both cylinders have piston stroke twice the entered crank throw, and their central axes are at right angles.
- Bores and stroke use one drawing scale. Support spacing and piston-rod lengths are generated for illustration, not design recommendations.
- No cutoff, valve timing, receiver pressure, steam properties, engine speed or material strength is entered.
- Receiver volume is the chosen multiple of HP swept volume; its pressure rating and physical dimensions are not calculated.
The cylinder barrels rock about fixed trunnions while sliding pistons and constant-length rods drive a shared crankpin. Compare bore, stroke and volume ratios. The pressure-area result is gross full-face force before back pressure and rod-area deductions.
How the rocking cylinders remain connected
The crankpin follows a circle. Each cylinder points along the line from its fixed trunnion to that pin. As the pin rotates, the barrel changes angle and the piston moves within the barrel. The piston rod keeps a constant length; neither the cylinder body nor rod stretches.
The two central cylinder axes are at right angles, following the arrangement in Hiscox Figure 227. A cutaway exposes the pistons and rods. Supports below the trunnions and the crankshaft connect the mechanism to the base.
The flow strip identifies supply, HP stage, receiver, LP stage and final exhaust. It does not represent port opening, cutoff or pressure changes through the working cycle. The animation is a geometric interpretation of the historical arrangement rather than a reconstruction of a specific engine.
What this comparison can be used for
The inputs let a reader explore how bore and crank throw change swept volume, how a selected receiver factor changes a volume target, and how a bore ratio changes the second cylinder’s size.
These geometric comparisons are useful for understanding the arrangement, but do not establish a practical engine design. Valve events, actual pressures, materials, sealing, bearing loads and operating conditions remain unspecified.
Geometry and gross force
Crank throw T gives piston stroke S = 2T. The HP swept volume is VHP = πDHP²S/4. Selecting receiver factor k gives Vreceiver = kVHP. Selecting bore ratio r gives DLP = rDHP. The LP/HP swept-volume ratio at equal stroke is therefore r², not r.
The gross pressure-area force is Fgross = PπDHP²/4. With supply pressure in psig, this is a full-face gross comparison before back-pressure or rod-area deductions. A working HP cylinder exhausts to a lower-pressure stage, so this gross quantity must not be described as its net working force or average power.
The animation solves the exact distance from each fixed trunnion to the crankpin and subtracts a constant rod length to position the piston. That construction gives a total piston travel of 2T.
Example: the default small-engine dimensions
For a 0.625-inch HP bore and 0.5-inch crank throw, stroke is 1 inch and HP swept volume is about 0.307 in³. A selected receiver factor of 1.5 gives about 0.460 in³. A bore ratio of 1.8 gives an LP bore of 1.125 inches.
At 80 psig, multiplying supply pressure by the full HP piston area gives about 24.54 lbf gross. Net force cannot be found without the opposing pressure and effective areas. The selected receiver volume is a geometric target, not a validated vessel size or pressure rating.
Limits of the illustration
Rocking the cylinder lets a straight piston rod follow the crank without a separate connecting rod and crosshead. The resulting angular motion must still be accommodated by real trunnions, seals and steam connections.
The page does not calculate those details or simulate thermodynamic performance. At extreme bore-to-stroke ratios, the animation shows the entered proportions without claiming they form a practical or efficient engine.
Questions about the oscillator
Why do the cylinders move?
Each barrel points from its fixed trunnion toward the moving crankpin so its straight piston rod stays aligned.
Do the cylinder bodies change length?
No. The barrels and piston rods are rigid in the model. Only the piston’s position within each barrel changes.
Why do both rods meet the same pin?
This is the shared-crankpin V arrangement represented in the historical reference, with the central cylinder axes at right angles.
Is the receiver factor an optimum?
No. It is a selected multiple of HP swept volume; no pressure or steam-flow optimization is performed.
Is the force output net engine force?
No. It is the gross pressure-area product before back pressure, rod area and losses are deducted.
Reference
Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances, printed page 69, Figure 227 (PDF page 79): compound oscillating engine with cylinders at right angles. Figure 226 above it illustrates a cylinder with central trunnions.
Frame dimensions and displayed support spacing are illustrative and are not copied as manufacturing dimensions from the reference.
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