An oscillating-cylinder engine rocks about a trunnion as its piston rod follows the crank. Explore the port-face principle and calculate ideal circumferential overlap between a moving port and fixed admission and exhaust slots at an entered angle.
Declared Port-Face Overlap Snapshot
Enter a declared cylinder angle, face radius, equal slot width and fixed-port angular offset. The worksheet calculates ideal one-dimensional circumferential overlap only; it does not predict timing, flow, pressure, leakage, torque or power.
Equation Used
- Cylinder angle phi and fixed-port offset beta are declared snapshot angles.
- R is the declared radius from trunnion center to the port-center arc and w is the equal circumferential width of the moving and fixed slots.
- Overlap is a one-dimensional arc-length intersection, not open area or discharge coefficient.
- Axial width, port shape, edge rounding, clearance, pressure, flow, cutoff, lead, compression, leakage, heat, friction, loads, speed, power and safety are excluded.
The schematic illustrates cylinder rocking and the port-face arrangement. The calculator separately evaluates equal-width slot overlap at the entered angle; animation phase does not set that angle.
The crank makes the cylinder rock across a stationary port face
In a simple oscillating-cylinder engine, the cylinder pivots on a trunnion while the piston rod is connected directly to a crank. As the crank rotates, the piston reciprocates and the cylinder axis follows the crank pin, so the whole cylinder rocks.
US Patent 4,114,514 documents an oscillating cylinder whose side slides on a stationary valve plate, alternately bringing the cylinder port into communication with supply and exhaust passages. Historical marine forms also conducted steam through hollow trunnions, as documented by ASME's John Penn engine landmark.
A historical steam-engine topology
ASME identifies the Diesbar engine, built by John Penn & Sons in 1841, as a two-cylinder oscillating steam engine and states that steam enters through outboard trunnions and exhausts through inboard trunnions.
This page explains topology and ideal port overlap only. It does not reproduce Penn valve gear or select, design, restore or operate a pressure vessel or steam engine.
Ideal circumferential interval overlap
s is the declared moving-port center arc, while the fixed admission and exhaust centers are +b and -b. Equal-width intervals overlap by w minus their center separation, floored at zero.
Declared snapshot example
For phi=12 degrees, R=50 mm, w=12 mm and beta=14 degrees, the moving center is 10.47 mm, each fixed center is 12.22 mm from neutral, admission overlap is 10.25 mm and exhaust overlap is 0.00 mm.
Those lengths do not predict port area, mass flow or engine timing.
What the snapshot and animation omit
The animation is a connected explanatory wobbler linkage, not a dimensioned Penn engine. The calculator is a local unwrapped-face interval model independent of the animation.
Overlap length alone does not determine open port area, flow, pressure or engine power. Those require the full port geometry and operating conditions, while actual valve timing requires the cylinder motion through the cycle.
Oscillating-cylinder questions
Does the cylinder itself move?
Yes. It rocks about the fixed trunnion as the crank-connected piston rod changes direction.
Does overlap equal open area?
No.
Does this predict admission cutoff or exhaust timing?
No.
Does it calculate leakage or power loss?
No.
Is this steam-engine construction or operating guidance?
No.
Oscillating-engine references
- US Patent 4,114,514, Oscillating steam engines: cylinder rocking on trunnions and a cylinder-side port sliding on a stationary valve plate to communicate alternately with supply and exhaust.
- ASME Landmark 245, Oscillating Steam Engine: authoritative history of the 1841 John Penn & Sons Diesbar engine and its inboard/outboard trunnion steam paths.
- US Patent 19,057, Oscillating steam-engine: primary patent description of ports positioned relative to the oscillation axis for admission and exhaust communication.
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